The Barley Glycosyltransferase Gene KOB1 Implicated in β-Glucan Biosynthesis by a Genome-Wide Association Study

β-glucan, a crucial trait in barley breeding programs, serves as a quality determinant of products intended for both human consumption and animal feed. Although genes involved in β-glucan synthesis have been reported, the genetic mechanisms regulating its accumulation in barley grain remain underexplored. In this study, we functionally characterized <i>KOB1</i>, a candidate gene identified from a genome-wide association study (GWAS) on barley seed β-glucan content, which encodes a glycosyltransferase. Haplotype analysis showed that haplotype E was associated with significantly elevated grain β-glucan levels compared to other haplotypes. Furthermore, overexpression of <i>KOB1</i> in rice led to a significant increase in grain β-glucan content relative to the wild-type Zhonghua 11, confirming its critical role in β-glucan biosynthesis. Our findings establish the glycosyltransferase gene <i>KOB1</i> as a valuable genetic resource for molecular breeding programs aimed at improving grain β-glucan content.