Infection by a foliar endophyte elicits novel arabidopside‐based plant defence reactions in its host, Cirsium arvense

Summary Endophytic fungi live asymptomatically within plants. They are usually regarded as nonpathogenic or even mutualistic, but whether plants respond antagonistically to their presence remains unclear, particularly in the little‐studied associations between endophytes and nongraminoid herbaceous plants. We investigated the effects of the endophyte C haetomium cochlioides on leaf chemistry in C irsium arvense . Plants were sprayed with spores; leaf material from both subsequent new growth and the sprayed leaves was analysed 2 wk later. Infection frequency was 91% and 63% for sprayed and new growth, respectively, indicating that C . cochlioides rapidly infects new foliage. Metabolomic analyses revealed marked changes in leaf chemistry with infection, especially in new growth. Changes in several novel oxylipin metabolites were detected, including arabidopsides reported here for the first time in a plant species other than A rabidopsis thaliana , and a jasmonate‐containing galactolipid. The production of these metabolites in response to endophyte presence, particularly in newly infected foliage, suggests that endophytes elicit similar chemical responses in plants to those usually produced following wounding, herbivory and pathogen invasion. Whether endophytes benefit their hosts may depend on a complex series of chemically mediated interactions between the plant, the endophyte, other microbial colonists and natural enemies.