Receptor-like protein 51 regulates brassinosteroid signaling by promoting the abundances of BRI1 and BAK1.

Abstract

Brassinosteroids (BRs) are essential phytohormones that broadly regulate plant growth, development, and adaptation to biotic and abiotic stresses. In Arabidopsis, apoplastic BR molecules are perceived by a plasma membrane-localized receptor complex comprising the ligand-binding receptor BRI1 and the co-receptor BAK1. While negative regulators of the BR receptor complex, such as BKI1, BIR3, and PUB12/13, have been well characterized, how BRI1 and BAK1 are positively modulated in the BR pathway remains largely unknown. In this study, a genetic screen involving overexpression of RLP genes in the bak1-3 bkk1-1 double mutant reveals that enhanced RLP51 expression partially suppresses the BR-deficient phenotypes of bak1-3 bkk1-1. RLP51 overexpression also partially rescues the weak bri1 mutant allele, bri1-301. Although the rlp51 single mutant exhibits wild-type-like phenotypes, it enhances BR-defective phenotypes in bri1-301 and bak1 serk1 mutants. RLP51 is next found to interact with both BRI1 and BAK1 without affecting BRI1-BAK1 interaction. Critically, co-expression of RLP51 with BRI1 or BAK1 significantly increases BRI1 and BAK1 protein abundances. RLP51 appears to promote protein synthesis rather than stabilize BRI1 and BAK1 proteins. Thus, our study identifies RLP51 as a positive regulator of BR signaling that enhances the protein levels of BRI1 and BAK1.