Fungal Friend or Foe? Arbuscular Mycorrhizal Fungi Supress Root Defences, Increasing Susceptibility to Invertebrate Enemies - ISRR 2018 10th Symposium for the International Society of Root Research

Adam Frew ^1,2 Jeff R Powell ² Gaetan Glauser ³ Alison E Bennett ⁴ Scott N Johnson ²

¹Graham Centre for Agricultural Innovation, School of Agricultural and Wine Sciences, Charles Sturt University, Australia
²Hawkesbury Institute for the Environment, Western Sydney University, Australia
³Neuchâtel Platform of Analytical Chemistry, University of Neuchâtel, Switzerland
⁴Department of Evolution, Ecology and Organismal Biology, The Ohio State University, USA

Most terrestrial plants form mutualistic associations with arbuscular mycorrhizal (AM) fungi whereby the fungi colonise the roots and increase plant uptake of nutrients, while the plant provides the fungus with carbon. Mycorrhizal fungi also alter plant defence chemistry which, along with nutritional quality, determine resistance to plant antagonists. However, outcomes of the AM symbiosis on host-plant interactions with invertebrates are highly variable and context dependent. Specifically, the response of plant-parasitic nematodes, which typically feed on and within plant roots, to AM fungi can range from positive to negative, with little understanding of the driving mechanisms. These nematodes represent an important component of ecosystem functioning, however these invertebrates collectively cause an estimated $US 80-125 billion a year in damage to agriculture globally. To understand the AM-plant-nematode relationship we took a metabolomics approach, comprehensively assessing primary and secondary compounds in the roots of wheat (Triticum aestivum), and tested the impacts of inoculation with AM fungi on the root metabolic profile and nutritional chemistry, and how this affected the nematode Pratylenchus neglectus. Despite an increase in nutrient concentrations in the roots, metabolomic profiling revealed unique changes in the metabolome in response to AM fungal colonisation of the roots. AM fungi reduced benzoxazinoid glucoside compounds by 20-36%, which was associated with an 85% increase in nematode populations in the soil. These results demonstrate that AM fungi can reduce plant growth and supress root defence compounds that have been implicated in plant resistance to pests and pathogens, yet mycorrhizal associations still benefit the host plants through increased nutrient acquisition. This highlights the context dependent nature of mycorrhizal-plant-nematode interactions, and that much remains to be understood about the ecology of the AM symbiosis before it can be adequately exploited in agriculture.