Effect of exogenous melatonin on resistance of cucumber to downy mildew

Downy mildew, caused by Pseudoperonospora cubensis, is a major threat to cucumber (Cucumis sativus) production. An increase in the disadvantages associated with chemical controls has prompted the need for alternative protecting agents. We analyzed the effects of melatonin pretreatment on resistance of cucumber to downy mildew. Application of 100 mu mol L-1 melatonin enhanced the activity of antioxidant enzymes and the expression of antioxidant genes, thereby improving the antioxidant ability of those plants following inoculation. This pretreatment effectively reduced the level of malondialdehyde and relative electrolytic leakage, thus protecting membrane integrity. Melatonin also increased enzyme activities and expression of genes related to this pathogen. Therefore, the disease index calculated for cucumber seedlings was significantly reduced after such pretreatment. In addition, melatonin supplementation improved photosynthetic efficiency and the level of chlorophyll. After inoculation, the capacity for nitrogen metabolism also rose. These responses demonstrate the benefit of melatonin in supporting fruit yields and enhancing the resistance of cucumber to downy mildew.