Molecular Genetic Analysis of the Xanthomonas oryzae pv. oryzae Avirulence Determinants

在感染的建立过程中，许多哺乳动物和植物细菌病原体产生效应分子，并将其传递给宿主细胞。这些效应器的传输是通过几种不同的机制完成的。例如，细菌III型分泌系统将效应物直接导入宿主细胞，而细菌I型分泌系统将效应物分泌到细胞外环境。在许多植物/病原体相互作用中，宿主的抗性反应是通过植物抗病基因编码的细胞内受体启动的，这些受体识别由细菌无毒（avr）基因编码的III型分泌效应分子。相比之下，植物细胞表面受体对细胞外效应物的反应机制尚不清楚。Xa21是一种水稻抗性基因，编码一种假定的受体激酶，赋予对水稻黄单胞菌pv的抗性。稻瘟病菌（Xoo）菲律宾6种，引起细菌性叶枯病，这是亚洲和非洲最具破坏性的水稻细菌性疾病。罗纳德博士已经证明，XA21细胞外富含亮氨酸的重复结构域负责对无毒菌株Xoo菲律宾种族6的特异性识别。罗纳德博士已经从这个菌株中鉴定出了600个基因，这些基因是无毒功能所必需的。其中三个基因（xooA, xooB, xooC）编码的蛋白质与革兰氏阴性菌I型分泌系统的成分有相似之处。本研究的长期目标是阐明这6个Xoo毒性决定因子的相互关系和功能，鉴定和表征效应分子，并分析效应分子与水稻受体激酶XA21的相互作用。由于几个原因，Xa21/Xoo相互作用对这些研究特别有利。Xa21是迄今为止发现的唯一一种编码受体激酶的抗病基因，可能识别细胞外效应。Xoo在遗传上是易处理的，编码I型分泌系统的基因是有特征的。最后，水稻是研究所有谷物的模型，因此预期的结果将对理解和控制谷物细菌性疾病产生广泛的影响。完成所提出的目标有望导致第一次详细了解1型分泌系统和硫酸化在确定植物致病菌宿主特异性中的作用。

In the establishment of infection, many mammalian and plant bacterial pathogens produce effector molecules that are delivered to host cells. Transport of these effectors is accomplished through several different mechanisms. For example, bacterial Type III secretion systems channel effectors directly into the host cell whereas bacterial Type I systems secrete effectors into the extracellular environment. In many plant/pathogen interactions, host resistance responses are initiated via intracellular receptors encoded by plant disease resistance genes that recognize Type III secreted effector molecules encoded by bacterial avirulence (avr) genes. In contrast, the mechanisms by which plant cell surface receptors respond to extracellular effectors are unknown. Xa21 is a rice resistance gene encoding a presumed receptor kinase that confers resistance to Xanthomonas oryzae pv. oryzae (Xoo) Philippine race 6, the causal agent of bacterial leaf blight, the most destructive bacterial disease of rice in Asia and Africa. Dr. Ronald has shown that the presumed extracellular leucine rich repeat domain of XA21 is responsible for specific recognition of the avirulent strain Xoo Philippine race 6. Dr. Ronald has identified 6 Xoo genes from this strain that are required for the avirulence function. Three of these genes (xooA, xooB, xooC) encode proteins that share similarity with components of Type I secretion systems of gram negative bacteria. The long-term goal of this proposal is to elucidate the relationship and function of these six Xoo avirulence determinants, identify and characterize the effector molecule and analyze the interaction between the effector and rice receptor kinase XA21. The Xa21/Xoo interaction is particularly advantageous for these studies for several reasons. Xa21 is the only disease resistance gene encoding a receptor kinase so far identified and likely recognizes an extracellular effector. Xoo is genetically tractable and the genes encoding Type I secretion systems are characterized. Finally, rice is a model for studies of all cereals and therefore the expected results will have a broad impact on understanding and controlling bacterial diseases of cereals. Accomplishment of the proposed objectives is expected to lead to the first detailed understanding of the roles of Type 1 secretion systems and sulfation in determining host specificity of phytopathogenic bacteria.