Molecular basis of virulence in tan spot of wheat

小麦腐斑病（Pyrenophora tritrii -repentis, Ptr）是小麦一种重要的斑叶病。Ptr侵染可导致易感寄主品种叶片变黄和/或死亡。这些症状反映了坏死性效应物（NEs）的活性，也称为宿主选择性毒素，由真菌的不同种族产生。迄今已鉴定出三种ne: Ptr ToxA、Ptr ToxB和Ptr ToxC。对这些效应物的敏感性通常与对产生这些效应物的真菌品种的敏感性共同分离，小麦/Ptr相互作用大致遵循宿主-寄生虫关系的反向基因对基因模型，其中病原体分泌的NE与匹配的宿主敏感基因之间的相互作用导致敏感性。然而，近年来也发现了几个数量性状位点和一个主要显性基因，这些基因赋予了黄斑病的种族非特异性抗性。虽然NEs与宿主敏感基因之间的关系在遗传学方面已经得到了很好的定义，但从分子角度来看，它们之间的相互作用还没有得到很好的理解。这些知识上的空白，再加上非特异性抗性的证据，表明需要更好地了解小麦/Ptr相互作用中涉及的分子反应。此外，虽然根据其感染一系列小麦品种（称为宿主差异集）的能力，已经描述了8个Ptr小种，但我们假设还有其他真菌小种，它们产生尚未表征的效应物，控制其毒力谱。我们最近发现了许多Ptr分离株，虽然根据它们在不同宿主上引起的症状与现有种族相似，但它们不编码预期的网元。随着Ptr新品种的鉴定，我们还假设存在新的效应物介导与特定寄主品种的相互作用。该研究的目标是：(1)通过双RNA-seq方法对小麦/Ptr相互作用的机制有更广泛的了解，同时检测宿主和寄生虫的转录本，不仅可以评估小麦的分子反应，还可以评估真菌的转录组；(2)通过评估295份硬粒小麦和面包小麦基因型的真菌毒力表型，对来自北非、新月沃土和中亚等病原体多样性中心的Ptr新小种进行鉴定；(3)鉴定真菌转录组和Ptr分离株培养滤液中的候选效应物，显示出新的毒力模式。这项工作将增加对病原菌效应物和毒力、寄主抗性机制以及Ptr小种结构的认识，为小麦褐斑病及相关疾病的管理和认识提供重要信息。

The fungus Pyrenophora tritici-repentis (Ptr) causes tan spot, an important leaf-spotting disease of wheat. Infection by Ptr results in the yellowing and/or death of the leaves of susceptible host varieties. These symptoms reflect the activity of necrotrophic effectors (NEs), also known as host-selective toxins, which are differentially produced by races of the fungus. Three NEs have been identified to date: Ptr ToxA, Ptr ToxB and Ptr ToxC. Sensitivity to these effectors generally co-segregates with susceptibility to the races of the fungus that produce them, and the wheat/Ptr interaction broadly follows the inverse gene-for-gene model of host-parasite relations, in which the interaction between a pathogen-secreted NE and a matching host sensitivity gene results in susceptibility. In recent years, however, several quantitative trait loci and a major dominant gene have also been identified, which confer race-nonspecific resistance to tan spot. While the relationship between the NEs and matching host sensitivity genes is well defined in genetic terms, the interaction is not as well understood from a molecular perspective. Such gaps in knowledge, combined with the evidence of nonspecific resistance, suggest the need for an improved understanding of the molecular responses involved in the wheat/Ptr interaction. Moreover, while eight races of Ptr have been described based on their ability to infect a suite of wheat varieties known as a host differential set, we hypothesize that there are additional races of the fungus, which produce as of yet uncharacterized effectors controlling their virulence profiles. We have recently identified numerous isolates of Ptr that, while resembling existing races based on the symptoms they cause on the differential hosts, do not encode for the expected NEs. Concomitant with the identification of new races of Ptr, we also hypothesize that there occur novel effectors mediating interactions with specific host varieties. The proposed research aims to: (1) develop a broader mechanistic understanding of the wheat/Ptr interaction via a dual RNA-seq approach, simultaneously detecting transcripts in the host and parasite and enabling an assessment not only of the molecular responses of wheat, but also of the fungal transcriptomes; (2) characterize novel races of Ptr from North Africa, the Fertile Crescent and Central Asia, centers of pathogen diversity, by evaluating fungal virulence phenotypes on a collection of 295 durum and bread wheat genotypes; and (3) identify candidate effectors in the fungal transcriptomes and in culture filtrates of isolates of Ptr showing novel virulence patterns. This work will increase knowledge of pathogen effectors and virulence, host resistance mechanisms, and the race structure of Ptr, providing vital information for the management and understanding of tan spot of wheat and related diseases.