Exogenous methyl jasmonate (MeJA) enhances the tolerance to cadmium (Cd) stress of okra (Abelmoschus esculentus L.) plants

Methyl jasmonate (MeJA), as a plant growth regulator, which is involves in many morphological and physiological processes, plays an important role in defense systems of plants under abiotic stress conditions. Our work revealed the physiological and biochemical mechanisms involved in enhancing cadmium (Cd) stress tolerance of okra (Abelmoschus esculentus L.) plants through the use of MeJA. The 50 mu M-MeJA okra plants had good growth and increased physical size and their fresh weight (FW) and dry weight (DW) were increased by 163 and 294%, respectively, under CdCl2 stress, while okra plants without using MeJA died based on pot-planting experiment. Okra plants with 50 mu M MeJA, 5 mM diphenyleneiodonium (NADPH oxidase inhibitor, DPI) and 50 mu M MeJA + 5 mM DPI, respectively, showed good growth and increased physical size and their FW and DW were increased by 144%, 164% and 178%, and 85%, 112% and 144%, respectively, under CdCl2 stress, while okra plants without using MeJA died. A higher accumulation of jasmonic acid (JA) and abscisic acid (ABA) level was observed in okra plants treated with MeJA under CdCl2 stress. The activities of lipoxygenase and nine-cis-epoxycarotenoid dioxygenase were significantly enhanced in okra plants treated with MeJA under CdCl2 stress. According to stress resistance related component analysis, okra plants after MeJA treatment showed a significant increase in osmotic adjustment substance (proline, soluble protein and soluble sugar), photosynthetic pigment (chlorophyll a and b, total carotenoids) and antioxidants, such as glutathione (GSH) and ascorbic acid (AsA) and a significant decrease in oxidative stress indicators, such as hydrogen peroxide (H2O2), superoxide radicals (O2-), malondialdehyde (MDA) content and electrolyte leakage rates. During CdCl2 stress, the MeJA-treatment okra plants showed a remarkably increase in the activities of antioxidant enzyme, such as superoxide dismutase, glutathione peroxidase, catalase (CAT), glutathione reductase (GR), ascorbate peroxidase (APX) and peroxidase. As a result, there was a marked reduction of H2O2, O2- and MDA content and a significant increase in CAT, GR and APX activities, as well as GSH and ASA content involved in the ASA-GSH cycle in MeJA-okra, DPI-okra and MeJA + DPI-okra plants under CdCl2 stress. These results demonstrate that exogenous MeJA increases the accumulation of JA, ABA and proline, which enhances Cd stress tolerance of okra plants, by regulating endogenous hormones metabolism, osmotic adjustment substances, photosynthesis pigment and ROS metabolism. It might provide breeders with practical tools to alleviate Cd toxicity in agricultural crops based on this knowledge.