Transcription factor FTD1 mediates brassinosteroid signaling and maintains MONOCULM 1 stability to regulate rice tillering.

Brassinosteroids (BRs) are polyhydroxylated steroid phytohormones that regulate important agronomic traits, including tiller number in rice (Oryza sativa L), although the underlying molecular mechanisms remain unclear. Here, we isolated the rice mutant fewer tillers and dwarf 1 (ftd1). Map-based cloning revealed that FTD1 encodes a plant-specific GRAS family protein harboring a conserved GRAS domain at its C-terminus. Genetic and biochemical analyses showed that FTD1 physically interacts with MONOCULM 1 (MOC1) and inhibits its degradation. Genetic analysis also indicated that FTD1 functions as a positive regulator in the BR signaling pathway. Furthermore, OsGSK2 interacts with and phosphorylates FTD1 for degradation via the 26S proteasome. We determined that BRs promote FTD1 accumulation by suppressing OsGSK2 phosphorylation, thus enhancing MOC1 stability for increased tiller number. Our findings reveal an OsGSK2-FTD1-MOC1 regulatory cascade that mediates BR signaling in regulating rice tiller development.