Molecular basis of virulence in Pyrenophora tritici-repentis

真菌Prenophora tritici-retentis会引起黄褐斑，这是一种叶斑病，在生长小麦发育过程中的任何时候都会损坏。尽管已有高达50%的损失报道，但据估计，Tan Spot将使产量减少2%-15%。小麦白粉病的侵染会在感病植物的叶片上形成病斑或斑点，其中包括死亡(坏死)和/或黄色(褪绿)组织。这些症状是由真菌产生的寄主选择性毒素造成的，这种毒素可以影响一些小麦品种，但不会影响其他品种。三种寄主选择性毒素，分别称为PTR ToxA、PTR ToxB和PTR ToxC，由小麦赤霉菌单独或联合产生。这三种毒素对黄褐斑的形成都很重要，这种真菌需要产生至少其中一种毒素才能在特定小麦品种上致病。然而，除了致病所需外，PTR ToxB还存在剂量效应，这意味着PTR ToxB水平较高会导致敏感小麦出现更严重的疾病症状。真菌菌株产生的PTR ToxB的数量反映了该菌株中ToxB基因(毒素的编码)的拷贝数，以及这些拷贝的表达活跃程度。尽管它在棕褐色斑点的形成中很重要，但目前尚不清楚为什么产生PTR ToxB的真菌菌株在加拿大极其罕见，而产生PTR ToxA的真菌菌株却非常常见。同样，目前尚不清楚ToxB基因的多个副本是发生在携带其他重要基因的染色体上，因此对真菌是必不可少的，还是发生在一条条件可选的染色体上，该染色体对真菌的生存不是必不可少的，但对于致病性或其他特殊功能是必要的。拟议的研究将有助于：1)通过比较产生PTR ToxB和PtR ToxA的真菌菌株的竞争能力，确定ToxB的流行病学意义；2)评估ToxB的拷贝是如何排列的，以及它们是否出现在必要的或可有可无的染色体上；3)确定彼此之间以及与密切相关物种之间的融合性质，这可以解释某些菌株最初是如何获得宿主选择性毒素的。这项工作的目的是增加对小黑麦致病力的基础和进化的了解，并提供对评估和减轻未来黑斑病流行风险至关重要的信息。通过了解真菌是如何致病的，以及它是如何致病的，我们可以制定更积极有效的疾病管理策略。这反过来将减少加拿大农民的产量和经济损失。

The fungus Pyrenophora tritici-repentis causes tan spot, a leaf spotting disease that can damage growing wheat anytime during its development. Tan spot is estimated to reduce yield by between 2-15%, although losses as high as 50% have been reported. Infection by P. tritici-repentis causes the formation of lesions or spots on the leaves of susceptible plants, which consist of dead (necrotic) and/or yellow (chlorotic) tissue. These symptoms result from the production, by the fungus, of host-selective toxins that can affect some wheat varieties but not others. Three host-selective toxins, termed Ptr ToxA, Ptr ToxB and Ptr ToxC, are produced alone or in combination by strains of P. tritici-repentis. All three toxins are important for tan spot development, and the fungus needs to produce at least one of them to be able to cause disease on specific wheat varieties. In addition to being required to cause disease, however, there is also a dosage effect of Ptr ToxB, meaning that higher levels of Ptr ToxB result in more severe disease symptoms on susceptible wheat. The amount of Ptr ToxB that is produced by a fungal strain reflects the number of copies of the ToxB gene (which codes for the toxin) in that strain, as well as how actively those copies are expressed.   Despite its importance in tan spot development, it is not clear why fungal strains that produce Ptr ToxB are extremely rare in Canada, while those that produce Ptr ToxA are very common. Similarly, it is not known whether the multiple copies of the ToxB gene occur on a chromosome that carries other important genes, and is therefore essential to the fungus, or whether it occurs on a conditionally dispensable chromosome that is not essential for survival of the fungus, but is necessary for pathogenicity or other specialized functions. The proposed research will: 1) help to determine the epidemiological significance of ToxB by comparing the competitive ability of fungal strains that produce Ptr ToxB and Ptr ToxA, 2) evaluate how copies of ToxB are arranged and if they occur on an essential or dispensable chromosome, and 3) determine the nature of fusion between strains of P. tritici-repentis with each other and with closely related species, which could explain how certain strains obtained the host-selective toxins in the first place. The aim of this work is to increase knowledge of the basis for and evolution of virulence in P. tritici-repentis, and to provide information vital to assessing and mitigating the risk of future tan spot epidemics. By understanding how a fungus causes disease, and how it can become pathogenic, we can develop more proactive and effective disease management strategies. This in turn will reduce the yield and economic losses incurred by Canadian farmers.