Melatonin improves the postharvest anthracnose resistance of mango fruit by regulating antioxidant activity, the phenylpropane pathway and cell wall metabolism

Anthracnose caused by Colletotrichum gloeosporioides is a major postharvest disease of mango. Melatonin (MT) is an endogenous plant hormone that plays a crucial role in both biotic and abiotic stress responses. The objective of this study was to explore the impact of MT treatment on anthracnose disease of "Tainong" mango fruit through both in vivo and in vitro experiments. In vivo tests showed that 0.2 mmol L-1 MT clearly postponed the occurrence of anthracnose and effectively reduced the lesion diameter on inoculated mango fruit. However, in vitro tests showed that MT had no significant effects on mycelium growth and spore germination of C. gloeosporioides. Further in vivo analysis demonstrated that MT significantly increased the activities of enzymes involved in phenylpropanoid metabolism and pathogenesis related proteins, as well as the contents of flavonoids, anthocyanins, lignin and total phenols in mango fruit. MT treatment caused an obvious production in the initial hydrogen peroxide (H2O2) and nitric oxide (NO) contents in inoculated mango fruit, while it decreased their content at later stages by enhancing the activities of antioxidant enzymes. Furthermore, MT significantly reduced the activity of cell-wall hydrolases and the soluble pectin content, and slowed down the loss of cellulose and protopectin. Principal constituent analysis (PCA) and correlation analysis showed that MT enhanced the resistance of mango fruit to C. gloeosporioides by enhancing defense enzyme activities and the contents of secondary metabolites and inhibiting pectin hydrolysis. In summary, MT can be an effective alternative to fungicides aimed at controlling postharvest anthracnose.