Hormone signaling in Arabidopsis: Analysis of the brassinosteroid enhanced expression genes.

摘要

Brassinosteroids (BRs) are a class of polyhydroxylated hormones, which are important regulators of plant growth and development. In Arabidopsis, BR mutants are dark green dwarves affecting all adult organs including leaves, inflorescences, flowers and fruits. One class of BR mutants contain mutations in BR biosynthesis enzymes, and exogenous application of BRs rescues their phenotype. In contrast, the bri1 mutants are insensitive to BR treatment suggesting that BRI1 acts as a BR signaling component. The BRI1 gene was previously cloned and encodes a leucine-rich repeat receptor serine/threonine kinase. This dissertation presents an analysis of the bri1 missense mutations, which highlight the importance of both the extracellular and kinase domain for BRI1 function. BRI1 expression is enhanced in actively growing young tissues, including the meristem, roots, shoots and hypocotyls of seedlings. In addition, BRI1-GFP is localized to the plasma membrane, suggesting BRI1 could be a component of the BR receptor complex.; While the BR signaling pathway between the BR receptor complex and BR-regulated genes is unknown, the BR Enhanced Expression genes (BEE1, 2, 3) potentially act as BR signaling intermediates. They are induced by BRs, require a functional BRI1 signal for this induction and encode putative basic helix-loop-helix (bHLH) transcription factors. Plants that overexpress BEE1 and bee1, 2, 3 triple knockout mutants have both developmental and hormone response phenotypes suggesting BEE1, 2, 3 are positive regulators of BR signaling. Abscisic acid (ABA) acts as an antagonist of BR signaling. ABA represses the expression of all three BEE genes, and mutant analysis indicates they act as negative regulators of ABA signaling. The opposing regulation of BEE1, 2, 3 by BRs and ABA is the first example of plant hormone cross talk mediated by altering the expression of signaling intermediates.