The G-protein γ subunit DEP1 facilitates brassinosteroid signaling in rice via a MYB–bHLH–ARF module

G-protein signaling and brassinosteroid (BR) phytohormones play important roles in regulating rice (Oryza sativa) yield-related plant architecture, such as leaf inclination and grain size. However, the relationship between G-proteins and BR signaling has not been fully elucidated in rice. The present study indicates that the G-protein Gγ subunit DENSE AND ERECT PANICLE 1 (DEP1) positively regulates BR signaling in rice and that BRs promote DEP1 nuclear entry through GRAIN NUMBER ASSOCIATED (GNA). Additionally, DEP1 interacts with and acts upstream of OsMYB86, an R2R3-MYB family transcription factor that positively regulates BR signaling by directly binding to the promoter of its downstream gene BRASSINOSTEROID UPREGULATED 1 (BU1), activating its expression in rice. In the nucleus, DEP1 interacts with OsMYB86 and GNA, significantly enhancing OsMYB86-mediated activation of BU1 expression. Furthermore, BU1 interacts with another HLH protein, INCREASED LEAF INCLINATION1 (ILI1), and a bHLH protein, ILI1 BINDING bHLH (IBH1). Interaction between ILI1 and BU1 facilitates translocation of BU1 from the cytoplasm to the nucleus, where they impede IBH1 binding to the promoter of the AUXIN RESPONSE FACTOR 11 (OsARF11) gene, which is involved in crosstalk between BR and auxin, thus effectively relieving the IBH1-repressed transcription of OsARF11. These findings reveal a DEP1-mediated signaling pathway that links G-proteins to the traditional BR signaling pathway, ensuring the efficient activation of BR responses in rice.