Brassinosteroid signal transduction: from receptor kinase activation to transcriptional networks regulating plant development

Abstract

Brassinosteroid (BR) signal transduction research has progressed rapidly from the initial discovery of the BR receptor to a complete definition of the basic molecular components required to relay the BR signal from perception by receptor kinases at the cell surface to activation of a small family of transcription factors that regulate the expression of more than a thousand genes in a BR-dependent manner. These mechanistic advances have helped answer the intriguing question of how a single molecule, such as a hormone, can have dramatic pleiotropic effects on a broad range of diverse developmental pathways and have shed light on how BRs interact with other plant hormones and environmental cues to shape the growth of the whole plant. This review summarizes the current state of BR signal transduction research and then examines recent articles uncovering gene regulatory networks through which BR influences both vegetative and reproductive development.

Figure 1.

Functional Analysis of BRI1 Phosphorylation Sites. The bri1-5 receptor kinase mutant is a weak allele with semidwarf phenotype, altered leaf structure, and shortened petioles. Expression of wild-type BRI1-Flag in bri1-5 rescues the mutant phenotype, while expression of a mutant construct in which the critical Thr-1049 kinase domain activation loop phosphorylation site is substituted with Ala leads to a dominant-negative effect with an extreme dwarf phenotype similar to bri1 null mutants. (Adapted from Figure 8 of Wang et al. [2005b] with permission.)

Figure 2.

Current Model of BR Signal Transduction. The active BR, brassinolide (BL), binds to the extracellular domain of the BRI1 receptor kinase, promoting a basal BRI1 kinase activity that phosphorylates the negative regulator BKI1 on Y211, releasing it from the membrane and allowing BRI1 to associate with BAK1 or its homologs BKK1 and SERK1. BRI1 and BAK1 transphosphorylate each other on specific residues, which enhances the signaling capacity of BRI1, leading to phosphorylation of BSK1 and its release from the receptor complex. Activated BSK1 associates with and activates BSU1, which dephosphorylates the BIN2 kinase on Tyr-200, inactivating it. The unphosphorylated forms of BES1 and BZR1 transcription factors then accumulate (with aid of PP2A) and bind to the promoters of BR-regulated target genes, eliciting a specific physiological response such as cell elongation. Positive regulators of the pathway are shown in black, with negative regulators in red. For simplicity, the nuclear or cytoplasmic localization of BSU1, BIN2, BES1, and BZR1 are not shown in this model but are discussed in more detail in the text.

Figure 3.

ATypical bHLH Transcription Factor in BR-Regulated Growth. (A) All six members of the PRE family of atypical bHLH transcription factors rescue the dwarf phenotype, altered leaf shape, and shortened petioles of the weak bri1-301 mutant allele when overexpressed, indicating a positive role in BR signaling. (B) Overexpression of the AIF1 bHLH transcription factor in wild-type Arabidopsis results in an extreme dwarf phenotype, suggesting that AIF1 is a negative regulator of BR signaling. The extent of the phenotype is correlated with the level of transgene expression. (Adapted with permission from Figures 4 and 7 of Wang et al. [2009].)