The pleiotropic effects of starch synthase IIIa (SSIIIa) mutation on the other SS isozymes in rice endosperm

Abstract

We analyzed the structure and composition of starch and the effects of the other SS isozymes derived from SSIIIa-deficiency in rice endosperm. Aactivities of isoamylase, pullulanase, branching enzyme IIa, BEIIb and phosphorylase in the mutants showed no obvious differences with those of the wild type. In constant, the amount of SSI protein was increased by 1.3-1.7-folds and it resulted in the increase of degree of polymerization (DP) 10-15 chains of the mutant amylopectin. Moreover, GBSSI protein was also increased by 1.4-1.7-folds and it resulted in the increase of amylose content and extra-long chains (DP≥500) of amylopectin. These changes in the structure and composition of starch of the mutants were caused directly by the reduction of long chains (DP≥33) of amylopectin by the SSIIIa-deficiency itself, and indirectly the enhancement of the activities of SSI and GBSSI. The combined effects brought about the dramatic reduction of pasting viscosity of the endosperm starch.