Receptor kinases (RKs) play an important role in the mechanism with which organisms perceive and respond to environmental signals, regulating growth and development in both animals and plants. The long-germ goal of this project is to elucidate the signal transduction cascade regulated by plant RKs. With the cloning of the rice disease resistance gene, Xa21, conferring resistance to Xanthomonas oryzae pv.oryzae(Xoo) (Song et al., 1995), this laboratory is in an excellent position to elucidate the molecular basis of RK mediated disease resistance in plants. Xa21 encodes a receptor-like kinase carrying a presumed glycosylated leucine rich repeat (LRR) domain, a single pass transmembrane domain, and a serine threonine kinase (STK domain). Based on the sequence of the Xa21 gene product (XA21) and information from receptor kinases in bacterial and animal systems, it is proposed that the LRR domain of the XA21 protein is extracellular and interacts with a ligand. Ligand binding induces a conformational change in the protein, activates the intracellular kinase domain, triggers a downstream defense response and restricts pathogen growth. In support of this model it has been demonstrated that the kinase domain is capable of autophosphorylation and that the LRR domain is sufficient for race-specific recognition. The specific objectives of this research are to: 1. Define the sub-cellular localization of the XA21 protein and 2. Clone the Xoo encoded avirulence gene (avrXa21). Accomplishment of these objectives will not only lead to the characterization of the determinants specifying disease resistance in the rice Xa21/avrXa21 interaction but will expand our knowledge of RK mediated signal transduction in plants. Information gained from these studies will facilitate the development of environmentally safe strategies for disease control in rice and other agronomically important crops.
