Melatonin delays softening of postharvest pepper fruits (Capsicum annuum L.) by regulating cell wall degradation, membrane stability and antioxidant systems

Melatonin plays an important role in maintaining the postharvest quality of fresh fruit. This study focused on the effect of exogenous melatonin on fruit softening and ROS metabolism in green pepper fruit under storage at 20℃. Our results indicated that melatonin treatment reduced the transcript levels of genes encoding polygalacturonase, β-glucanase (associated with hydrolysis of cell wall components), lipoxygenase and phospholipase enzymes, associated with membrane stability, thereby preserving the firmness of harvested green pepper fruits; the decreased transcription of the phospholipase gene was associated with melatonin-induced downregulation of expression of a corresponding transcription factor gene. In addition, melatonin treatment induced the transcription of genes encoding the antioxidant enzymes catalase and peroxidase, upregulated the activities of the same enzymatic antioxidants and increased the ratios of glutathione-to-glutathione disulfide and of ascorbic acid to dehydroascorbic acid. As a consequence, the concentrations of intracellular superoxide anions, H2O2 and malondialdehyde were decreased, reducing the accumulation of reactive oxygen species and the lipid peroxidation of cell membranes. In summary, this work proved that exogenous melatonin application delays the softening of pepper fruits postharvest and hence extends their storage period and shelf life.