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

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 constructed an OsGSK3-mediated regulatory network.

Within this network, we identified 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.