Sesquiterpene Biosynthetic Gene vir4 from Trichoderma virens Enhances Direct Herbivore Resistance while Maintaining Indirect Defense

Trichoderma species are widely used as root-colonizing biocontrol agents that enhance plant resistance to biotic and abiotic stresses while promoting growth. These fungi produce diverse volatile and non-volatile metabolites that mediate interactions with plants. Trichoderma can influence both direct and indirect plant defenses, including the release of herbivore-induced plant volatiles (HIPVs) that attract natural enemies of herbivores. In this study, we examined the effects of T. virens and its vir4 gene (regulating terpenoid synthesis) knockout-mutant on maize (Zea mays), the herbivore Helicoverpa armigera, and its predator Macrolophus pygmaeus. Previous research has shown that T. virens differentially modulates maize root gene expression and specialized metabolite concentrations. Here, we found that caterpillars feeding on maize seedlings colonized by wild-type T. virens gained significantly less weight than those feeding on maize colonized by the vir4 knockout mutant or uncolonized plants, suggesting that the vir4 gene cluster contributes to herbivore resistance. Although fungal colonization led to moderate changes in HIPV composition, total volatile emissions remained unchanged. In Y-tube assays, M. pygmaeus preferred caterpillar-infested maize over healthy plants, but fungal colonization did not significantly affect predator behavior. Our findings demonstrate that T. virens enhances direct plant defense against herbivores while maintaining indirect defense through a mechanism regulated by terpenoid synthesis depending on vir4 gene. Further research is needed to elucidate the metabolic changes in maize induced by T. virens that contribute to reduced herbivore performance.