Brassinosteroid signaling in plants

Abstract

Brassinosteroids (BRs) are a class of plant hormones comprising more than 70 polyhydroxylated sterol derivatives with structural similarity to animal steroid hormones. BRs are widely distributed across the plant kingdom and play essential roles in regulating multiple physiological processes and developmental programs including cell and organ elongation, cell division and differentiation, vegetative and reproductive development, and responses to the environment. BR signaling is initiated by binding of the BR ligand to the extracellular domain of a membrane-bound receptor kinase, BRASSINOSTEROIDINSENSITIVE 1 (BRI1). BRI1 forms heterooligomers with its co-receptor, BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1), and its close homologs to initiate a phosphorelay that ultimately results in changes in nuclear gene expression. Specific phosphorylation sites of BRI1 and BAK1 have been analyzed by biochemical and genetic approaches, and phosphorylation at individual residues has been shown to have profound effects on BR signaling and overall plant growth. Downstream substrates of BRI1 have been identified and characterized as well as intermediate signaling components including kinases, phosphatases, and transcription factors. BR signaling generates pleiotropic responses by regulating transcription factors that target genes involved in cell elongation and cell wall metabolism, cell division and differentiation, development of numerous plant organs, environmental responses to light and abiotic and biotic stress, and genes involved in the biosynthesis, transport, and signaling of other hormones. Specific components of BR, gibberellin (GA), and light signaling pathways directly interact to form a core transcription module that coordinately regulates seedling morphogenesis.