Molecular basis of Arabidopsis susceptibility to Pseudomonas syringae infection

在植物和微生物病原体的共同进化中，大多数植物已经发展出复杂的免疫反应来抵御大多数（但不是全部）潜在的病原体。当植物防御失败，病原体携带适当的正毒力因子时，疾病就发生了。在过去的二十年里，人们投入了大量的精力来研究植物如何在分子水平上抵御病原体。相比之下，人们对植物如何无法防御并最终屈服于有毒病原体的感染知之甚少。该项目的长期目标是有助于了解拟南芥对细菌病原体丁香假单胞菌（Pseudomonas syringae pv）敏感性的分子基础。番茄（Pst） DC3000。在过去的二十年中，对易感植物-病原体相互作用的研究（主要是从病原体方面）揭示了病原体攻击寄主植物的复杂机制的丰富信息。现在很清楚，大多数细菌病原体使用专门的蛋白质分泌系统- III型分泌系统-主动将毒性效应蛋白“注射”到宿主细胞中。影响该蛋白分泌系统的突变消除了Pst DC3000感染拟南芥的能力，表明III型效应蛋白在调节拟南芥易感性中起着至关重要的作用。最近的研究提供了令人兴奋的证据，表明一些III型效应蛋白可以抑制各种形式的宿主免疫。例如，在拟南芥中，效应物HopPtoM最近被证明可以抑制水杨酸介导的宿主基础免疫，并直接或间接地促进宿主细胞死亡。然而，III型效应蛋白调节宿主免疫和其他宿主细胞过程的确切机制尚不清楚，并且仍然是生物学中的一个基本问题。本基金中描述的分子遗传学、细胞生物学、基因组学和转基因实验将鉴定和表征HopPtoM效应的拟南芥细胞靶标。了解hopptom介导的植物免疫抑制和促进宿主细胞死亡可能对了解多种植物中的细菌发病机制具有广泛的意义。HopPtoM在功能上对另一个效应器AvrE是冗余的。HopPtoM和AvrE家族的效应物广泛存在于丁香假单胞菌和/或其他植物致病菌中。最重要的是，III型效应物的HopPtoM/AvrE家族突变通常会导致病原体毒力的急剧丧失。因此，阐明HopPtoM失活宿主防御和促进宿主细胞死亡的机制将是我们了解植物对细菌病原体易感性的分子基础的重要一步。拟议中的研究还对社会产生了更广泛的影响。本项目将培养本科生、研究生和博士后科研人员；它将扩大代表性不足群体对研究的参与；加强PI的教学活动；它还将通过科学期刊上的出版物和在区域、国家和国际会议上的发言以及向立法官员及时传播知识。本项目所获得的基础知识将为发明新的植物病害防治策略奠定基础，从而提高食品质量和产量，减少有害化学品的使用，增强国家生物安全和食品安全。

In the co-evolution between plants and microbial pathogens, most plants have developed sophisticated immune responses to ward off the majority of (but not all) potential pathogens. When a plant fails to defend and the pathogen carries appropriate positive virulence factors, disease occurs. In the past two decades, significant efforts have been invested in studying how plants defend against pathogens at the molecular level. In contrast, little is known about how a plant fails to defend and ultimately succumbs to infection by virulent pathogens. The long-term goal of this project is to contribute to the understanding of the molecular basis of Arabidopsis susceptibility to bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Research on susceptible plant-pathogen interactions in the past two decades (mainly from the pathogen side) has revealed a wealth of information about intricate mechanisms used by pathogens to attack host plants. It is now clear that the majority of bacterial pathogens use a specialized protein secretion system-the type III secretion system-to actively 'inject' toxic effector proteins into the host cell. Mutations affecting this protein secretion system eliminate the ability of Pst DC3000 to infect Arabidopsis, suggesting a crucial role of type III effector proteins in modulating Arabidopsis susceptibility. Recent research provides exciting evidence that some type III effector proteins function as inhibitors of various forms of host immunity. For example, effector HopPtoM has recently been shown to inhibit salicylic acid-mediated host basal immunity and, directly or indirectly, to promote host cell death in Arabidopsis. However, the precise mechanisms by which the type III effector proteins modulate host immunity and other host cellular processes are not understood and remain a fundamental question in biology. Molecular genetic, cell biological, genomic, and transgenic experiments described in this grant will identify and characterize Arabidopsis cellular targets of effector HopPtoM. Understanding HopPtoM-mediated suppression of plant immunity and promotion of host cell death likely has broad implications for understanding bacterial pathogenesis in diverse plants. HopPtoM is functionally redundant to another effector, AvrE. Effectors of the HopPtoM and AvrE families are widespread in P. syringae strains and/or other plant-pathogenic bacteria. Most importantly, mutations in the HopPtoM/AvrE families of type III effectors often result in drastic loss of pathogen virulence. Elucidation of the mechanisms by which HopPtoM functions to inactivate the host defenses and to promote host cell death will therefore represent a significant step forward in our understanding of the molecular basis of plant susceptibility to bacterial pathogens. The proposed research also has a broader impact on society. This project will provide training of undergraduate, graduate, and postdoctoral scientists; it will broaden the participation in research of underrepresented groups; it will enhance the PI's teaching activities; and it will enable timely dissemination of knowledge through publications in scientific journals and presentations at regional, national and international conferences and to legislative officers. The basic knowledge gained from this project will provide a foundation for invention of novel strategies for controlling plant diseases, thus improving food quality and yield, reducing use of hazardous chemicals, and increasing national biosecurity and food safety.