Hydrogen peroxide produced by NADPH oxidase: a novel downstream signaling pathway in melatonin-induced stress tolerance in Solanum lycopersicum

The beneficial effects of melatonin on abiotic stress have been demonstrated in several plants. However, little is known about the signal transduction pathway of melatonin involved in the plant stress response. Here, we manipulated the melatonin levels in tomato plants through a chemical approach. The roles of melatonin in stress tolerance were studied by assessing the symptoms, chlorophyll fluorescence and stress-responsive gene expression. Moreover, both chemical and genetic approaches were used to study the roles of hydrogen peroxide (H2O2) in melatonin-induced signal transduction in tomato plants. We found that melatonin activates NADPH oxidase (RBOH) to enhance H2O2 levels by reducing its S-nitrosylation activity. Furthermore, melatonin-induced H2O2 accumulation was accompanied by obtainable stress tolerance. Inhibition of RBOH or chemical scavenging of H2O2 significantly reduced the melatonin-induced defense response, including reduced expression of several stress-related genes (CDPK1, MAPK1, TSPMS, ERF4, HSP80 and ERD15) and reduced antioxidative enzyme activity (SOD, CAT and APX), which were responsible for the stress tolerance. Collectively, these results revealed a novel mechanism in which RBOH activity and H2O2 signaling are important components of the melatonin-induced stress tolerance in tomato plants.