Abstract Background Salicylic acid (SA) is an important plant hormone but is also produced by microorganisms. Contrary to the well-described roles and biosynthetic pathways of SA in plants, its role in fungal physiology and its biosynthesis within fungi remains largely unclear. Here, we sought to investigate the role of SA in the physiology of Trichoderma spp. and to identify fungal genes responsible for SA biosynthesis in Trichoderma virens , while applying and optimizing a transformation approach recently adapted for Trichoderma atroviride . Results Significant strain- and species-dependent differences in both SA biosynthesis and growth in the presence of exogenous SA were observed. Furthermore, in certain Trichoderma species SA biosynthesis turned out to be induced by the presence of plant volatile organic compounds (VOCs). Based on plant SA biosynthesis pathways, candidate fungal SA biosynthesis genes were screened and respective T. virens gene deletion mutants generated through application and optimization of an enhanced transformation approach. Gene deletion did not result in a decrease in SA biosynthesis, providing evidence that SA biosynthesis in T. virens is distinct from the canonical plant pathways. Conclusions Although we were not able to identify genes responsible for SA biosynthesis in T. virens , we uncovered how certain Trichoderma and fungal phytopathogen species are affected by SA in their environment and how SA release by Trichoderma spp. can be affected by the presence of a plant host. Furthermore, we were able to optimize an approach to measuring phytohormones produced by Trichoderma spp. in plate culture and proved the applicability of an optimized transformation approach in T. virens .
Pierson et al. (2026) studied this question.