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.     

油菜素内酯调控作物农艺性状和非生物胁迫响应的研究进展

植物对不利环境的适应依赖于将外部胁迫信号传递到内部信号通路中,在进化过程中形成一系列的胁迫响应机制。其中,油菜素内酯(brassinosteroids, BRs)是一种类固醇激素,广泛参与植物生长发育和逆境响应过程。BRs被包括受体BRI1和共受体BAK1在内的细胞表面受体感知,继而触发信号级联,导致蛋白激酶BIN2的抑制和转录因子BES1/BZR1的激活,BES1/BZR1可直接调控数千个下游响应基因的表达。在模式植物拟南芥中的研究表明,BR的生物合成和信号转导通路成员,特别是BIN2和其下游的转录因子BES1/BZR1,可以被各种环境因子广泛地调节。本文系统总结了BR相关的最新研究进展,对BR的生物合成和信号转导是如何被复杂的环境因子所调节,以及BR与环境因子如何协同调控作物重要农艺性状、冷胁迫和盐胁迫的响应进行了综述。     

受体激酶介导的油菜素内酯信号转导途径

油菜素内酯（brassinosteroids,BRs）是一类重要的类固醇激素,参与调控植物生长发育的许多过程。结合应用遗传学、生物化学以及蛋白质组学等研究手段现已基本阐明了BR信号转导的主要过程。BRI1作为受体在细胞表面感知BR,BRI1抑制子BKI1从质膜上解离下来,使BRI1与其共受体BAK1结合。BRI1和BAK1通过顺序磷酸化将BR信号完全激活。活化的BRI1将BSK磷酸化激活,BSK活化BSU1,BSU1将BIN2去磷酸化使其失活,解除BIN2对BES1/BZR1的抑制功能。PP2A可以将BES1/BZR1去磷酸化激活,又可以将受体BRI1去磷酸化促使其降解。BR信号的传递最终使去磷酸化状态的BES1/BZR1在细胞内累积,激活BR信号通路下游的转录调控。     

BRing it on: new insights into the mechanism of brassinosteroid action

Several recent breakthroughs have filled in key details of the brassinosteroid (BR) response. Identification of BAK1, a BRI1 interacting protein, the negative regulator BIN2, as well as direct targets of BIN2, BZR1 and BES1, provide a link between BR perception at the cell surface and regulation of gene expression in the nucleus. Global expression studies further defined the downstream events in this pathway, confirming the role of several factors acting in negative feedback regulation on BR levels. New links to the plant hormone, auxin, were also uncovered.     

Nuclear-localized BZR1 mediates brassinosteroid-induced growth and feedback suppression of brassinosteroid biosynthesis

Plant steroid hormones, brassinosteroids (BRs), are perceived by a cell surface receptor kinase, BRI1, but how BR binding leads to regulation of gene expression in the nucleus is unknown. Here we describe the identification of BZR1 as a nuclear component of the BR signal transduction pathway. A dominant mutation bzr1-1D suppresses BR-deficient and BR-insensitive (bri1) phenotypes and enhances feedback inhibition of BR biosynthesis. BZR1 protein accumulates in the nucleus of elongating cells of dark-grown hypocotyls and is stabilized by BR signaling and the bzr1-1D mutation. Our results demonstrate that BZR1 is a positive regulator of the BR signaling pathway that mediates both downstream BR responses and feedback regulation of BR biosynthesis.     

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.