Regulator Of Brassinosteroids Signaling Research Articles

Advances in Understanding Brassinosteroid Signaling

Brassinosteroids (BRs) function as signaling molecules in plants and are involved in processes such as stem elongation, vascular differentiation, male fertility, timing of senescence and flowering, leaf development, and resistance to biotic and abiotic stresses. Unlike animal steroids that are perceived by nuclear receptors, BRs are perceived by transmembrane receptor kinase complexes that initiate a phosphorylation-mediated signaling cascade to transduce the steroid signal. BR binding to the extracellular domain of the receptor BRI1 induces kinase activation and hetero-oligomerization with the second transmembrane kinase BAK1. Activated BRI1 then dissociates from the BRI1-interacting protein BKI1, a newly identified negative regulator of BR signaling. In the presence of BR, the kinase BIN2, which is the Arabidopsis homolog of GSK3 (glycogen synthase kinase 3), is inhibited by an unknown mechanism, leading to dephosphorylation of BES1 and BZR1 inside the nucleus. This allows BES1 and BZR1 to homodimerize or combine with other transcription factors to bind to promoters of BR-responsive genes. These studies of BR signaling in plants have revealed signaling pathways that are distinctly different from related ones operating in animal cells.

Functional analysis of the BIN2 genes of cotton

Brassinosteroids (BR) promote the elongation of cotton fibers and may be a factor in determining their final length. To begin to understand the role of BR-mediated responses in the development of cotton fibers we have characterized the BIN 2 genes of cotton. BIN 2 is a member of the shaggy-like protein kinase family that has been identified as a negative regulator of BR signaling in Arabidopsis. Sequence analyses indicate that the tetraploid cotton genome includes four genes with strong sequence similarity to BIN 2. These genes fall into two distinct subclasses based on sequence and expression patterns. Sequence comparisons with corresponding genes from cotton species that have the diploid A and D genomes, respectively, shows that each pair of genes comprises homeologs derived from the A and D sub-genomes. Transgenic Arabidopsis plants that express these cotton BIN 2 cDNAs show reduced growth and similar phenotypes to the semi-dominant bin 2 mutant plants. These results indicate that the cotton BIN 2 genes encode functional BIN 2 isoforms that can inhibit BR signaling. Further analyses of the function of BIN 2 genes and their possible roles in determining fiber yield and quality are underway.

It runs in the family: regulation of brassinosteroid signaling by the BZR1–BES1 class of transcription factors

Research of the molecular events underlying brassinosteroid (BR) signaling is emerging as a new paradigm for studying plant hormone signal transduction pathways. Two recent reports have shown that the BZR1-BES1 family of proteins is a class of novel plant-specific transcription factors that directly bind to and regulate BR-responsive genes, thus filling in key details of the link between hormonal signal transmission in the cytoplasm and transcriptional status change in the nucleus.

Regulating Plant Steroids

Plant hormones include small peptides, complex chemicals, and steroids known as brassinosteroids. Brassinosteroids regulate, among other aspects, the plant's response to light conditions, its growth habit, and flowering patterns. The signaling pathways controlled by brassinosteroids are likely to be complex and highly branched. Li et al. have cloned and analyzed the BIN2 gene, which encodes a protein that resembles the SHAGGY-type kinases well known for controlling a variety of metabolic pathways in Drosophila , yeast, and mammalian cells. The BIN2 protein product function seemingly early in the brassinosteroid signaling response pathway, but just how closely linked its function is to the initial response of brassinosteroid and its receptor remains unclear. J. Li, K. H. Nam, Regulation of brassinosteroid signaling by a GSK3/SHAGGY-like kinase. Science 295 , 1299-1301 (2002). [Abstract] [Full Text]

Regulation of brassinosteroid signaling by a GSK3/SHAGGY-like kinase.

GSK3/SHAGGY is a highly conserved serine/threonine kinase implicated in many signaling pathways in eukaryotes. Although many GSK3/SHAGGY-like kinases have been identified in plants, little is known about their functions in plant growth and development. Here we show that the Arabidopsis BRASSINOSTEROID-INSENSITIVE 2 (BIN2) gene encodes a GSK3/SHAGGY-like kinase. Gain-of-function mutations within its coding sequence or its overexpression inhibit brassinosteroid (BR) signaling, resulting in plants that resemble BR-deficient and BR-response mutants. In contrast, reduced BIN2 expression via cosuppression partially rescues a weak BR-signaling mutation. Thus, BIN2 acts as a negative regulator to control steroid signaling in plants.