Effects of low light during grain-Filling stage on starch biosynthesis and related gene expression in rice

During the grain-filling stage of rice, overcast and rainy conditions with low sunlight reduce both yield and quality. Grain filling in rice primarily involves starch synthesis and accumulation, yet the molecular mechanisms by which low light affects starch biosynthesis remain poorly understood. In this study, shading treatments were applied during the grain-filling stage to simulate low-light conditions, and its effects on starch synthesis and related gene expression in rice grains were investigated. The result demonstrated that low light significantly decreased the 1000-grain weight while increasing chalky grain rate and chalkiness. It also reduced grain weight and starch content during grain filling. The endosperm starch structure displayed reduced compactness, with loosely arranged compound starch granules exhibiting intergranular gaps and surface adherents. Furthermore, the expression of key starch biosynthesis genes (OsSuS3, OsBT1, GBSSI, and SSIII-2) was downregulated under low light. Additionally, the expression of the light-signaling gene OsPHYB declined during the active grain-filling phase, whereas OsPIL13/OsPIL14 increased. In osphyb mutants, increased chalkiness, elevated OsPIL13/OsPIL14, and reduced SSIII-2/GBSSI mRNA abundance were observed. Yeast one-hybrid assays confirmed OsPIL13/OsPIL14 bind to SSIII-2 and GBSSI promoters. Thus, OsPHYB likely regulates starch synthesis via OsPIL13/OsPIL14-mediated suppression of SSIII-2 and GBSSI. These findings indicate that low light affects starch synthesis in rice grains through multiple pathways: by limiting substrate supply, reducing catalytic efficiency, and disrupting light-mediated signaling. This study provides new insights into the mechanisms by which low light influences rice yield and quality from both energetic and signaling perspectives.