Undermining effector-targeted susceptibility factors to provide late blight resistance

世界人口的增加和气候变化的影响对世界粮食供应提出了越来越大的要求。全球粮食安全的一个主要制约因素是植物病虫害造成的作物损失。随着农业化学品审批条件的日益严格，在不久的将来，有效的杀菌剂和杀虫剂的选择将变得更加有限。此外，由于渗入的寄主抗性基因在田间被病原体和害虫迅速击败，迫切需要探索开发新的、持久的、可持续的作物病害防治手段。植物面对大量的微生物威胁，并通过采用两层诱导防御反应来保护自己。第一种涉及细胞表面受体对基本的、广泛保守的分子（称为PAMPs）的识别，这些分子在感染期间被病原体暴露。成功的（适应的）病原体分泌并传递一种叫做效应物的蛋白质来抑制这些防御。第二层防御涉及免疫受体、抗性蛋白对效应器的识别，以唤起效应器触发的免疫。该项目重点研究马铃薯和番茄这两种世界主要粮食作物的主要病原菌——晚疫病菌。在征服寄主的武器中，有一种是RXLR效应物，它被传递到植物细胞内以促进易感性。RXLR效应物已被证明可靶向和抑制免疫的阳性调节因子。相比之下，这里我们提出了大量未发表的信息，揭示了两种针对宿主易感性(S)因子的RXLR效应物的发现；促进疾病的蛋白质。一种效应物与植物蛋白磷酸酶1 （PP1）的异构体相互作用，形成全酶，可以使宿主蛋白去磷酸化以激活或灭活它们。另一种效应物与NRL相互作用，NRL是光信号质膜相关调节家族的成员。NRL通过抑制由P. infestans PAMP感知触发的免疫来促进易感性。这些因素为有效和持久地破坏感染提供了非常有吸引力的机会。例如，抗性蛋白对病原体施加选择压力，使其改变或失去同源无毒基因以逃避检测，而S因子的修饰或有条件地去除施加了更具挑战性的选择压力，因为感染需要相关的宿主蛋白活性。关键的是，降低这些S因子（PP1c同型型和NRL）的内源性水平可以减轻鼠疫杆菌感染，这表明联合降低这两种S因子可能为对抗晚疫病提供了一种持久的策略。人们对S因子如何促进疾病知之甚少。这是一个新兴的，令人兴奋的植物病害研究领域，使我们的建议及时。我们将详细了解1)pp1c效应全酶去磷酸化的毒力靶点，以及去磷酸化的机制后果（目标1）；2) NRL抑制pamp触发的免疫的手段（目标2）。它还将揭示这些S因子是否存在不能被相关效应物靶向的自然变异（目的3C），暗示S因子的进化以逃避效应物介导的利用。这些研究将为植物免疫负调控提供新的见解。它们还提供了对抗植物疾病，特别是马铃薯枯萎病的新方法。业界热衷于利用对S因子的理解，作为提供抗病能力的持久手段。该项目与工业合作伙伴JR Simplot合作，将提供病原体诱导去除S因子的新方法，以提供晚疫病抗性（目标3A； 3B）。

An increasing world population and impacts of climate change place ever-greater demands on the world food supply. A major constraint to global food security is crop loss due to plant pests and diseases. With the increasing stringency of conditions under which chemicals are approved for agriculture, the choice of effective fungicides and pesticides will become more limited in the near future. Furthermore, as introgressed host resistance genes are rapidly defeated by pathogens and pests in the field, there is an urgent need to explore the development of novel, durable and sustainable means to combat crop diseases.Plants face a barrage of microbial threats and defend themselves by employing two layers of inducible defence responses. The first involves recognition by cell surface receptors of essential, widely conserved molecules (called PAMPs) that are exposed by pathogens during infection. Successful (adapted) pathogens secrete and deliver proteins called effectors to suppress these defences. The second defence layer involves recognition of effectors by immune receptors, resistance proteins, to evoke effector-triggered immunity. This project focusses on the late blight pathogen Phytophthora infestans, the major pathogen of potato and tomato, two of the world's main food crops. Amongst the weapons P. infestans has to subjugate its hosts are RXLR effectors which are delivered inside plant cells to promote susceptibility. RXLR effectors have been shown to target and suppress positive regulators of immunity. In contrast, here we present considerable unpublished information revealing the discovery of two RXLR effectors that target host susceptibility (S) factors; proteins whose activity promotes disease. One effector interacts with isoforms of plant protein phosphatase 1 (PP1), forming holoenzymes that may dephosphorylate host proteins to activate or inactivate them. The other effector interacts with NRL, member of a family of plasma membrane-associated regulators of light signalling. NRL promotes susceptibility by suppressing immunity triggered by perception of a P. infestans PAMP. S factors present very attractive opportunities to undermine infection effectively and durably. For example, whereas a resistance protein imposes a selection pressure on a pathogen to alter or lose the cognate avirulence gene to evade detection, modification or conditional removal of an S factor imposes a far more challenging selection pressure, in that the associated host protein activity is required for infection. Critically, reducing endogenous levels of these S factors (PP1c isoforms and NRL) attenuates P. infestans infection, suggesting that combined reduction of both S factors may provide a durable strategy to combat late blight.Little is understood about how S factors promote disease. It is an emerging, exciting area of plant disease research, making our proposal timely. We will provide detailed understanding of 1) the virulence targets dephosphorylated by effector-PP1c holoenzymes, and the mechanistic consequences of dephosphorylation (objective 1); and 2) the means by which NRL suppresses PAMP-triggered immunity (objective 2). It will also reveal whether natural variants exist of these S factors that cannot be targeted by the associated effectors (objective 3C), implicating evolution of S factors to evade effector-mediated exploitation. These studies will provide novel insights into negative regulation of plant immunity. They also provide novel means with which to combat plant disease, and potato blight specifically.Industry is keen to exploit understanding of S factors as a durable means to provide disease resistance. This project, with industrial partner JR Simplot, will deliver novel approaches for pathogen-inducible removal of S factors to provide late blight resistance (objectives 3A; 3B).