The pretreatment of cucumber with methyl jasmonate regulates antioxidant enzyme activities and protects chloroplast and mitochondrial ultrastructure in chilling-stressed leaves

To elucidate further the physiological mechanism of chilling stress mitigation by exogenous methyl jasmonate (MeJA), seedlings of Cucumis sativus cv. Jinchun no. 4 were pretreated with 100 μM MeJA for 1 d and then were subjected to normal (25/18 °C) or chilling (15/8 °C) temperatures for 2 d. It was investigated to determine whether MeJA could protect chloroplast and mitochondrial ultrastructure from chilling and whether this protection was associated with regulation of antioxidant enzymes. The chilling stress inhibited plant growth and disintegrated the membranes of 86.1% chloroplasts and 79.0% mitochondria in leaf cells, and it also increased the levels of superoxide radicals (O2−) and hydrogen peroxide (H2O2) in leaves. When the MeJA-pretreated cucumber seedlings were exposed to chilling, the growth inhibition was alleviated, the disintegrated membranes of chloroplasts and mitochondria were separately decreased to 20.2% and 21.3%, and the increases of H2O2 and O2− levels were repressed as well, while the decrease in the activities of superoxide dismutase (SOD, EC 1.15.1.1), guaiacol peroxidase (GPX, EC 1.11.1.7), glutathione peroxidase (GSH-Px, EC 1.11.1.9), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) and glutathione reductase (GR, EC 1.6.4.2) were inhibited. The MeJA pretreatment may help regulate the antioxidant enzyme levels in chilling-stressed cucumber leaves, which represses the increase of H2O2 and O2− levels, protects ultrastructure of chloroplasts and mitochondria at chilling temperatures, and enhances the chilling tolerance of cucumber seedlings.

► MeJA pretreatment alleviates growth inhibition of cucumber caused by chilling.
► MeJA protects chloroplasts and mitochondria ultrastructure in chilling-stressed cucumber leaves.
► MeJA represses increases in O2− and H2O2 levels at chilling temperature.
► MeJA inhibits decreases in antioxidant enzyme activities under chilling stress.
► Exogenous MeJA enhances chilling tolerance of cucumber seedlings.