Reverse chemical ecology to study the defense of the plant host Sextonia rubra and the chemical mediators of its endophyte Fusarium falciforme against phytopathogen Trametes versicolor

Sextonia rubra is a tropical tree endemic to the Guiana Shield and the Brazilian Amazon. Despite its renowned wood durability, it remains susceptible to degradation by white-rot fungi such as Trametes versicolor. To mitigate biotic stresses, plants can rely on their associated microbial communities, including endophytes, which play a crucial role in their defense mechanisms. In this study, we explored the cultivable microbiota of S. rubra, considering it a holobiont. Endophytic strains were isolated from the bark, sapwood and heartwood of S. rubra, and metabolome were extracted. We used a reverse chemical ecology approach to elucidate the mechanisms underlying these extracts' fungicidal activity. In this context, glutathione-S-transferases (GST), key detoxification enzymes of the lignivorous fungus T. versicolor, were chosen as targets. GST tests confirmed the presence of antifungal compounds in extracts from 13 of the 152 endophytes. Two isolates of Fusarium falciforme and one isolate of Fusarium graminearum were selected for co-culture experiments with T. versicolor. A comprehensive metabolic analysis of the confrontation zones using RPLC-ESI(+)-HRMS/MS and molecular networking revealed that the antifungal activity against T. versicolor was primarily mediated by cyclopeptides, and the observed contact inhibition was attributed to fusarins. These findings shed new light on the role of endophytic fungi in the chemical defense strategies of S. rubra, highlighting their potential as a source of bioactive compounds with antifungal properties.