OsGSK2-OsBSK3 feedback loop controls rice BR signaling

Brassinosteroids (BRs) are essential plant hormones that regulate growth and development, including leaf angle regulation and grain size determination. Maintaining appropriate BR signaling requires precise coordination between pathway activation and attenuation. BR signaling is initiated by OsBRI1 activation, which triggers a phosphorylation cascade that ultimately inhibits OsGSK2 kinase activity, releasing OsBZR1 from repression and activating downstream responses. However, whether additional regulatory mechanisms fine-tune BR signaling remains unclear. Here, we identify OsBSK3 as a direct substrate of OsGSK2 and uncover a phosphorylation-dependent feedback mechanism within the BR signaling pathway. OsGSK2-mediated phosphorylation enhances the interactions of OsBSK3 with OsBRI1, OsGSK2, and OsPPKL1 while attenuating the regulatory activity of OsBSK3. Genetic analyses revealed that overexpression of wild-type OsBSK3 or the phosphorylation-deficient variant OsBSK3SA enhances BR responses and fully rescues the leaf angle and grain size defects of the osbsk3 mutant, whereas the phosphomimetic variant OsBSK3SD fails to restore these phenotypes. Together, our findings establish a phosphorylation-dependent OsGSK2-OsBSK3 negative feedback mechanism that fine-tunes BR signaling homeostasis in rice.