Relationships among Genetic, Structural, and Functional Properties of Rice Starch

We determined the relationships among the structural properties, in vitro digestibility, and genetic factors in starches of 14 rice cultivars. Weight-based chain-length distributions in amylopectin ranged from 18.07% to 24.71% (fa, DP 6-12), 45.01% to 55.67% (fb1, DP 13-24), 12.72% to 14.05% (fb2, DP 25-36), and 10.80 to 20.72% (fb3, DP > 36), respectively. The contents of rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) ranged from 78.5% to 87.5%, 1.2% to 6.0%, and 10.1% to 18.0%, respectively. AAC was negatively correlated with RDS content but positively correlated with RS content in rice starch. The proportion of short chains in amylopectin, i.e. the amount of fraction IIa (FrIIa) fractionated by gel permeation chromatography (GPC), was positively correlated with RDS. Starch synthase IIa (SSIIa) gene controlled the degree of crystallinity, the amount of fa chains of amylopectin. SSIIIa gene controlled the amount of fb1 chains. Wx gene controlled the FrI, FrIIa, RDS, and RS. Starch debranching enzyme isoamylase II (ISA2) gene also controlled the RDS, which may suggest that RDS was also affected by amylopectin structure, although no correlation between them was found. This study indicated that genetics (i.e., starch biosynthesis related genes) controlled the structural properties of starch, and both amylose content and amylopectin fine structure determined functional properties of rice starch (i.e., the digestion), each in a different way. Understanding the "genetics-structure-function" relationships in rice starches will assist plant breeders and food processors in developing new rice varieties and functional foods.