Abstract
Lipid transfer from host plants to arbuscular mycorrhiza fungi was hypothesized for several years because sequenced arbuscular mycorrhiza fungal genomes lack genes encoding cytosolic fatty acid synthase ( Wewer et al., 2014 ; Rich et al., 2017 ). It was finally shown by two independent experimental approaches ( Jiang et al., 2017 ; Keymer et al., 2017 ; Luginbuehl et al., 2017 ). One approach used a technique called isotopolog profiling ( Keymer et al., 2017 ). Isotopologs are molecules, which differ only in their isotopic composition. For isotopolog profiling an organism is fed with a heavy isotope labelled precursor metabolite. Subsequently, the labelled isotopolog composition of metabolic products is analysed via mass spectrometry. The detected isotopolog pattern of the metabolite(s) of interest yields information about metabolic pathways and fluxes ( Ahmed et al., 2014 ). The following protocol describes an experimental setup, which enables separate isotopolog profiling of fatty acids in plant roots colonized by arbuscular mycorrhiza fungi and their associated fungal extraradical mycelium, to elucidate fluxes between both symbiotic organisms. We predict that this strategy can also be used to study metabolite fluxes between other organisms if the two interacting organisms can be physically separated.