The GSK3 / SHAGGY ‐like OsGSK3 phosphorylates and inhibits phase separation of OsFCA at Ser‐43 and Ser‐45 to regulate brassinosteroid signaling and rice architecture

Brassinosteroid (BR) signaling plays a critical role in rice (Oryza sativa L.) grain development. GLYCOGEN SYNTHASE KINASE 3 (OsGSK3), a negative regulator of BR signaling, suppresses the transcriptional activity of OsBZR1 through phosphorylation. In this study, we employed a phosphoproteomic approach to construct an OsGSK3-mediated regulatory network. Within this network, we identified FLOWERING CONTROL LOCUS A (OsFCA) as a positive regulator of BR signaling and grain length, with m-Osfca mutants exhibiting significantly lower sensitivity to brassinolide treatment. Importantly, OsGSK3 interacts with and phosphorylates OsFCA on serine residues S43 and S45, and then forms condensates via liquid-liquid phase separation. Phosphorylated OsFCA promotes the translocation of the OsGSK3-OsFCA complex into the cytoplasm. Within the cytoplasm, OsGSK3 and OsFCA no longer exist in a condensate state. This mechanism provides precise regulation of grain length in rice. Notably, Osfca mutants are late-flowering, with OsFCA promoting heading under long-day conditions by repressing the expression of Grain number, plant height and heading date 7 (Ghd7) while activating that of Early heading date 1 (Ehd1), Heading date 3a (Hd3a), and RICE FLOWERING LOCUS T 1 (RFT1), which is potentially modulated by OsGSK3. This study clarifies the BR signaling transduction network by identifying OsFCA as a positive BR signaling component that regulates grain development and heading date, providing theoretical foundations for molecular breeding design in rice.