Brassinosteroid receptor mutation influences starch granule size distribution in barley grains

Brassinosteroids (BR) are plant-based steroids which influence several morphogenetic and developmental processes. A barley (Hordeum vulgate L.) genotype Kinai Kyoshinkai-2 (KK-2) carrying the uzu mutation exhibited altered starch granule size distribution. Hybridizing KK-2 with a barley genotype CDC Kendall with bi-modal starch granules produced progeny lines (116, 144 and 168) with almost uni-modal starch granules. Bioassays correlated uzu mutation with defective BR perception. DNA sequence analysis of the BR receptor-1 (BRI-1) gene detected a single-nucleotide A > G substitution at the position 2612 in the kinase domain which resulted in the change of His (CAC) to Arg (CGC) at residue 857 in subdomain IV of the kinase domain of the respective polypeptide. The study focused on the development of barley grain, accumulation of starch and composition influenced by defective BR perception due to the mutation detected in KK-2 and three other barley-breeding lines (116, 144 and 168). Aberrant BRI-1 delayed grain development, amylose synthesis and starch accumulation in the endosperm. The barley breeding lines 116, 144 and 168 carrying the aberrant BRI-1, exhibited altered granule size distribution with significant shift in the diameter maxima, but insignificant differences in amylose concentration. The BRI-1 mutation also altered amylopectin fine structure in both B- and C- type small starch granules, resulting in an increased fraction of short A-type glucan chains (< 10 DP) and decreased fraction of B2 chains (25-36 DP) in genotypes carrying the BRI-1 mutation. The results show the influence of BR on barley grain development, starch accumulation, granule size distribution and amylopectin structure.