Abstract
Wheat has been cultivated for 10000 years and ever since the origin of hexaploid wheat it has been exempt from natural selection. Instead, it was under the constant selective pressure of human agriculture from harvest to sowing during every year, producing a vast array of varieties. Wheat has been adopted globally, accumulating variation for genes involved in yield traits, environmental adaptation and resistance. However, one small but important part of the wheat genome has hardly changed: the regulatory regions of both the x- and y-type high molecular weight glutenin subunit (HMW-GS) genes, which are alone responsible for approximately 12% of the grain protein content. The phylogeny of the HMW-GS regulatory regions of the Triticeae demonstrates that a genetic bottleneck may have led to its decreased diversity during domestication and the subsequent cultivation. It has also highlighted the fact that the wild relatives of wheat may offer an unexploited genetic resource for the regulatory region of these genes. Significant research efforts have been made in the public sector and by international agencies, using wild crosses to exploit the available genetic variation, and as a result synthetic hexaploids are now being utilized by a number of breeding companies. However, a newly emerging tool of genome editing provides significantly improved efficiency in exploiting the natural variation in HMW-GS genes and incorporating this into elite cultivars and breeding lines. Recent advancement in the understanding of the regulation of these genes underlines the needs for an overview of the regulatory elements for genome editing purposes.
Highlights
Hexaploid wheat only exists in a cultivated form, and it is derived from a cross between the cultivated Triticum turgidum subsp. dicoccum and a wild goat grass (Aegilops tauschii)
Among the domesticated crops only hexaploid wheat went through speciation, while all other plants retained their genetic relations to their wild types (Harlan et al, 1973)
The high similarity between the regulatory region of the homeologous high molecular weight glutenin subunit (HMW GS) genes raises many questions: (i) What genetic variability is available in the wild species? (ii) How and when were the paralogs (x and y type) separated? (iii) Which gene was the original copy and how has the evolution of the regulatory regions changed during the duplication? (iv) What are the signs of functional shift that are usually characteristic of duplicated genes?
Summary
Hexaploid wheat only exists in a cultivated form, and it is derived from a cross between the cultivated Triticum turgidum subsp. dicoccum and a wild goat grass (Aegilops tauschii). Application of genome editing is currently not without challenges and difficulties, it is widely anticipated that these can be overcome by further development of the technology as reviewed recently by Schaeffer and Nakata (2015) Based on this assumption, it is essential to have a clear understanding of the genetic options offered as templates by the wild relatives (Charmet, 2011) in order to obtain novel genetic variability for cultivated wheat. Any attempt to improve this ratio by increasing the activity of the y-type HMW GS genes may positively affect the protein composition of the wheat grain and the bread-making quality of the dough. We propose a possible strategy for gene/promoter replacement via studying the phylogeny of the regulatory regions of HMW GS genes of Triticeae and exploring the potential advantages of a new type of promoter
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