Changes in the Activities of Protective Enzymes Induced by Mepiquat Chloride (DPC) in Cotton to Defend Against Aphids

Abstract

Background: Mepiquat chloride (DPC) enhances the resistance of cotton plants, and it is widely used as a growth regulator. DPC can stimulate photosynthesis, stabilize the structure of cotton leaves, and affect population reproduction and energy substances in cotton aphids, but interactions between DPC and cotton aphids remain unclear. In this study, we analyzed the physiological responses of cotton to DPC, and the toxicity of DPC toward cotton aphids, before and after feeding, to explore the DPC-induced defense mechanism against cotton aphids.Results: Measurements of protective enzyme activity in cotton showed that the soluble protein contents, peroxidase (POD) activity, and catalase (CAT) activity in cotton treated with different concentrations of DPC were higher than in the control. Superoxide dismutase (SOD) activity was higher than that of controls when the concentration of DPC was <0.1 g/L. Under aphid stress, POD activity of cotton treated with a low insect population density was significantly lower than that of controls, but the reverse was true for cotton treated with a high insect population density, and SOD activity was positively correlated with population density. The activities of detoxification enzymes in field and laboratory experiments showed that DPC promoted the specific activity of glutathione S-transferase (GST) in cotton aphids, while the specific activities of carboxylesterase and acetylcholinesterase were decreased.Conclusions: DPC enhanced the resistance of cotton by increasing the activity of protective enzymes. It also had a toxic effect on cotton aphids by increasing GST activity (the main DPC target) and lowering carboxylesterase and acetylcholinesterase activities. DPC increased the soluble protein content and SOD activity in cotton under aphid stress, and thereby enhanced tolerance to cotton aphids.