Various mechanisms are behind seed dormancy and pre-harvest sprouting control in cereals, notably the abscisic acid to gibberellins balance, regulation of water entry by seed coat structures and activity of the MKK3 kinase, an unusual MAP2K containing a longer C-terminal domain. An international team has now dissected the MKK3 locus in barley and shown that its role in dormancy control is a bit more complex than previously thought. The first discovery was that the MKK3 locus contains a single copy of MKK3 in most wild barleys, while domesticated lines can carry multiple copies with identical or slightly different variants, leading to higher expression level. A low-activity variant (MKK3-T260) linked to high dormancy is mostly found in regions with high precipitation where pre-harvest sprouting is a significant risk. Conversely, two variants (MKK3-Q165 and MKK3-V79) with higher kinase activity leading to low dormancy correlate with dry weather at the time of harvest. Finally, controlled field trials compared dormancy and pre-harvest sprouting susceptibility of various barley lines with different MKK3 haplotypes to validate the findings.
The conclusion is that MKK3 is the major controller of dormancy, notably through the post-domestication combinatorial selection of different variants in the MKK3 locus. The authors also discuss the link between dormancy needs and specific or historical usage of barley grains in various regions of the world, such as its use as staple food in Tibet, or for brewing by the Vikings. They end by speculating that this type of variability and complexity through gene copy number and modified enzymatic activity is probably behind other important quantitative traits selected during domestication.
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