Study On The Biocontrol Efficacy Of Pichia Caribbica Combined With Oligochitosan Against Postharvest Diseases Of Tomato And The Involved Mechanisms

Tomatoes are easily susceptible to pathogens caused postharvest diseases during storage,transportation and sales,which results fruit decay and causes huge economic losses.Alternaria alternata is the pathogen causing black spot of postharvest tomatoes,which not only causes fruit decay,but also produces mycotoxins which is harmful to human health.The use of antagonistic microorganisms for biological control of postharvest diseases in fruits and vegetables has broad application prospects due to its advantages such as safety and environmental protection.However,antagonistic microorganisms alone often fail to achieve the biocontrol efficacy of chemical fungicides,and thus it is difficult to achieve commercial application.Therefore,we investigate the control efficacy of antagonist of Pichia caribbica in combination with oligosaccharides（OCH）against black spot and natural decay of postharvest tomatoes,further study the physiological and molecular mechanisms of the antagonistic yeast combined with OCH in controlling postharvest diseases of tomatoes.The main results of the paper are as follows:（1）The results of biocontrol experiments in vivo showed that different concentrations of OCH had no significant control effect on the black spot of postharvest tomatoes.P.caribbica combined with OCH at the concentration of 2 mg/m L showed significant control efficacy against black spot of fruits compared with the control group and the group treated with antagonistic yeast alone,and the control efficacy against postharvest natural decay was also significantly improved with increasing the storage quality of tomatoes.（2）The physiological mechanisms involved in controlling postharvest disease of tomatoes with the treatment of P.caribbica combined with OCH include two aspects:on the one hand,OCH can promote the colonization and growth of P.caribbica in tomato wounds and reduce the accumulation of reactive oxygen species（ROS）in antagonistic yeast,and thus improving the biocontrol efficacy of the antagonistic yeast,on the other hand,compared with the treatment with antagonistic yeast alone,P.caribbica combined with OCH can further increase the activities of the enzymes related to resistance（polyphenoloxidase,phenylalanine ammonia lyase,β-1,3-glucanase and chitinase）,the contents of resistance-related substances.Furthermore,the activities of enzymes related to ROS production（NADPH oxidase）and ROS scavenging（superoxide dismutase,peroxidase,catalase,ascorbate peroxidase,mono-dehydroascorbate reductase,dehydroascorbate reductase,glutathione peroxidase,glutathione reductase and glutathione S-transferase）,as well as the contents of substances related to ROS scavenging（ascorbic acid and glutathione）were further enhanced,which can further reduce the accumulation of ROS（O₂ˉ·and H₂O₂）and the degree of membrane lipid peroxidation,and thus further enhancing tomatoe resistance to diseases.（3）The molecular mechanism involved in controlling postharvest disease of tomatoes with the treatment of P.caribbica combined with OCH was analyzed based on transcriptome technology.Compared with the control,4552 differentially expressed genes（DEGs）in tomatoes treated with P.caribbica combined with OCH were screened out.According to the function and pathway analysis of these DEGs,antagonist combined with OCH could enhance the fruit disease resistance through inducing the expression of genes participating in the signal transduction pathways such as Ca²⁺,MAPK,plant hormones（such as auxin,ethylene,jasmonic acid and salicylic acid）signaling pathways and downstream defense response,ROS scavenging and the synthesis of secondary metabolites related to resistance in tomatoes.Fuethermore,compared to the treatment with P.caribbica alone,there were 292 DEGs in tomatoes treated with P.caribbica combined with OCH.According to the function and pathway analysis of these DEGs,the treatment of antagonsit in combination with OCH could further improve the fruit resistance through inducing the expression of genes involving inα-linolenic acid metabolism,the signal transduction pathways such as Ca²⁺,MAPK and plant hormones（such as auxin,ethylene,jasmonic acid and salicylic acid）signaling pathways and the synthesis of secondary metabolites related to resistance in tomatoes.