Molecular Mechanisms of Brassinosteroid Action

9420016油菜素类固醇(BRs)是一种广泛存在的植物化合物，其结构与动物类固醇激素相似。BRs具有多种生理效应，包括促进细胞伸长，但对这些植物生长调节剂的作用机制知之甚少。用一种综合的方法研究了与BR促进的茎组织伸长相关的一些生理和分子参数。从BR处理的大豆(Glyine Max L)上胚轴基因文库中，克隆了BR处理2小时内主要受BRs调控的基因(BRU1)。生长素、细胞分裂素、ABA或Ba3的微摩尔浓度不会增加BRU1的转录水平，并且BRU1在茎组织中的表达最高。BRS导致Instron测量的塑料延伸率显著增加，这表明它们可能部分通过改变细胞壁的机械性能(壁松动)来促进延伸率。随着BRU1基因表达的增加，细胞壁的松弛动力学也随之增强。这表明BRU1蛋白可能在BR刺激的伸长中发挥作用。这个项目的目的是检验这一假说，并进一步研究植物类固醇对基因表达的调节。尽管BRS对植物有明显的促进生长作用，并与其他生长调节剂相互作用，但对BRS作用的分子机制知之甚少。了解BR活性在基础发育生物学和实用农业中具有重要的潜在意义。伸长过程中管壁松动的确切生化机制仍不清楚。拟议实验的完成可能会增加我们对控制茎伸长的分子事件的了解。关于BRS调节BRU1基因水平升高的研究将增加我们对转录后机制的了解，事实上，这将是对植物类固醇调控基因机制的第一个详细研究。BRU1和其他受BRS调控的基因的分析也可能对未来生长特性改变的转基因作物的发展产生影响。

9420016 Clouse Brassinosteroids (BRS) are widely occurring plant compounds with structural similarity to animal steroid hormones. BRS have numerous physiological effects, including enhanced cell elongation, but little is known about the mechanism of action of these plant growth regulators. An integrative approach was used to investigate a number of physiological and molecular parameters associated with BR-promoted elongation of stem tissue. From a cDNA library of BR- treated soybean (Glycine max L) epicotyl mRNA, a gene (BRU1) was cloned which is regulated primarily by BRS within two hours of treatment. The BRU1 transcript levels are not increased by micromolar concentrations of auxins, cytokinins, ABA or BA3, and BRU1 expression is highest in stem tissue. BRS caused a marked increase in Instron- measured plastic extensibility, suggesting that they may promote elongation in part by altering mechanical properties of the cell wall (wall loosening). The kinetics of wall- loosening closely followed the increase in BRU1 expresssion. This suggests a possible role for the BRU1 protein in BR- stimulated elongation. The objective of this project is to test this hypothesis and to further examine the regulation of gene expression by plant steroids. Despite the obvious growth-promoting effects of BRS on plants and its demonstrated interactions with other growth regulators, little is known about the molecular mechanisms of BRS action. Understanding BR activity has great potential importance in basic developmental biology and practical agriculture. The precise biochemical mechanisms of wall loosening during elongation remain elusive. The completion of the proposed experiments may increase our knowledge of the molecular events controlling stem elongation. The proposed studies on BRS-regulated increases in BRU1 mRNA levels should add to our knowledge of posttranscriptional mechanisms and will, in fact, be the first detailed study of the mechanisms of gene regulat ion by plant steroids. The analysis of BRU1, and other BRS- regulated genes may also have future impact on development of transgenic crop plants with altered growth properties.