Progress 01/01/06 to 01/01/07
This application describes experiments designed to understand how plant cells respond to brassinosteroids, the steroid hormones of plants. Recent studies from our laboratory and others indicate that both the biosynthetic and response pathways of brassinosteroids (BRs) may be manipulated to affect plant stature. Previously, we showed that BRI1, a leucine-rich repeat receptor kinase, binds BRs directly through a novel region in its extracellular domain. During the past year, we have continued to study the activation mechanism of BRI1, as well as its regulated trafficking from the cell surface to endosomal compartments. A model for BRI1 activation is emerging. In the model, BRI1 kinase activity is kept at a basal level by its C-terminal tail. Upon BR binding, a conformational change of BRI1's intracellular domain allows BRI1's kinase to phosphorylate the C-terminal tail, which is correlated with the dissociation of BKI1, a novel phosphoprotein, from the plasma membrane. Release of BKI1 may then allow BRI1 to interact with its co-receptor. During the next funding period, we will continue these studies and refine the model. Of note, we now have crystals of BRI1's extracellular domain, and we hope to determine its structure. We have also continued work on the signaling pathway, refining the mechanism by which BRs regulate gene expression in the nucleus. In addition, new mutant screens have identified novel components of BRI1's signaling pathway, which we plan to characterize during the upcoming year.
Identification and modification of the activities of BR biosynthetic and response pathway genes should allow an alternative strategy for modifying stature and yield of plants. In the long term, these transgenic plant varieties could circumvent the need for expensive chemical syntheses, as well as the requirement for manual applications of the hormone. Thus research on the brassinosteroids might lead to changes in agriculture that would be both economically and environmentally beneficial.