OsGREA Coordinates Yield Formation and Grain Quality via Regulation of Protein Metabolism in Rice

Background/Objectives: FK506-binding proteins (FKBPs) act in protein folding, signaling and stress responses in plants. OsGREA (OsFKBP12) interacts with the amino acid permease OsAAP6, a key regulator of grain protein content, but its own role in rice yield and grain quality is unknown. We aimed to define the function and natural variation of OsGREA. Methods: Two OsGREA knockout lines were generated by CRISPR/Cas9 in Oryza sativa L. ssp. japonica cv. Zhonghua 11 and compared with the wild type for gene expression, agronomic traits, grain quality, glume cytology, grain filling and endosperm proteome. Haplotypes were analyzed in 533 accessions. Results: OsGREA was constitutively expressed, most highly in leaves. Knockout lines had higher seed-setting rate and grain yield per plant but lower thousand-grain weight. Their grains were shorter and narrower, with smaller and fewer outer-glume cells and slower early grain filling. Mutant grains contained less total protein, amino acids and free fatty acids and had a lower gelatinization temperature, whereas starch content and milling quality were unchanged. Proteomics revealed 238 differentially expressed proteins in mutant endosperm, enriched in amino acid and carbohydrate metabolism and protein processing in the endoplasmic reticulum, and storage protein genes including GluA2 and GluB1 were downregulated. Five major haplotypes showed indica–japonica differentiation; the japonica-predominant Hap1 was associated with greater grain width, thickness and weight. Conclusions: OsGREA influences both yield components and grain quality through glume cell development, grain filling and storage protein accumulation. Its loss of function raises yield per plant but reduces grain size and grain protein content. OsGREA and Hap1 are potential targets for gene editing and marker-assisted breeding.