Mechanistic Study On The Strengthening Of Bacterial Soft Rot Resistance In Lettuce By Elemental Sulfur Nanomaterials

Crop diseases seriously threaten global food security,causing a 20-40% reduction in global food production.At the same time,global climate change provides a more suitable environment for crop pathogens,which will aggravate the occurrence of crop diseases.In recent years,traditional metal-based Nanomaterials(NMs)have emerged in the control of crop diseases,but the long-term application of metal-based NMs will not only increase the drug resistance of pathogenic bacteria,but also cause environmental risks and human health risks.Therefore,it is an urgent problem to develop environment-friendly NMs to control crop diseases.It has been found that environmentally friendly elemental sulfur(S)NMs can control tomato fungal diseases(Fusarium wilt).However,compared with fungal diseases,crop bacterial diseases are more destructive.In this study,lettuce was used as the test crop,and Pectobacterium carotovorum subsp.carotovorum was used as the test pathogen to explore the performance and potential mechanism of S NMs in regulating bacterial soft rot of lettuce.The main results are as follows:(1)Through hydroponic experiment,it was found that the incidence of lettuce soft rot could be significantly reduced by applying 10-100 mg/L S NMs to leaves.Among them,leaf application of 100 mg/L S NMs had the best inhibition rate(decreased by 94%).In addition,leaf application of 100 mg/L S NMs can significantly increase lettuce biomass(shoot increased by 67%,root increased by 16%)and chlorophyll content(chlorophyll a increased by 8%,chlorophyll b increased by 16 %).Therefore,in this study,100 mg/L S NMs was used as the optimal concentration to control bacterial soft rot of lettuce.(2)At the concentration of 100 mg/L leaf application,the effects of different S-based materials on lettuce soft rot were compared,including: S NMs,large particle sulfur(S BPs),sulfur ions(S ions)and sulfur-containing pesticides(thiophanate-methyl,the guiding concentration was 1000 mg/L).The performance of different S-based materials in controlling lettuce soft rot is in the order of S NMs ≈ pesticide > S BPs > S ions.Among them,the disease control efficiency of S NMs is 1.33 times and 3.20 times that of S BPs and S ions,respectively,and the aboveground and underground biomass of S NMs treatment group is1.25 times and 1.17 times that of pesticide treatment,respectively.To sum up,S NMs is superior to other S-based materials in controlling lettuce soft rot.(3)Subsequently,this study explored the specific mechanism of S NMs regulating lettuce soft rot.The main mechanisms were as follows:(I)S NMs activated systemic induced resistance and systemic acquired resistance mediated by jasmonic acid(JA)and salicylic acid(SA),thus up-regulating the expression of disease-related genes(up-regulating by 29-260%);(II)S NMs enhanced the activity of antioxidant enzymes and the expression of antioxidant-related genes(increased by 68-327%),and alleviated the oxidative stress induced by pathogenic bacteria;(III)S NMs has direct bacteriostatic ability.In addition,compared with the infected control,the levels of total flavonoids,vitamin C and soluble protein in S NMs treatment group were significantly increased by 102%,192% and 21% respectively.(4)At the concentration of 100 mg/L,this paper continued to explore the effects of different sulfur-based materials and traditional preservative(sodium metabisulfite)on lettuce preservation.The results showed that S NMs significantly reduced the weight loss rate of lettuce(decreased by 25%)and increased the soluble protein and total phenol content of lettuce(6%-48%),which showed a good effect in prolonging the fresh-keeping of lettuce.To sum up,this study found that foliar application of S NMs can inhibit the occurrence of bacterial soft rot of lettuce by up-regulating crop disease resistance.At the same time,S NMs can improve the fresh-keeping quality of lettuce.Therefore,leaf application of S NMs not only improves the nutritional quality,yield and production economy of lettuce,but also reduces the environmental risks related to the use of conventional pesticides.The results of this study provide useful insights for sustainable management strategies of bacterial diseases.