Tomato is one of the most important food crops immensely affected by water scarcity. Therefore, it is vital to find biocontrol agents for improving the yield of tomato crop in arid areas. The fungal genus, Trichodermais widely used as an eco-friendly, biocontrol agent in commercial formulas because of the various beneficial
effects it has on plants including the resistance to biotic and abiotic stresses. In the present study, the ef- fects of an isolate of Trichoderma atroviride ID20G (Ta) on tomato (Solanum lycopersicum L.) seedlings were investigated under drought stress. The isolated fungus was identified using ITS (internal transcribed spacer) sequences. Root colonization by Ta induced changes in growth performance indexes such as root growth, root branching, and leaf number as compared to the untreated seedlings. Chlorophyll and carotenoid contents of the untreated tomato seedlings decreased after drought stress along with extensive membrane deterioration, whereas seed colonization by Ta prevented lipid oxidation and ameliorated the harmful effects of drought on pigment contents. Antioxidant enzyme activity was elevated and hydrogen peroxide (H2O2) concentration was found to decrease under drought stress in the Ta treated seedlings. These observations suggest that coloniza- tion of tomato seedlings by Ta is effective in counteracting the injurious effects of drought, and therefore, may have a prominent role in increasing the drought tolerance of tomato plant by decreasing H2O2 concentration and activating the antioxidant enzymes. Furthermore, fungus-based biocontrol agent formulation of Ta might serve as a potential tool in tomato agriculture owing to its low cost, effectiveness, and characteristics required for balancing the natural ecology.