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Brassinosteroid Signaling Pathway

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Science's STKE  05 Dec 2006:
Vol. 2006, Issue 364, pp. cm4
DOI: 10.1126/stke.3642006cm4

Abstract

Plant growth is regulated by an intricate network of hormonal signaling pathways. These small-molecule hormones cause changes in gene expression that are associated with cell expansion and division and changes in development. Paradoxically, six of these hormones appear to have largely overlapping functions, yet the loss of response to any one hormone cannot be compensated by the action of another plant hormone. Among these hormones are the brassinosteroids (BRs), the polyhydroxylated steroid hormones of plants. The emerging picture of BR signal transduction diverges radically from the paradigms of animal steroid signaling, which generally involve the action of members of the nuclear receptor superfamily. BRs bind the extracellular domain of a small family of leucine-rich-repeat receptor kinases to activate intracellular signal transduction cascades that regulate the expression of hundreds of genes. The signaling pathway involves a cell surface receptor complex, a glycogen synthase kinase 3, a kelch-containing serine/threonine phosphatase, and a novel family of basic helix-loop-helix and Myc-like plant specific transcription factors. The receptor and each of the signaling components were identified in Arabidopsis thaliana, and knowledge of their sequences allowed identification of orthologs in rice, tomato, barley, and pea.

Description

This record contains general information about the Brassinosteroid Signaling Pathway collected across species.

Plants are sessile organisms and use a plethora of small molecule hormones to finely tune their growth and development in response to environmental changes. Among these hormones are the brassinosteroids (BRs), the polyhydroxylated steroid hormones of plants. Here, we focus on the signaling mechanism of BRs in the context of the biological processes that they regulate (1).

The role of BRs in plant development has been inferred from the study of biosynthesis and response mutants. BRs act in a multitude of processes throughout the plant life cycle, ranging from germination, root and stem elongation, seedling photomorphogenesis, vascular development, senescence, and reproductive development (2). BRs have similar structures to animal steroid hormones, and several BR biosynthesis enzymes share sequence identity with mammalian steroid biosynthetic enzymes. For example, the Arabidopsis thaliana DET2 locus encodes a plant ortholog of mammalian steroid 5α-reductases (3). DET2 catalyzes the conversion of (24R)-24-methylcholest-4-En-3-one to (24R)-24-methyl-5α-cholestan-3-one, early in the BR biosynthetic pathway, and in det2 mutants, this activity can be rescued by transgenic plants harboring either the type I or type II human steroid 5α-reductase. Conversely, AtDET2 is also capable of reducing both progesterone and testosterone in vitro (4).

Despite this conservation of steroid biosynthetic pathways between the plant and animal kingdoms, steroid signaling pathways share no common components. In metazoans, steroid hormones are perceived by the nuclear receptor superfamily of transcription factors (5), and signaling involves transcriptional reprogramming of hormone-responsive genes to promote the desired physiological response. A second, "nongenomic steroid signaling" pathway is poorly understood and may be mediated by receptors located at the plasma membrane, which may be members of the G protein–coupled receptor family. The Arabidopsis genome does not encode members of the nuclear receptor superfamily. Rather, a small family of plasma membrane-localized receptors initiates all known effects of BRs (611).

BR receptors signal changes in gene expression through a signaling pathway that involves phosphorylation and dephosphorylation of a plant-specific family of transcription factors (Fig. 1). In Arabidopsis (Table 1,) BRs activate a plasma membrane receptor complex consisting of BRI1 (the ligand-binding subunit) and BAK1 (its proposed co-receptor), both of which are leucine-rich repeat (LRR)-receptor serine/threonine kinases (12). In the absence of BRs, a protein called BKI1 binds to the kinase domain of BRI1 and prevents the interaction of BAK1 and BRI1, which in turn prevents receptor activation (13). As a result, BIN2 (14), also known as UCU1 (15) and DWARF12 (16), an Arabidopsis homolog of glycogen synthase kinase 3 (GSK-3), phosphorylates the plant-specific brassinosteroid response factors (BRFs) (17), BES1 and BZR1, to keep them in an inactive state (1821), reviewed in (22). In the presence of BRs, BKI1 dissociates from the receptor, allowing the kinase domains of BRI1 and BAK1 to interact and transphosphorylate (12, 13). Consequently, the activity of BIN2 is inhibited and, through the activity of a nuclear-localized kelch-containing protein phosphatase (KPP, known as BSU1 in Arabidopsis), BES1 (23), is dephosphorylated. Dephosphorylation allows BES1 (and presumably other BRFs) to dimerize or hetero-dimerize with members of the Myc family of transcription factors, bind elements in the promoters of BR-responsive genes, and activate or repress their expression (20, 24).

Fig. 1.

Pathway image captured from the dynamic graphical display of the information in the Connections Maps available 20 November 2006. For a key to the colors and symbols and to access the underlying data, please visit the pathway (About Connections Map).

Table 1.

Specific examples of the brassinosteroid signaling pathway in the Database of Cell Signaling. These specific pathways are based on the canonical Brassinosteroid Signaling Pathway.

Pathway Details

URL: About Connections Map

Scope: Canonical

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