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.
