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Grain size is an important yield related trait in rice.Intensive arti cial selection for grain size during domestication is evidenced by the larger grains of most of today’s cultivars compared with their wild relatives.However,the molecular genetic control of rice grain size is still not well characterized.Here,we report the identi cation and cloning of Grain Size 6(GS6),which plays an important role in reducing grain size in rice.A premature stop at the t348 position in the coding sequence(CDS)of GS6 increased grain width and weight signi cantly.Alignment of the CDS regions of GS6 in 90 rice materials revealed three GS6alleles.Most japonica varieties(95%)harbor the Type I haplotype,and 62.9%of indica varieties harbor the Type II haplotype.Association analysis revealed that the Type I haplotype tends to increase the width and weight of grains more than either of the Type II or Type III haplotypes.Further investigation of genetic diversity and the evolutionary mechanisms of GS6showed that the GS6gene was strongly selected in japonica cultivars.In addition,a"ggc"repeat region identi ed in the region that encodes the GRAS domain of GS6 played an important historic role in the domestication of grain size in rice.Knowledge of the function of GS6might aid efforts to elucidate the molecular mechanisms that control grain development and evolution in rice plants,and could facilitate the genetic improvement of rice yield.