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

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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).