The GRAIN SIZE 8-GRAIN LENGTH 10 module controls the grain size in rice by regulating the expression of GA- and CTK-related genes.

Rice grain size is an important agronomic trait that affects the grain weight and yield of rice (Oryza sativa). In this study, genome-wide association studies (GWASs) were performed regarding grain length (GL), grain width (GW), grain thickness (GT), the grain length-width ratio (GLWR) and the 1000-grain weight (TGW) across 172 rice accessions in 2020, 2021 and 2022. Among the 31 QTLs identified across all 3 years, the QTL qGL8/qGW8/qGT8/qGLWR8 on chromosome 8 was newly discovered in our study. OsFBX291 was validated as the key candidate gene for GRAIN SIZE8 (GS8) through CRISPR/Cas9-mediated knockout analysis. GS8 encodes a nucleus-localized protein containing an F-box domain that is constitutively expressed across multiple tissues. The A7444 allele (GS8) promotes cell proliferation and expansion in grain glumes by increasing the CTK and GA contents in young panicles. Moreover, RNA sequencing analysis revealed that GS8 regulates the expression of GA and CTK signalling pathway-related genes, such as GL10, OsGA20ox1, OsGA20ox2, OsCKX1 and WLG, which participate in the regulation of cell proliferation and hull expansion. Multiple sets of experimental data have collectively demonstrated that GS8 and GL10 exhibit protein-protein interaction. These findings suggest that the GS8-GL10 functional module plays a regulatory role in determining rice grain size by modulating the expression of genes associated with gibberellin (GA)- and cytokinin (CTK)-signaling pathways. The elite GS8 haplotype has a large grain size in the germplasm. These results provide important genetic information that can be used to further improve grain size through molecular breeding.