A Single Amino Acid Substitution in STKc_GSK3 Kinase Conferring Semispherical Grains and Its Implications for the Origin of Triticum sphaerococcum Perc.

Map-based cloning of a gene underlying grain shape in wheat suggests that modest genetic changes induce dramatic phenotypic variations associated with a new wheat subspecies during evolution. Six subspecies of hexaploid wheat (Triticum aestivum) have been identified, but the origin of Indian dwarf wheat (Triticum sphaerococcum), the only subspecies with round grains, is currently unknown. Here, we isolated the grain-shape gene Tasg-D1 in T. sphaerococcum via positional cloning. Tasg-D1 encodes a Ser/Thr protein kinase glycogen synthase kinase3 (STKc_GSK3) that negatively regulates brassinosteroid signaling. Expression of TaSG-D1 and the mutant form Tasg-D1 in Arabidopsis (Arabidopsis thaliana) suggested that a single amino acid substitution in the Thr-283-Arg-284-Glu-285-Glu-286 domain of TaSG-D1 enhances protein stability in response to brassinosteroids, likely leading to formation of round grains in wheat. This gain-of-function mutation has pleiotropic effects on plant architecture and exhibits incomplete dominance. Haplotype analysis of 898 wheat accessions indicated that the origin of T. sphaerococcum in ancient India involved at least two independent mutations of TaSG-D1. Our results demonstrate that modest genetic changes in a single gene can induce dramatic phenotypic changes.