Wheat

Technological advances in soil preparation andseed placement at planting time, use of crop rotation and fertilizers to improve plant growth,and advances in harvesting methods have all combined to promote wheat as a viable crop. When the use ofseed drillsreplaced broadcasting sowing of seed in the 18th century, another great increase in productivity occurred.Yields of pure wheat per unit area increased asmethods ofcrop rotationwere applied to long cultivated land, and the use offertilizersbecame widespread. Improved agricultural husbandry has more recently included threshing machines and reaping machines (the'combine harvester'),tractor-drawn cultivators and planters, and better varieties (seeGreen RevolutionandNorin 10 wheat). Great expansion of wheat production occurred as new arable land was farmed in the Americas and Australia in the 19th and 20th centuries.GeneticsWheat genetics is more complicated than that of most other domesticated species. Some wheat species arediploid, with two sets ofchromosomes, but many are stablepolyploids, with four sets of chromosomes (tetraploid) orsix (hexaploid).[26]*.Einkornwheat (T. monococcum) is diploid (AA, two complements of seven chromosomes, 2n=14).[3]*.Most tetraploid wheats (e.g.emmeranddurum wheat) are derived fromwild emmer,T. dicoccoides. Wild emmer is itself the result of a hybridization between two diploid wild grasses,T. urartuand a wild goatgrass such asAegilops searsiiorAe. speltoides. The unknown grass has never been identified among now surviving wild grasses, but the closest living relative isAegilops speltoides.[27]The hybridization that formed wild emmer (AABB) occurred in the wild, long before domestication,[26]and was driven by natural selection.*.Hexaploid wheats evolved in farmers' fields. Either domesticated emmer or durum wheat hybridized with yet another wild diploid grass (Aegilops tauschii) to make thehexaploidwheats,speltwheat andbread wheat.[26]These havethreesets of paired chromosomes, three times as many as in diploid wheat.The presence of certain versions of wheat genes has been important for crop yields. Apart from mutant versions of genes selected in antiquity during domestication, there has been more recent deliberate selection ofallelesthat affect growth characteristics. Genes for the 'dwarfing' trait, first used byJapanese wheat breedersto produce short-stalked wheat, have had a huge effect on wheat yields worldwide, and were major factors in the success of theGreen Revolutionin Mexico and Asia, an initiative led byNorman Borlaug. Dwarfing genes enable the carbon that is fixed in the plant during photosynthesis to be diverted towards seed production, and they also help prevent the problem of lodging. 'Lodging' occurs when an ear stalk falls over in the wind and rots on the ground, and heavy nitrogenous fertilization of wheat makes the grass grow taller and become more susceptible to this problem. By 1997, 81% of the developing world's wheat area was plantedto semi-dwarf wheats, giving both increased yields and better response to nitrogenous fertilizer.Wild grasses in the genusTriticumand related genera, and grasses such asryehave been a source of many disease-resistance traits for cultivated wheatbreedingsince the 1930s.[28]Heterosis, or hybrid vigor (as in the familiar F1 hybrids of maize), occurs in common (hexaploid) wheat, but it is difficult to produce seed of hybrid cultivars on a commercial scale (as is done withmaize) because wheat flowersare perfect and normallyself-pollinate. Commercial hybrid wheat seed has been produced using chemical hybridizing agents; these chemicals selectively interfere with pollen development, or naturally occurring cytoplasmic male sterility systems. Hybrid wheat has been a limited commercial success in Europe (particularlyFrance), the United States and South Africa.[29]F1 hybrid wheat cultivars should not be confused with the standard method of breeding inbred wheat cultivars by crossing two lines using hand emasculation, then selfing or inbreeding the progeny many (ten or more) generations before release selections are identified to be released as a variety or cultivar.Synthetic hexaploids made by crossing the wild goatgrass wheat ancestorAegilops tauschiiand various durum wheats are now being deployed, and these increase the geneticdiversity of cultivated wheats.[30][31][32]Stomata(or leaf pores) are involved in both uptake of carbon dioxide gas from the atmosphere and water vapor losses from the leaf due to watertranspiration. Basic physiological investigation of these gas exchange processes has yielded valuable carbonisotopebased methods that are used for breeding wheat varieties with improved water-use efficiency. These varieties can improve crop productivity in rain-fed dry-land wheat farms.[33]In 2010, a team of UK scientists funded byBBSRCannounced they had decoded the wheat genome for the first time (95% of the genome of a variety of wheat known as Chinese Spring line 42).[34]This genome was released in a basic format for scientists and plant breeders to use but was not a fully annotated sequence which was reported in some of the media.[35]On 29 November 2012, an essentially complete gene set of bread wheat was published.[36]Random shotgun libraries of total DNA and cDNA from theT. aestivumcv. Chinese Spring (CS42) were sequenced in Roche 454 pyrosequencer using GS FLX Titanium and GS FLX+ platforms to generate 85 Gb of sequence (220 million reads), equivalent to 5X genome coverage and identified between 94,000 and 96,000 genes.[36]This sequence data provides direct access to about 96,000 genes, relying on orthologous gene sets from other cereals. and represents an essential step towards a systematic understanding of biology and engineering the cereal crop for valuable traits. Its implications in cereal genetics and breeding includes the examination of genome variation, association mapping using natural populations, performingwide crosses and alien introgression, studying the expression and nucleotide polymorphism in transcriptomes, analyzing population genetics and evolutionary biology, and studying the epigenetic modifications. Moreover, the availability of large-scale geneticmarkers generated through NGS technology will facilitate trait mapping and make marker-assisted breeding much feasible.[37]Moreover, the data not only facilitate in deciphering the complex phenomena such as heterosis and epigenetics, it may also enable breeders to predict which fragment of a chromosome is derived from which parent in the progeny line, thereby recognizing crossoverevents occurring in every progeny line and inserting markers on genetic and physical maps without ambiguity. In due course, this will assist in introducing specific chromosomalsegments from one cultivar to another. Besides, the researchers had identified diverse classes of genes participating in energy production, metabolism and growth that were probably linked with crop yield, which can now be utilized for the development of transgenic wheat. Thus whole genome sequence of wheatand the availability of thousands of SNPs will inevitably permit the breeders to stride towards identifying novel traits, providing biological knowledge and empowering biodiversity-based breeding.
Read less