Epiphytic ecology and nutrition for control of a wheat pathogen

我的研究涉及一种真菌，ZymosepVictoria tritici，它攻击小麦植株，导致一种名为SepVictoria tritici blotch(STB)的疾病。STB每年给英国造成约3亿GB的小麦产量损失和作物上使用的杀菌剂成本。更糟糕的是，这种真菌开始对治疗它的杀菌剂产生抗药性。这意味着我们需要新的方法来控制感染。为了开发控制小麦纹枯病菌的新方法，有必要充分了解这种真菌与小麦植株的相互作用方式，以及这种相互作用如何受到环境条件的影响。在以前的工作中，我证明了一些小麦纹枯病菌菌株在入侵之前可以在叶表面生长大约10天。不同真菌分离物的叶面生长量和持续时间不同，同一分离物侵染不同小麦品种时也不同。如果特定小麦品种对特定的小麦纹枯病菌分离物具有抗性，那么该分离物将永远不会侵入叶片。然而，这些“无毒”菌株似乎可以留在叶表面，甚至在那里繁殖，产生新的孢子，以便扩散到更敏感的小麦植株上。相比之下，大多数植物病原真菌无法从叶表面获得足够的营养物质，无法存活超过24小时。因此，我想首先确定，这个叶表面生长阶段对小麦赤霉菌有多重要，以及它是否与该真菌在致病方面的有效程度有关。我还想找出一些分离物是否比其他分离物更有可能在叶表面停留较长时间，以及这种行为差异是否可以归因于真菌分离物基因组的差异。其次，我的目标是确定真菌在叶表面使用的是什么营养物质。例如，真菌可能依赖于内部的脂肪储存，或者利用从叶子中渗出的营养，或者利用化肥等农业投入。或者，它可能能够分泌酶来消化树叶的结构成分，如蜡质，以从这些结构成分中获得营养。这种真菌还可能利用叶表面上其他微生物的活动，这些微生物分泌这种酶，或者通过破坏叶表面导致营养物质从叶中泄漏。第三，因此，我打算在田间采样小麦叶片，并使用元基因组学来研究叶表面存在哪些微生物。然后，我将比较这些微生物群落，注意到小麦受到小麦赤霉菌的严重影响，以寻找特定微生物的存在与促进真菌感染之间的关系。在获得了这些关于小麦纹枯病菌叶表面生长的数据后，我打算利用这些数据来构建一张详细的图片，了解这种真菌在进入叶片之前，在感染的第一个时期需要什么才能存活下来；或者，如果小麦是抗性的，它不能进入，那么它就会在叶片表面存活和繁殖。这将使我能够识别这种真菌所具有的任何漏洞，我们可能能够利用这些漏洞来控制疾病。例如，如果化肥的存在促进了叶表面的生长，那么通过将化肥和杀菌剂的使用时间相互关联起来，就有可能增加杀菌剂的有效性。或者，如果真菌依赖于特定的代谢途径来获得营养，那么该途径可能成为新形式的化学或其他控制的目标。或者，如果真菌获得了与特定细菌共享叶表面的巨大优势，那么控制该细菌，也许是通过与无法促进真菌生长的竞争细菌进行生物控制，可能会间接控制真菌。

My research concerns a fungus, Zymoseptoria tritici, which attacks wheat plants, causing a disease known as Septoria tritici blotch (STB). STB costs the UK around £300 Million per year in lost wheat yields and in the cost of the fungicide used on the crops. Worse, the fungus is beginning to develop resistance to the fungicides that are available to treat it. This means that we need new methods to control the infection. To develop new ways to control Z. tritici, it is necessary to gain a full understanding of the ways in which the fungus interacts with the wheat plant, and how that interaction can be affected by environmental conditions. In previous work, I showed that some isolates of Z. tritici can grow on the leaf surface for around ten days before invading. The amount and duration of leaf surface growth varies between fungal isolates, and also when the same isolate infects different wheat varieties. If the particular wheat variety is resistant to a particular Z. tritici isolate, then that isolate will never invade the leaf. However, it appears that such 'avirulent' isolates can persist on the leaf surface instead, and even reproduce there, making new spores for dispersal to more susceptible wheat plants. Most plant pathogenic fungi, by contrast, can't obtain enough nutrients on the leaf surface to survive for more than 24 hours. I therefore want to determine, firstly, how important this leaf surface growth phase is for Z. tritici, and whether it is related to how effective the fungus is at causing disease. I also want to find out whether some isolates are more likely than others to spend a prolonged period of time on the leaf surface, and whether such differences in behaviour can be attributed to differences in the genomes of the fungal isolates. Secondly, I aim to determine what nutrients the fungus is using when it is on the leaf surface. For example, the fungus might be relying on internal lipid stores, or taking advantage of nutrients that are exuded from the leaf, or of agricultural inputs like fertilisers. Alternatively, it might be able to secrete enzymes which digest structural components of the leaf such as waxes, to obtain nutrients from those. The fungus might also be able to take advantage of the activities of other microbes on the leaf surface which secrete such enzymes, or which cause nutrients to leak from the leaf by damaging the leaf surface. Thirdly, therefore, I intend to sample wheat leaves in the field and use metagenomics to study which microbes are present on the leaf surface. I will then compare these microbial communities, taking note of how severely affected the wheat in each field was by Z. tritici, to look for correlations between the presence of particular microbes and the promotion of fungal infection. Having obtained these data about leaf surface growth in Z. tritici, I intend to use them to build a detailed picture of what the fungus needs to survive throughout this first period of infection, before it enters the leaf; or to persist and reproduce on the leaf surface if the wheat is resistant and it cannot enter. This will allow me to identify any vulnerabilities the fungus has that we might be able to exploit in order to control the disease. For instance, if leaf surface growth is boosted by the presence of fertiliser, then it may be possible to increase the usefulness of fungicides by inter-relating the timings of fertiliser and fungicide application. Alternatively, if the fungus relies on a particular metabolic pathway to obtain nutrients, then that pathway could be targeted for new forms of chemical or other control. Or, if the fungus gains a large advantage sharing the leaf surface with a particular bacterium, then controlling that bacterium, perhaps via bio-control with a competing bacterium that is not able to promote fungal growth, might indirectly control the fungus.