Melatonin delays leaf senescence of postharvest Chinese flowering cabbage through ROS homeostasis

Reactive oxygen species (ROS) trigger and accelerate leaf senescence. Melatonin, a low molecular compound with several biological functions in plants, is known to delay leaf senescence in different species, including Chinese flowering cabbage. However, the mechanism(s) underpinning melatonin-delayed leaf senescence remains unclear. Here, we found that melatonin lowered the expression of chlorophyll catabolic genes (BrPAO and BrSGR1) and senescence-associated genes (BrSAG12 and BrSEN4), decreased chlorophyll loss, minimized the alteration in Fv/Fm ratio and remarkably delayed senescence of Chinese flowering cabbage after harvest. Moreover, the over-accumulation of O₂^•-, hydrogen peroxide (H₂O₂) and malondialdehyde contents and the expression of respiratory burst oxidase homologues (RBOH) genes (BrRbohB, BrRbohC, BrRbohD, BrRbohD2 and BrRbohE) were significantly inhibited by melatonin treatment. Melatonin-treated cabbages also showed higher O₂^•-, OH^• and DPPH radical scavenging capacity and enhanced activities of peroxidase (POD), superoxide dismutase (SOD) and their gene expressions. Up-regulation of key components of ascorbate-glutathione (AsA-GSH) cycle, the metabolic pathway that detoxify H₂O₂, was also observed in melatonin-treated cabbages. These findings suggest that melatonin-delayed postharvest leaf senescence of postharvest Chinese flowering cabbage may be mediated, at least in part, by maintaining ROS homeostasis through restraining RBOHs-catalyzed ROS production and enhancing the activity of ROS-scavenging system including major antioxidant enzymes and AsA-GSH cycle.