A Single Amino Acid Substitution in STKc_GSK3 Kinase Conferring Semispherical Grains and Its Implications for the Origin of <i>Triticum</i><i>sphaerococcum</i>

Abstract

Six subspecies of hexaploid wheat (<i>Triticum</i> <i>aestivum</i>) have been identified, but the origin of Indian dwarf wheat (<i>Triticum sphaerococcum</i>), the only subspecies with round grains, is currently unknown. Here, we isolated the grain-shape gene <i>Tasg-D1</i> in <i>T</i> <i>sphaerococcum</i> via positional cloning. <i>Tasg-D1</i> encodes a Ser/Thr protein kinase glycogen synthase kinase3 (STKc_GSK3) that negatively regulates brassinosteroid signaling. Expression of <i>TaSG-D1</i> and the mutant form <i>Tasg-D1</i> in Arabidopsis (<i>Arabidopsis thaliana</i>) 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 <i>T</i> <i>sphaerococcum</i> in ancient India involved at least two independent mutations of <i>TaSG-D1</i> Our results demonstrate that modest genetic changes in a single gene can induce dramatic phenotypic changes.