Brassinosteroids are plant hormones that perform a fundamental role in coordinating growth and development. The Arabidopsis gene, Brassinosteroid Insensitive 1 (BRI1), encodes a receptor-like protein kinase (RLK), which is a critical component in brassinosteroid-mediated signaling. Using activation tagging, we identified a gene can suppress the growth defects of a mutant bri1 allele. This bri1 suppressor, BRS1, is predicted to encode a secreted serine carboxypeptidase. Elevated expression of BRS1 suppresses two bri1 alleles that have mutations in their extracellular domain, but not a kinase domain mutation. Genetic and physiological data strongly suggest BRS1 regulates an early step in brassinosteroid signaling. In many signal transduction pathways proteolytic processing plays an important role in regulating signal perception and transduction. We hypothesize that BRS1 is involved in a proteolytic processing step in the BRI1 signaling pathway. To test this hypothesis, we propose to undertake a detailed characterization of BRS1, including its cellular and subcellular localization, and analysis of its enzymatic activity. In addition, BRS1 is a member of a gene family. To examine a possible involvement of BRS1 and the BRS1-like genes in BRI1 and other RLK signaling pathways, we will use genetic approaches to investigate their function. These studies will contribute to our understanding of brassinosteroid signaling and other RLK mediated processes that control growth and development in plants.
