Molecular Determinants Governing the Rice XA21/Xoo Interaction

受体激酶（Receptor kinases，RKs）在生物体感知和响应环境信号的机制中发挥重要作用，调节动物和植物的生长和发育。 本项目的长期目标是阐明植物RKs调控的信号转导级联。 随着水稻抗病基因Xa 21的克隆，赋予对黄单胞菌致病变种（Xoo）的抗性（Song等人，1995），该实验室是在一个极好的位置，以阐明RK介导的植物抗病性的分子基础。 Xa 21编码一种受体样激酶，携带一个假定的糖基化富亮氨酸重复序列（LRR）结构域、一个单程跨膜结构域和一个丝氨酸苏氨酸激酶（STK结构域）。 基于Xa 21基因产物（XA 21）的序列和来自细菌和动物系统中的受体激酶的信息，提出XA 21蛋白的LRR结构域是胞外的并且与配体相互作用。 配体结合诱导蛋白质的构象变化，激活细胞内激酶结构域，引发下游防御反应并限制病原体生长。 为了支持这个模型，已经证明激酶结构域能够自磷酸化，并且LRR结构域足以进行种族特异性识别。 本研究的具体目标是：1.定义XA 21蛋白的亚细胞定位和2.克隆Xoo编码的无毒基因（avrXa 21）。 这些目标的实现将不仅导致在水稻Xa 21/avrXa 21相互作用中指定抗病性的决定因素的表征，而且将扩展我们对RK介导的植物信号转导的知识。 从这些研究中获得的信息将有助于制定对环境安全的水稻和其他重要农业作物病害控制战略。

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.