Molecular basis of virulence in Pyrenophora tritici-repentis

黑斑病是一种斑叶病，可以在小麦生长过程中的任何时候对其造成损害。据估计，黄褐斑会使产量降低2-15%，尽管有报道称损失高达50%。受小麦螺旋体的感染会在易感植物的叶片上形成病变或斑点，这些病变或斑点由死亡（坏死）和/或黄色（褪绿）组织组成。这些症状是由真菌产生的寄主选择性毒素引起的，这种毒素可以影响某些小麦品种，但不会影响其他品种。三种宿主选择性毒素，称为Ptr ToxA， Ptr ToxB和Ptr ToxC，是单独或联合由P. tritric -repentis菌株产生的。这三种毒素对棕斑病的发展都很重要，真菌需要产生至少一种毒素才能对特定的小麦品种致病。然而，除了必须引起疾病之外，Ptr ToxB还存在剂量效应，这意味着较高水平的Ptr ToxB会导致易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（编码毒素）的拷贝数，以及这些拷贝表达的活跃程度。尽管它在棕斑发育中很重要，但尚不清楚为什么产生Ptr ToxB的真菌菌株在加拿大极为罕见，而产生Ptr ToxA的真菌菌株却非常常见。同样，我们也不知道弓形虫基因的多个拷贝是否发生在携带其他重要基因的染色体上，因此对真菌来说是必不可少的，或者它是否发生在有条件的可缺性染色体上，这对真菌的生存来说不是必不可少的，但对致病性或其他特殊功能来说是必要的。拟议的研究将：1)帮助确定的流行病学意义ToxB通过比较真菌菌株的竞争力产生Ptr ToxB Ptr ToxA, 2)的评估副本ToxB排列,如果他们出现在一个重要或可有可无的染色体,和3)确定融合菌株之间的性质p tritici-repentis彼此密切相关的物种,这可能解释为什么某些菌株获得了host-selective毒素在第一时间。这项工作的目的是增加对小锥虫毒力的基础和进化的认识，并为评估和减轻未来斑点流行的风险提供至关重要的信息。通过了解真菌如何引起疾病，以及它如何成为致病性，我们可以制定更积极有效的疾病管理策略。这反过来又会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.