Combining metabolomics and gene expression analysis reveals that propionyl- and butyryl-carnitines are involved in late stages of arbuscular mycorrhizal symbiosis

The arbuscular mycorrhizal (AM) symbiosis is a widespread mutualistic association between soil fungi (Glomeromycota) and the roots of most plant species. AM fungi are obligate biotrophs whose development is partially under the control of their plant host. We explored the possibility to combine metabolomic and transcriptomic approaches to find putative mycorrhiza-associated metabolites regulating AM fungal development. Methanol extracts of Medicago truncatula roots colonized or not with the AM fungus Rhizophagus irregularis were analyzed and compared by ultra-high-performance liquid chromatography (UHPLC), high-resolution mass spectrometry (Q-TOF), and multivariate statistical discrimination. We detected 71 mycorrhiza-associated analytes exclusively present or at least 10-fold more abundant in mycorrhizal roots. To identify among these analytes those that could regulate AM fungal development, we fractionated by preparative and semi-preparative HPLC the mycorrhizal and non-mycorrhizal root extracts and established how the 71 analytes were distributed among the fractions. Then we tested the activity of the fractions on germinating spores of R. irregularis by quantifying the expression of 96 genes known for their diverse in planta expression patterns. These investigations reveal that propionyl- and butyryl-carnitines accumulated in mycorrhizal roots. The results suggest that these two molecules regulate fungal gene expression in planta and represent interesting candidates for further biological characterization.