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PubMed2005

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.

Arabidopsis↗
PubMed2006

Regulation of brassinosteroid signaling.

Both animal and plant steroids mainly rely on transcriptional factors to bring about specific physiological responses; however, the signaling mechanisms that regulate these transcriptional factors are different. Steroid binding inside an animal cell directly alters the transcriptional activity of intracellular steroid receptors, whereas brassinosteroid (BR) binding to a cell surface receptor activates a phosphorylation-mediated signaling cascade that changes the amount, subcellular localization, and/or DNA-binding activity of a family of novel transcription factors. Genetic and molecular studies conducted over the past several years have uncovered several crucial BR signaling components that have dramatically increased our understanding of the signaling process of plant steroid hormones. In this review, we discuss the biochemical mechanisms of these signaling proteins for regulating the activity of the membrane BR receptor and the transduction of the BR signal from the cell surface to the nucleus.

Arabidopsis Proteins↗
PubMed2004

EXORDIUM regulates brassinosteroid-responsive genes.

In a screen for potential mediators of brassinosteroid (BR) effects, the EXORDIUM (EXO) protein was identified as a regulator of BR-responsive genes. The EXO gene was characterized as a BR-up-regulated gene. EXO overexpression under the control of the 35SCaMV promoter resulted in increased transcript levels of the BR-up-regulated KCS1, Exp5, delta-TIP, and AGP4 genes, which likely are involved in the mediation of BR-promoted growth. 35S::EXO lines grown in soil or in synthetic medium showed increased vegetative growth in comparison to wild-type plants, resembling the growth phenotype of BR-treated plants. Thus, the EXO protein most likely promotes growth via the modulation of gene expression patterns.

Arabidopsis↗
PubMed2000

Biosynthesis of brassinosteroids in cultured cells of Catharanthus roseus.

Precursor administration experiments with 2H-labeled 6-oxocampestanol, 6-deoxocastasterone and 6alpha-hydroxycastasterone in cultured cells of Catharanthus roseus were performed and the metabolites were analyzed by GC-MS. [2H6]Cathasterone was identified as a metabolite of [2H6]6-oxocampestanol, whereas [2H6]6alpha-hydroxycastasterone and [2H6]castasterone were identified as metabolites of [2H6]6-deoxocastasterone, and [2H6]castasterone was identified as a metabolite of [2H6]6alpha-hydroxycastasterone, indicating that 6-deoxocastasterone is converted to castasterone via 6alpha-hydroxycastasterone. In addition, 6-deoxocathasterone, a putative biosynthetic intermediate in the late C6-oxidation pathway, was identified as an endogenous brassinosteroid. These studies provide further evidence supporting our proposed biosynthetic pathways for brassinolide.

Cells, Cultured↗
PubMed2000

Synthesis and bioactivity evaluation of brassinosteroid analogs.

Four new analogs of 28-homocastasterone have been synthesized and completely characterized for the first time from stigmasterol. (22R, 23R,24S)-3beta-acetoxy-22,23-dihydroxy-5alpha-stigmastan+ ++-6-one (17), (22R,23R,24S)-3beta-bromo-22,23-dihydroxy-5alpha-stigmast an-6-one (18), (22R,23R,24S)-3beta-acetoxy-5,22, 23-trihydroxy-5alpha-stigmastan-6-one (20), and (22R,23R, 24S)-3beta-bromo-5,22,23-trihydroxy-5alpha-stigmastan-6-one (21), were obtained through a synthetic route based on regioselective Delta(5) epoxidation. Compounds 17 and 18, bearing a 5alphaH moiety, were prepared through a reductive opening of the 5beta,6beta epoxy precursor, and compounds 20 and 21, analogs with a 5alphaOH moiety were obtained by hydrolytic opening of a mixture of 5alpha,6alpha and 5beta,6beta epoxy precursors. Known compounds 19 and 22 were also obtained following the described synthetic routes, respectively. The new compounds were tested with the traditional auxin-like bioassay for brassinosteroids with 19 and 22 as standards. All compounds were comparatively evaluated for their inhibitory effect on the replication of DNA (HSV-1) virus.

Animals↗
PubMed2002

BES1 accumulates in the nucleus in response to brassinosteroids to regulate gene expression and promote stem elongation.

Plant steroid hormones, known as brassinosteroids (BRs), signal through a plasma membrane localized receptor kinase BRI1. We identified bes1, a semidominant suppressor of bri1, which exhibits constitutive BR response phenotypes including long and bending petioles, curly leaves, accelerated senescence, and constitutive expression of BR-response genes. BES1 accumulates in the nucleus in response to BRs. BES1 is phosphorylated and appears to be destabilized by the glycogen synthase kinase-3 (GSK-3) BIN2, a negative regulator of the BR pathway. These results establish a signaling cascade for BRs with similarities to the Wnt pathway, in which signaling through cell surface receptors leads to inactivation of a GSK-3 allowing accumulation of a nuclear protein that regulates target gene expression.

Amino Acid Sequence↗
PubMed2002

BRI1/BAK1, a receptor kinase pair mediating brassinosteroid signaling.

The Arabidopsis BAK1 (BRI1 Associated receptor Kinase 1) was identified by a yeast two-hybrid screen as a specific interactor for BRI1, a critical component of a membrane brassinosteroid (BR) receptor. In yeast, BAK1/BRI1 interaction activates their kinase activities through transphosphorylation. BAK1 and BRI1 share similar gene expression and subcellular localization patterns and physically associate with each other in plants. Overexpression of the BAK1 gene leads to a phenotype reminiscent of BRI1-overexpression transgenic plants and rescues a weak bri1 mutant. In contrast, a bak1 knockout mutation gives rise to a weak bri1-like phenotype and enhances a weak bri1 mutation. We propose that BAK1 and BRI1 function together to mediate plant steroid signaling.

Amino Acid Sequence↗
PubMed1998

A tale of dwarfs and drugs: brassinosteroids to the rescue.

Brassinosteroids (BRs), a specific class of low abundance plant steroids, are capable of eliciting strong growth responses and a variety of physiological changes through exogenous application to plants. Recently, BRs gained general acceptance as important regulators (hormones) of plant growth and development through genetic and molecular identification and characterization of genes involved in BR biosynthesis or response in Arabidopsis, pea and tomato. This major advance in the molecular genetics of BR biosynthesis and mode of action disclosed another case of amazing functional conservation of signalling molecules utilized in plants and animals, and provided a set of extremely valuable tools for the functional analysis of BRs.

Mutation↗
PubMed1998

Recent advances in brassinosteroid molecular genetics.

The importance of brassinosteroids (BRs, a specific class of ecdysone-like plant steroids) as essential endogenous regulators of growth and development is demonstrated through a growing number of well characterised Arabidopsis, pea, and tomato mutants deficient in BR biosynthesis or BR response. Thus, a rapid advancement in understanding the molecular genetics of BR biosynthesis and mode of action can be witnessed, which will be further enhanced through the availability of a set of extremely valuable molecular tools for the analysis of the biological function of BRs.

Mutation↗
PubMed2004

Microarray and proteomic analysis of brassinosteroid- and gibberellin-regulated gene and protein expression in rice.

Brassinosteroid (BR) and gibberellin (GA) are two groups of plant growth regulators essential for normal plant growth and development. To gain insight into the molecular mechanism by which BR and GA regulate the growth and development of plants, especially the monocot plant rice, it is necessary to identify and analyze more genes and proteins that are regulated by them. With the availability of draft sequences of two major types, japonica and indica rice, it has become possible to analyze expression changes of genes and proteins at genome scale. In this review, we summarize rice functional genomic research by using microarray and proteomic approaches and our recent research results focusing on the comparison of cDNA microarray and proteomic analyses of BR- and GA-regulated gene and protein expression in rice. We believe our findings have important implications for understanding the mechanism by which BR and GA regulate the growth and development of rice.

Gene Expression Profiling↗
PubMed2002

Uncoupling brassinosteroid levels and de-etiolation in pea.

The suggestion that brassinosteroids (BRs) have a negative regulatory role in de-etiolation is based largely on correlative evidence, which includes the de-etiolated phenotypes of, and increased expression of light-regulated genes in, dark-grown mutants defective in BR biosynthesis or response. However, we have obtained the first direct evidence which shows that endogenous BR levels in light-grown pea seedlings are increased, not decreased, in comparison with those grown in the dark. Similarly, we found no evidence of a decrease in castasterone (CS) levels in seedlings that were transferred from the dark to the light for 24 h. Furthermore, CS levels in the constitutively de-etiolated lip1 mutant are similar to those in wild-type plants, and are not reduced as is the case in the BR-deficient lkb plants. Unlike lip1, the pea BR-deficient mutants lk and lkb are not de-etiolated at the morphological or molecular level, as they exhibit neither a de-etiolated phenotype or altered expression of light-regulated genes when grown in the dark. Similarly, dark-grown WT plants treated with the BR biosynthesis inhibitor, Brz, do not exhibit a de-etiolated phenotype. In addition, analysis of the lip1lkb double mutant revealed an additive phenotype indicative of the two genes acting in independent pathways. Together these results strongly suggest that BR levels do not play a negative-regulatory role in de-etiolation in pea.

Journal Article↗
PubMed2006

Downstream nuclear events in brassinosteroid signalling.

Brassinosteroids (BRs) are steroid hormones that control many aspects of plant growth and development. BRs bind to the plasma membrane receptor kinase BRI1, and act through a signalling pathway that involves a glycogen synthase kinase-3-like kinase (BIN2) and a serine/threonine phosphatase (BSU1). Previous models proposed that BIN2 negatively regulates BR signalling by controlling the stability and subcellular localization of the related transcription factors BES1 and BZR1 by phosphorylation, in a manner reminiscent of the canonical Wnt signalling pathway of metazoans. Here we present strong evidence for a different mode of regulation of BR signalling. We show that BES1 is localized constitutively to the nucleus, where its activity is modulated by nuclear-localized BIN2 kinase. BIN2-mediated phosphorylation of BES1 inhibits its DNA-binding activity on BR-responsive target promoters and its transcriptional activity through impaired multimerization. Our observations demonstrate that phosphorylation-dependent inhibition of DNA binding and trans-activation is the key primary mechanism of BES1 regulation.

Arabidopsis↗
PubMed2000

Lesions in the sterol delta reductase gene of Arabidopsis cause dwarfism due to a block in brassinosteroid biosynthesis.

The brassinosteroid (BR) biosynthetic pathway, and the sterol pathway which is prerequisite to the BR pathway, are rapidly being characterized because of the availability of a large number of characteristic dwarf mutants in Arabidopsis. Here we show that the Arabidopsis dwarf5 mutants are disrupted in a sterol Delta7 reduction step. dwf5 plants display the characteristic dwarf phenotype typical of other BR mutants. This phenotype includes small, round, dark-green leaves, and short stems, pedicels, and petioles. Metabolite tracing with 13C-labeled precursors in dwf5 verified a deficiency in a sterol Delta7 reductase activity. All six independent alleles contain loss-of-function mutations in the sterol Delta7 reductase gene. These include a putative mRNA instability mutation in dwf5-1, 3' and 5' splice-site mutations in dwf5-2 and dwf5-6, respectively, premature stop codons in dwf5-3 (R400Z) and dwf5-5 (R409Z), and a mis-sense mutation in dwf5-4 (D257N). The dwf5 plant could be restored to wild type by ectopic overexpression of the wild-type copy of the gene. Both the Arabidopsis dwf5 phenotype and the human Smith-Lemli-Opitz syndrome are caused by loss-of-function mutations in a sterol Delta7 reductase gene, indicating that it is required for the proper growth and development of these two organisms.

Alleles↗
PubMed2000

Tissue-specific induction of the mRNA for an extracellular invertase isoenzyme of tomato by brassinosteroids suggests a role for steroid hormones in assimilate partitioning.

Brassinosteroids (BRs) induce various growth responses when applied exogenously to plant tissues, and the analysis of biosynthetic mutants reveals an essential role for plant growth and development. Only a few BR-regulated genes have been identified so far, and the corresponding gene products are assumed to be involved in cell elongation. The present study shows that BR growth responses are linked to the regulation of carbohydrate metabolism by induction of the mRNA for the key enzyme of an apoplastic phloem-unloading pathway. Addition of BRs to autotrophic tomato suspension culture cells specifically elevates the activity of cell-wall-bound invertase, whereas the intracellular invertase activities were not affected. This enhanced enzyme activity was shown to correlate with the induction of the mRNA of extracellular invertase Lin6, whereas the mRNA levels of the other three extracellular invertase isoenzymes were not affected. The induction level induced by different BRs correlates with their growth-promoting activity. The physiological significance of this regulation is further supported by the low concentrations and short incubation times required to induce Lin6 mRNA. This regulatory mechanism results in an elevated uptake of sucrose via the hexose monomers, and thus an increased supply of carbohydrates to the BR-treated cells. Experiments with tomato seedlings showed that the localized BR-dependent growth response of the hypocotyl elongation zone was accompanied by a specific induction of Lin6 mRNA that is restricted to the corresponding tissues. This study demonstrates a role of BRs in tissue-specific source/sink regulation.

Blotting, Northern↗
PubMed2001

Overexpression of DWARF4 in the brassinosteroid biosynthetic pathway results in increased vegetative growth and seed yield in Arabidopsis.

Plants unable to synthesize or perceive brassinosteroids (BRs) are dwarfs. Arabidopsis dwf4 was shown to be defective in a steroid 22alpha hydroxylase (CYP90B1) step that is the putative rate-limiting step in the BR biosynthetic pathway. To better understand the role of DWF4 in BR biosynthesis, transgenic Arabidopsis plants ectopically overexpressing DWF4 (AOD4) were generated, using the cauliflower mosaic virus 35S promoter, and their phenotypes were characterized. The hypocotyl length of both light- and dark-grown AOD4 seedlings was increased dramatically as compared to wild type. At maturity, inflorescence height increased >35% in AOD4 lines and >14% in tobacco DWF4 overexpressing lines (TOD4), relative to controls. The total number of branches and siliques increased more than twofold in AOD4 plants, leading to a 59% increase in the number of seeds produced. Analysis of endogenous BR levels in dwf4, Ws-2 and AOD4 revealed that dwf4 accumulated the precursors of the 22alpha-hydroxylation steps, whereas overexpression of DWF4 resulted in increased levels of downstream compounds relative to Ws-2, indicative of facilitated metabolic flow through the step. Both the levels of DWF4 transcripts and BR phenotypic effects were progressively increased in dwf4, wild-type and AOD4 plants, respectively. This suggests that it will be possible to control plant growth by engineering DWF4 transcription in plants.

Arabidopsis↗
PubMed2003

The LKA gene is a BRASSINOSTEROID INSENSITIVE 1 homolog of pea.

Brassinosteroids (BRs) are growth-promoting plant steroid hormones, and in garden pea (Pisum sativum L.), the lka mutant is defective in BR perception. Here, we show that LKA encodes P. sativum BRI1 (PsBRI1), a homolog of BRI1, which is the Arabidopsis leucine-rich repeat receptor-like kinase/BR receptor. PsBRI1 was isolated by screening a pea cDNA library using Arabidopsis BRI1 cDNA as the probe. PsBRI1 is predicted to encode a 1188-amino-acid protein that has 78% similarity with Arabidopsis BRI1. Sequence analysis of PsBRI1 in the lka mutant led to the identification of a missense mutation that converts the highly conserved aspartic acid residue to asparagine, which is located in the leucine-rich repeat, just before the island domain that may bind BR or a BR-protein complex. The mutation identified in PsBRI1 co-segregated with the semi-erectoide lka phenotype. Transcript analysis of LKA/PsBRI1 indicates that it is ubiquitously expressed in pea and that the expression was downregulated by exogenous BR. The lka mutant was then utilized in further studies to analyze the independent actions of BR and gibberellin (GA) through the characterization of BR response on GA mutants and GA response on BR mutants.

Amino Acid Sequence↗
PubMed2003

A dwarf mutant strain of Pharbitis nil, Uzukobito (kobito), has defective brassinosteroid biosynthesis.

Japanese morning glory (Pharbitis nil) is a model plant characterized by a large stock of spontaneous mutants. The recessive mutant Uzukobito shows strong dwarfism with dark-green rugose leaves. The phenotype was rescued by the application of brassinolide, a bioactive brassinosteroid (BR), indicating that Uzukobito was a BR-deficient mutant. A detailed analysis of the endogenous BR levels in Uzukobito and its parental wild-type plant showed that Uzukobito had a lower level of BRs downstream of (24R)-24-methyl-5alpha-cholestan-3-one and (22S, 24R)-22-hydroxy-24-methyl-5alpha-cholestan-3-one than those in wild-type plants, while their immediate precursors (24R)-24-methylcholest-4-en-3-one and (22S, 24R)-22-hydroxy-24-methylcholest-4-en-3-one accumulated relatively more in Uzukobito. These results indicate that Uzukobito had a defect in the conversion of (24R)-24-methylcholest-4-en-3-one and (22S, 24R)-22-hydroxy-24-methylcholest-4-en-3-one to their 5alpha-reduced forms, which is catalyzed by de-etiolated2 (DET2) in Arabidopsis. The P. nil ortholog of the DET2 gene (PnDET2) was cloned and shown to have the greatest similarity to DET2 among all the putative genes in Arabidopsis. Uzukobito had one amino acid substitution from Glu62 to Val62 in the deduced amino acid sequence of PnDET2. Recombinant PnDET2 expressed in COS-7 cells was found to be a functional steroid 5alpha-reductase (S5alphaR) converting (24R)-24-methylcholest-4-en-3-one to (24R)-24-methyl-5alpha-cholestan-3-one, while PnDET2 with the mutation did not show any catalytic activity. This shows that a plant S5alphaR can convert an intrinsic substrate. All these results clearly demonstrate that the Uzukobito phenotype resulted from a mutation on PnDET2, and a morphological mutant has been characterized at the molecular level among a large stock of P. nil mutants.

Amino Acid Sequence↗
PubMed2006

Molecular analysis of brassinosteroid action.

Brassinosteroids (BRs) are steroidal plant hormones with important regulatory roles in various physiological processes, including growth, xylem differentiation, disease resistance, and stress tolerance. Several components of the BR signal transduction pathway have been identified. The extracellular domains of receptor kinases such as BRI1 perceive BRs and transduce the signal via intracellular kinase domains. Within the cell further kinases and phosphatases determine the phosphorylation status of transcription factors such as BES1 and BZR1. These factors mediate major BR effects. Studies of BR-regulated genes shed light on the molecular mode of BR action. Genes encoding cell-wall-modifying enzymes, enzymes of the BR biosynthetic pathway, transcription factors, and proteins involved in primary and secondary metabolism are subject to BR-regulation. Gene expression data also point at interactions with other phytohormones and a role of BR in stress responses. This article gives a survey of the BR-signaling pathway. Two BR-responsive genes, OPR3 and EXO, are described in detail.

Arabidopsis↗