Induction effects of jasmonic acid on tannin content and defense-related enzyme activities in conventional cotton plants
茉莉酸对棉花单宁含量和抗虫相关酶活性的诱导效应

Cotton (Gossypium hirsutum L.) is an important crop worldwide challenged by a number of pests which cause enormous yield loss. Although extensive cultivation of transgenic Bt cotton has decreased the population growth of the most destructive pest, cotton bollworm (Helicoverpa armigera), the secondary pest like mirid bug has become a primary pest. In addition to the evolution of Bt toxin resistance in cotton bollworm, it is urgent to develop environment friendly and plant-derived pesticides for the integrated pest management. Induction of plant defense against pests is very common among higher plants. Jasmonic acid (JA) is a kind of plant growth regulator with a variety of biological function except for induced plant defense against herbivores. The objectives of the current study were to investigate the dosage- and exposure time-dependent induction of defense-related secondary metabolites and antinutritive/antidigestive enzymes in cotton seedlings by exogenous application of JA. The potential pest control nature of induced defense in seedlings of conventional cotton plants by JA was also discussed in terms of the fecundity, developmental time, mortality and parasitism of insects. We hypothesized that, as a ubiquitous phytohormone, JA should regulate the induced plant defense in cotton like those in some model plants including mustard, tobacco and tomato by increasing the contents of some feeding deterrent secondary metabolites and the activity of enzymes responsible for their synthesis, as well as the activities of antinutritive and antidigestive enzymes. Results of the present study suggested that 0.01 mmol/L, 0.1 mmol/L, and 1.0 mmol/L JA were effective concentrations in priming the pest resistance nature of six-foliage cotton seedlings within two weeks in terms of enhanced contents of cotton tannin and protease inhibitors (PIs) and increased activities of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD) and catalase (CAT). In comparison, 0.1 mmol/L jasmonic acid was more effective in inducing PIs, PPO, POD and CAT than the other two concentrations tested, while 0.1 mmol/L and 1.0 mmol/L JA were equally powerful in priming tannin and PAL, and both JA concentrations were more effective than 0.01 mmol/L JA. We also found that JA concentration, time after JA exposure and their interaction have significant effects on tannin content, PPO activity and POD activity, and that JA concentration and time after JA exposure have significant effect on PIs content and PAL activity, while JA concentration and its interaction with exposure time after JA exposure have significant effects on CAT activity. In addition, analysis of the duration of JA-mediated defense suggested that the optimal induction effects occurred one day after JA treatment for POD, 7 d for PIs, 14 d for tannin, 1 d and 7 d for PPO, 7 d and 14 d for PAL, while no significance was observed for CAT. In summary, JA can enhance cotton defense against insects by the ind