Signal Pathways-Mediated by the Rice Receptor-Like Kinase XA21

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The long-term goal of this project is to gain an understanding of receptor serine/threonine kinase-mediated signaling in plants. Receptor kinases, including receptor tyrosine kinases and receptor serine/threonine kinases, are major transmembrane signal transducers of many processes such as cell proliferation, differentiation, migration, apoptosis, and defense. In animal systems, receptor tyrosine kinases appear to be predominant in cell signaling. In plants, almost all receptor-like kinases (RLK) identified have serine/threonine specificity, making plants particularly amenable to studying receptor serine/threonine kinase-mediated signaling. The first rice disease resistance gene Xa21, which encodes a RLK, was cloned. The putative intracellular domain of XA21 can autophosphorylate multiple serine and threonine residues and interacts with seven proteins (XBs) in a yeast two-hybrid system. One of the seven binding proteins, XB1, physically interacts with and acts as a substrate of the XA21 kinase. XB1 consists of two domains: an ankyrin domain involved in interacting with XA21 and a RING finger motif belonging to the newly defined R-box family that may play a universal role in ubiquitin-mediated protein degradation. These studies provide an excellent system to study mechanisms underlying initiation, propagation, and termination of RLK-mediated signaling. The specific goals of this project are: 1. Investigate the mode of regulation of the XA21-mediated signaling2. Investigate the role of XBs in the XA21-mediated signaling Achieving these goals will provide insights into RLK-mediated signaling and will advance our understanding of plant disease resistance at the molecular level. Information from these studies would provide new approaches for the engineering of broad-spectrum, durable disease resistance in rice and other crop plants, thereby reducing the dependency of crop production on environmentally unfriendly chemicals.