Brassinosteroids (BRs) are plant-specific steroid hormones that control plant growth and development. Recent studies have identified key components of the BR signaling pathway in and in rice (); however， the mechanism of BR signaling in rice， especially downstream of GSK3/SHAGGY-like kinase (GSK2)， remains unclear. Here， we identified a BR-insensitive rice mutant， ()， and cloned the corresponding gene. was identical to the previously reported ()， which was cloned from another allele. encodes a transcription factor with an APETALA2 DNA binding domain. Genetic analysis indicated that RLA1/SMOS1 functions as a positive regulator in the BR signaling pathway and is required for the function of BRASSINAZOLE-RESISTANT1 (OsBZR1). In addition， RLA1/SMOS1 can interact with OsBZR1 to enhance its transcriptional activity. GSK2 can interact with and phosphorylate RLA1/SMOS1 to reduce its stability. These results demonstrate that RLA1/SMOS1 acts as an integrator of the transcriptional complex directly downstream of GSK2 and plays an essential role in BR signaling and plant development in rice.