Roles of Endogenous Melatonin in Resistance to Infection in an Model.

Melatonin is an important bioactive molecule in plants. Two synthetases, N-acetylserotonin methyltransferase (ASMT) and serotonin N-acetyltransferase (SNAT) are involved in the final two steps of melatonin synthesis. Melatonin participates in responses to a variety of biotic and abiotic stresses in plants, but few studies have addressed the roles of endogenous melatonin in pathogen resistance. We investigated the role of endogenous melatonin in resistance to infection in an model system. Plant lines that overexpressed or through genetic manipulation showed upregulated expression of resistance genes and , transcription factor gene , and jasmonic acid (JA) defense pathway marker gene , and downregulated transcription factor gene in JA signaling pathway. Higher melatonin content also enhanced the activity of antioxidant enzymes superoxide dismutase (SOD) and peroxidase (POD), increased JA content, reduced plant disease symptoms, and reduced lesion size in leaves. These findings indicate that endogenous melatonin enhances plant resistance to infection. In contrast, and gene silencing lines showed opposite results and were more susceptible to . Thus, it can be demonstrated that melatonin functions as an effective regulator of plant stress resistance at the genetic level. A schematic model is presented for its role in resistance to infection. Our findings also helped to elucidate the associated signal transduction pathways and interactions between melatonin and other plant hormones.