SSIIIa-RNAi suppression associated changes in rice grain quality and starch biosynthesis metabolism in response to high temperature.

High temperature (HT) is a main environmental restraint that affects rice yield and grain quality. In this study, SSIIIa-RNAi and its wild-type (WT) were used to investigate the effect of HT exposure on the isozyme-specific variation of several key starch biosynthesis enzymes in developing endosperms and its relation to starch properties. SSIIIa-RNAi had minimal impact on grain chalky occurrence under normal temperature growth, but it could up-grade the susceptibility of grain chalky occurrence to HT exposure, due to the relatively sensitive response of AGPase and SSI to HT exposure. Different from WT, SSIIIa-RNAi had the relatively enriched proportion of chains with DP 13-16 under HT, and HT-induced decline in the proportion of DP < 12 became much larger for SSIIIa-RNAi relative to WT. SSIIIa-RNAi significantly enhanced the expression of SSI isozyme and total SS activity, whereas SSI-RNAi deficiency had little impact on the expression of SSIIIa isozyme. In this regard, the compensatory increase in SSI isozyme as a result of SSIIIa deficiency occurred only in a one-way manner. SSIIIa-RNAi caused a striking elevation in BEIIa expression, and the effect of SSIIIa deficiency on the chain length distribution in relation to HT exposure was closely associated with the participation of BEIIa, SSI, and their interaction in amylopectin biosynthesis.