COMT1-dependent melatonin biosynthesis mediates Trichoderma asperellum-induced resistance to Cladosporium fulvum leaf mold in tomato.

Tomato leaf mold, caused by Cladosporium fulvum, is a destructive foliar disease in protected cultivation. Biological control using Trichoderma and plant defense elicitors such as melatonin offers a sustainable alternative to chemical fungicides. However, the synergistic effect of combining Trichoderma with melatonin and the role of endogenous melatonin in Trichoderma-induced resistance to tomato leaf mold remain unclear. In this study, we screened ten Trichoderma strains and identified T. asperellum T141 strain as the most effective antagonist against C. fulvum in dual culture assays. Moreover, exogenous melatonin (100 μmol/L) resulted in the lowest disease index and significantly reduced malondialdehyde content. The combined application of T. asperellum and melatonin prior to pathogen inoculation reduced the disease index by 77.57% and promoted plant growth compared with pathogen-only controls. The combination also decreased H2O2 and O2 ·-, elevated antioxidant enzyme (SOD, POD, CAT, and APX) activities, and restored photosynthetic parameters, pigment contents, Rubisco activity, FBPase activity, and expression of photosynthesis-related genes (FBPase, SBPase, FBPA, and TPI). Virus-induced gene silencing of the COMT1 gene, a key melatonin biosynthesis gene, drastically reduced endogenous melatonin, and largely compromised T. asperellum-induced resistance, along with attenuated antioxidant defense and photosynthetic recovery. Collectively, our results demonstrate that T. asperellum and melatonin synergistically protect tomato against C. fulvum by mitigating oxidative stress and preserving photosynthetic function, and that COMT1-dependent endogenous melatonin synthesis is essential for T. asperellum-induced resistance. This study provides a theoretical basis for developing Trichoderma-melatonin biopreparations as an eco-friendly strategy for the management of tomato leaf mold.