Mechanisms of nuclear suppression of host immunity by Arabidopsis downy mildew effectors

农作物容易受到各种微生物引起的疾病的影响，这些疾病会造成大量的产量损失。农民喷洒农药来控制疾病；这是昂贵的，需要燃料和劳动力，并导致二氧化碳排放和土壤压实。作物最好是抗病的，这样就不需要使用杀菌剂了。实现这一目标需要更多的知识。育种家利用抗病基因改良作物。R基因赋予病原体毒力促进分子（所谓的“效应物”）的识别能力，这些分子有助于增强毒力。为了能够预测并（在未来）设计出最持久的R基因，我们需要充分了解病原体效应物的作用，以及它们所针对的宿主过程。我们研究了一个具有良好特征的模型宿主/病原体系统，拟南芥霜霉病；这是马铃薯晚疫病病菌疫霉的近亲。已经确定了这种霜霉病的基因组序列，并预测了100多种可能的影响因子。我们已经测试了其中的许多，并表明它们抑制了宿主的防御。其中许多主要针对宿主细胞核，在那里它们可以干扰宿主基因表达。我们把这个项目的重点放在核局部效应上。多亏了两位优秀的欧洲资助博士后，对两种效应物的分析已经非常先进，研究其他许多效应物的试剂也很先进。效应物HaRxL106与携带蛋白质从细胞质到细胞核的宿主成分相互作用，并与响应氧化还原应激的转录调节因子相互作用。表达该蛋白的拟南芥株系也表现出光响应的发育变化，表明该效应物针对一些基本的宿主机制。拟南芥具有良好的遗传特性，利用对拟南芥生物学的深入了解，我们希望能够准确揭示HaRxL106与植物成分的相互作用如何导致对疾病的易感性增强。HaRxL44与中介复合物的组分相互作用，中介复合物在连接改变基因表达的dna结合蛋白和转录机制本身方面起着重要的基础作用。表达HaRxL44的拟南芥品系表现出增强的疾病易感性和卷曲叶片，再次与对宿主基因调控的深刻影响相一致。这种卷曲叶表型经常出现在被小RNA分子改变其调节的植物中。对于这两种效应物，我们的目标是充分了解它们如何抑制宿主防御。我们正在分析更多的核效应物，并致力于推进这些研究。HaRxL62是最深刻地抑制免疫的Hpa效应蛋白，但我们迄今尚未发现相互作用的宿主蛋白；我们打算这样做。HaRxL2、HaRxL73和HaRxL3在植物中表达也对宿主免疫产生深远影响，并引起发育紊乱；我们的目标是通过识别它们在植物中相互作用的蛋白质来推进我们对它们的作用机制的理解。这些知识将如何发挥作用？通过了解效应器是如何结合和干扰宿主成分的，我们应该能够设计出这些宿主成分的形式，使其对效应器的作用不起作用；携带这些新形式的植物应该表现出更强的抗病性。

Crop plants are subject to diseases caused by various microbes that can cause substantial yield losses. Farmers spray agrochemicals to control disease; this is expensive, requires fuel and labour, and leads to CO2 emissions and soil compaction. It would be preferable for crops to be disease resistant, so that no fungicide applications are required. More knowledge is required to achieve this goal. Breeders use disease resistance (R) genes for crop improvement. R genes confer recognition of pathogen virulence-promoting molecules (so-called "effectors") that contribute to virulence. In order to be able to predict and (in future) design the most durable R genes, we need to fully understand what pathogen effectors do, and the host processes that they target.We work with a well-characterized model host/pathogen system, downy mildew of Arabidopsis; this is a close relative the potato late blight pathogen Phytophthora infestans. The genome sequence of this downy mildew has been determined, and over 100 likely effectors have been predicted. We have tested many of these and shown that they suppress host defences. Many of them are predominantly targeted to the host cell nucleus, where they could interfere with host gene expression. We are focusing this project on nuclear localized effectors. Thanks to two excellent European funded postdoctoral fellows, analysis of two effectors is already well advanced, and reagents to study many others are also well advanced. Effector HaRxL106 interacts with host components that carry proteins from the cytoplasm to the nucleus, and with a transcriptional regulator that responds to redox stress. Arabidopsis lines that express this protein also show developmental changes in light responses, suggesting that this effector targets some fundamental host mechanisms. Arabidopsis is well defined genetically, and using this deep knowledge of Arabidopsis biology, we expect to be able to reveal exactly how HaRxL106 interactions with plant components results in enhanced susceptibility to disease.HaRxL44 interacts with components of the Mediator complex, which plays an important fundamental role linking DNA-binding proteins that alter gene expression, to the transcription machinery itself. HaRxL44 expressing Arabidopsis lines show enhanced disease susceptibility and curly leaves, again consistent with a profound effect on host gene regulation. This curly leaf phenotype is often seen in plants that are altered in their regulation by small RNA molecules. For both of these effectors we aim to fully understand how they suppress host defence.We are analysing additional nuclear effectors and aim to advance these studies too. HaRxL62 is the Hpa effector that most profoundly suppresses immunity, but we have so far not identified interacting host proteins; we aim to do so. HaRxL2, HaRxL73 and HaRxL3 also show profound effects on host immunity and cause developmental perturbations when expressed in planta; we aim to advance our understanding of their mechanisms of action by identifying proteins with which they interact in planta.How will this knowledge be useful? By understanding how effectors bind to and interfere with host components, we should be able to design forms of these host components that are refractory to the action of the effectors; plants carrying these novel forms should exhibit enhanced disease resistance.