Update on Phosphorylation-Mediated Brassinosteroid Signaling Pathways

Abstract

Protein phosphorylation is a universal mechanism that regulates cellular activities. The brassinosteroid (BR) signal transduction pathway is a relay of phosphorylation and dephosphorylation cascades. It starts with the BR-induced activation of the membrane receptor kinase brassinosteroid insensitive 1 (BRI1), resulting in the dephosphorylation of transcription factors such as BZR1/BES2 and BZR2/BES1 followed by BR-induced gene expression. Brassinosteroid signal transduction research has progressed rapidly by identifying the phosphorylation/dephosphorylation site(s) of the BR-regulated kinase and phosphatase substrates with a simultaneous pursuit of mutant phenotypes. Autophosphorylation, transphosphorylation, and serine/threonine and tyrosine phosphorylation of the receptor protein kinases BRI1 and BRI1-associated kinase (BAK1) have increased the understanding of the regulatory role of those kinases during physiological and developmental processes in plants. The phosphorylation event initiated by BR is also found in the regulation of receptor-mediated endocytosis and the subsequent degradation of the receptor. However, the basic molecular links of the BR signal transduction pathway are not well understood regarding this phosphorylation/dephosphorylation event. This review summarizes the current state of BR signal transduction research to uncover the phosphorylation/dephosphorylation networks and suggests directions for future research on steroid signal transduction to gain a more comprehensive understanding of the process. IntroductionBrassinosteroids are the only group of steroid hormones discovered in plants, which are different from steroid hormones in the animal system that has several different steroid hormones. Brassinosteroids regulate plant growth and development such as cell elongation, vascular development, senescence, stress responses, and photomorphogenesis [10,11,68]. BR-related mutants of the brassinosteroid biosynthesis pathway and signal transduction exhibited a short hypocotyl, opened cotyledons, and de-etiolation in the dark. Under light conditions, they exhibited severe dwarfism, aggregated and dark-green rosette leaves with epinasty, decreased elongation of the petiole, defects in the fertility of pollens, apical dominance, and senescence [9,69]. Therefore, brassinosteroids have pivotal roles in plant growth and differentiation [11,18]. In animal systems, steroid ligands bind to nuclear binding receptors that act as transcriptional regulators [65]. However, in plant systems, BRs bind to BRI1, an LRR-RLK on the plasma membrane, and this binding induces a signal, which is transduced to the nucleus through several signaling components in the cytoplasm [33,63].For the past few years, information on brassinosteroid-mediated phosphorylation has increased enormously. Not only serine/threonine phosphorylation but also tyrosine phosphorylation have been found, and the significance of autophosphorylation and transphosphorylation became known. Recently, many phosphorylation sites on protein kinases in ...