Suppression of starch synthase I (SSI) by RNA interference alters starch biosynthesis and amylopectin chain distribution in rice plants subjected to high temperature

Based on known cDNAs of rice starch synthase isoforms, we constructed dsRNA interference vectors for starch synthase I (SSI) to produce transgenic plants containing starch with a moderately high amylose content. We investigated the effect of SSI suppression on grain quality traits, starch biosynthesis, and amylopectin chain distribution in rice plants exposed to two different temperature regimes. The activities and transcripts of BEs, DBEs, and other SS isoforms were further investigated to clarify the effect of SSI suppression on these key enzymes and their specific isoforms under different temperature treatments. Suppression of SSI by RNAi altered grain starch component and amylopectin chain distribution, but it exerted only a slight effect on total starch content (%) and accumulation amount (mg kernel−1) and on starch granule morphology and particle size distribution. Under normal temperature (NT), insignificant differences in kernel weight, chalky kernel proportion, chalky degree, and starch granule morphology between SSI-RNAi line and its wild type (WT) were observed. However, amylose content (AC) level and granule-bound starch synthase (GBSS) activity in rice endosperms were markedly increased by SSI-RNAi suppression. The chalky kernel proportion and chalky degree of SSI-RNAi lines were significantly higher than those of WT under high temperature (HT) exposure at filling stage. Inhibition of SSI by RNAi affected amylopectin chain distribution and raised starch gelatinization temperature (GT) in two ways: directly from the SSI deficiency itself and indirectly by reducing BEIIb amounts in an SSI-deficient background. The deficiency of SSI expression led to an alteration in the susceptibility of grain chalkiness occurrence and starch gelatinization temperature to HT exposure, owing to a pleiotropic effect of SSI deficiency on the expression of other genes associated with starch biosynthesis.