Molecular Mechanisms of Brassinosteroid Action

North Carolina State University, Raleigh NC

Investigators

Abstract

Brassinosteroids (BRs) are essential growth-promoting natural products found at low levels in pollen, seeds and young vegetative tissues throughout the plant kingdom. The P.I.'s group has identified a BR-insensitive mutant in Arabidopsis thaliana (bri1) that confers severe pleiotropic phenotypic effects inhibiting plant development. BRI1 has been shown by others to encode a membrane-bound leucine-rich repeat Ser/Thr receptor kinase. In animals, the Transforming Growth Factor-b (TGF-b) family of peptides, acts via receptor kinases to prominently impact several pathways involved in animal development and adult homeostasis. TGF-b Receptor Interacting Protein (TRIP-1) is an intracellular substrate of the TGF-b Type II receptor kinase that plays an important role in TGF-b signaling. TRIP-1 is a WD-repeat protein that also has a dual role as an essential subunit of the eukaryotic translation initiation factor eIF3 in animals, yeast and plants, thereby revealing a putative link between a developmental signaling pathway and the control of protein translation. The P.I.'s group recently found that transcript levels of TRIP-1 homologs in plants are regulated by BR treatment under a variety of conditions, and that antisense TRIP-1 plants exhibited a broad range of developmental defects including some that resemble the phenotype of BR-deficient and -insensitive mutants. Furthermore, recombinant BRI1 kinase domain phosphorylates recombinant TRIP-1 in vitro. These findings suggest that TRIP-1 may mediate some of the molecular mechanisms underlying the regulation of plant growth and development by BRs. The P.I. will test this hypothesis by performing the following two objectives: OBJECTIVE 1: TRIP-1 protein levels will be monitored under a variety of conditions, in both wild-type and BR mutants, and the effect of BR on translation initiation will be examined. The possible nuclear localization of TRIP-1 protein, in addition to its role in the eIF3 complex, will also be studied. OBJECTIVE 2: Immunoprecipitation followed by mass spectrometry will be used to identify in vivo phosphorylation sites of TRIP-1 and their dependence on an active BRI1 receptor kinase. This project should increase our understanding of BR signal transduction and the role that this hormone plays in controlling plant growth and development. Given that BRs are found throughout the plant kingdom, greater understanding of the molecular mechanisms of BR action could have practical impact on generating crop plants with altered growth properties.