Melatonin alleviates cold-induced oxidative damage by regulation of ascorbate-glutathione and proline metabolism in melon seedlings (Cucumis melo L.)

Cold stress is one of the most detrimental environmental factors affecting plant growth and development. Melatonin (MEL), a natural indoleamine compound, responds to various environmental cues. To explore the role of MEL in the response of melon (Cucumis melo L.) seedlings to cold stress, the effects of exogenous MEL on the ascorbate-glutathione (AsA-GSH) cycle and proline metabolism were investigated. Melon seedlings were sprayed with various concentrations of MEL (0, 50, 100, 200, or 400 mu M), then exposed to cold stress, 12/6 degrees C (day/night) for 7 d, followed by recovery at 28/18 degrees C for another 7 d. The results showed that MEL, especially the 200M treatment, dramatically alleviated growth inhibition caused by cold stress, manifested by increased plant growth and decreased O-2(center dot-) production rate and malondialdehyde content. Importantly, exogenous application of MEL enhanced the ratios of reduced and oxidized forms of AsA (AsA/DHA) and GSH (GSH/GSSG), and the activities of ascorbate peroxidase (APX), glutathione reductase (GR), and dehydroascorbate reductase (DHAR) involved in the ascorbate-glutathione (AsA-GSH) cycle in melon seedings grown under cold stress. Besides, MEL pretreatment further increased the contents of proline and soluble protein under cold stress. The results reveal that protective effects of MEL against cold stress in melon seedlings are most likely associated with the regulation of the AsA-GSH cycle and proline metabolism as an effective antioxidant system.