Establishing the role of IAA and related plant indole compounds in nematode infection.

Plant parasitic nematodes are microscopic worms in soil that feed on plant roots, stunt their growth and make root systems much less efficient at withdrawing nutrients and water from soil. The yields of nematode-infected crop plants are reduced and annual losses world wide are estimated to cost US$100b, despite control measures. In the UK the major nematode pest, potato cyst nematodes, causes £50m of yield losses each year to the potato crop. Nematodes usually do not cause characteristic symptoms in plants and the general stunting of infected crops is often attributed to other causes; nematodes are frequently referred to as the hidden enemy. In intensive cropping systems growers depend on host resistance and chemicals (nematicides) to manage nematode pests. Nematicides are some of the most toxic products used in crop protection and their use has been banned in several European countries because of environmental concerns and public pressures to reduce dependence on agro-chemicals. There is an urgent need to develop other control methods to replace current nematicides. This research proposal builds on recent work at Rothamsted Research that has demonstrated that root-knot nematodes, which include the most important nematode pests in world agriculture, may use indole acetic acid and other plant-derived indole compounds to locate and invade host-plant roots in soil. We will confirm this observation and identify the most active compounds. We will also investigate how these compounds cause changes in nematode behaviour and guide them to infection sites. The output of this project will provide nematode targets for chemical or genetic intervention. Interfering with host recognition processes, offers great promise for the design of efficient, specific, safe and environmentally friendly management tactics to limit plant damage due to major groups of nematode pests, including the potato cyst nematodes.

We have shown previously that the plant hormone IAA and related indoles cause behavioural changes in nematodes and act as attractants. The aims of the project are to establish whether such compounds function as signals to direct nematode invasion of roots and to identify the nematode genes involved in the response to these compounds. The research will focus on the interaction between the root-knot nematode Meloidogyne incognita and the model plant species Arabidopsis thaliana, which is a host for this nematode. Sensitive mass spectrometric-based analytical methods will be used to determine whether IAA and other indoles are present in the rhizosphere at concentrations that can induce behavioural changes in the nematodes. An electrochemical method will be developed for measuring IAA concentrations on the root surface and within the root to test the hypothesis that nematodes follow an IAA gradient during infection. Arabidopsis mutants with altered IAA transport will be used to determine the importance of IAA for root infection. The mechanism for the effect of IAA and other indoles on nematode behaviour will be probed using cDNA-AFLP to determine changes in the gene expression profile in nematodes treated with IAA. We will utilise the genetic resources available for C. elegans, which also changes its behaviour in response to IAA treatment, by identifying mutants that do not respond to IAA. These approaches will provide an insight into the nematode genes involved in the response mechanism. The anatomical localisation of these genes in M. incognita will be established by in situ hybridisation and RNAi will be used to confirm their involvement in the chemical recognition process and/or the induced nematode behavioural changes. Information from these studies could lead to the development of compounds that interfere with nematode chemoreception of host signals or alternatively, provide markers for nematode resistance in crop species.