SSI allelic variation influences grain quality and starch structure in rice subjected to high growth temperatures

Elevated temperatures are a major environmental stress factor that impairs rice productivity and compromises grain quality. This study investigated the effects of natural allelic variation in soluble starch synthase I (SSI) on grain quality and starch structural properties under high-temperature (HT) conditions. Four near-isogenic lines (NILs) differing in SSI alleles and carrying either the Wx^b or wx backgrounds were cultivated under HT stress. A range of analytical techniques revealed that HT significantly reduced grain appearance quality, protein content, apparent amylose content (AAC), and total starch content across all NILs. In addition, rapid visco-analysis (RVA) profiles of rice flour showed a marked reduction in viscosity under HT. Starch fine structure analysis demonstrated a decrease in short-chain amylopectin and an increase in long-chain amylopectin under HT, which was associated with enhanced starch crystallinity and elevated gelatinization temperatures. Notably, rice lines carrying the SSIⁱ allele, in both Wx^b and wx backgrounds, exhibited milder declines in grain quality traits compared to those with the SSI^j allele. These findings provide new insights into the role of SSI allelic variation in maintaining rice grain quality under HT stress and offer a genetic basis for breeding heat-tolerant rice varieties.