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.

Brassinosteroids regulate organ boundary formation in the shoot apical meristem of Arabidopsis

“…Finally, the pattern of BZR1 accumulation in the SAM is consistent with its inhibitory role in boundary formation and its positive role in promoting cell growth. Nuclear accumulation of BZR1 is tightly controlled by upstream BR signaling (12,32), whereas transcription of the BZR1 gene seems ubiquitous as indicated by both pBZR1::bzr1-1D-CFP and pBZR1::GUS. Thus, the discrete pattern of BZR1 accumulation in the SAM may reflect a distribution pattern of either BR itself or a key upstream BR signaling component.…”

Functional importance of BAK1 tyrosine phosphorylation in vivo

“…8 The Leucine-Rich Repeat Receptor-Like Kinases (LRR-RLKs) with 223 members is the largest subfamily and several of these plant receptor kinases have proven functional roles in the regulation of plant growth, morphogenesis, disease resistance and responses to environmental stress signals, [14][15][16][17] but the functions of the vast majority of this large and important family of signal transduction molecules remains unknown. In addition to BRI1 and BAK1, which are essential for BR sensing and signaling, 18,19 several other LRR-RLK members have identified functions including CLAVATA1, which controls meristematic cell fate, 20 ERECTA, which specifies organ shape, 17 HAESA, which plays role in floral organ abscission, 21 TOAD2, which plays a role in embryonic pattern formation, 22 and BAK1/AtSERK3 and FLAGELLIN-SENSITIVE2 (FLS2), 23 which function together in innate immunity after binding the bacterial flagellin peptide (flg22) ligand.…”

Deactivation of the Arabidopsis BRASSINOSTEROID INSENSITIVE 1 (BRI1) receptor kinase by autophosphorylation within the glycine-rich loop

“…Animal receptor kinases are predominantly tyrosine kinases, whereas plant receptor kinases are generally classified as Ser/Thr kinases, although recent work suggests that some plant receptor kinases are dual-specificity kinases that can also autophosphorylate on tyrosine residues (2)(3)(4)(5). One of the best-studied plant receptor kinases is BRASSINOSTEROID INSENSITIVE 1 (BRI1), which functions with its coreceptor, BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1), in brassinosteroid (BR) signaling (6)(7)(8)(9)(10)(11). Current thinking is that BRI1 and BAK1 are in their unphosphorylated forms and inactive in the absence of BR, whereas in the presence of the BR ligand, BRI1 and BAK1 heterodimerize and become activated via auto-and transphosphorylation (12).…”