OsPUB9 modulates leaf angle and grain size through the brassinosteroid signaling pathway in rice

Plant U-box (PUB) E3 ubiquitin ligases are well known for their diverse functions in plant growth and development through physical link to cell signaling hubs to activate regulatory networks. Brassinosteroid (BR) signaling is negatively regulated by a protein kinase GSK2, which interacts with and phosphorylates OsOFP8 (OVATE family protein 8) to regulate BR signaling. Here we identified OsPUB9, a functional E3 ubiquitin ligase, which acts as a negative factor in BR signaling to regulate leaf inclination angle and grain size in rice. OsPUB9 displays autoubiquitination activity and is degraded in response to BR treatment. Interaction with OsUBC13, a rice E2 ubiquitin-conjugating enzyme, suppresses OsPUB9 degradation. OsPUB9 interacts with GSK2, and the interaction reduces autoubiquitination of OsPUB9. Coexpression of OsPUB9 and GSK2 in rice protoplasts suppresses degradation of OsPUB9 but promotes degradation of GSK2. OsPUB9 also interacts with OsOFP8, a positive regulator in BR signaling, and the interaction suppresses degradation of OsPUB9 but facilitates OsOFP8 degradation. Our study reveals that OsPUB9, GSK2, and OsOFP8 form a regulatory network in BR signaling to mediate gene expression, leaf angle, and grain size in rice.