Epiphytic ecology and nutrition for control of a wheat pathogen

我的研究涉及一种真菌，小麦酵母，它会攻击小麦植株，引起一种被称为小麦酵母斑疹病（STB）的疾病。STB每年给英国造成约3亿英镑的小麦产量损失和作物杀菌剂成本。更糟糕的是，这种真菌开始对可用来治疗它的杀菌剂产生耐药性。这意味着我们需要新的方法来控制感染。为了开发新的防治小麦黑穗病的方法，有必要充分了解这种真菌与小麦植物的相互作用方式，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，我发现一些分离的小麦偃麦草芽孢杆菌在入侵前可以在叶片表面生长10天左右。叶片表面生长的数量和持续时间在不同的真菌分离株之间是不同的，而且当同一菌株感染不同的小麦品种时也是不同的。如果特定的小麦品种对特定的小麦弧菌分离株具有抗性，那么该分离株将永远不会入侵叶片。然而，这种“无毒”的分离株似乎可以在叶片表面持续存在，甚至在那里繁殖，产生新的孢子传播到更敏感的小麦植株。相比之下，大多数植物病原真菌不能在叶片表面获得足够的营养来存活超过24小时。因此，我想确定，首先，这个叶片表面生长阶段对小麦黑穗病菌有多重要，以及它是否与真菌致病的有效性有关。我还想弄清楚是否有些菌株比其他菌株更有可能在叶片表面停留更长时间，以及这种行为上的差异是否可以归因于真菌菌株基因组的差异。其次，我的目标是确定真菌在叶子表面时使用的营养物质。例如，真菌可能依赖于内部脂质储存，或利用从叶子中渗出的营养物质，或农业投入物，如肥料。或者，它可能能够分泌酶来消化叶子的结构成分，如蜡，从中获取营养。真菌也可能利用其他微生物在叶子表面的活动，这些微生物分泌这种酶，或者通过破坏叶子表面导致营养物质从叶子中泄漏。第三，因此，我打算在田间取样小麦叶片，利用宏基因组学研究哪些微生物存在于叶片表面。然后，我将比较这些微生物群落，并注意到每一块田地的小麦受小麦黑穗病弧菌影响的严重程度，以寻找特定微生物的存在与促进真菌感染之间的相关性。在获得了这些关于小麦小麦叶片表面生长的数据后，我打算用它们来建立一个详细的图片，说明真菌在进入叶片之前，在感染的第一个时期存活需要什么；或者在小麦具有抗性而不能进入的情况下，在叶片表面持续繁殖。这能让我找出真菌的任何弱点我们或许可以利用这些弱点来控制疾病。例如，如果肥料的存在促进了叶片表面的生长，那么通过将肥料和杀菌剂的施用时间相互关联，就有可能增加杀菌剂的用途。或者，如果真菌依赖于特定的代谢途径来获取营养，那么该途径可以针对新形式的化学或其他控制。或者，如果真菌与一种特定的细菌共享叶子表面获得了很大的优势，那么控制这种细菌，也许是通过与一种不能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.