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Article Contents

Introduction, origin and evolution of wheat, cultivated wheats today, why has wheat been so successful, wheat gluten proteins and processing properties, wheat in nutrition and health, adverse reactions to wheat, the future for wheat.

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P. R. Shewry, Wheat, Journal of Experimental Botany , Volume 60, Issue 6, April 2009, Pages 1537–1553, https://doi.org/10.1093/jxb/erp058

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Wheat is the dominant crop in temperate countries being used for human food and livestock feed. Its success depends partly on its adaptability and high yield potential but also on the gluten protein fraction which confers the viscoelastic properties that allow dough to be processed into bread, pasta, noodles, and other food products. Wheat also contributes essential amino acids, minerals, and vitamins, and beneficial phytochemicals and dietary fibre components to the human diet, and these are particularly enriched in whole-grain products. However, wheat products are also known or suggested to be responsible for a number of adverse reactions in humans, including intolerances (notably coeliac disease) and allergies (respiratory and food). Current and future concerns include sustaining wheat production and quality with reduced inputs of agrochemicals and developing lines with enhanced quality for specific end-uses, notably for biofuels and human nutrition.

Wheat is counted among the ‘big three’ cereal crops, with over 600 million tonnes being harvested annually. For example, in 2007, the total world harvest was about 607 m tonnes compared with 652 m tonnes of rice and 785 m tonnes of maize ( http://faostat.fao.org/ ). However, wheat is unrivalled in its range of cultivation, from 67º N in Scandinavia and Russia to 45º S in Argentina, including elevated regions in the tropics and sub-tropics ( Feldman, 1995 ). It is also unrivalled in its range of diversity and the extent to which it has become embedded in the culture and even the religion of diverse societies.

Most readers will be aware of the significance of bread in the Judaeo-Christian tradition including the use of matzo (hard flat bread) at the Jewish Passover and of bread to represent the ‘host’ at the Christian Eucharist (Holy Communion). The latter may be a thin unleavened wafer, similar to the Jewish matzo, in the Roman Catholic Church and some Protestant denominations, or leavened in other Protestant denominations and the Eastern Orthodox Church. But how many readers are aware that bread is treated as sacred in everyday life in the largely Muslim communities of Central Asia, such as Uzbekistan and Kyrgyzstan? In this culture, the leavened round breads (nan) are stamped before baking and must be treated with respect, including being kept upright and never left on the ground or thrown away in public. These customs almost certainly originate from earlier indigenous religions in the Middle East in which wheat played a similar role and was sometimes equated with the sun and its god.

Although such cultural and religious traditions are fascinating and will certainly reward further study, they are essentially outside the scope of this article which will examine why wheat has developed and continues to be so successful as a crop and food source.

The first cultivation of wheat occurred about 10 000 years ago, as part of the ‘Neolithic Revolution’, which saw a transition from hunting and gathering of food to settled agriculture. These earliest cultivated forms were diploid (genome AA) (einkorn) and tetraploid (genome AABB) (emmer) wheats and their genetic relationships indicate that they originated from the south-eastern part of Turkey ( Heun et al. , 1997 ; Nesbitt, 1998 ; Dubcovsky and Dvorak, 2007 ). Cultivation spread to the Near East by about 9000 years ago when hexaploid bread wheat made its first appearance ( Feldman, 2001 ).

The earliest cultivated forms of wheat were essentially landraces selected by farmers from wild populations, presumably because of their superior yield and other characteristics, an early and clearly non-scientific form of plant breeding! However, domestication was also associated with the selection of genetic traits that separated them from their wild relatives. This domestication syndrome has been discussed in detail by others, but two traits are of sufficient importance to mention here. The first is the loss of shattering of the spike at maturity, which results in seed loss at harvesting. This is clearly an important trait for ensuring seed dispersal in natural populations and the non-shattering trait is determined by mutations at the Br ( brittle rachis ) locus ( Nalam et al. , 2006 ).

The second important trait is the change from hulled forms, in which the glumes adhere tightly to the grain, to free-threshing naked forms. The free forms arose by a dominant mutant at the Q locus which modified the effects of recessive mutations at the Tg ( tenacious glume ) locus ( Jantasuriyarat et al. , 2004 ; Simons et al. , 2006 ; Dubkovsky and Dvorak, 2007 ).

Cultivated forms of diploid, tetraploid, and hexaploid wheat all have a tough rachis apart from the spelt form of bread wheat. Similarly, the early domesticated forms of einkorn, emmer, and spelt are all hulled, whereas modern forms of tetraploid and hexaploid wheat are free-threshing.

Whereas einkorn and emmer clearly developed from the domestication of natural populations, bread wheat has only existed in cultivation, having arisen by hybridization of cultivated emmer with the unrelated wild grass Triticum tauschii (also called Aegilops tauschii and Ae . squarosa ). This hybridization probably occurred several times independently with the novel hexaploid (genome AABBDD) being selected by farmers for its superior properties. The evolution of modern wheats is illustrated in Fig. 1 which also shows examples of spikes and grain.

The evolutionary and genome relationships between cultivated bread and durum wheats and related wild diploid grasses, showing examples of spikes and grain. Modified from Snape and Pánková (2006) , and reproduced by kind permission of Wiley-Blackwell.

The genetic changes during domestication mean that modern wheats are unable to survive wild in competition with better adapted species. This was elegantly demonstrated by John Bennet Lawes in the 1880s when he decided to allow part of the famous long-term Broadbalk experiment at Rothamsted to return to its natural state ( Dyke, 1993 ). He therefore left part of the wheat crop unharvested in 1882 and monitored the growth in successive years. After a good crop in 1883 the weeds dominated and in 1885 the few remaining wheat plants (which were spindly with small ears) were collected and photographed.

The A genomes of tetraploid and hexaploid wheats are clearly related to the A genomes of wild and cultivated einkorn, while the D genome of hexaploid wheat is clearly derived from that of T . tauschii . In fact, the formation of hexaploid wheat occurred so recently that little divergence has occurred between the D genomes present in the hexaploid and diploid species. By contrast, the B genome of tetraploid and hexaploid wheats is probably derived from the S genome present in the Sitopsis section of Aegilops , with Ae . speltoides being the closest extant species. The S genome of Ae . speltoides is also closest to the G genome of T . timopheevi , a tetraploid species with the A and G genomes ( Feldman, 2001 ).

The spread of wheat from its site of origin across the world has been elegantly described by Feldman (2001) and is only summarized here. The main route into Europe was via Anatolia to Greece (8000 BP) and then both northwards through the Balkans to the Danube (7000 BP) and across to Italy, France and Spain (7000 BP), finally reaching the UK and Scandanavia by about 5000 BP. Similarly, wheat spread via Iran into central Asia reaching China by about 3000 BP and to Africa, initially via Egypt. It was introduced by the Spaniards to Mexico in 1529 and to Australia in 1788.

Currently, about 95% of the wheat grown worldwide is hexaploid bread wheat, with most of the remaining 5% being tetraploid durum wheat. The latter is more adapted to the dry Mediterranean climate than bread wheat and is often called pasta wheat to reflect its major end-use. However, it may also be used to bake bread and is used to make regional foods such as couscous and bulgar in North Africa. Small amounts of other wheat species (einkorn, emmer, spelt) are still grown in some regions including Spain, Turkey, the Balkans, and the Indian subcontinent. In Italy, these hulled wheats are together called faro ( Szabó and Hammer, 1996 ) while spelt continues to be grown in Europe, particularly in Alpine areas ( Fossati and Ingold, 2001 ).

The recent interest in spelt and other ancient wheats (including kamut, a tetraploid wheat of uncertain taxonomy, related to durum wheat) as healthy alternatives to bread wheat ( Abdel-Aal et al. , 1998 ) may also lead to wider growth for high value niche markets in the future.

Despite its relatively recent origin, bread wheat shows sufficient genetic diversity to allow the development of over 25 000 types ( Feldman et al. , 1995 ) which are adapted to a wide range of temperate environments. Provided sufficient water and mineral nutrients are available and effective control of pests and pathogens is ensured, yields can exceed 10 tonnes ha −1 , comparing well with other temperate crops. However, deficiencies in water and nutrients and the effects of pests and pathogens cause the global average yield to be low, at about 2.8 tonnes ha −1 . Wheat is also readily harvested using mechanical combine harvesters or traditional methods and can be stored effectively indefinitely before consumption, provided the water content is below about 15% dry weight and pests are controlled.

There is no doubt that the adaptability and high yields of wheat have contributed to its success, but these alone are not sufficient to account for its current dominance over much of the temperate world. The key characteristic which has given it an advantage over other temperate crops is the unique properties of doughs formed from wheat flours, which allow it to be processed into a range of breads and other baked products (including cakes and biscuits), pasta and noodles, and other processed foods. These properties depend on the structures and interactions of the grain storage proteins, which together form the ‘gluten’ protein fraction.

Transcriptomic studies have shown that over 30 000 genes are expressed in the developing wheat grain ( Wan et al. , 2008 ) while proteomic analysis of mature grain has revealed the presence of about 1125 individual components ( Skylas et al. , 2000 ). However, many of these components are present in small amounts and have little or no impact on the utilization of the grain, with one protein fraction being dominant in terms of amount and impact. This fraction is the prolamin storage proteins, which correspond to the gluten proteins. The precise number of individual gluten protein components has not been determined, but 2D gel analyses suggest that about 100 is a reasonable estimate. Together they have been estimated to account for about 80% of the total grain protein in European wheats ( Seilmeier et al. , 1991 ).

Gluten was one of the earliest protein fractions to be described by chemists, being first described by Beccari in 1728 (see translation by Bailey, 1941 ). It is traditionally prepared by gently washing wheat dough in water or dilute salt solution, leaving a cohesive mass which comprises about 80% protein, the remainder being mainly starch granules which are trapped in the protein matrix.

The ability to prepare the gluten proteins in an essentially pure state by such a simple procedure depends on their unusual properties. Firstly, they are insoluble in water or dilute salt solutions but are soluble in alcohol/water mixtures (as discussed below) and were hence defined as ‘prolamins’ by TB Osborne in his classic studies of plant proteins carried out at the end of the 19th century and the start of the 20th century ( Osborne, 1924 ). Secondly, the individual gluten proteins are associated by strong covalent and non-covalent forces which allow the whole fraction to be isolated as a cohesive mass.

What is the origin of gluten?

In common with other seed storage proteins, the gluten proteins are secretory proteins, being synthesized on the rough endoplasmic reticulum and co-translationally transported into the lumen of the ER. Once within the ER lumen, cereal seed storage proteins may follow two routes: a Golgi-dependent route leading to deposition within protein bodies of vacuolar origin or a Golgi-independent route in which protein deposits formed within the ER lumen may ultimately fuse with protein bodies of vacuolar origin (see Kumamaru et al. , 2007 , for a review).

Work carried out by Galili and colleagues ( Levanany et al. , 1992 ; Galili et al. , 1995 ; Galili, 1997 ) indicated that wheat gluten proteins may follow both routes, and this has recently been confirmed using epitope tags and specific antibodies to follow individual proteins and groups of proteins in cells of developing grain ( Tosi et al. , 2009 ). It is also clear that the protein deposits fuse to form a continuous matrix as the cells of the starchy endosperm dry and die during the later stages of grain maturation ( Fig. 2A ). Thus a proteinaceous network is present in each endosperm cell ( Fig. 2B ) and these networks are brought together when flour is mixed with water to form a continuous network in the dough. Washing the dough to remove non-gluten components therefore allows the network to be recovered as the cohesive mass which is called gluten ( Fig. 2C ).

The origin of wheat gluten. (A) Transmission electron microscopy of the developing starchy endosperm cells at 46 d after anthesis shows that the individual protein bodies have fused to form a continuous proteinaceous matrix. Taken from Shewry et al. , 1995 , ( Biotechnology 13, 1185–1190) and provided by Dr M Parker (IFR, Norwich, UK). (B) Digestion of a flour particle with amylases to remove starch reveals a continuous proteinaceous network. Taken from Amend and Beauvais (1995) and reproduced by kind permission of Getreidetechnologie. (C) After kneading, dough can be washed to recover the gluten network as a cohesive mass which is stretched in the photograph to demonstrate its viscoelastic properties.

The biochemical and molecular basis for gluten functionality

Humankind has been aware for many centuries that wheat dough has unusual properties which are shared to a limited extent by doughs made from rye flour but not by those from other cereal flours. These properties, which are usually described as ‘viscoelasticity’, are particularly important in making leavened bread, as they allow the entrapment of carbon dioxide released during leavening. However, they also underpin a range of other uses including making unleavened breads, cakes, and biscuits, pasta (from durum wheat), and noodles (from bread wheat). They are also exploited in the food industry where gluten proteins may be used as a binder in processed foods.

The volume of research carried out on wheat gluten is vast, with a simple search of the Web of Science database showing almost 20 000 papers since 1945. This volume not only reflects the commercial importance of wheat processing, but also the complexity of the system which remains incompletely understood. They include studies at the genetic, biochemical, biophysical, and functional (ie processing) levels.

Genetic studies have exploited the extensive polymorphism which exists between the gluten protein fractions present in different genotypes to establish genetic linkages between either groups of gluten proteins, or allelic forms of these, and aspects of processing quality. Similarly, studies at the biochemical and biophysical levels have demonstrated a relationship between dough strength and the ability of the gluten proteins to form polymeric complexes (called glutenins). Combining results from these two approaches highlighted the importance of a specific group of gluten proteins, called the high molecular weight (HMW) subunits of glutenin.

Cultivars of bread wheat express between three and five HMW subunit genes, with the encoded proteins accounting for up to about 12% of the total grain protein ( Seilmeier et al. , 1991 ; Halford et al. , 1992 ). The HMW subunits are only present in high molecular mass polymers and allelic variation in both the number of expressed genes and the properties of the encoded proteins results in effects on the amount and size of the polymers and hence dough strength (reviewed by Payne, 1987 ; Shewry et al. , 2003 b ). These glutenin polymers are known to be stabilized by inter-chain disulphide bonds, but it is apparent that non-covalent hydrogen bonds are also important in stabilizing the interactions between glutenin polymers and monomeric gluten proteins (called gliadins) ( Belton, 2005 ). Hence, the individual gliadins and glutenin polymers can be separated using solvents which disrupt hydrogen bonding (such as urea) but reducing agents (such as 2-mercaptoethanol or dithiothreitol) are required to break down the glutenin polymers to release the individual subunits.

Although the HMW subunits are the main determinants of glutenin elasticity relationships between other gluten proteins and functional properties have also been reported (reviewed by Shewry et al. , 2003 a ).

The relationship between the HMW subunits and dough strength was first established over 25 years ago ( Payne et al. , 1979 ) and allelic forms associated with good processing quality have been selected by plant breeders for over two decades, using simple SDS-PAGE separations. The established relationships between the number of expressed HMW subunit genes, the total amount of HMW subunit protein and dough strength have also resulted in the HMW subunit genes being an attractive target for genetic transformation, in order to increase their gene copy number and hence dough strength.

The first studies of this type were reported over 10 years ago ( Altpeter et al. , 1996 ; Blechl and Anderson, 1996 ; Barro et al. , 1997 ) and many studies have since been reported (reviewed by Shewry and Jones, 2005 ; Jones et al. , 2009 ). It is perhaps not surprizing that the results have been ‘mixed’, but some conclusions can be drawn. Firstly, expression of an additional HMW subunit gene can lead to increased dough strength, even when a modern good quality wheat cultivar is used as the recipient (see Field et al. , 2008 ; Rakszegi et al. , 2008 , as recent examples, and reviews of earlier work cited above). However, the effect depends on the precise HMW subunit gene which is used and on the expression level, with the transgenes resulting in over-strong (ie too elastic) gluten properties in some studies. Thus, although transgenesis is a realistic strategy to increase dough strength in wheat, it is also necessary to have an understanding of the underlying mechanisms in order to optimize the experimental design.

Wheat is widely consumed by humans, in the countries of primary production (which number over 100 in the FAO production statistics for 2004) and in other countries where wheat cannot be grown. For example, imported wheat is used to meet consumer demands for bread and other food products in the humid tropics, particularly those with a culinary tradition dating back to colonial occupation. Statistics are not available for the total volume of wheat which is consumed directly by humans as opposed to feeding livestock, although figures for the UK indicate about one-third of the total production (approximately 5.7 m tonnes per annum are milled with home production being 15–16 m tonnes). Globally there is no doubt that the number of people who rely on wheat for a substantial part of their diet amounts to several billions.

The high content of starch, about 60–70% of the whole grain and 65–75% of white flour, means that wheat is often considered to be little more than a source of calories, and this is certainly true for animal feed production, with high-yielding, low-protein feed varieties being supplemented by other protein-rich crops (notably soybeans and oilseed residues).

However, despite its relatively low protein content (usually 8–15%) wheat still provides as much protein for human and livestock nutrition as the total soybean crop, estimated at about 60 m tonnes per annum (calculated by Shewry, 2000 ). Therefore, the nutritional importance of wheat proteins should not be underestimated, particularly in less developed countries where bread, noodles and other products (eg bulgar, couscous) may provide a substantial proportion of the diet.

Protein content

Although wheat breeders routinely select for protein content in their breeding programmes (high protein for breadmaking and low protein for feed and other uses), the current range of variation in this parameter in commercial cultivars is limited. For example, Snape et al. (1993) estimated that typical UK breadmaking and feed wheats differed in their protein content by about 2% dry weight (eg from about 12–14% protein) when grown under the same conditions, which is significantly less than the 2-fold differences which can result from high and low levels of nitrogen fertilizer application. This limited variation in conventional wheat lines has led to searches for ‘high protein genes’ in more exotic germplasm.

Early studies of the USDA World Wheat Collection showed approximately 3-fold variation in protein content (from 7–22%), with about one-third of this being under genetic control ( Vogel et al. , 1978 ). However, the strong environmental impact on protein content (accounting for two-thirds of the variation) underpins the difficulty of breeding for this trait. Nevertheless, some success has been achieved by incorporating sources of variation from exotic bread wheat lines or related wild species.

The former include Atlas 50 and Atlas 66, derived from the South American line Frandoso, and Nap Hal from India. These lines appear to have different ‘high protein genes’ and both were extensively used in breeding programmes in Nebraska with the Atlas 66 gene being successfully incorporated into the commercial variety Lancota ( Johnson et al. , 1985 ). Frandoso and related Brazilian lines have also been successfully exploited in other breeding programmes in the USA ( Busch and Rauch, 2001 ). The Kansas variety, Plainsman V, also contained a high protein gene(s) from a related Aegilops species ( Finney, 1978 ).

The most widely studied source of ‘high protein’ is wild emmer (tetraploid Tr . turgidum var. dicoccoides ) wheats from Israel. One accession, FA15-3, accumulates over 40% of protein when grown with sufficient nitrogen ( Avivi, 1978 ). The gene in this line was mapped to a locus on chromosome 6B (called Gpc-B1 ), which accounted for about 70% of the variation in protein content in crosses ( Chee et al. , 2001 ; Distelfeld et al. , 2004, 2006 ). More recent studies have shown that the gene Gpc-B1 encodes a transcription factor which accelerates senescence in the vegetative parts of the plant, resulting in increased mobilization and transfer to the grain of both nitrogen and minerals (notably iron and zinc) ( Uauy et al. , 2006 ). However, it remains to be shown whether this gene can be incorporated into high-yielding and commercially viable lines.

Protein composition

Of the 20 amino acids commonly present in proteins, 10 can be considered to be essential in that they cannot be synthesized by animals and must be provided in the diet. Furthermore, if only one of these is limiting the others will be broken down and excreted. There has been much debate about which amino acids are essential and the amounts that are required, with the most recent values for adult humans being shown in Table 1 . This table includes a combined value for the two aromatic amino acids, tyrosine and phenylalanine, which are biosynthetically related, and both single and combined values for the two sulphur-containing amino acids: methionine, which is truly essential, and cysteine which can be synthesized from methionine. Comparison with the values for whole wheat grain and flour shows that only lysine is deficient, with some essential amino acids being present in considerably higher amounts than the requirements. However, the lysine content of wheat also varies significantly with the values shown in Table 1 being typical of grain of high protein content and the proportion increasing to over 30 mg g −1 protein in low protein grain ( Mossé and Huet, 1990 ). This decrease in the relative lysine content of high protein grain results from proportional increases in the lysine-poor gluten proteins when excess N is available (for example, when fertilizer is applied to increase grain yield and protein content) and also accounts for the lower lysine content of the white flour (the gluten proteins being located in the starchy endosperm tissue).

Recommended levels of essential amino acids for adult humans compared with those in wheat grain and flour (expressed as mg g −1 protein)

FAO/WHO/UNU (2007) .

Calculated from literature values as described in Shewry (2007) .

The amino acid requirements for infants and children vary depending on their growth rate, being particularly high in the first year of life. Similarly, higher levels of essential amino acids are required for rapidly growing livestock such as pigs and poultry.

Wheat as a source of minerals

Iron deficiency is the most widespread nutrient deficiency in the world, estimated to affect over 2 billion people ( Stoltzfus and Dreyfuss, 1998 ). Although many of these people live in less developed countries, it is also a significant problem in the developed world. Zinc deficiency is also widespread, particularly in Sub-Saharan Africa and South Asia, and has been estimated to account for 800 000 child deaths a year (Micronutrient Initiative, 2006 ), in addition to non-lethal effects on children and adults. Wheat and other cereals are significant sources of both of these minerals, contributing 44% of the daily intake of iron (15% in bread) and 25% of the daily intake of zinc (11% in bread) in the UK ( Henderson et al. , 2007 ). There has therefore been considerable concern over the suggestion that the mineral content of modern wheat varieties is lower than that of older varieties.

This was initially suggested by Garvin et al. (2006) who grew 14 red winter wheat cultivars bred between 1873 and 2000 in replicate field experiments and determined their mineral contents. Plants were grown at two locations in Kansas and significant negative correlations were found between grain yield, variety release date, and the concentrations of zinc in material from both of these sites and of iron in materials from only one site. Similar trends were reported by Fan et al. (2008 a , b ) who took a different approach. Rather than carrying out direct comparisons of varieties in field trials, they analysed grain grown on the Rothamsted Broadbalk long-term wheat experiment. This experiment was established in 1843 and uses a single variety which is replaced by a more modern variety at regular intervals. Analysis of archived grain showed significant decreases in the contents of minerals (Zn, Fe, Cu, Mg) since semi-dwarf cultivars were introduced in 1968. A similar difference was observed between the cultivars Brimstone (semi-dwarf) and Squareheads Master (long straw) which were grown side by side in 1988–1990, the concentrations of Zn, Cu, Fe, and Mg being 18–29% lower in Brimstone. A more recent comparison of 25 lines grown also showed a decline in the concentrations of Fe and Zn since semi-dwarf wheats were introduced ( Zhao et al. , 2009 ) ( Fig. 3 ). Although the decrease in the mineral content of modern wheats is partly due to dilution, resulting from increased yield (which was negatively correlated with mineral content), it has been suggested that short-strawed varieties may be intrinsically less efficient at partitioning minerals to the grain compared with the translocation of photosynthate.

The relationship between the iron content of wholemeal flours from 25 wheat cultivars grown on six trial sites/seasons and their release dates. Taken from Zhao et al. (2009) and reproduced by kind permission of Elsevier.

Such genetic differences in mineral content are clearly relevant to international efforts to increase the mineral content of wheat to improve health in less developed countries. Thus, increasing iron, zinc, and vitamin A contents are a major focus of the HarvestPlus initiative of the Consultative Group on International Agricultural Research (CGIAR) which is using conventional plant breeding ( Ortiz-Monasterio et al. , 2007 ) while other laboratories are using genetic engineering approaches (reviewed by Brinch-Pedersen et al. , 2007 ).

These initiatives are focusing not only on contents of minerals but also on their bioavailability. Iron is predominantly located in the aleurone and as complexes with phytate ( myo -inositolphosphate 1,2,3,4,5,6-hexa-kisphosphate). These complexes are largely insoluble, restricting mineral availability to humans and livestock. The use of transgenesis to express phytase in the developing grain can result in increased mineral availability, particularly when a heat-stable form of the enzyme is used to allow hydrolysis to occur during food processing (reviewed by Brinch-Pedersen et al. , 2007 ).

Guttieri et al. (2004) also reported an EMS-induced low phytate mutant of wheat. This mutation resulted in 43% less phytic acid in the aleurone, but has not so far been incorporated into commercial cultivars. However, previous experience with low phytic acid mutants of maize, barley, and soy bean has shown that they may also have significant effects on yield and germination rates (reviewed by Brinch-Pedersen et al. , 2007 ).

Wheat as a source of selenium

Selenium is an essential micronutrient for mammals (but not plants), being present as selenocysteine in a number of enzymes. However, it is also toxic when present in excess (above about 600 μg d −1 ; Yang and Xia, 1995 ). Cereals are major dietary sources of selenium in many parts of the world, including China ( FAO/WHO, 2001 ), Russia ( Golubkina and Alfthan, 1999 ), and the UK (MAFF, 1997). However, the content of selenium in wheat varies widely from about 10 μg kg −1 to over 2000 μg kg −1 ( FAO/ WHO, 2001 ; Combs, 2001 ).

The concentration of selenium in wheat is largely determined by the availability of the element in the soil. Consequently, wheat produced in Western Europe may contain only one-tenth of the selenium that is present in wheat grown in North America. Thus, a survey of 452 grain samples grown in the UK in 1982 and 1992 showed a mean value of 27 μg Se kg −1 fresh weight ( Adams et al. , 2002 ) compared with 370 μg SE kg −1 fresh weight for 290 samples from the USA ( Wolnik et al. , 1983 ).

Because the import of wheat from North America into Europe has declined over the last 25 years, the intake of selenium in the diet has also decreased, which has resulted in concern in some European countries. One response to this is to apply selenium to the crops in fertilizer (called biofortification), which is practised in Finland ( Eurola et al. , 1991 ).

Unlike iron, selenium is not concentrated in the aleurone, being present wherever sulphur is present. The concentration of selenium in grain from the Broadbalk continuous wheat experiment also appeared to be determined principally by the sulphur availability in the soil (which competes to prevent selenium uptake), with no evidence of decreased levels over time ( Fan et al. , 2008 b ). However, sulphur fertilizer is often applied to wheat to improve the grain quality ( Zhao et al. , 1997 ) and this could clearly have negative impacts on selenium in grain.

The reader is referred to a recent review article by Hawkesford and Zhao (2007) for a detailed review of selenium in wheat.

Wholegrain wheat and health

The consumption of white flour and bread have historically been associated with prosperity and the development of sophisticated roller mills in Austro-Hungary during the second part of the 19th century allowed the production of higher volumes of whiter flour than it was possible to produce by traditional milling based on grinding between stones and sieving (see Jones, 2007 , for a fascinating account of the history of roller milling). However, the increased consumption of bread made from highly refined white flour was not accepted universally, leading to what we would today recognize as a movement to increase the consumption of wholegrain products.

In 1880, May Yates founded the Bread Reform League in London to promote a return to wholemeal bread, particularly to improve the nutrition of the children of the poor, and suggested in 1909 that an official minimum standard of 80% flour extraction rate should be adopted. This was called ‘Standard Bread’. Although we now appreciate the nutritional advantages of wholegrain products, this was not supported by the science of the time and clearly conflicted with the tastes of consumers as well as the economics of bread production. Nevertheless, the League continued to campaign and received scientific support in 1911 when Gowland Hopkins agreed that ‘Standard Bread’ may contain ‘unrecognized food substances’ which were vital for health: these were subsequently called vitamins ( Burnett, 2005 ).

By contrast, Thomas Allinson (1858–1918) had a much greater impact by marketing and vigorously promoting his own range of wholemeal products. He can therefore be regarded as the father of the wholegrain movement and remains a household name to this day in the UK ( Pepper, 1992 ).

We now know that wholegrain wheat products contain a range of components with established or proposed health benefits which are concentrated or solely located in the bran. Hence they are either present in lower amounts or absent from white flour which is derived almost exclusively from starchy endosperm cells. They vary widely in their concentrations. For example, lignans, a group of polyphenols with phytoestrogen activity, are present at levels up to about 10 μg g −1 in wholemeal wheat and twice this level in bran ( Nagy-Scholz and Ercsey, 2009 ), while total phenolic acids in wholemeal range up to almost 1200 μg g −1 ( Li et al. , 2008 ).

The most detailed study of wheat phytochemicals which has so far been reported was carried out as part of the EU Framework 6 HEALTHGRAIN programme ( Poutanen et al. , 2008 ; Ward et al. , 2008 ). This study determined a range of phytochemicals in 150 wheat lines grown on a single site in one year, meaning that the levels of the components may have been influenced by environmental as well as genetic effects. The lines were selected to represent a broad range of dates and places of origin. The choice of phytochemicals focused on those which have putative health benefits and for which cereals are recognized dietary sources. For example, cereals are considered to account for about 22% of the daily intake of folate (vitamin B12) in the UK ( Goldberg, 2003 ) and 36% and 43% of the daily intake in Finnish women and men, respectively ( Findiet Study Group, 2003 ). In the HEALTHGRAIN study the contents of folates in wholemeal varied from 364 to 774 ng g −1 dry weight in 130 winter wheats and from 323 to 741 ng g −1 dry weight in 20 spring wheats, with the content in the former being positively correlated with bran yield and negatively correlated with seed weight (indicating concentration in the bran) ( Piironen et al. , 2008 ).

The quantitatively major group of phytochemicals in the wheat grain is phenolic acids, derivatives of either hydroxybenzoic acid or hydroxycinnamic acid. Epidemiological studies indicate that phenolic acids have a number of health benefits which may relate to their antioxidant activity; the total antioxidant activities of grain extracts and their phenolic acid contents being highly correlated ( Drankham et al. , 2003 ; Beta et al. , 2005 ; Wende et al. , 2005 ).

Cereals are also significant sources of tocols (which include vitamin E) (27.6–79.7 μg g −1 in the HEALTHGRAIN study) ( Lampi et al. , 2008 ) and sterols (670–959 μg g −1 ) ( Nurmi et al. , 2008 ).

The HEALTHGRAIN study also determined the levels of dietary fibre. In wheat, this mainly derives from cell wall polymers: arabinoxylans (approximately 70%) with lower amounts of (1-3)(1-4)β- D -glucans (approximately 20%) and other components. The arabinoxylans also occur in soluble and insoluble forms, with the latter being rich in bound phenolic acids which form oxidative cross-links. These bound phenolic acids account, on average, for 77% of the total phenolic acid fraction and are predominantly ferulic acid. Soluble fibre is considered to have health benefits ( Moore et al. , 1998 ; Lewis and Heaton, 1999 ) which are not shared by insoluble fibre and these may therefore be reduced by the phenolic acid cross-linking. However, insoluble fibre may also have benefits in delivering phenolic antioxidants into the colon: these benefits may include reduction in colo-rectal cancer ( Vitaglione et al. , 2008 ).

The HEALTHGRAIN study showed wide variation in the contents of total and water-extractable arabinoxylans in both white flour and bran fractions ( Gebruers et al. , 2008 ) ( Fig. 4 ). Similarly, Ordaz-Ortiz et al. (2005) showed variation from 0.26% to 0.75% dry weight in the content of water-extractable arabinoxylan in 20 French wheat lines and from 1.66% to 2.87% dry weight in total arabinoxylans. A high proportion of the variation in water-extractable arabinoxylans is also heritable ( Martinant et al. , 1999 ).

Contents of arabinoxylan (AX) fibre in flour and bran of 150 wheat cultivars grown on a single site as part of the EU FP6 HEALTHGRAIN project. (A) Total AX in flour (mg g −1 ); (B) water-extractable AX in flour (%); (C) total AX in bran (mg g −1 ), and (D) water-extractable AX in bran. Prepared from data reported by Gebruers et al. (2008) with permission of the authors.

It is clear from these and other studies that there is sufficient genetically determined variation in the phytochemical and fibre contents of wheat to be exploited in breeding for varieties with increased nutritional benefits.

Allergy to wheat

Both respiratory and food allergies to wheat have been reported.

Respiratory allergy (bakers' asthma) has been known since Roman times (when slaves handling flour and dough were required to wear masks) and is currently one of the most important forms of occupational allergy. For example, it is the second most widespread occupational allergy in the UK and has been reported to affect over 8% of apprentice bakers in Poland after only 2 years exposure ( Walusiak et al. , 2004 ). A wide range of wheat grain proteins have been shown to react with immunoglobulin (Ig)E in sera of patients with bakers' asthma, including gliadins, glutenins, serpins (serine proteinase inhibitors), thioredoxin, agglutinin, and a number of enzymes (α- and β-amylases, peroxidase, acyl CoA oxidase, glycerinaldehyde-3-phosphate dehydrogenase and triosephosphate isomerase) (reviewed by Tatham and Shewry, 2008 ). However, it is clear that the predominant wheat proteins responsible for bakers' asthma are a class of α-amylase inhibitors, also known as CM proteins due to their solubility in chloroform:methanol mixtures ( Salcedo et al. , 2004 ). Furthermore, their activity has been demonstrated by a range of approaches including skin pricks and RAST (radioallergosorbent test) as well as immunoblotting, ELISA, and screening expression libraries with IgE fractions.

The CM proteins comprise monomeric, dimeric, and tetrameric forms, with subunit masses ranging between about 10 000 and 16 000. They differ in their spectrum of activity but all inhibit mammalian and insect α-amylases (including those in some pest organisms) rather than endogenous wheat enzymes. Hence, they are considered to be protective rather than regulatory in function. Eleven individual subunits have been shown to play a role in bakers' asthma (using one or more of the assays listed above) but they differ in their activity, with a glycosylated form of CM16 being particularly active.

Wheat is listed among the ‘big eight’ food allergens which together account for about 90% of all allergic responses. However, the incidence of true (ie IgE-mediated) food allergy is, in fact, fairly infrequent in adults, although it may affect up to 1% of children ( Poole et al. , 2006 ). A number of wheat proteins have been reported to be responsible for allergic responses to the ingestion of wheat products but only one syndrome has been studied in detail. Wheat-dependent exercise-induced anaphylaxis (WDEIA) is a well-defined syndrome in which the ingestion of a product containing wheat followed by physical exercise can result in an anaphylactic response. Work carried out by several groups has clearly established that this condition is associated with a group of ω-gliadins (called ω5-gliadins) which are encoded by genes on chromosome 1B ( Palosuo et al. , 2001 ; Morita et al. , 2003 ; Battais et al. , 2005 ). Mutational analysis has also identified immunodominant epitopes in the ω5-gliadins: short glutamine-rich and proline-rich sequences present in the repetitive domains of the proteins ( Matsuo et al. , 2004 , 2005 ; Battais et al. , 2005 ). However, a number of other proteins have also been shown to react with IgE from patients with WDEIA, including gliadins, glutenin subunits, and related proteins from barley and rye (reviewed by Tatham and Shewry, 2008 ).

Other allergic responses to wheat proteins include atopic dermatitis, urticaria, and anaphylaxis. Not surprizingly, these symptoms have been associated with a number of wheat proteins, most notably gluten proteins but also CM proteins, enzymes, and lipid transfer protein (LTP) (reviewed by Tatham and Shewry, 2008 ).

Comparison of the proteins identified as responsible for the respiratory and food allergy shows significant overlap in their functions (most being storage or protective) and identities (notably gluten proteins and CM proteins).

Intolerance to wheat

Dietary intolerance to wheat is almost certainly more widespread than allergy, notably coeliac disease (CD) which is estimated to affect 1% of the population of Western Europe ( Feighery, 1999 ), and dermatitis herpetiformis which has an incidence between about 2-fold and 5-fold lower than CD ( Fry, 1992 ).

CD is a chronic inflammation of the bowel which leads to malabsorption of nutrients. Like bakers' asthma, CD has been known since classical times but it was only defined in detail in 1887 and its relationship to wheat established by Dicke in the late 1940s ( Losowsky, 2008 ).

A series of elegant studies carried out over the past decade, particularly by Sollid, Koning and co-workers, have established that CD results from an autoimmune response which is triggered by the binding of gluten peptides to T cells of the immune system in some (but not all) individuals with the human leucocyte antigens (HLAs) DQ2 or DQ8, expressed by specialized antigen-presenting cells. The presented peptides are then recognized by specific CD4+ T cells which release inflammatory cytokines which lead to the flattening of the intestinal epithelium. It has also been demonstrated that tissue transglutaminase enzyme present in the epithelium of the intestine plays an important role, generating toxic peptides by deamidation of glutamine residues to give glutamate.

The HLA-DQ2 antigen is present in about 95% of coeliac patients ( Karell et al. , 2003 ) and detailed studies have identified the peptide sequences which are recognized by intestinal T cell lines, using either peptide fractions produced from gluten proteins or synthetic peptides. This has led to the definition of two overlapping immunodominant epitopes corresponding to residues 57–68 (α-9) and 62–75 (α-2) of A gliadin (a form of α-gliadin) ( Arentz-Hansen et al. , 2000 , 2002 ; Anderson et al. , 2000 ; Ellis et al. , 2003 ). Related epitopes were similarly defined in γ-gliadins, corresponding to residues 60–79, 102–113, 115–123, and 228–236 ( Sjöström et al. , 1998 ; Arentz-Hansen et al. , 2002 ; Vader et al. , 2002 a ). Furthermore, Vader et al. (2002 b ) showed that the spacing between glutamine and proline residues determined the specificity of glutamine deamidation and hence peptide activation, and developed algorithms to predict the presence of novel T cell stimulatory peptides in gluten proteins and in related proteins from other cereals.

Less work has been carried out on the determinants of the HLA8-DQ8 associated coeliac disease, which affects only about 6% of patients without HLA-DQ2 and 10% of patients with HLA-DQ2 ( Karell et al. , 2003 ). This has again allowed immunodominant epitopes to be identified in gliadins and glutenin subunits ( van der Wal et al. , 1998 , 1999 ; Mazzarella et al. , 2003 ; Tollefsen et al. , 2006 ) although detailed structural studies indicate that the HLA-DQ2 and HLA-DQ8-mediated forms of the disease may differ in their molecular mechanisms ( Henderson et al. , 2007 ).

The possibility of producing wheat which lacks the coeliac toxic peptides has been discussed for many years but interest in the strategy tended to decline as it became clear that most, if not all, gluten proteins are toxic to at least some susceptible individuals, rather than only the α-gliadins as initially thought. However, Spaenij-Dekking et al. (2005) and van Herpen et al. (2006) have shown that it is possible to identify natural forms of gliadin which have few or no coeliac toxic epitopes, raising the possibility of selecting for less toxic lines of wheat by classical plant breeding. RNA interference (RNAi) technology has also been used to silence the α-gliadin ( Becker et al. , 2006 ; Wieser et al. , 2006 ) and γ-gliadin ( Gil-Humanes et al. , 2008 ) gene families, although some effects on grain-processing properties were observed.

The combination of these two approaches may therefore allow the production of less toxic, if not non-toxic, wheat for coeliac patients without significant loss of the processing properties conferred by the gluten proteins.

Dermatitis herpetiformis is a skin eruption resulting from ingestion of gluten, and is associated with the deposition of IgA antibodies in dermal papillae. These include IgA antibodies to epidermal transglutaminase which is considered to be an important autoantigen in disease development ( Hull et al. , 2008 ).

Other medical conditions related to gluten proteins

There are many reports of the association of wheat, and particularly wheat proteins, with medical conditions, ranging from improbable reports in the popular press to scientific studies in the medical literature. Not surprisingly, they include autoimmune diseases such as rheumatoid arthritis which may be more prevalent in coeliac patients and relatives ( Neuhausen et al. , 2008 ). It is perhaps easier to envisage mechanisms for relationships between such diseases which have a common immunological basis ( Hvatum et al. , 2006 ) than to explain a well-established association between wheat, coeliac disease, and schizophrenia ( Singh and Roy, 1975 ; Kalaydiian et al. , 2006 ) Other reported associations include ones with sporadic idiopathic ataxia (gluten ataxia) ( Hadjivassiliou et al. , 2003 ), migraines ( Grant, 1979 ), acute psychoses ( Rix et al. , 1985 ), and a range of neurological illnesses ( Hadjivassiliou et al. , 2002 ). An association with autism has also been reported ( Lucarelli et al. , 1995 ) with some physicians recommending a gluten-free, casein-free diet ( Elder, 2008 ).

Some of these effects may be mediated via the immune system but effects which are not immune-mediated are notoriously difficult to define and diagnose. However, they could result from the release within the body of bioactive peptides, derived particularly from gluten protein. Thus, gluten has been reported to be a source of a range of such peptides including opioid peptides (exorphins) ( Takahashi et al. , 2000 ; Yoshikawa et al. , 2003 ) and an inhibitor of angiotensin I-converting enzyme ( Motoi and Kodama, 2003 ) (see also reviews by Dziuba et al. , 1999 ; Yamamoto et al. , 2003 ). However, these activities were demonstrated in vitro and their in vivo significance has not been established.

There is little doubt that wheat will retain its dominant position in UK and European agriculture due to its adaptability and consumer acceptance. However, it may also need to adapt to face changing requirements from farmers, food processors, governments, and consumers.

Reducing inputs

Currently grown wheat cultivars require high inputs of nitrogen fertilizer and agrochemicals to achieve high yields combined with the protein content required for breadmaking. For example, UK farmers currently apply 250–300 kg N ha −1 in order to achieve the 13% protein content required for the Chorleywood Breadmaking Process, which is the major process used for breadmaking in the UK. Since a 10 tonnes ha −1 crop containing 13% protein equates to about 230 kg N ha −1 , this means that 50–70 kg N ha −1 may be lost. As fertilizer N currently costs about £1 kg −1 this represents a significant financial loss as well as a loss of the energy required for fertilizer production and may also have environmental consequences.

A number of projects worldwide are therefore focusing on understanding the processes that determine the efficiency of uptake, assimilation, and utilization of nitrogen in order to improve the efficiency of nitrogen recovery in the grain (reviewed by Foulkes et al. , 2009 ).

Reducing the nitrogen requirement of wheat does not only relate to the grain protein content, as an adequate supply of nitrogen is also essential for high wheat yields in order to build a canopy and fix carbon dioxide by photosynthesis. Furthermore, a substantial proportion of this nitrogen is remobilized and redistributed to the developing grain during canopy senescence ( Dalling, 1985 ). Hawkesford and colleagues at Rothamsted Research have targeted this process in order to develop a strategy for improving the recovery of N in the grain, using a combination of biochemical analysis and metabolite and transcript profiling to identify differences in metabolites and gene expression which are associated with efficient mobilization and redistribution ( Howarth et al. , 2008 ). Some of the genes identified in these and similar studies are suitable candidates for manipulation to increase the proportion of the total nitrogen recovered in the grain.

Stability of quality

The increases in temperature and carbon dioxide concentration associated with climate change are expected to have effects on crop development and yield, although the magnitude of these is difficult to predict due to interactions with other factors which may also be affected, notably water availability and populations of pests and pathogens ( Coakley et al. , 1999 ; Semenov, 2008 ). Similarly, although it is generally accepted that higher growth temperatures result in greater dough strength, the precise effects are not clearly understood (see review by Dupont and Altenbach, 2003 ) with heat stress (ie above 30–33 °C) actually resulting in dough weakening and reduced quality ( Blumenthal et al. , 1993 ). A recent review of the effects of CO 2 concentration on grain quality also failed to draw clear conclusions ( Högy and Fangmeier, 2008 ).

Of more immediate interest to wheat breeders and grain-utilizing industries are year-to-year fluctuations in growth conditions, and the frequency and magnitude of such fluctuations are also predicted to increase in the future ( Porter and Semenov, 2005 ). Although some cultivars are generally considered to be more consistent in quality than others, this is largely anecdotal with no detailed scientific comparisons.

Given the recent advances in ‘omics’ technologies it should now be possible to dissect the effects of G×E on grain development and quality, and to establish markers suitable for use in plant breeding. However, this will require substantial resources and a multi-disciplinary approach: by growing mapped populations and lines in multi-site/multi-year trials and determining aspects of composition and quality from gene expression profiling to pilot scale breadmaking.

Wan et al. (2009) have reported the application of this approach using a limited set of seven doubled haploid lines to identify a number of transcripts whose expression profile was associated with quality traits independently of environmental conditions. Millar et al. (2008) also reported a larger study in which three doubled haploid populations of wheat were used to map novel QTLs (quantitative trait loci) for breadmaking and pastry making which were stable over two years field trials, but did not relate quality traits to gene expression profiles.

Wheat is an attractive option as a ‘first generation biofuel’ as the high content of starch is readily converted into sugars (saccharification) which can then be fermented into ethanol. Murphy and Power (2008) recently reported that the gross energy recovered in ethanol using wheat was 66 GJ ha −1 a −1 , but that this only corresponds to 50% energy conversion and that the net energy production is as low as 25 GJ ha −1 a −1 . The same authors also calculated that the net energy production could be increased to 72 GJ ha −1 a −1 if the straw was combusted and the residue after distillation, called stillage or distillers grain and solubles (DGS), was converted to biogas (biomethane).

A major concern about using wheat grain for biofuel production is the high energy requirement for crop production, including that required to produce nitrogenous fertilizer. It is therefore necessary to develop new crop management strategies to reduce inputs ( Loyce et al. , 2002 ) as well as exploiting wheats with low N input requirements combined with high starch contents ( Kindred et al. , 2008 ).

The second major concern is, of course, the impact on international grain prices which may exacerbate problems of grain supply to less affluent populations.

New benefits to consumers

The increasing awareness of the important role of wheat-based products in a healthy diet has been discussed above, focusing on the identification and exploitation of natural variation in bioactive components. However, in some cases the natural variation in a trait may be limited in extent or difficult to exploit and, in this case, other approaches may be required. Currently, the most important target of this type of approach is resistant starch.

Most of the starch consumed in the human diet, including wheat starch, is readily digested in the small intestine, resulting in a rapid increase in blood glucose which may contribute to the development of type 2 diabetes and obesity ( Sobal, 2007 ). However, a fraction of the starch may resist digestion and pass through the small intestine to the colon, where it is fermented to short chain fatty acids, notably butyrate, which may have health benefits including reduction of colo-rectal cancer (as discussed by Topping, 2007 ).

Although the proportion of resistant starch (RS) depends on a number of factors including the effects of food processing, the most widely studied form is high amylose starch. In most species, amylose accounts for 20–30% of starch and amylopectin for 70–80%. However, mutant lines have been identified in a number of species in which the proportion of amylose is increased up to about 40% (e.g. Glacier barley; Yoshimoto et al. , 2000 ).

Selection for high amylose mutants is relatively easy in a diploid species such as barley, but more painstaking approaches are required in hexaploid wheat as mutations in homoeologous genes on all three genomes may be required to have a significant effect on the phenotype. This has been demonstrated very elegantly by Yamamori et al. (2000) who combined mutations in the gene encoding the starch synthase II enzyme (also called starch granule protein 1) that catalyses the synthesis of amylopectin. The resulting triple mutant line contained about 37% amylose.

However, the complexity of starch biosynthesis means that similar high amylose phenotypes can result from changes in other enzymes, with a notable example being the use of RNA interference technology to down-regulate the gene encoding starch-branching enzyme IIa ( Regina et al. , 2006 ). The resulting transgenic lines had up to 80% amylose and increased RS as measured in rat feeding trials. This study demonstrates the power of GM technology, although it remains to be shown that lines with such high levels of amylose will have acceptable yields and properties for milling and processing.

It also remains to be shown that consumers will be prepared to eat bread and other foods produced from GM wheat. The wheat grain and its products have been treated with reverence by humans for millennia and GM wheat may just be regarded as a step too far, even in countries in which other GM crops are currently accepted.

I wish to thank all of my colleagues and collaborators who have contributed to the work discussed in this article, Professor John Snape and the John Innes Institute for providing Fig. 1 and Dr Jane Ward (Rothamsted) for preparing Fig. 4 . Rothamsted Research receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the UK.

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  • Journal of Economic Perspectives
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Wheat from Chaff: Meta-analysis as Quantitative Literature Review

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Review article, wheat quality: a review on chemical composition, nutritional attributes, grain anatomy, types, classification, and function of seed storage proteins in bread making quality.

literature review on wheat

  • 1 Department of Biochemistry, University of Jhang, Jhang, Pakistan
  • 2 Nuclear Institute for Agriculture and Biology (NIAB), Faisalabad, Pakistan

Wheat ( Triticum aestivum L.) belonging to one of the most diverse and substantial families, Poaceae, is the principal cereal crop for the majority of the world’s population. This cereal is polyploidy in nature and domestically grown worldwide. Wheat is the source of approximately half of the food calories consumed worldwide and is rich in proteins (gluten), minerals (Cu, Mg, Zn, P, and Fe), vitamins (B-group and E), riboflavin, niacin, thiamine, and dietary fiber. Wheat seed-storage proteins represent an important source of food and energy and play a major role in the determination of bread-making quality. The two groups of wheat grain proteins, i.e., gliadins and glutenins, have been widely studied using SDS-PAGE and other techniques. Sustainable production with little input of chemicals along with high nutritional quality for its precise ultimate uses in the human diet are major focus areas for wheat improvement. An expansion in the hereditary base of wheat varieties must be considered in the wheat breeding program. It may be accomplished in several ways, such as the use of plant genetic resources, comprising wild relatives and landraces, germplasm-assisted breeding through advanced genomic tools, and the application of modern methods, such as genome editing. In this review, we critically focus on phytochemical composition, reproduction growth, types, quality, seed storage protein, and recent challenges in wheat breeding and discuss possible ways forward to combat those issues.

1. Overview

Wheat is the most extensively cultivated cereal grain around the globe and holds a crucial place in agriculture ( 1 – 4 ). It is a principal nutriment for 36% of the world’s populace and is propagated in 70% of the world’s cultivated regions. ( 5 , 6 ). Internationally, wheat supplies approximately 55% of the carbohydrates and 21% of food calories consumed worldwide ( 6 – 8 ). It beats every other single grain crop (including rice, maize, etc.) in production and acreage and is grown across a broad range of climatic situations ( 9 ); it is therefore the most significant grain crop on the entire planet ( Table 1 ).

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Table 1 . Classification of Triticum aestivum ( 10 ).

Wheat is of supreme importance among cereals mainly because of its grains, which comprise protein with exclusive physical and chemical attributes. It also encompasses other useful components, such as minerals (Cu, Mg, Zn, Fe, and P), protein, and vitamins (riboflavin, thiamine, niacin, and alpha-tocopherol), and is also a valuable source of carbohydrates ( 11 ). However, wheat proteins have been found to lack vital amino acids; for example, lysine and threonine ( 12 – 14 ).

Wheat production and quality could possibly be enhanced through the development of new and improved varieties that are able to produce a superior yield and perform better under various agro-climatic stresses and conditions ( 15 ). It is the common consensus that the diversity of germplasm in breeding material is an essential component in plant breeding ( 16 , 17 ).

1.1. Wheat background

Wheat was first cultivated approximately ten thousand years ago during the ‘Neolithic Revolution’, which saw a shift from hunting and collecting food to stable land management. Diploid, i.e., genome AA, einkorn, and tetraploid, i.e., genome AABB, emmer, were the first types of wheat to be grown and, according to their hereditary relationship, they originated in the southeastern regions of Turkey ( 18 , 19 ). Cultivation expanded to the Nearby East almost nine thousand years ago with the first appearance of hexaploid wheat ( 20 , 21 ). The evolutionary and genome relationships between cultivated bread and durum wheat and related wild diploid grasses, showing examples of spikes and grain, are shown in Figure 1 ( 20 ).

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Figure 1 . The evolutionary and genome relationships between cultivated bread and durum wheat and related wild diploid grasses, showing examples of spikes and grain.

The previously grown forms of wheat were essentially landraces from wild populations that were carefully chosen by farmers, probably because of their higher yields. However, domestication is also linked with the genetic trait selection of wheat, which is detached from that of its wild ancestors. Two characteristics are of significant importance, the first being spike-shattering loss at maturity, which causes a loss of seeds at the time of harvest ( 22 ). It is a vital characteristic for certifying the dissemination of seeds in genuine populations. The second is the non-shattering characteristic, which has been deduced through alterations at the brittle rachis (Br) locus ( 23 ) and the conversion from husked to free-threshing nude forms, in which the outer sterile husk attaches firmly to the seeds. The unrestricted configurations merge from a deviant on the Q locus that alters the influence of receding mutations at the pertinacious grain husk (Tg) locus ( 18 , 24 , 25 ).

The haploid content of DNA regarding wheat’s six sets of chromosomes ( Triticum aestivum L. em Thell, 2n = 42, AABBDD) is almost 1.7 × 1,010 base pair. It is approximately 100× greater than that of the genome of Arabidopsis , 40× that of rice, and nearly 6× that of maize ( 20 , 26 ). The majority of the DNA sequence of bread wheat is derived from polyploidy, with substantial duplication, in which, repetitive DNA sequences make up 80% of the entire genome ( 27 , 28 ). The typical wheat chromosome is approximately 810 MB, 25× greater than the usual rice chromosome. The developmental history of wheat is illustrated in Figure 2 ( 29 ).

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Figure 2 . Evolutionary history of wheat.

At present, approximately 95% of wheat cultivated throughout the world is hexaploid bread wheat, and the residual 5% is tetraploid durum wheat ( 21 ). The latter is better adapted to the arid Mediterranean environment than to bread wheat and is frequently referred to as pasta wheat to manifest its ultimate specific usage ( 30 ). Small quantities of other species of wheat, like emmer, spelt, and einkorn, are cultivated in few areas, including the Balkans, Spain, Turkey, and the Indian subcontinent ( 20 , 31 ).

1.2. Taxonomic classification

See Table 1 .

1.3. Types of wheat

The genus name for wheat, i.e., Triticum , is derived from the Latin word ‘tero’ (I thresh). The modern name, Triticum aestivum , represents hexaploid bread wheat with genomes A, B, and D, differentiating it from tetraploid macaroni wheat, which is Triticum durum, comprising genomes A and B, and is consumed predominantly for the production of pasta. Nowadays, bread wheat ( Triticum aestivum ) is the most extensively grown wheat. It is a hexaploid type of free-threshing wheat (genome AABBDD). According to Nesbitt and Samuel, it stemmed from the recent hybridization of the diploid (DD) Aegilops tauschii var. strangulate and an allotetraploid wheat (AABB) no longer than 8,000 years ago ( 32 ).

Triticum aestivum and Triticum durum consist of seven chromosome pairs (2n =14). Wheat has been cultivated in the form of spring or winter crops. In extremely cold areas, spring varieties of wheat have been propagated during spring so that they can grow and ripen rapidly and can be harvested before the arrival of the autumn snowfall. Within more temperate areas, winter wheat is propagated prior to the onset of the winter snowfall that otherwise covers the saplings, resulting in vernalization and allowing quick growth when the snow thaws in the spring. In warm environments, peculiarity in spring and winter wheat is almost futile. The point of significant difference is early or delayed maturity ( 33 ). Types of wheat have been frequently differentiated according to endosperm texture, seed coat, dough strength, color, and planting season. These are concisely explained as follows ( 34 ).

1.3.1. White and red wheat

Red wheat variants usually have greater latency than white variants and have therefore been preferred in environments that are favorable to harvesting before germination. White wheat variants are suitable for growth in regions that are arid throughout the course of ripening and harvesting and are ideal for manufacturing flat noodles and bread ( 35 ).

1.3.2. Soft and hard wheat

Although there are several wheat varieties grown around the world, they all fall into two essential categories ( Table 2 ) with distinct properties: hard wheat and soft wheat ( 37 ). Variety and seed stability in hard and soft wheat are related to resistance to being crushed ( 35 ).

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Table 2 . Principal cultivars of soft and hard wheat cultivated in the world ( 36 ).

1.3.3. Weak and strong wheat

The production of leavened bread is chiefly restricted to the full DNA sequence code for the proteins required for making a strong and elastic dough that is appropriate for capturing gas bubbles during fermentation, allowing the dough to upsurge. The exclusive pliable attributes of dough are principally the result of the amount and type of gluten present. Varieties with high gliadin glutenin contents are viscous and produce expansible doughs, which are appropriate for preparing cookies, for example, while varieties with a small gliadin glutenin content have greater strength and elasticity, which is ideal for bread making.

The difference in alleles in high-molecular-weight glutenins is nearly associated with the quality of bread making and the ability of the dough to withstand. Bread wheat varieties have three or five main high-molecular-weight glutenin subunits ( 38 , 39 ). Glu-D1 genes encode two of these subunits, one or two are encoded by Glu-B1, and either none or one may be encoded by Glu-A1 ( 35 , 40 ). More than 50% of the difference in the baking potential and viscoelastic attributes of dough depends on the wheat’s composition of high-molecular-weight subunits of glutenin ( 35 ).

1.3.4. Spring and winter wheat

These varieties are diverse in their need for a frozen phase to allow normal development and reproductive growth. This need for vernalization has been vigorously affected by changes at the Vrn-1 position (present on the long arms of group 5 chromosomes), such as Vrn-B1, Vrn-A1, and Vrn-D1, and their superficial adjustment by negligible flower-inducing genes ( 41 ). Spring habits result from the dominant Vrn-la on any of the three genomes of wheat and the presence of the recessive, while winter habits result from alleles on Vrn-1b on all three genomes. However, Vrn-1 genes have a close association with the genes providing resistance to cold ( 42 ) and, thus, persist in winter ( 34 ).

Wheat development is largely determined by temperature, the requirement of a cold phase, variety, and plant responses to the corresponding lengths of dark and light periods during their developmental phase. As previously stated, winter wheat variety maturation was found to be accelerated due to the flowering process in plants, i.e., with low-temperature exposure, usually 3–10°C, for 6 to 8 weeks. Growth has also been enhanced through long day exposure which meaning growth is enhanced through longer period of light as the days lengthen in spring. As the varieties differ in their responses to vernalization, temperature, photoperiod, and the extent of interaction between certain factors, they vary continuously in their maturation rate and, therefore, contribute to the broader distribution and adaptation of wheat in agriculture globally ( 34 , 43 ).

1.4. Vegetative growth

1.4.1. development of wheat seed.

Wheat seeds require moisture levels of 35–45% for germination ( 35 , 44 , 45 ). During propagation ( Figure 3 ) ( 46 ), the adventitious side root outspreads earlier than the coleoptile. Seminal roots are generated in relation to the node of the coleoptile. When coleoptile arises from the soil, its development halts and the first true leaf propels to its end. Seedlings rely on nutrients and energy supplied through the endosperm until their first leaf is photo-synthetically efficient ( 35 ).

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Figure 3 . Life cycle of wheat.

1.4.2. Root growth

More than one node can grow in the soil based on sowing depth, all exhibiting roots ( 47 ). The root axis grows during expected periods in association with shoot growth, and the overall number of roots generated is linked with the number of leaves present on lateral branches and the extent of tillering ( 45 ). Roots emerging from lateral branches usually spread once the tillers have developed three leaves. A variety’s root development is analogous to its apex extension ( 35 ).

1.4.3. Leaf growth

After germination, the apex of a vegetative shoot gives rise to secondary leaf primordia. Leaf primordial count can differ from seven to fifteen ( 35 ) and is influenced by light strength, the nutritional level of the plant, and temperature. Temperature imparts a significant effect on the emergence of leaves and expansion. The lowest temperature withstood for the expansion of leaves is approximately 0°C, the optimal temperature is 28°C, and the maximum is 38°C ( 35 , 48 ).

1.4.4. Stem development

Stem elongation overlaps with the growth of tillers, leaves, inflorescence, and roots ( 49 ). Stem elongation initiates when the maximum number of florets are present on the evolving spike, introducing the stamen’s initial identifiable stage, which resembles almost terminal spikelet development. The fourth internode, with nine leaves, is the first to extend in spring wheat, whereas the stem’s lower internodes remain short. Once an internode is extended partly to its ultimate extent, the internode above it starts to extend. This continues until the elongation of the stem is complete, generally close to anthesis.

The peduncle is the last segment to extend. The height of the wheat plant extends from 30 to 150 cm depending on the variety and the propagating state. Alterations in plant stature are generally attributed to the differences in internode dimensions and not to the internode number ( 35 , 50 ).

1.4.5. Tiller growth

The first lateral branches to arise are formed between the coleoptile axils and the first true leaf. Generally, three intervals between two successive leaves divide the leaf emergence and its subtended tiller. In winter wheat, small numbers of tillers develop in winter or autumn if circumstances are moderate. The main shoot and initially developed tillers fulfill their growth and develop granules in spring and winter wheat ( 51 ).

1.5. Reproduction

Wheat is principally an intra-floral pollinated crop. However, the rate of outcrossing is up to 10% or greater on the basis of genotype, population density, and environmental conditions. Cross-fertilization due to wind depends greatly on physical aspects such as excessive humidity and warm climates ( 52 ). Dry, warm climates give rise to increased cross-fertilization rates, i.e., 3.7–9.7% in comparison to the insignificant cross-fertilization rates of 0.1% under high-moisture conditions ( 53 ). Allogamy in wheat has been observed as high as 1–2%. Flowering time and duration depend on geographical location. Sunny climates and temperatures of at least 11–13°C are necessary for blooming ( 54 ).

1.5.1. Spike growth prior to anthesis

The shift to propagative growth takes place close to apical cupola elongation, once the core shoot has almost three complete leaves. Floret division initiates in the crucial portion of the spike and continues both up and down as spikelet induction is completed. It creates a growth pyramid inside the prickle, which continues throughout grain growth and anthesis. Terminal spikelet instigation indicates the completion of spikelet formation ( 55 ).

During pre-anthesis, various developmental phases synchronize with one another ( 46 ). Kirby identified a difference of several weeks in the instigation of numerous shoots on a plant, which is decreased to just a few days in the period of spike appearance. Likewise, variation in the spikelet initiation period between the two early clusters in a fused flower could span 2 days; however, the difference in the duration of meiosis of these flowerets is around 6 h ( 56 ). Once the pollen-comprising stamen part elongates up to 1 mm and is green, meiosis takes place instantaneously in the pistil and anthers ( 55 ). The duration for which wheat flowers remain open varies from 8 to 60 min depending on environmental conditions and genotype ( 57 ).

1.5.2. Kernel growth

The ratio of multiplication of the endosperm cell is affected by water stress, light intensity, genotype, and temperature ( 58 , 59 ). The accumulation of starch starts at 1 to 2 weeks following anthesis and begins a 2 to 4-week period of direct rise in a dry mass of kernel ( 60 , 61 ). The development and ultimate mass of a single kernel are determined by spikelet and floret site; grains that are developed in proximal florets and middle spikelets are generally very large ( 56 , 60 , 62 ). When rain coincides with harvesting, germination takes place. Seeds ripened in cold conditions are more latent compared to seeds matured in warm environments ( 63 ).

1.6. Grain anatomy

Wheat grain is divided into three main segments, all structurally and chemically distinguished from erstwhile. These are: the germ, also called the embryo, which is located at a single end of the grain in the form of a tiny, yellow mound, simply differentiated from the rest of the kernel; the endosperm, which covers a larger part of the whole grain and supplies nutrition to the developing plant as the kernel evolves; and the external seed crust and cover lying underneath, which contains protein cells that cover the whole kernel and protects the embryo and the endosperm on or after injury during the grain’s subsistence (latent phase) ( 64 ). Regarding the unique roles of all three parts, a significant difference exists in the chemical composition of their constituents and, therefore, a broader variation is found in their nutritional value ( 65 ).

Wheat kernels are usually elliptical, though different types of wheat have kernels that vary from virtually long, trampled, slender, and spherical in shape. The length and mass of the kernel are typically around 5–9 mm and 35–50 mg. It features a crinkle below the lateral side and it was therefore initially associated with the wheat flower. The wheat kernel ( Figure 4 ) ( 65 ) encompasses 2–3% of the germ, 13–17% of the bran, and 80–85% of the mealy endosperm (entire elements altered to dehydrated material) ( 66 ).

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Figure 4 . Chemical constituents in different parts of wheat grain.

Wheat fiber is made up of several layers of cells which, when the seed is dry, adhere so firmly to one another that they are removed by the milling process in comparatively large pieces. By shifting and other mechanical means, almost all the embryo and endosperm are removed. However, as the separation is never perfect, even the purest commercial bran always contains a little endosperm and possibly traces of embryo ( 67 ). Chemically, as well as structurally, bran differs significantly from the embryo and endosperm and, consequently, its nutritive value also differs. The longitudinal and transverse section of the wheat grain is shown in Figure 5 ( 68 ).

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Figure 5 . Wheat grain structure in longitudinal and transverse section.

The bran, also known as the seed coat, which is present on the external layers of the wheat kernel and composed of numerous layers, is responsible for providing protection to the central part of the kernel. The seed coat is enriched with minerals and vitamin B ( 14 ). The bran is detached from the endosperm containing starch during the initial step of milling. The bran consists of fiber that is not soluble in water so as to protect the kernel and endosperm. It contains 53% of the cellulosic components.

Wheat fiber has a complicated chemical configuration; however, it comprises pentosans and cellulose, polymers founded on arabinose, and xylose firmly attached to the proteins. These elements are typical polymers found in the cell layers, like the aleuronic layer and the wheat cell wall. Carbohydrates and proteins both signify 16% of the bran’s entire dry mass. The value of minerals is somewhat high, at 72%.

The two outer strata of the grain, the pericarp and the seed cover, are composed of inactive hollow cells. The internal bran sheet, the aleuronic sheet, is packed with active contents of plant cells ( 69 ). This somewhat illustrates the elevated concentrations of carbohydrates and protein in the bran. Significant variation exists in the particular level of amino acids that is present in the flour and the aleurone layer. The level of proline and glutamine is nearly half, whereas arginine is triple, and histidine, asparagine, lysine, alanine, and glycine are double the level in flour ( 65 ).

The endosperm forms approximately 84% of the entire seed, its proportions varying with the plumpness of the grain. This part of the seed provides food for the growing embryo. Unlike the constituents of the embryo, those of the endosperm are relatively stable substances, designed by nature to remain unchanged until the germinating embryo draws on them for its first supply of food ( 21 ).

The endosperm is enclosed via the fused seed coat and the pericarp. In the external endosperm, the aleuronic sheet holds a unique configuration ( 70 ). It is made up of a single sheet of cubical cells. Proteins and enzymes are abundant in the aleuronic sheet and perform a critical role during propagation. The internal endosperm lacking an aleuronic sheet is observed to be the mealy endosperm. In contrast to the other parts of the seed, the endosperm is characterized by its very high starch content, which, together with the protein, equals nearly 89% of its whole composition ( 71 ). The endosperm largely encloses food assets required for the development of seedlings and is full of starch. Besides carbohydrates, the starchy endosperm consists of 15% fats and 13% proteins, such as globulins, albumins, glutenins, and gliadins. The amount of nutritional fibers and minerals is low, at 0 or 5% and 1 or 5%, respectively ( 66 ).

The germ on one side of the kernel is enriched with 25% proteins and 8–13% lipids. The level of minerals is relatively high (4–5%). Wheat germ is obtained as a by product during wheat milling ( 65 ).

1.7. Chemical constituents of Triticum aestivum

The main chemical components of wheat are given below.

1.7.1. Vitamins and minerals

Vitamin B5, B1, B6, B3, B8, B2, B12, K, E, and A; ascorbic acid; boran; dry ascorbic acid; iodine; sodium; carotene; group 2 metals of the periodic table; magnesium; molybdenum; potassium; zinc; aluminium; copper; phosphorus; sulfur; Iron; and selenium ( 72 ).

1.7.2. Enzymes

Superoxide dismutase, protease, amylase, lipase, transhydrogenase, cytochrome oxidase ( 73 ).

1.7.3. Supplementary constituents

Amino acids, e.g., valine, asparagine, aspartic acid, alanine, glutamine, proline, methionine, glycine, phenylalanine, threonine, leucine, arginine, tryptophan, isoleucine, tyrosine, serine, histidine, and lysine; mucopolysaccharides; chlorophyll; P4D1, i.e., glucoprotein; bioflavonoids, such as apigenin, luteolin, and quercitin; laetrile; indole complexes; and choline, i.e., amygdalin ( 74 – 76 ).

1.8. Nutritional attributes of wheat

Wheat grains and their products are significant constituents of our daily diet. The average wheat consumption is 318 grams per person each day, making up 83% of the overall cereal consumption ( 72 ).

Wheat contributes a larger percentage of protein than energy to the nutritional requirement of an adult male. Alone, it can fulfill the daily requirement of niacin and thiamine. The majority of the daily riboflavin and iron requirement is fulfilled by the quantity of wheat recommended for an adult male ( Table 3 ).

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Table 3 . Nutrients and calories supplied by the wheat as % of suggested daily allowance for adult male ( 77 ).

Wheat is predominantly considered a source of protein, vitamins, calories, and minerals. It is comparable with various cereals in nutritional content. Its protein content is higher than sorghum, rice, and maize and about equivalent to that of other cereals. The protein content is influenced by a variety of cultural and environmental conditions, such as soil temperature, moisture, availability of nitrogen, and method of cultivation. The percentage of protein in wheat can be influenced to a certain extent by the time of fertilizer application and fertilizer type ( 72 ).

The nutritional content of protein is estimated not simply based on the concentration of protein but also the amino acid equilibrium within the protein. During human digestion, protein is broken down into its constituents, absorbed by the bloodstream, and then assembled again to form different types of protein required by the body for growth, maintenance, and repair ( 21 ). Eight amino acids are vital for humans as the body is unable to produce them and must take them from food.

The biological significance of wheat is determined by limiting essential amino acids. These amino acids become deficient due to the body’s increased requirements. Lysine is the deficient amino acid in wheat ( 64 ). During the process of milling, one-third of the total protein is removed along with lysine as the majority of the protein and lysine is present in the bran and the germ ( 14 ). There is an inverse relationship between the quantity of protein in grain and the quantity of lysine/grams of protein ( 77 ).

1.9. Wheat quality

Wheat quality has two main characteristics: external quality and internal quality. External quality involves freedom from foreign material and weather damage, type, and purity of color. These factors are used to separate wheat into visual grades ( 72 ). Internal factors involve parameters such as density, which is determined by evaluating the test weight; chemical composition, which includes protein content; moisture; and processing potential, which comprises milling quality, end-use quality, and enzyme activity ( 5 ).

1.9.1. Classification and function of wheat proteins

Protein is regarded as the most significant nutrient for animals and humans, as the name of its origin indicates (“proteios,” meaning primary in Greek). The protein content varies from 10–18% of the entire dry mass of the wheat grain ( 72 ). Proteins determine the capability of wheat flour, which can be dispensed into diverse foodstuffs. Wheat proteins have an important role in carbon dioxide retention, dough development, and baking quality due to their quantitative qualitative and quantitative attributes ( 78 ). Mature wheat grains contain 8–20% protein. The proteins in wheat display great intricacy and diverse collaboration with one another, rendering them hard to describe ( 79 , 80 ).

Wheat proteins have been classified ( Figure 6 ) ( 81 ) by their enforceability and solubility in various diluents. Cataloging was conducted on the basis of Thomas. D. Osborne’s work from the shift of the previous era ( 82 ). According to his method, serial withdrawal of crushed wheat kernels gives rise to protein properties as follows:

• Water soluble albumins

• Globulins, not soluble in natural water but soluble in diluted solution of sodium chloride while

• insoluble at high NaCl concentrations

• Gliadins, soluble in 70% ethanol

• Glutenins, soluble in diluted NaOH or acid solutions

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Figure 6 . Types of wheat proteins.

Albumins and globulins are the smallest wheat proteins. The partition of globulins and albumins was not clear as originally recommended by Osborne. Gliadins and glutenins represent complex proteins of high molecular weight ( 83 ). The maximum number of wheat kernel proteins that are physiologically active has been found in globulin and albumin sets. In mueslis, they are stored in the germ, seed coverings, and aleuronic cells, with a low concentration in the starchy endosperm. Globulin and albumin constitute nearly 25% of all kernel proteins ( 79 ).

Traditionally, protein in wheat grains has been divided into prolamins and non-prolamins. The prolamins consist of gliadins and glutenins, while the non-prolamins include salt and water-soluble globulins. Albumin and globulin proteins concentrate during the initial phase of grain development, after which, the content of these proteins remains constant from 10–15 days after flowering (DAF) onwards, the albumins and globulins tend to accumulate in the emerging starchy endosperm from 10 to 15 DAF, involving primarily trypsin inhibitors, α, β-amylase, and triticins. The characteristics of wheat flour quality depend on the prolamin content and composition in the endosperm, whereas the role of albumins and globulins in the development of flour quality is not defined as well as that of prolamins ( 66 ).

Albumins and globulins are primarily metabolic enzymes, which have a role in numerous metabolic events during the course of grain filling, comprising starch synthesis, protein synthesis, folding, and energy metabolism. Storage proteins (gliadins and glutenins) constitute approximately 75% of the overall protein content. Wheat crops accumulate proteins in this way for seedling usage in advance. They are usually found in the starchy endosperm, not in the germ or seed coat sheet. Wheat storage proteins are technically dynamic. They lack enzyme action, but they perform a role in dough development; for example, these proteins are able to hold gas, generating soft baked foodstuff ( 66 ).

Albumins and the wheat endosperm’s globulin cover 20–25% of the total grain protein. Globulins and albumins have an excellent amino acid equilibrium with regard to nutrition. Several of these such proteins (enzymes) are involved in metabolic actions ( 79 ).

Wheat is exclusive among the palatable kernels, as its flour possesses a complex protein known as gluten, which, when prepared as dough, has viscous and elastic characteristics and is essential for manufacturing leavened bread. The rheological characteristics of gluten are required not merely for bread manufacture but for a broad range of foodstuff that is only prepared using wheat, like cookies, pastries, pitta bread, pasta, noodles, etc. The proteins in gluten include monomeric and polymeric gliadins and glutenins. Glutenins and gliadins are considered the main storing proteins of wheat, representing around 75–85% of the overall seed proteins, with a ratio of nearly 1:1 in bread and common wheat. They are enriched with proline, glutamine, asparagine, and arginine, but nutritionally significant amino acids, like tryptophan, lysine, and methionine, are present in small amounts ( 84 ).

The gliadins, which represent 30–40% of all total proteins in flour, are a polymorphic blend of proteins that are soluble in 70% alcohol. They are separated into alpha, beta, gamma, and omega gliadins, with an MW of 30–80 kilo Daltons, as defined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The omega gliadin MW is in the range of 46–74 kilo Daltons, while alpha, beta, and gamma are low molecular weight gliadins, ranging from 30–45 kDa by amino acid sequencing and SDS-PAGE. Recent methodology has revealed a close link between alpha and beta gliadins and, thus, these are frequently called alpha-type gliadins ( 82 ).

Gliadins are freely soluble in dilute alcohols, except glutenin polymers; however, their subunits have the ability to be dissolved in a similar way to the gliadins. The subunits of glutenin can be acquired through the treatment of glutenin using a disulfide reducing agent; for example, β-mercapto-ethanol or dithiothreitol. Gliadins and glutenin subunits both have unexpectedly high levels of glutamine and proline. Hence, it has been suggested that these storing proteins be called ‘prolamins’, as they display strong similarities with most storage proteins in associated cereals, such as rye or barley. Residues of cysteine have an important role in the structure of gliadins and glutenin subunits. These residues have a role in either disulfide bonds inside similar or different polypeptides, i.e., intra-chain disulfide bonds, or inter-chain disulfide bonds ( 85 ).

Gliadins have shown an extremely varied fusion of a monomeric form of gluten proteins. Three anatomically different gliadins, alpha, gamma, and omega, can be illustrated. The evaluation of amino acid sequences has shown that α-and γ-gliadins are linked to low-molecular-weight glutenin subunits. For that reason, they have been categorized as ‘prolamins enriched with sulfur’. Residues of cysteines are situated at alpha type six remains of cysteine, and gamma type eight remains of cysteine gliadins have been found at extremely preserved sites and have a role in preserved intra-chain bonds of disulfide. Conversely, cysteine residues are absent in ω-type gliadins and possess a very small amount of methionine. So, these gliadins have been termed ‘sulfur-poor prolamins’ ( 73 ).

Polymers of glutenin are composed only of polypeptides related through the disulfide bonds present between the molecules, which account for approximately 45% of the total protein inside the kernel endosperm. Wheat protein consists of two types of subunits, the LMW 10,000–70,000 Da and the HMW subunits of glutenin 80,000 to 130,000 Da. Studies on glutenin genetics of wheat have shown the presence of high-molecular-weight glutenin subunit genes on the 1A, 1B, and 1D (extended chromosome arm) at the Glu-B1, Glu-D1, and Glu-A1 positions, respectively. Tightly linked genes (two) are found in each Glu-1 locus encoding x or y subunit types. In Triticum aestivum , the Glu-A1 locus encodes null (N) subunit and 1Ax, while the Glu-B1 locus commonly codes for 1Bx and 1By. Occasionally, Glu-B1 codes for 1Bx or 1By subunits, whereas the Glu-D1 locus codes for 1Dx and 1Dy subunits ( 85 ). As a result, for hexaploid wheat, three to five HMW-GS are usually produced by each genotype ( 85 ).

Electrophoretic studies have shown a significant alteration in mobility and the number of HMW-G subunits in pasta and bread wheat. LMW-GS constitutes around one-third of all the protein in the seeds and 60% of all the gluten protein ( 86 ). The LMW-S looks like gamma gliadins in sequence and consists of roughly 20–30% of the total protein, whereas the high-molecular-weight subunits constitute around 5–10% of the total protein ( 85 ).

Low molecular weight proteins that are abundant in cysteine might affect the viscoelastic characteristics of dough through disulfide or sulfhydryl exchange reactions with the proteins of gluten. Proteins capable of binding lipids can influence gluten-lipid protein interactions, and consequently, the functionality of protein in gluten ( 87 ). According to the evidence, stowage globulins that are polymeric in nature are related to few bread-making functions. In contrast to non-gluten proteins, gluten proteins are sparingly soluble in water or dilute salt solutions ( 85 ). A low quantity of ionizable side chain amino acids and an elevated level of non-polar amino acids and glutamine are the factors that contribute to its low solubility. The latter has better hydrogen bonding capabilities ( 73 ).

1.9.2. Gluten proteins and wheat flour’s bread-making function

Proteins of gluten principally define the bread-making capability of wheat flour. A gluten protein allows the formation of cohesive viscoelastic dough when the flour is mixed with water and is able to retain gas produced in the process of fermentation or baking, forming bread’s exposed form configuration after baking. The viscoelastic attributes of dough that are crucial for bread manufacture are mainly regulated by the gluten proteins of wheat, but the collaboration between the gluten protein medium and the additional constituents of flour, such as lipids in flour ( 88 ), arabinoxylans ( 89 ), and non-gluten proteins, also have an influence on its viscoelastic properties. These properties of wheat gluten are altered further by the addition of oxidants, proteases, and reducing agents, which immediately modifies the gluten proteins, or by the addition of emulsifiers, lipids, and hemicelluloses, which alters gluten protein interactions.

The bread manufacturing ability of wheat flour is directly associated with the protein content in flour ( 90 ) and, therefore, with the gluten protein content, as this type of protein rises more than non-gluten protein due to its increased grain protein content. Therefore, an increased ‘amount’ of proteins in gluten is crucial. Nevertheless, the direct correlation between breadmaking performance and protein content relies on the genotype of the wheat, indicating that the quality of ‘gluten’ protein is also of significance in the overall quality of the wheat ( 82 , 85 ).

A sufficient viscoelasticity equilibrium or the potential thereof is mandatory for excellent breadmaking. Inadequately flexible gluten will lead to decreased loaf size. Improved elasticity indicates increased loaf volume; however, excessively elastic gluten inhibits gas cell expansion ( 91 ), also causing decreased loaf size. Glutenin polymers are responsible for the strength and elasticity of dough ( 85 ). The glutenin elasticity is thought to be influenced by flexible stretching of actively and more favorably folded conformation of glutenin. Belton ( 92 ) suggested that gluten elasticity is due to non- covalent interaction mediate gluten elasticity primarily hydrogen bonds, inside and between single glutenin chains. Conversely, gliadins are plasticizers that deteriorate glutenin chain interaction ( 93 ), thus increasing the viscosity of dough. Therefore, the proportion of monomeric gliadin-polymeric glutenin defines the equilibrium in the viscoelasticity of dough and consequently influences the gluten protein value. Today, it is usually thought that quality differences are strongly influenced by alterations in the quality of glutenin ( 85 ).

1.10. Current challenges in wheat breeding programs and improvement approaches

An increase in the world’s population of almost 10 billion is expected by 2050, which will result in an increase in wheat demand at a rate of approximately 1.7 annually ( 94 ). Therefore, creating a sufficient supply response will persist as a policy challenge throughout the 21st century. Wheat breeders’ responsibility and their role in developing better varieties of wheat are becoming more significant in the improvement of crop production ( 95 ). Wheat productivity is vulnerable to newly developed diseases and pests, inadequate water resources, limited arable land, and quickly altering climatic situations ( 94 , 96 ).

Numerous diseases, like rusts (stripe, leaf and stem rust, powdery mildew, spot blotch, Karnal bunt, and Fusarium head blight) severely hamper wheat productivity ( 97 ). Several researchers have described rusts as a major biotic stress in wheat, causing a wheat loss of 10–100%, depending upon virulence factor, resistant/susceptible cultivar genotype, initial time of infection, environment, pathogenesis ratio, and disease duration ( 98 ).

Puccinia graminis f. sp. tritici (stem rust) can result in a yield loss of up to 100%, and the sudden rise and spread of stem rust in Africa, known as Ug99, to Iran, the Middle East, and other countries is a severe concern for wheat production worldwide. Tilletia indica (Karnal bunt) disease is not only responsible for yield loss but also affects the quality of grain due to the infection of kernels. This disease was detected in various other countries, such as Mexico, Pakistan, India, Iran, Nepal, Afghanistan, Iraq, South Africa, and the United States ( 94 ).

The Fusarium culmorum and Fusarium graminearum species of Fusarium Head Blight/head scab cause grain to become infected with mycotoxins, such as nivalenol (NIV), deoxynivalenol (DON), and zearalenone (ZON). Yield losses occur due to shriveled grain, low test weight, and failure of seed formation ( 99 ). Spot blotch (SB), a vicious wheat leaf disease initiated by Cochliobolus sativus can cause a 70% yield loss. Composite quantitative inheritance of SB resistance has reduced breeding progress ( 100 ). Another harmful disease is the biotrophic fungus Blumeria graminis , a powdery mildew (PM), which is a universally occurring wheat foliar disease responsible for terrible yield loss ( 101 ).

In contrast to biotic stress resistance, the resistance gene plays a small role in defending wheat against insects due to the substantial effect of temperature and light on the existence and performance of the insects ( 94 ). Continuous damage to crop production caused by pests and diseases is one of the key limitations in wheat breeding and, therefore, food sufficiency globally. Aphids, termites, wheat midges, wheat weevil armyworms, Hessian flies, and cereal cyst nematodes (CCN) are the main arthropods feeding on wheat among various pests ( 102 ), making it essential to recognize novel genes and know their interactions and functions in resistance to CCN ( 103 ).

Abiotic stresses like drought, salt, and terminal heat stress are important as they limit wheat production and pose a substantial challenge to wheat breeding programs internationally ( 104 ). Climate change is another factor that has resulted in a wheat loss of 33% globally because of temperature increases and water shortages in wheat-growing areas ( 105 , 106 ). The induction of chromosome restitution in meiosis at the time of male gamete development is a major problem caused by climate variation. Heat stress at the terminal stage of the wheat crop halts plant growth and the accumulation of starch, causing yield fickleness ( 94 ). On the other hand, global warming brutally disturbs weather patterns, ensuing temperature extremes, frequent frost, drought, and snowfall ( 94 ). Complex interactions of cellular and molecular mechanisms with whole-plant adaptation have limited breeding approaches to heat resistance ( 104 ).

The complex wheat genome and barriers in hybridization pose major challenges in identifying and understanding various gene functions, thus making the manipulation and characterization of traits of concern very difficult in the development of better varieties ( 107 ). Therefore, in order to understand several networks of genes and their jobs in the wheat genome, the continued characterization of traits of landraces and wild relatives is still needed for rapid progress in the improved development of cultivars ( 98 ).

Plant breeders need to find new resistance genes by manipulating wheat germplasm, which is essential in combatting such insect/pest diseases. Genome-wide association studies (GWAS) or QTL mapping can be employed to find genes with drought resistance in unexplored germplasm. The use of genes/transcription factors from wheat germplasm like DREB, NHX2, AVP1, and SHN1 and their associated markers is a viable method for producing salt-tolerant wheat genotypes ( 104 ). QTLs in combination with R-specific resistant genes provide effective and durable resistance in different environments ( 108 ). Both approaches are suggested to deal with climate change.

Precise pre-breeding and selection approaches need to be carefully designed and followed for the identification and exploitation of the most effective and resilient loci, pyramiding, and partially tolerant gene accumulation. Identification, cloning and modification, and transfer of various R genes to diverse crop species through conventional breeding methods, molecular Marker Assisted Selection (MAS), and biotechnological tools, such as OMICS (genomics, transcriptomics, proteomics, metabolomics, etc.), can be used to combat pests and diseases and achieve long-lasting resistance ( 108 ).

Tools in tissue culture techniques, like micropropagation, gametic embryogenesis ( 103 , 109 ), somatic embryogenesis, cell suspension, and protoplast fusion facilitate the fast, large scale-cloning of high-value plants to produce pure lines ( 109 ). Apart from plant breeding technologies, some abiotic factors can be reversed through environmental management practices and developing microbe linkage to plants to biologically control pathogens ( 110 , 111 ). Diethyl aminoethyl hexanoate application is found to increase plant tolerance to abiotic stress, such as cold ( 112 ) and salt stress, ( 113 ) whereas applying proline amino acid during plant adaptation increases tolerance to salinity stress ( 114 ).

The incorporation of in vitro approaches, like protoplast fusion ( 109 ), gametic embryogenesis ( 103 ), somatic embryogenesis, mutagenesis, and plant cell/tissue culture, and the current biotechnology practices, like synthetic biology, transgenic plants, gene editing ( 115 ), OMICS technologies, and interference RNAs ( 116 , 117 ), can enhance tolerance to biotic and abiotic factors and control the depth of plant roots ( 103 ). Another way to increase resistance is by using microbial biotechnology to enhance plant nutrition and/or promote biocontrol against pathogens ( 110 ), applying diethyl aminoethyl hexanoate to achieve resistance to abiotic stress ( 116 , 118 ), and using proline to increase salt tolerance ( 119 ) and improve poor water conditions ( 114 ).

Gene stacking can be used to combat disease-resistant genes and inherit them as a sole trait ( 120 ). The incorporation of even more disciplines is needed for breeding to reach the ultimate ‘deterministic’ phase and catch up with the situation in genomics, in silico breeding and phenomics, etc. Therefore, public sectors must incorporate novel technologies into Mendelian genetics and the principles of quantitative genetics in order to make dynamic alterations in crop production ( 94 ).

Author contributions

AK: write-up and revision of manuscript. AH: planning, finalization of basic idea, and revision. MT: revision of manuscript. All authors contributed to the article and approved the submitted version.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Keywords: wheat, HMW-GS, LMW-GS, grain anatomy, nutritional quality

Citation: Khalid A, Hameed A and Tahir MF (2023) Wheat quality: A review on chemical composition, nutritional attributes, grain anatomy, types, classification, and function of seed storage proteins in bread making quality. Front. Nutr . 10:1053196. doi: 10.3389/fnut.2023.1053196

Received: 25 September 2022; Accepted: 26 January 2023; Published: 24 February 2023.

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Copyright © 2023 Khalid, Hameed and Tahir. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Anam Khalid, ✉ [email protected]

Deep learning in wheat diseases classification: A systematic review

  • Published: 14 February 2022
  • Volume 81 , pages 10143–10187, ( 2022 )

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  • Deepak Kumar 1 &
  • Vinay Kukreja   ORCID: orcid.org/0000-0002-9760-0824 1  

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The main goal of this paper is to review systematically the recent studies that have been published and discussed WD prediction models. The literature analysis is performed based on studies published from January 1997 to February 2021 by following Kitchenham instructions. After inclusion/exclusion and quality assessment criteria screening, a total of 74 studies have been selected. The literature shows that WD is categorized into three (fungal diseases, bacterial diseases, and insect diseases) types. The research analysis shows that most of the work in the literature has been found on wheat stripe rust (60.81%) disease and the most used prediction technique is ANN (13.32%). The results show that accuracy (67%) is the most prominent performance metric and in the year 2020, a maximum number of papers are published on WD. Also, only five studies have used hybrid approaches which are the combination of SVM and NN techniques.

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Kumar, D., Kukreja, V. Deep learning in wheat diseases classification: A systematic review. Multimed Tools Appl 81 , 10143–10187 (2022). https://doi.org/10.1007/s11042-022-12160-3

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Bread wheat: a role model for plant domestication and breeding

Eduardo venske.

1 Plant Genomics and Breeding Center, Crop Science Department, Eliseu Maciel College of Agronomy, Federal University of Pelotas, Capão do Leão Campus, Capão do Leão, Rio Grande do Sul 96010-610 Brazil

Railson Schreinert dos Santos

Carlos busanello, perry gustafson.

2 Plant Sciences Division, 1–32 Agriculture, University of Missouri, Columbia, MO 65211 USA

Antonio Costa de Oliveira

Associated data.

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.

Bread wheat is one of the most important crops in the world. Its domestication coincides with the beginning of agriculture and since then, it has been constantly under selection by humans. Its breeding has followed millennia of cultivation, sometimes with unintended selection on adaptive traits, and later by applying intentional but empirical selective pressures. For more than one century, wheat breeding has been based on science, and has been constantly evolving due to on farm agronomy and breeding program improvements. The aim of this work is to briefly review wheat breeding, with emphasis on the current advances.

Improving yield potential, resistance/tolerance to biotic and abiotic stresses, and baking quality, have been priorities for breeding this cereal, however, new objectives are arising, such as biofortification enhancement. The narrow genetic diversity and complexity of its genome have hampered the breeding progress and the application of biotechnology. Old approaches, such as the introgression from relative species, mutagenesis, and hybrid breeding are strongly reappearing, motivated by an accumulation of knowledge and new technologies. A revolution has taken place regarding the use of molecular markers whereby thousands of plants can be routinely genotyped for thousands of loci. After 13 years, the wheat reference genome sequence and annotation has finally been completed, and is currently available to the scientific community. Transgenics, an unusual approach for wheat improvement, still represents a potential tool, however it is being replaced by gene editing, whose technology along with genomic selection, speed breeding, and high-throughput phenotyping make up the most recent frontiers for future wheat improvement.

Final consideration

Agriculture and plant breeding are constantly evolving, wheat has played a major role in these processes and will continue through decades to come.

Bread wheat ( Triticum aestivum L.) is one of the most important crop species, responsible for the emergence and development of agriculture and has fed, and continues to feed, a large part of the world’s population across many centuries [ 97 , 106 ]. Wheat has been improved by man over the last 8000 to 10,000 years ago when the species first arose. Initially it happened in an unconscious way, then intentionally, but empirically, and then, for more than a century, based on scientific knowledge [ 18 , 64 ].

Wheat breeding, as for many other crops, has been evolving fast, both in terms of basic science, methods and tools. The literature on wheat breeding is vast, including countless scientific papers, reviews and even dense book collections already published. Therefore, all relevant aspects and examples cannot be covered in a single text. On the contrary, we do encourage readers to go through this bibliographic ever growing wealth for a deeper understanding on any given topic. Thus, the objective of this review is to provide a brief and valuable synthesis on some selected aspects related to the history, but especially, current advances in wheat breeding, devoted especially to students and researchers with little or even no knowledge on the theme. Through this review, the reader can have a quick and general overview on the discussed topics and, when necessary, get a direction to start searching for further literature, as we have tried to cite the most important and recent papers on each topic. Therefore, in the next sections we show the origin of this species and how it became so important with a brief history of wheat cultivation and breeding. Priorities and particularities of wheat breeding are presented. Special consideration is given to new approaches and tools that are currently under development, and the ones that lately reappeared. Finally, the promising future and perspectives are discussed.

Origin and importance

One of the fathers and lifelong ally of agriculture.

Bread or common wheat is undoubtedly one of the most important cultivated plants, in fact, in addition to its ancestry, the cereal represents a large part of the history of agriculture itself [ 8 , 18 , 44 , 58 , 93 , 97 ].

Today, wheat is the basis of a significant part of the world’s diet, being an important source of energy (providing ca. of 20% of world population demand), and protein (also providing ca. 20%), as well as vitamins and other beneficial compounds, not only for humans, but also as animal feed [ 42 , 106 ].

It is grown from 67° North to 45° South, including a wide range of altitudes, but it is less cultivated in tropical regions [ 33 ]. In 2016, more than 749 million tonnes of this cereal were produced on 220 million hectares around the world, which puts wheat in second place regarding production among the cereal crops (behind maize - Zea mays L.) but in the first position regarding area harvested amongst all crops [ 32 ]. Approximately 95% of wheat cultivated is hexaploid with the remaining 5% being durum wheat ( T. turgidium L.) and few other less important types [ 106 ].

The origin of the species

Bread wheat is an allohexaploid species (2 n  = 6× = 42, AABBDD genomes), resulting from the combination of 3 interrelated diploid genomes [ 28 , 66 , 79 , 83 ]. Donors of the A genome ( T. urartu ) and B genome (closely related to Aegilops speltoides ), diverged from a common ancestor about 7 million years ago. These two species first generated (~ 5.5 million years ago) the donor of the D genome ( Ae. tauschii ), through hybridization and homoploid speciation. Less than one million years ago emmer wheat ( T. turgidum ), an allotetraploid with AABB genomes became into existance. Finally, from 8000 to 10,000 years ago, probably in the Fertile Crescent, in a region that nowadays comprises Northern Iran, the hybridization between T. turgidum and Ae. tauschii gave rise to the hexaploid T. aestivum , which after domestication and centuries of cultivation and selection, resulted in the bread wheat that is cultivated today [ 27 , 28 , 53 , 67 , 68 , 79 , 83 , 98 ].

Unlike other cultivated species, hexaploid wheat was not selected from a wild species, but arose from the hybridization between a species already cultivated by man that time (emmer wheat), so it is possible to say that maybe there was never any T. aestivum in the wild [ 106 ]. The reasons why this cereal became so widely adopted by man include its high environmental adaptability, thanks to its allopoliploid nature, which has conferred to wheat the so-called “genomic plasticity”. Also, due to its excellent food/feed qualities, not only regarding carbohydrates, proteins and vitamin content, but also for the unique elastic property of its gluten, which provided a more diverse use for its flour [ 27 , 106 ].

The beginning and evolution of wheat cultivation and breeding

The emergence of modern T. aestivum occurred due to agriculture. Thanks to growing its ancestor (emmer) in an area with spontaneous occurrence of Ae. tauschii , the inter-specific hybridization that generated this species occurred [ 27 ]. After its emergence, cultivation gradually began to predominate around its center of origin and then expanded to several regions of the globe, improved by natural selection and man in an unintentional way [ 18 ].

The “intentional” breeding, even if empirical, began at the end of the XVIII century. The first reported attempts to allow for cross-fertilization of different types of plants was made by Knight (1787) in England. These crosses allowed for the observation of improvements especially for disease resistance [ 64 ]. At the end of the XIX century, Vilmorin, in France, and Rimpau in Germany, amongst other breeders, made important contributions in the development of superior wheat genotypes by man-made hybridization or simply selection, motivated by Darwin [ 22 , 23 ], but occurred without a clear understanding of important foundations of their work [ 64 ]. Breeding from a solid scientific base began only after the rediscovery of Mendel’s findings, at the beginning of the last century. Biffen’s classic work [ 7 ] was probably the first to validate such knowledge in wheat, once again focusing on disease resistance. Nilsson-Ehle [ 76 ] greatly contributed to the study of quantitative traits involving grain color in wheat.

Other advances took place gradually over the decades, until a major leap was made with the so-called “Green Revolution” of the mid-1960s, when wheat and rice ( Oryza sativa L.) were protagonists [ 9 , 29 , 80 , 91 ]. This revolution consisted in the development of “modern” cultivars - those of wheat mainly by CIMMYT, the International Center for Maize and Wheat Improvement, Mexico. Those were short statured (semi-dwarf), photoperiod insensitive and high yielding spring cultivars. This was only possible due to the incorporation of the genes Reduced height ( Rht ) and Photoperiod ( Ppd ), which have had extremely important effects on the adaptability of this species. Ppd-D1a, which is an insensitive allele to the photoperiod that reduces flowering time, and Rht-B1b and Rht-D1b , which makes the cereal insensitive to gibberellin, shortened plant’s stature. These genes are today widespread in the wheat elite germplasm all around the world and new alleles are still under study, with potential to contribute to this trait [ 10 , 125 , 128 ].

These new genotypes became widely adopted, especially in developing countries, and generated an impact on the reduction of hunger and poverty, with huge repercussions [ 9 , 29 , 78 , 80 ]. The Nobel Peace Prize awarded to Dr. Norman E. Borlaug deserves a special mention here, due to his decisive role in this revolution [ 9 ].

Since then, wheat breeding has advanced even further with new technologies such as molecular markers, the recent availability of a reference genome sequence and annotation, and even the recent use of techniques such as genome editing, genomic selection, speed breeding and high-throughput phenotyping. The evolution of wheat breeding accross time is briefly illustrated in Fig.  1 , highlighting phases and important events.

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Wheat breeding timeline. Three main phases can be defined in wheat breeding history: the “unconscious”, the “empirical” and the “scientific” breeding, this latter is illustrated with several important events

Wheat breeding: priorities and some general aspects

The priorities in wheat breeding.

The main objectives of wheat breeding have been similar over many decades. Increasing the yield potential has been prioritized in order to meet the food requirements of an ever increasing population [ 9 , 80 ].

Probably the second most important trait is disease resistance, as from the first breeding attempts by Knight in 1787 until today, in different countries [ 64 ]. For instance, “old diseases”, such as the rusts, are still a cause of concern for wheat cultivation, but new ones are appearing, such as wheat blast, considered one of the most recent and concerning threat for wheat cultivation worldwide [ 127 ].

Third, is tolerance to abiotic stresses, especially drought and heat – the latter is a borderline to cereal crop expansion, cold and acid soils (aluminum), and various quality traits. Finally, all the others must come, such as resistance to insects, lodging, double-purpose (forage and grain), and improved nutrient use and grain biofortification efficiency, among numerous others. This ranking is based on a general overview on the vast available literature, however this order of priority more than certainly varies within each environmental region and over time.

As already mentioned, publications on wheat breeding are vast, fortunately there has been a number of reviews already published, which summarize the most important steps already taken for different traits, ie., yield potential [ 29 , 91 ], stem rust resistance [ 107 ], drought tolerance [ 74 ] and biofortification, which should grow in importance over the next few years [ 129 ].

New priorities in wheat breeding

Most future priorities in wheat breeding should remain the same, but the need for faster development and accumulation of knowledge from different fields should provide new strategies and paths to reach these goals. Increasing photosynthetic capacity has been shown to be one of the most important barriers to improve wheat yield potential and there is theoretical evidence that it could be enhanced by the insertion of genes for C4 carbon fixation, whose strategy has merited investment [ 87 , 90 ].

Wheat grain is known to be rich in gluten, a trait that is critical for baking, but negative for consumption by celiac, and also non-celiac gluten-sensitive people has been a largely discussed topic among nutritionists [ 15 , 37 ]. This may lead to a potential reduction in wheat consumption in the coming decades, unless we can provide grain that does not possess this disadvantage. Fortunately, there is evidence of some wheats that possess a gluten, but of a chemically different type, which can be consumed by people with celiac disease, potentially becoming an important target for wheat breeding in forthcoming years [ 95 , 111 ].

Special aspects on wheat breeding

Wheat is a self-pollinated species. Therefore, the conventional structure of its breeding programs do not differ much from other autogamous plants. It includes the use of artificial hybridizations between previously selected genotypes, something already performed for more than two centuries, and different forms of selection within segregating populations [ 64 , 100 ]. It is recognized that these processes were, and will continue to be, the main responsiblity for the development of wheat cultivars worldwide. However, new tools and approaches are assisting this process, increasing its success rate and diminishing costs, time and labour.

Improving wheat may be more difficult than for many other crops, since the breeder needs to “match” quantity and quality, allying yield with grain and flour quality, which needs are not a constant concern for crops like soybean ( Glycine max L.) or maize ( Zea mays L.), which can, for the most part, focus on yield [ 106 ]. Also, it is a species with restricted genetic variability when compared to most of other crops. Moreover, its genome size, complexity and polyploid nature constitute a challenge when applying some biotechnological techniques.

The restricted genetic diversity

Wheat is recognized to have restricted genetic variability, when compared to most other crops [ 18 , 20 ]. This is due to several reasons: 1) it is an allohexaploid generated by crosses involving three highly interrelated diploid species, and poplyploidization is a force which restricts itself genetic variability; 2) another reason, suggests that few plants of the ancestral species were involved in the formation of wheat, also restricting its initial genetic variability [ 27 , 58 ]; 3) Finally, it is a young species, ca. 8000 to 10,000 years old, which is insufficient time for the species to accumulate mutations or to receive genes or alleles by natural or artificial interspecific cross-breeding processes [ 20 , 28 , 66 ].

Domestication, centuries of cultivation, and modern breeding have further restricted the genetic variability of several cultivated species, and wheat is among them [ 34 , 71 , 89 , 119 ]. It is important to remember that wheat was one of the first species to be domesticated and cultivated, further decreasing its variability due to constant selection cycles since then [ 18 , 58 , 93 ]. The impact of the narrowing of wheat variability is visible through current projections, which show that the cereal might not meet its demand in few decades [ 88 ], unless measures are taken in order to broaden its genetic base.

To broaden the genetic diversity available for wheat breeding, different techniques will need to be applied, including induce mutation, genetic transformation, genome editing, and introgressions from species of the secondary and tertiary gene pools.

Resurgent and current approaches in wheat breeding

Introgressions.

Among all crop species, wheat is probably the one in which most research has been invested regarding the use of wild and cultivated relatives as source of variability for its improvement. The attempt to incorporate traits of related species into wheat germplasm is not new. In fact, the attempts in this sense began long ago, as early as plant breeding itself [ 6 ]. If, on one hand, wheat is restricted in variability within its germplasm, there is an immeasurable richness in variation found in related species belonging to its secondary and tertiary gene pools [ 25 , 102 , 131 ].

The most important introgression to date in wheat involved a chromosomal translocation 1RS-1BL between wheat and rye ( Secale cereale L.), generated in the first third of the last century, which increased wheat yield potential and resistance/tolerance to biotic and abiotic stresses. This segment is still present in many of important cultivars currently used [ 21 , 85 , 101 ]. The researcher E.R. Sears deserves also a special mention here, due to his great contribution to this field. Today, there are several excellent chromosome manipulation studies in progress (e.g. [ 54 ]). However, there is a consensus that the practical use of introgressed genes in the development of superior cultivars has in the past been very limited and should be further explored [ 132 ].

Another strategy in this field is the development of synthetic wheat, repeating the interspecific crosses that occurred in nature that led to the formation of hexaploid wheat [ 61 , 130 ]. In this method, different accessions of the species T. monococcum , T. turgidum, and Ae. tauschii can be used for the formation of new genetic constitutions of wheat, greatly increasing the genetic variability of the primary gene pool [ 73 ]. Numerous synthetic wheat germplasm pools have been developed by CIMMYT [ 130 ]. This illustrates an advantage that wheat possesses, as an allohexaploid, when compared to diploid species.

The use of other species in wheat pre-breeding programs has been an important field of research (for a complete review, see [ 72 ]). Recently, however, it seems to be reaching a new momentum, driven by a remarkable shortage of genetic diversity in wheat, accompanied by an increased need for improved adaptability for the crop. This adaptability is needed to counteract the unfavorable conditions brought by the ongoing climate changes. Enhanced technologies for introgression detection, such as high-throughput genotyping, have motivated investiments in this field. Other potential approaches, such as gene editing will be further discussed in a dedicated section [ 12 , 54 , 131 ].

Mutagenesis

Mutation induction, whether via chemical or physical mutagens, has been widely used in order to increase the genetic variability in several cultivated species, including wheat [ 77 ]. The polyploid nature of wheat confers a kind of buffer effect, in which mutations in one of its genomes can be compensated by homoeologous genes masking their effect making them difficult to be detected [ 77 ]. Fortunately, TILLING methods [ 108 , 114 ] and high-resolution melting analysis [ 26 ] have proven to be efficient for the detection of mutations in the different genomes of hexaploid wheat.

From 1960 to 2017, 256 wheat cultivars were generated by mutagenesis in different countries and have been registered in the FAO/IAEA database ( https://mvd.iaea.org ). In this repository [ 31 ], all cultivars are described with information about how the mutations were induced and focuses on the value-added attributes. Among the many examples of agronomically important mutations are resistance to herbicides of the imidazolinones group [ 84 ] and increases in amylose content and starch resistance [ 109 ].

Molecular markers and new genotyping approaches

The use of molecular markers for QTL mapping and marker-assisted selection (MAS), such as for resistance to fusarium head blight [ 13 ] and drought [ 39 ] has been growing and the accumulation of data generated during the past decades has allowed us to perform different meta-analyses [ 39 ]. From the 1990s to 2000s, AFLP, RFLP, and SSR were the most used markers [ 17 , 40 , 46 , 75 , 110 ]. However, recently a revolution occurred, in which science changed from the use of a few markers, from the types mentioned above, to thousands of single nucleotide polymorphism (SNP) markers using high-throughput platforms. This was initiated with DArT markers [ 1 ] and then with SNPs evaluated through genotyping arrays such as Illumina ® 9 K iSelect Beadchip Assay [ 16 ], Illumina ® iSelect 90 K SNP Assay [ 121 ] and Axiom ® 820 K SNP array [ 126 ], in which respectively 9000 to nearly 820,000 SNPs can be evaluated in a single analysis. Also, using genotyping by sequencing (GBS), thanks to the arrival of next generation sequencing technologies, maps containing 20 to 450 K loci have already been generated for wheat [ 82 , 96 ].

Similarly to other crops, genetic mapping also evolved from mapping populations generated from crosses between only two contrasting parents to genome-wide association studies (GWAS), in which hundreds of diverse accesses are evaluated on each study, thus allowing the capture of a larger genetic diversity and a deeper look in the causal variation between agronomically interesting phenotypes [ 3 , 14 , 38 , 56 , 60 , 81 ].

Genomic selection

Although Marker Assisted Selection (MAS) has proven to be useful in a number of situations in wheat breeding, it has the limitation of being only able to aid the selection for a few genes or alleles at a time. However, it is well known in crop breeding that most agronomic traits present a quantitative nature, are governed by numerous genes, most of these with very small effect on the phenotype. In this regard, genomic selection (GS) came as a revolutionizing ally, also in animal breeding [ 69 ]. The approach aims ultimately to perform selection and prediction of breeding values based only on genotyping, within a model calibrated with phenotypic values, and with a whole genome perspective, i.e., taking into account genomic polymorphisms in linkage disequilibrium with as many as possible genes with effect on a given trait [ 51 ].

The number of studies applying GS in wheat breeding are at an increasing rate. One of the main measures to assay the effectiveness of GS is its accuracy, i.e., how much the prediction compares with the real phenotypes. Applying genotyping by sequencing, GS for wheat yield under irrigated and drought conditions showed accuracies of 0.28 and 0.45, respectively, which are low to moderate values [ 81 ]. On the other hand, GS for fusarium head blight resistance showed moderate to high accuracies, being 0.82 the highest value found, for fusarium damaged kernels trait [ 4 ]. High accuracies are pursued in this approach, and many factors affect its value, such as the heritability of the trait, the number and quality of the markers, the GS statistical model adopted, among others [ 43 ]. In this regard, Bassi et al. [ 5 ] proposed different schemes dedicated to the implementation of GS in wheat breeding.

The reference genome sequence and annotation

In 2005, efforts to generate a reference genome of wheat for the scientific community began, with the establishment of the International Wheat Genome Sequencing Consortium (IWGSC). Nine years latter, in 2014, the first version of this sequence, still considered as a draft, was published for the hexaploid wheat cultivar Chinese Spring [ 47 ]. This huge and complex sequence, estimated in 16 to 17 Gb in total, has been gradually assembled, improved and made available through the repository of the consortium ( https://www.wheatgenome.org ). Finally, after another 3 years, a first version of the annotation has been made available [ 48 ], which has also been continuosly improved [ 49 ]. In addition to IWGSC, another research group was responsible for the first near-complete assembly of the hexaploid bread wheat genome, with a total of 96% of its sequence, also of Chinese Spring [ 136 ].

Now these reference genomes, especialy the one made available by IWGSC, through its platform for public access, are a powerful tool for breeding and other genetic studies on this crop, being used to better understand wheat evolution [ 28 , 66 ] and for genome wide association studies [ 3 ], among many other examples of use.

The completion of the first wheat reference genome of the Chinese Spring cultivar has been considered a step-change by researchers. However, it is obvious that more representatives from the species should also be sequenced, for a more effective use of genomics in breeding. It motivated the establishment of 10+ Wheat Genomes Project \ ( http://www.10wheatgenomes.com/ ). This global partnership aims to characterize the wheat ‘pan genome’, and will generate at high quality wheat genome assemblies and develop strategies and resources to compare multiple wheat genome sequences from around the world.

Hybrid breeding

In some crops, such as maize and rice, the development and cultivation of hybrid cultivars is common, not recent and with clear advantages over the cultivation of open pollinated populations or inbred lines. For wheat, however, less than 1% of the area is cultivated with hybrids [ 52 , 63 ]. After unsuccessful attempts during the past decades, research in the development and cultivation of hybrids seems to be becoming one priority in wheat breeding [ 63 , 124 ].

This is due to a huge accumulation of knowledge and new technologies, and recent results are promising. The use of genomic tools to analyze the heterotic pattern among large groups of lines has proved to be efficient in obtaining highly productive hybrids [ 135 ], with genome wide selection being the most advantageous method of prediction [ 60 ]. In this sense, several hybrids have shown to be highly advantageous regarding yield [ 62 ] and resistant to diseases [ 70 ], while several difficulties associated with seed production are being overcome [ 124 ].

Genetic transformation (transgenics)

The cultivation of transgenics is still a debate topic in our society. Its acceptance is not unanimous around the world, either because of social or religious reasons [ 106 ]. The scientific results have not been able to overcome the fear on its potential effects on human health [ 45 , 65 ]. This is why there are not many records of the use of transgenic wheat cultivars [ 116 ], not allowing its comparison with crops such as soybean, maize or cotton, even after 27 years of the first transformed wheat [ 117 ]. Indeed, authors have termed wheat as the cereal abandoned by GM [ 127 ]. Research results, however, have been encouraging, generating genotypes with improved resistance to powdery mildew ( Blumeria graminis ) [ 134 ], leaf spot caused by Bipolaris sorokiniana [ 50 ] and fusarium head blight (caused mainly by Fusarium graminearum ) [ 59 ]. Also, tolerance to drought [ 118 ], salinity and freezing [ 35 ] and even improvement in baking traits [ 86 ] have been achieved, among other traits [ 116 ]. Another alternative tool is the creation of cisgenic plants, where transferred genes come from the same species, something that has proven to be more easily accepted by society [ 113 ]. Despite these considerations, genetic transformation has been quickly replaced by genome editing, a very powefull approach, as presented in the next topic.

Genome editing

Among the most recent and promising innovations in terms of biotechnology and plant breeding involves genome or gene editing [ 11 , 99 ]. This technique can accurately target segments of the genome for modification, either by deletion, insertion or substitution of nucleotides [ 99 ]. In wheat, despite the great complexity of its extensive, redundant, and polyploid genome, several attempts have proven to be successful [ 105 , 115 , 122 , 133 ]. Even a specific protocol for this species has already been established using the CRISPR/Cas9 system [ 104 ]. Among the most exciting results obtained with this technique is the simultaneous modification of three homoeo-alleles of the same gene, i.e., being capable of modifying this gene in all three different genomes, demonstrating the precision that these methods have been able to reach [ 122 ].

Gene editing can also be applied as a tool for gene introgression from wild relatives into wheat background, in which the linkage drag can be mitigated by precise gene replacement [ 120 ].

Meiotic recombination manipulation

Crop breeding relies largely on meiotic recombination, which allows for recombination of genes/alleles in different new genetic compositions, thus allowing selecting new improved cultivars [ 57 ]. Controlling this process would be of high interest for breeders. In bread wheat, the Ph1 locus is a well-characterized regulator of this process, whose main role is allowing only homologous chromosomes (belonging to the same genome) to pair and recombine during meiosis [ 57 , 94 , 103 ]. In this regard, there are mutant lines that harbor an alternative allele for this locus, for instance ph1 , which is not functional, thus allowing homoeologous chromosomes to pair and recombine [ 132 ]. These homoeologous chromosomes include the ones from wheat, but also chromosomes from species from the secondary and tertiary gene pools of the cereal, during the process of gene introgression, being this a powerfull mechanism for this approach [ 132 ]. Since other genes appear to contribuite on this mechanism, other studies are being carried out to better elucidate it.

Speed breeding

Crop breeding is, or, has been, a process which requires considerable time, usualy several years - as for wheat - until a new improved cultivar can be released. The current increasing demand for food added to a number of other factors, such as the ongoing climate change, put pressure on breeding to accelerate the process. Growing segregating lines out of season, at different locations, and the double haploid method have contribuited in this regard, but speed breeding has come as a game-changer to accelerate the plant improvement. It is a very recent approach which ultimately aims to shorten plant’s generation time, accelerating breeding and research programmes, in which wheat has been protagonist, among few other crops [ 123 ]. It is basically based on photoperiod, light and temperature manipulation (artificially), in growth chambers and glasshouses, and allows one to achieve up to six generations per year - from seed to seed, for spring wheat [ 36 , 123 ]. The method not only allows for generation advancing, but also for faster phenotyping for numerous traits, such as flowering time, plant height and disease resistance in wheat [ 36 ].

High-throughput phenotyping

The use of high-throughput phenotyping, aims to evaluate several traits in a large number of plants over a short period of time. This technique is comprised of several highly optimized and automated steps, and emerged also in an attempt to follow the performance achieved through genotyping towards the increasing demands of breeding [ 2 , 14 , 24 ].

This can be done under controlled conditions, such as in growth chambers or greenhouses, using plant-manipulating robots and photographic cameras with temperature sensors, CO 2 meters and scales for weighing live plants [ 30 , 92 ]. At field level, tractor-coupled or self-propelled platforms, drones or even satellite imagery can perform the tasks [ 19 , 41 , 55 , 112 ]. After data collection, analysis is also differentiated, requiring specific software, such as for image processing [ 30 , 55 ].

Final considerations and future perspectives

Agriculture has the challenge of meeting the increasing demand for food by an ever growing world population, and these days in an adverse scenario of climate change, restricted availability of arable land and water and constant evolution of pathogens, among other obstacles. Moreover, the demand for food goes beyond quantity, as quality is also required, especially regarding nutritional aspects. Bread wheat and plant breeding have a crucial role on this task.

Breeding has been responsible for increasing wheat yields and improving many other traits, such grain quality, resistance to biotic stresses, etc. However, the cereal mean genetic gain has to be doubled in the next few decades, in order to meet its global demand. Thus, efforts in the development and implementation of improved strategies must continuously take place in wheat breeding programs.

Classical breeding, which is largelly based on crosses and phenotypic selection has been the most used plant breeding method around the globe for more than one century and is still the main approach these days, responsible for the release of the largest number of cultivars. This approach will still be applied as the main or even unique strategy for several years to come, specially in developing countries. It will be gradually replaced to a certain extent by improved methods, again firstly in developed countries, next, in developing ones. Crosses may be replaced by direct insertion of a gene of interest through gene editing and phenotypic selection by GS. However, the complete extinction of the classical breeding cannot be even conceived. Instead, combined approaches will probably predominate in breeding programs. Crosses followed by speed breeding practices and high-throughput phenotyping for selection or GS is a simple example of a combined scheme.

Gene editing and GS are the current cutting-edge approaches in plant breeding. Both can still be improved to deliver more effective results, which will probably happen within the next decade. However, the most important “improvement” required from these methods resides on the reduction of their costs, which is especially true for GS, as genotyping is still considerably expensive. As science and technology continue to move towards., it is difficult to even predict which advance will become available for breeders in two or three decades.

Plant breeding has experienced innovations and revolutions throughout its existence and wheat has been witness to most, if not all, of these transformations and probably will continue as an ally of the transformations to come.

Acknowledgements

We would like to thank the Brazilian funding agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS).

Abbreviations

Authors’ contributions.

ACO and EV contributed for the conception and design of the review; EV wrote the first draft of the manuscript; ACO, RSS, CB and PG performed revisions and wrote parts of the manuscript; ACO performed the final revision of the manuscript. All authors contributed to manuscript revision, read and approved the submitted version.

Grants and fellowships were received from the Brazilian Institutes: Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul - FAPERGS.

Availability of data and materials

Ethics approval and consent to participate.

Not applicable. The experiments described in this manuscript do not involve human participants.

Consent for publication

All the authors involved in this study declared their consent to publish the manuscript.

Competing interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Eduardo Venske, Email: [email protected] .

Railson Schreinert dos Santos, Email: [email protected] .

Carlos Busanello, Email: moc.liamg@azzubsolrac .

Perry Gustafson, Email: ude.iruossim@sugp .

Antonio Costa de Oliveira, Phone: +55-53-3275-7263, Email: rb.moc.arret@lotsoca .

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Systematic Literature Review for Classificatoin of Wheat Grians

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Using a computer to capture and analyze images. Also called "machine vision" (MV) . The ultimate goal of computer vision is to model and replicate human vision using computer software and hardware at different levels. It combines knowledge in computer science, electrical engineering, mathematics, physiology, biology, and cognitive science. It needs knowledge from all these fields in order to understand and simulate the operation of the human vision system . Computer vision is the automated extraction of information from images. Information can mean anything from 3D models, camera position, object detection and recognition to grouping and searching image content [6].

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Danny M Vaughn, Ph.D., CMS

literature review on wheat

Encyclopedia of Data Warehousing and Mining, Second Edition

Generally speaking, classification is the action of assigning an object to a category according to the characteristics of the object. In data mining, classification refers to the task of analyzing a set of pre-classified data objects to learn a model (or a function) that can be used to classify an unseen data object into one of several predefined classes. A data object, referred to as an example, is described by a set of attributes or variables. One of the attributes describes the class that an example belongs to and is thus called the class attribute or class variable. Other attributes are often called independent or predictor attributes (or variables). The set of examples used to learn the classification model is called the training data set. Tasks related to classification include regression, which builds a model from training data to predict numerical values, and clustering, which groups examples to form categories. Classification belongs to the category of supervised learning, ...

srikrishna sudarsan

Ahmadullah Ahmadullah

Image classification refers to the task of extracting information classes from a multiband raster image.  Image classification is the process of assigning land cover classes to pixels.  Classification process is to categorize all pixels in a digital image into several land cover classes.

Journal of Cereal Science

Harry Sapirstein

Erik Holst , Helle Rootzen

Emilio Cervantes

Modern automated and semi-automated methods of shape analysis depart from the 16 coordinates of the points in the outline of a figure and obtain, based on artificial vision algorithms, 17 descriptive parameters (i.e. the length, width, area, and circularity index). These methods omit an 18 important factor: the resemblance of the examined images to a geometric figure. We have described 19 a method based on the comparison of the outline of seed images with geometric figures. The J index 20 is the percentage of similarity between a seed image and a geometric figure used as a model. This 21 allows the description and classification of wheat kernels based on their similarity to geometric 22 models. The figures used are the ellipse and the lens of different major/minor axis ratios. Kernels of 23 different species, subspecies and varieties of wheat adjust to different figures. A relationship is 24 found between their ploidy levels and morphological type. Kernels of diploid einkorn and anc...

Punnarai Siricharoen

www.ijarst.com

Smruti Smaraki Sarangi

Classification is the problem of identifying to which of a set of categories (sub-populations) a new observation belongs, on the basis of a training set of data containing observations (or instances) whose category membership is known. An example would be assigning a given email into "spam" or "nonspam" classes or assigning a diagnosis to a given patient as described by observed characteristics of the patient (gender, blood pressure, presence or absence of certain symptoms, etc.).In the terminology of machine learning, [1] classification is considered an instance of supervised learning, i.e. learning where a training set of correctly identified observations is available. The corresponding unsupervised procedure is known as clustering, and involves grouping data into categories based on some measure of inherent similarity or distance. An algorithm that implements classification, especially in a concrete implementation, is known as a classifier. The term "classifier" sometimes also refers to the mathematical function, implemented by a classification algorithm that maps input data to a category.

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COMMENTS

  1. The contribution of wheat to human diet and health

    In addition to being a major source of starch and energy, wheat also provides substantial amounts of a number of components which are essential or beneficial for health, notably protein, vitamins (notably B vitamins), dietary fiber, and phytochemicals.

  2. Wheat

    Introduction Wheat is counted among the 'big three' cereal crops, with over 600 million tonnes being harvested annually. For example, in 2007, the total world harvest was about 607 m tonnes compared with 652 m tonnes of rice and 785 m tonnes of maize ( http://faostat.fao.org/ ).

  3. (PDF) Role of Nutrients in Wheat: A Review

    Wheat (Triticum aestivum L.) is an important cereal crop that provides ample nutritious calories for humans and animals. The nutrient plays a vital role in the production of wheat. In this...

  4. A systematic literature review of life cycle ...

    According to this paper's authors' opinion, this supports the rationale of conducting a systematic literature review of Life Cycle Assessments (LCAs) in the durum wheat (DW) sector, to highlight environmental hotspots and improvement potentials in the phases of cultivation and processing into finished products like pasta and bread.

  5. Heat stress effects and management in wheat. A review

    Increasing temperature and consequent changes in climate adversely affect plant growth and development, resulting in catastrophic loss of wheat productivity. For each degree rise in temperature, wheat production is estimated to reduce by 6%. A detailed overview of morpho-physiological responses of wheat to heat stress may help formulating appropriate strategies for heat-stressed wheat yield ...

  6. Frontiers

    Separating the Empirical Wheat From the Pseudoscientific Chaff: A Critical Review of the Literature Surrounding Glyphosate, Dysbiosis and Wheat-Sensitivity. Jacqueline A. Barnett 1 Deanna L. Gibson 1,2* 1 Department of Biology, The University of British Columbia, Kelowna, BC, Canada;

  7. Wheat From Chaff: Meta-Analysis As Quantitative Literature Review

    Wheat From Chaff: Meta-Analysis As Quantitative Literature Review T.D. Stanley I n an era characterized by the expansion of research publications and an avalanche of information, balanced and critical literature reviews serve a crucial function.

  8. Plants

    Factors Affecting the Nutritional, Health, and Technological Quality of Durum Wheat for Pasta-Making: A Systematic Literature Review by Silvia Zingale 1, Alfio Spina 2, Carlo Ingrao 3,*, Biagio Fallico 1, Giuseppe Timpanaro 1, Umberto Anastasi 1,* and Paolo Guarnaccia 1 1

  9. (PDF) THE WHEAT CROP

    Wheat (Triticum aestivum L) is the most extensively grown cereal crop in the world, covering about 237 million hectares annually, accounting for a total of 420 million tonnes (Isitor et al.,...

  10. Wheat production and marketing in Ethiopia: Review study

    SOIL & CROP SCIENCES Wheat production and marketing in Ethiopia: Review study Adugnaw Anteneh & Dagninet Asrat | Manuel Tejada Moral (Reviewing editor) Article: 1778893 | Received 24 Jan 2020, Accepted 15 May 2020, Published online: 17 Jun 2020 Cite this article https://doi.org/10.1080/23311932.2020.1778893 In this article Full Article

  11. Wheat from Chaff: Meta-analysis as Quantitative Literature Review

    B41 Economic Methodology. Wheat from Chaff: Meta-analysis as Quantitative Literature Review by T. D. Stanley. Published in volume 15, issue 3, pages 131-150 of Journal of Economic Perspectives, Summer 2001, Abstract: This paper presents and develops a quantitative method of literature reviewing and evaluating empirical resea...

  12. Sensory attributes of wheat bread: a review of influential factors

    Bread is one of the main bakery products which is consumed all over the world in large amounts. It has specific role in nutrition and providing energy to human, due to possessing macronutrients like carbohydrates and proteins as well as micronutrients including iron, calcium and group B' vitamins. In spite of high global production of bread, there is extensive extent of wheat bread waste ...

  13. Frontiers

    Wheat (Triticum aestivum L.) belonging to one of the most diverse and substantial families, Poaceae, is the principal cereal crop for the majority of the world's population. This cereal is polyploidy in nature and domestically grown worldwide. Wheat is the source of approximately half of the food calories consumed worldwide and is rich in proteins (gluten), minerals (Cu, Mg, Zn, P, and Fe ...

  14. Remote sensing of quality traits in cereal and arable production

    Related papers published between 2002 and 2022 were retrieved from the ISI Web of Knowledge in the Core Collection Database for a systematic literature review. In all the relevant studies conducted so far, the focus of this review is on the utilization of RS technology at the field scale for predicting grain quality.

  15. Factors Affecting the Nutritional, Health, and Technological Quality of

    2. Materials and Methods. This systematic review was conducted following the PRISMA guidelines [].To perform the literature search, Science Direct, Scopus, and Web of Science were screened to retrieve published studies regarding the factors affecting the nutritional, technological, and health properties of durum wheat grain, semolina, and pasta.

  16. Deep learning in wheat diseases classification: A systematic review

    The main goal of this paper is to review systematically the recent studies that have been published and discussed WD prediction models. The literature analysis is performed based on studies published from January 1997 to February 2021 by following Kitchenham instructions. After inclusion/exclusion and quality assessment criteria screening, a total of 74 studies have been selected. The ...

  17. Drivers of female farmers' adoption of improved bread wheat varieties

    According to the model results, educational level, family labour, oxen ownership, training access, membership in cooperatives, and credit access positively influenced, while the age of the female farmers negatively influenced the adoption of bread wheat production by female farmers.

  18. Wheat From Chaff: Meta-Analysis As Quantitative Literature Review

    Wheat From Chaff: Meta-Analysis As Quantitative Literature Review. T.D. Stanley. I. n an era characterized by the expansion of research publications and an avalanche of information, balanced and critical literature reviews serve a crucial function. They act as intelligent agents searching through mountains of potentially contradictory research ...

  19. Bread wheat: a role model for plant domestication and breeding

    Bread wheat ( Triticum aestivum L.) is one of the most important crop species, responsible for the emergence and development of agriculture and has fed, and continues to feed, a large part of the world's population across many centuries [ 97, 106 ]. Wheat has been improved by man over the last 8000 to 10,000 years ago when the species first arose.

  20. Systematic Literature Review for Classificatoin of Wheat Grians

    Systematic Literature Review of Classification of Wheat Citations 1021 Textural features for image classification - Haralick, Shanmugam, et al. - 1973 551 Statistical and Structural Approaches to Textures - Haralick - 1979 152 Simultaneous structure and texture image inpainting - Bertalmío, Vese, et al. - 2003 Classification of cereal grains ...