Molecular Mechanism by Which OsSUT2 Regulates Chalkiness Formation in Rice Grains

Rice chalkiness is a key constraint in breeding high-quality rice, and unbalanced sucrose transport and starch metabolism are its primary causes. To clarify the molecular mechanism by which OsSUT2 regulates rice grain chalkiness formation, the rice cultivar TP309 was used as material, and ossut2 homozygous mutants were generated via CRISPR/Cas9. Systematic studies were performed using genetic complementation, phenotypic identification, cytological observation, transcriptome sequencing, and haplotype analysis. The results show that loss of OsSUT2 function significantly increased grain chalkiness, deteriorated agronomic traits, induced carbon assimilate accumulation in leaves, blocked sugar transport and starch synthesis in grains, and destroyed starch fine structure; the mutant phenotype was largely restored by functional complementation with wild-type OsSUT2. OsSUT2 was expressed in both source and sink organs, with the strongest inhibition detected in the panicles. Mutation of OsSUT2 disrupted sucrose and starch metabolic pathways. Three main haplotypes of OsSUT2 were identified in natural populations, with significant indica–japonica differentiation. OsSUT2 is confirmed as a key regulator of rice chalkiness, providing gene resources and theoretical support for rice quality improvement.