Multiple mechanisms modulate brassinosteroid signaling

Abstract

Brassinosteroids are essential hormones for plant growth and development. Genetic studies have identified key components of the BR signaling pathway, including the cell-surface receptor kinases that perceive BR, an intracellular kinase and a phosphatase, and nuclear transcription factors. Subsequent biochemical studies have revealed many details about signaling events from BR perception at the cell surface to gene expression in the nucleus. Recent studies have identified the 14-3-3 proteins as BR signaling components and elucidated multiple mechanisms by which phosphorylation modulates the BR transcription factors. In addition, BRI1 signaling from the endosomes and BR-independent functions of BAK1 have been observed. However, a major gap still exists in the current BR signaling pathway between the receptor complex and downstream signaling events.

Figure 1

Proposed functions of BAK1. Receptors exist as independent monomers in the absence of any ligand. BAK1 promotes BR and flagellin signaling through ligand-induced interaction with BRI1 and FLS2. BAK1 is likely also involved in signaling of other PAMPs to induce innate immunity responses and signaling of unknown endogenous signals to restrict cell death.

Figure 2

Phosphorylation and dephosphorylation of BZR1 and BZR2/BES1 (BZR) are mediated by BIN2 and BSU1, either one of which might be regulated by BR. BSU1 likely needs an intermediate protein for function in planta (Purple). Phosphorylation (p) at multiple sites on BZR proteins regulates activity of the proteins through multiple mechanisms. Phospho-mediated 14-3-3 binding regulates cytoplasmic retention of the protein. Phosphorylation also inhibits the DNA binding activity and promotes proteasomal degradation, possibly through E3-mediated ubiquitination. When dephosphorylated upon BR signaling, BZR proteins are able to bind DNA and activate or repress BR-regulated genes. It is unclear how BR signaling from the receptor complex regulates BZR phosphorylation, though upstream signaling might regulate the activity of either BIN2 or BSU1, or modify BZR to change its affinity for BIN2 or BSU1.