How do interactions between herbivores and mycorrhizal fungi regulate production of plant signalling compounds and parasitoid behaviour?

当植物受到食草动物的攻击时，最令人着迷的适应性反应之一是向大气中释放化学物质。这些挥发性的信号化合物可以传播很远的距离，并被拟寄生蜂检测到，利用它们来定位和寄生在叶类食草动物身上。在许多情况下，已经证明某些拟寄生物只对特定的半化学物质有反应，它们是由植物系统地产生的。这种半化学物质释放机制有可能成为植物控制蚜虫种群的有效策略。植物芽、蚜虫和拟寄生物之间的三方相互作用是比较清楚的。然而，关于这个故事的另一个有趣的方面——地下通道的半化学物质转移和叶食草动物、植物根和根际有益微生物之间的关系的研究很少。高度简化的水培系统有证据表明，植物有可能向根际释放半化学物质，这些物质可被邻居吸收。在自然土壤中，有可能通过菌根真菌在植物之间直接转移半化学物质。这些真菌大量寄生在几乎所有陆地植物的根部，并产生大量的菌丝体，将单个植物连接成一个共同的网络。有人认为，它们可能是将半化学物质转移到散装土壤和未被蚜虫感染的邻近植物的管道，这一假设尚未得到证实。虽然迄今为止进行的许多实验已经确定了特定生态成分的重要性，但很明显，需要采取更全面的方法来确定自上而下和自下而上控制植物信号的相对重要性，以及间接和直接的符号化学转移途径（Bruce & Pickett, 2007）。因此，该博士研究生将首先寻求确定叶片食草动物对菌根真菌关键功能性状（营养通量）和多样性的影响，其次，确定植物之间传递半化学物质的地下途径。这些目标将通过测试以下假设来实现：蚜虫侵染植物影响菌根真菌功能多样性；2. 蚜虫侵染改变植物根际和菌根际的化学释放；3. 不同植物种类间存在根际和菌根际间的半化学物质间接转移；4. 常见的菌根菌丝网络可以实现植物间半化学物质的直接转移；5. 丛枝菌根真菌与蚜虫相互作用的结果取决于每种生物的定植程度和真菌的物种组成。这项工作将使用基于增加生物复杂性的微观系统的组合。这种方法意味着我们可以识别关键机制，同时控制潜在的混杂因素，管理项目风险，并建立更生态相关的系统。这项工作将首先使用蚕豆（Vicia faba L.）作为试验品种。这种植物很容易形成丛枝菌根，当蚜虫侵染时产生半化学物质。将开展盆栽试验，使这些植物融入共同的菌丝体网络，选定的个体将被蚜虫侵染，并测量拟寄生物对这种处理的反应。这些测量将在CASE合作伙伴的实验室使用嗅觉计和摄像机技术进行。利用13CO2和33P脉冲追踪实验、分子群落分析（TRFLP）来确定蚜虫对菌根真菌的影响。

One of the most fascinating adaptive responses by plants when they are subjected to attack by leaf herbivores is the release of semiochemicals into the atmosphere. These volatile signalling compounds can travel significant distances and are detected by parasitoid wasps that use them to locate and parasitise leaf herbivores. In many cases, it has been shown that certain parasitoids respond only to particular semiochemicals and that they are produced systemically by the plant. This mechanism of semiochemicals release has potential to be an effective strategy by plants for control of aphid populations. The three-way interaction among plant shoots, aphids and parasitoids is relatively well understood. However, there has been very little research on an intriguing additional dimension to the story - that of below ground pathways of semiochemicals transfer and the relationship between leaf herbivores, plant roots and beneficial microorganisms in the rhizosphere. There is evidence from highly simplified hydroponic systems that plants have the potential to release semiochemicals into the rhizosphere, which are available for uptake by neighbours. In natural soils, there is the possibility of a direct transfer process of semiochemicals between plants via mycorrhizal fungi. These fungi heavily colonise the roots of virtually all land plants and produce vast lengths of mycelium that interlink individual plants into a common network. It has been suggested that they may be able to act as conduits for transfer of semiochemicals into bulk soil and to neighbouring plants uninfected by aphids, a hypothesis that remains untested. While many of the experiments undertaken to date have identified the importance of particular ecological components, it is clear that a more holistic approach needs to be undertaken to determine the relative importance of top-down and bottom-up controls of plant signalling, and indirect and direct pathways of semiochemical transfers (Bruce & Pickett, 2007). This PhD studentship therefore will, firstly, seek to determine the impact of leaf herbivores on the key functional traits (nutrient fluxes) and diversity of mycorrhizal fungi, and secondly, identify the below ground pathways by which semiochemicals are transferred between plants. These aims will be achieved by testing the following hypotheses: 1. Aphid infestation of plants affects the functional diversity of mycorrhizal fungi; 2. Aphid infestation of plants modifies semiochemical release into the rhizosphere and mycorrhizosphere; 3. Indirect transfer of semiochemicals in the rhizosphere and mycorrhizosphere occurs between different plant species; 4. Common mycorrhizal mycelial networks enable direct plant-to-plant transfer of semiochemicals; 5. The outcome of interactions between arbuscular mycorrhizal fungi and aphids is dependent on the degree of colonisation of each organism, and the species composition of the fungi. The work will use a combination of microcosm based systems of increasing biological complexity. This approach means that we can identify key mechanisms while controlling potentially confounding factors, manage project risk and build up to more ecologically relevant systems. The work will in the first instance use broad bean (Vicia faba L.) as a test species. This plant readily forms arbuscular mycorrhizas and produces semiochemicals when infested with aphids. Pot experiments will be developed to enable the plants to become integrated into common mycelial networks, and selected individuals will be infested with aphids and the response of parasitoids to this treatment measured. These measurements will be made at the CASE partner's laboratory using olfactometer and video camera techniques. The impacts of aphids on mycorrhizal fungi will be determined using 13CO2 and 33P pulse chase experiments, molecular community profile analysis (TRFLP).