The RLA1/SMOS1 Transcription Factor Functions with OsBZR1 to Regulate Brassinosteroid Signaling and Rice Architecture

Abstract

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 <i>Arabidopsis thaliana</i> and in rice (<i>Oryza sativa</i>); 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, <i>reduced leaf angle1</i> (<i>rla1</i>), and cloned the corresponding gene. <i>RLA1</i> was identical to the previously reported <i>SMALL ORGAN SIZE1</i> (<i>SMOS1</i>), which was cloned from another allele. <i>RLA1/SMOS1</i> 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.