Mechanism Of Chitosan Nanoparticles Promoting Sesame Plant Resistance Against Spodoptera Litura

Plant diseases and insect pests pose a serious threat to crop production and sustainable agricultural development.Although traditional chemical pesticides have made great contributions to pest management,the long-term overuse and abuse of pesticides will not only increase the resistance of pests to pesticides,but also easily drain into the environment,which results in adverse effects on the environmental ecosystem and even human health.With the development of nanotechnology,safe and efficient engineered nanomaterials have great potential in crop pest control.Among them,chitosan nanoparticles（CNPs）have attracted extensive attention in agricultural pest control due to its renewable,low cost,low toxicity,and biodegradability.It has been reported that CNPs could improve plants defenses against abiotic and biotic stresses,but the effect and mechanism of CNPs on plant defenses against phytophagous insect was still unclear.Therefore,a meta-analysis of 58 existing studies was firstly performed to evaluate the effect of chitosan-mediated effects on plant defense against oxidative stress,and identified the key influence factors on the pest control effectiveness of chitosan.Subsequently,sesame plant,Spodoptera litura,chitosan nanoparticles were selected as the research objects to construct the optimal culture system for improving the resistance of sesame plant to S.litura.Finally,the mechanism of CNPs promoting sesame plant to S.litura was elucidated.The main research results are as follows:（1）Meta-analysis results showed that chitosan overall increased the antioxidant enzyme activity（40.6%）,antioxidant metabolite content（24.6%）,defense enzyme activity（77.9%）,and defense-related gene expression（103.2%）of plants under stress,thereby reducing oxidative stress（32.2%）,and increasing plant biomass（28.1%）,and yield（15.7%）.In addition,the effect of chitosan-mediated plant resistance against oxidative stress is mainly affected by the application method and dose of chitosan.（2）In the optimal system experiment for CNPs improving the resistance of sesame plant against S.litura,the results showed that 100 mg/L was the optimal concentration for improving the resistance of sesame plant against S.litura,such effec was better than that of bulk chitosan.Compared with the control,foliar spray with 100 mg/L CNPs increased the shoot biomass（56.4%）,shoot height（60.2%）and chlorophyll content（62.3%）of sesame plant,while significantly reduced the growth weight of S.litura（32.3%）.Moreover,this anti-insect effect was not caused by the direct toxicity of CNPs to S.litura.（3）In the mechanism of CNPs promoting the resistance of sesame plant against S.litura experiment,the results showed that the inhibition effect of CNPs on the growth of S.litura was equivalent to that of traditional pesticide acetamiprid（100 mg/L）.The results showed that CNPs could alleviate the oxidative stress caused by insect by increasing the activities of antioxidant enzymes including superoxide dismutase（SOD）（70.2%）,peroxidase（POD）（88.5%）and catalase（CAT）（78.9%）,respectively,as compared with the control.In addition,CNPs increased plant resistance to S.litura by activating Ca²⁺influx（166.9%）and phytohormone including jasmonic acid（JA）（54.3%）,salicylic acid（SA）（76.4%）and abscisic acid（ABA）（33.8%）defense signals.Also,CNPs up-regulated the expression of key genes involved in the biosynthesis of the insect resistant substances（sesamolin and shanzhiside methyl ester）,and promoted the content of sesamolin and shanzhiside methyl ester,ultimately improving the resistance of seame plants to S.litura.Finally,the results of field experiments showed that although CNPs did not significantly increase the yield of sesame,it increased the nutritional substances in sesame seeds（such as sesamin,sesamolin,proline,valine）,thereby improving the nutritional quality of sesame seeds.