Brassinosteroid Signaling in Arabidopsis
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Brassinosteroid Signaling in Arabidopsis

Plant growth and development are ruled by environmental and endogenous signals, which are integrated through genetic networks that finally act on the division, expansion, and differentiation of cells to generate specific developmental patterns. BRs (Brassinosteroids) are steroid hormones belonging to the group of endogenous signals required for plant growth and organogenesis, controlling processes such as Cell Expansion, Vascular Differentiation, Etiolation and Reproductive Development. In addition to a role in Plant growth and development, BR has been also implicated in the modulation of Plant Stress Responses, including Enhancement of Chilling, Thermo, Salt-tolerance and Protection of Plant from the Mild Drought Injury and Pathogen Attack. Plants those are defective in BR biosynthesis or perception display characteristic mutant phenotypes that include a Dwarf Stature, Delayed Flowering and Senescence, Reduced Apical Dominance and Male Fertility, and Photomorphogenesis in the dark (Ref.1). The Signaling Pathways controlled by BRs are believed to be complex and highly branched. In Arabidopsis thaliana, BR signaling constitutes several signaling components. These include the cell-surface receptor kinases BRI1 (Brassinosteroid Insensitive-1) and BAK1 (BRI1-Associated Receptor Kinase-1), which perceive the BR signal and initiate the signal transduction cascade; the nuclear protein BZR1 (Brassinazole-Resistant-1) and its homolog BZR2 (Brassinazole-Resistant-2) /BES1 (BRI1-EMS-Suppressor-1 Protein), which activate growth responses and are dephosphorylated and stabilized by BR signaling; the BIN2 (Brassinosteroid Insensitive-2) kinase, which negatively regulates BR responses by phosphorylating BZR1 and BZR2/BES1 and the BSU1 (BRI1 Suppressor Protein-1) Phosphatase, which positively regulates BR responses by dephosphorylating  and stabilizing the BZR2/BES1 protein (Ref.2).

BR perception is initiated by two Leucine-rich Repeat Receptor-like Kinases, BRI1 and BAK1. BRI1 is an LRR-RLK (Leucine Rich Repeat-Receptor-Like Kinase) that consists of an LRR (Leucine Rich Repeat)-containing Extracellular Domain, connected by a single-pass transmembrane segment to an Intracellular Serine/Threonine Kinase Domain. The Extracellular Domain of BRI1 consists of several discrete regions, including an N-terminal Signal Peptide, Leucine-Zipper motif, two pairs of Cysteine residues flanking 25 tandem LRRs, and a 70-Amino Acid) Island buried between LRR21 and 22. BRI1 perceives the BR signal through its Extracellular Domain and initiates a signal transduction cascade through its Cytoplasmic Kinase activity. Like BRI1, BAK1 is also an LRR-containing Receptor Kinase that functions with BRI1 in BR signaling. BAK1 has a shorter Extracellular Domain, with only five LRRs, and it lacks the 70–Amino Acid Island Domain. BAK1 and BRI1 have the same expression pattern, and both proteins are Plasmamembrane localized so that physiological interaction is possible. In addition, BAK1 and BRI1 phosphorylate one another, and BAK1 shows an increased level of phosphorylation in the presence of BRI1 (Ref.3, Ref.4 & 5). Recent molecular genetic studies have implicated a GSK3 (Glycogen Synthase Kinase 3)-like protein BIN2 as a negative regulator of BR responses. BIN2 encodes a cytoplasmic Ser/Thr Kinase that displays 70% sequence identity within the Catalytic Domain to those of the Mammalian GSK3 and the Drosophila Shaggy kinases, which play key roles as negative regulators in a wide variety of signaling processes controlling Cell Proliferation, Cell Differentiation, Cytoskeleton Dynamics, and PCD (Programmed Cell Death). The discovery of the cytoplasmic kinase BIN2 provided a great opportunity for identifying additional BR signaling components. In the absence of BR signal BIN2 phosphorylates downstream targets to block the further transduction of BR signaling in the Cytosol. Two nuclear proteins, BZR1 and BES1, are recently identified as positive regulators of the BR signaling pathway downstream of BIN2 (Ref.1 & 6).

BRI1 and BAK1 mainly exist as inactive monomers that are in equilibrium with a tiny pool of BRI1/BAK1 heterodimers. In the absence of BR, the constitutively active BIN2 phosphorylates BZR1 and BZR2/BES1 and targets them for degradation by the Ubiquitin-dependent Proteasome pathway. As a result, no BR signal reaches the nucleus and cell elongation is inhibited. BR binding stabilizes or promotes the formation of the BRI1/BAK1 heterodimer, leading to activation of both receptor kinases via transphosphorylation. BR either binds directly to BRI1 or first complexes with an unidentified SBP (Steroid Binding Protein), which require processing by a Serine Carboxypeptidase. BR binding to the cell-surface receptor BRI1 activates BRI1 kinase and leads to inhibition of the BIN2 kinase, allowing dephosphorylation and accumulation of BZR1 and BZR2/BES1 proteins in the nucleus. BZR2/BES1 activates BR-induced genes such as Tch4, CDC2B (Cell Division Control Protein-2 homolog B) and Kor (Korrigan), and then affects cell elongation and some responses. BES1 does not act alone but associates with BIM proteins (BES1-Interacting Myc-like proteins), which belong to a different family of DNA binding proteins. BZR1 not only activates the BR-induced genes and promotes cell elongation but also suppresses BR biosynthetic genes such as CPD, leading to feedback inhibition of BR biosynthesis. Thus, BZR1 and BZR2/BES1 define two overlapping branches of the BR signal transduction pathway. Parallel to BRI1, BR might interact with an unknown receptor or component to regulate expression of some other BR response genes such as CycD3 (Cyclin Delta-3) and Rav1, resulting in the promotion of cell division and other subsequent physiological responses in Plants. Studies of the BR signaling pathway and BR gene-regulating properties indicate that there is cross talk between BRs and other hormones, including those with established roles in plant defense responses such as ABA (Abscisic Acid), JA (Jasmonic Acid), and Ethylene. Recent advances in elucidation of the BR signaling pathway, although still incomplete, make this pathway one of the best-understood signal transduction cascades in plants. Understanding how BRI1 and BAK1 influence the activities of additional components in the BR pathway will lead to a better understanding of hormonal signaling and the roles of the receptor protein kinases in plant growth and development (Ref.7 & 8).