The regulatory effect of melatonin on physiological, biochemical and molecular parameters in cold-stressed wheat seedlings

We investigated the possible mediatory role of melatonin in protecting wheat plants from cold stress. Ten-day-old wheat seedlings were pretreated with 1 mmol l⁻¹ melatonin for 12 h and subsequently exposed to stress conditions at 5/2 °C (day/night) for 3 days. Cold stress caused serious reductions in leaf surface area, water content, and photosynthetic pigment content, whereas melatonin application attenuated these reductions. Accumulation of reactive oxygen species (ROS), including superoxide and hydrogen peroxide, was very high in cold-stressed plants and caused lipid peroxidation in membranes. Concomitantly, ROS damaged the DNA profile and negatively influenced expression and/or activity of many enzymes, including RuBisCo. When compared to controls, cold-stressed plants had higher activities of the antioxidant enzymes superoxide dismutase, guaicol peroxidase, ascorbate peroxidase, and glutathione reductase and higher levels of the antioxidant compounds total ascorbate, reduced ascorbate, total glutathione, reduced glutathione, and phenolic substances; however, this elevation could not cope with the destructive effects of cold stress. Melatonin-pretreated plants exhibited greater increases in these parameters comparison with untreated cold-stressed plants. Isozyme bands monitored in native gel and RuBisCo expression supported these changes. Also, due to the cold-induced increase in dehydroascorbate and oxidized glutathione, the corrupted redox status in the cell was ameliorated by melatonin application. Similarly, levels of the osmoprotectants total soluble protein, carbohydrate, and proline were also increased by cold stress; however, melatonin-applied seedlings had a higher content of these solutes in comparison to untreated cold-stressed plants. We suggest that melatonin can improve plant resistance to cold stress in wheat seedlings by directly scavenging ROS and by modulating redox balance and other defence mechanisms.