Root exudates of stressed plants stimulate and attract Trichoderma soil fungi

Plant roots release complex mixtures of bioactive molecules including compounds that affect the activity and modify the composition of the rhizosphere microbiome. In this work, we investigated the initial phase of the interaction between tomato and an effective biocontrol strain of Trichoderma harzianum (T22). We found that root exudates (RE), obtained from plants grown in a split root system and exposed to a choice of biotic and abiotic stress factors (wounding, salt, pathogen attack), stimulate the growth and act as chemoattractants of the biocontrol fungus. On the other hand, some of the treatments did not result in an enhanced chemotropism on Fusarium oxysporum f. sp. lycopersici, indicating a mechanism that may be selective for non-pathogenic microbes. The involvement of peroxidases and oxylipins, both known to be released by roots in response to stress, was demonstrated by using RE fractions containing these molecules and their commercial purified analogues, testing the effect of a specific inhibitor, and characterizing the complex pattern of these metabolites released by tomato roots both locally and systemically.