Comparative exo-proteomic analysis of Trichoderma antagonists induced by oomycete mycelium highlights diverse antagonism strategies

Plant-beneficial fungi from the genus Trichoderma (Hypocreales, Ascomycota) can control oomyceteous plant-pathogenic Pythium myriotylum (Peronosporales, Oomycota) and thus serve as bioeffectors for the eco-friendly products of crop protection. However, the underlying mechanisms of microbe-microbe interactions have yet to be fully understood. In this study, we focused on the role of the Trichoderma secretome induced by P. myriotylum mycelia. For this purpose, we selected strains showing strong (T. asperellum, T. atroviride, T. virens), moderate (T. cf. guizhouense, T. reesei), and weak (T. parepimyces) activities, respectively, and cultured with the sterilized P. myriotylum mycelia. Secreted proteins were analyzed using label-free LC-MS/MS, bioinformatic localization prediction, gene ontology (GO) annotation, and ortholog analysis. The exoproteomic analysis quantified proteins in the six Trichoderma spp., suggesting unequal antagonistic mechanisms among the strong and weak strains, respectively, with different proportions of putative cellulases, proteases, redox enzymes, and extracellular proteins of unknown function. Notably, proteolysis-related proteins were abundant, while the abundant proteases tended not to be conserved across the species (i.e., non-orthologous). Putative cellobiohydrolases were detected abundantly in all Trichoderma species except for the weak antagonist T. parepimyces, even though its genome encodes for these proteins. Notably, secretomes of the most potent anti-Pythium bioeffectors tended to have higher endo-cellulase activity. Cellulose and other glucans are major components of the oomycete cell wall, which was partly reflected in the cellulases produced by the Trichoderma species. The varying abundances of orthologous proteins suggested the evolution of differing transcription regulation mechanisms across the Trichoderma genus in response to the ubiquitous presence of Oomycota.