Molecular analysis of brassinosteroid action.

Brassinosteroids (BRs) are steroidal plant hormones with important regulatory roles in various physiological processes, including growth, xylem differentiation, disease resistance, and stress tolerance. Several components of the BR signal transduction pathway have been identified. The extracellular domains of receptor kinases such as BRI1 perceive BRs and transduce the signal via intracellular kinase domains. Within the cell further kinases and phosphatases determine the phosphorylation status of transcription factors such as BES1 and BZR1. These factors mediate major BR effects. Studies of BR-regulated genes shed light on the molecular mode of BR action. Genes encoding cell-wall-modifying enzymes, enzymes of the BR biosynthetic pathway, transcription factors, and proteins involved in primary and secondary metabolism are subject to BR-regulation. Gene expression data also point at interactions with other phytohormones and a role of BR in stress responses. This article gives a survey of the BR-signaling pathway. Two BR-responsive genes, OPR3 and EXO, are described in detail.