Effects Of ε-poly-L-lysine (ε-PL) On The Pathogenicity Of Penicillium Expansum And Penicillium Digitatum Based On Organic Acid Metabolism

Postharvest rot of fruits caused by fungal infections is a global problem that causes huge economic losses every year.Penicillium expansum and Penicillium digitatum both belong to the genus Penicillium,which are two important postharvest pathogenic fungi.The blue mold caused by P.expansum causes the rot of apples,pears,cherries,kiwifruit and other fruits,and the green mold caused by P.digitatum leads to the rot of citrus fruits.Both of these bacteria are acidifying fungi,which can acidify the host tissue by secreting acidic substances to achieve the purpose of rapid infection.ε-poly-L-lysine(ε-PL)is an efficient biosynthetic fungicide with good effect and high safety,and is widely used in food sterilization.The antibacterial mechanism of ε-PL on fungi has been reported,but the effect of ε-PL on fungal pathogenicity based on organic acid metabolism has not been reported yet.In this thesis,P.expansum and P.digitatum were used as the target strains.First,the optimal growth and acid production environment of the two strains was clarified under laboratory simulation conditions.Secondly,the effects of ε-PL on acid production and pathogenicity of P.expansum and P.digitatum were explored from two aspects of in vitro growth and in vivo infection.Finally,the effects of ε-PL treatment on the growth and pathogenicity of P.expansum and P.digitatum were explored by proteomic method.Differences in pathogenic effects.The main results are as follows:1.The two Penicillium species have different adaptations to environmental conditions.Under the given conditions of the experiment,the most suitable medium for P.expansum growth and acid production is PDA medium with temperature of 25 °C and20 °C,p H of 4 and 10,respectively,in dark environment.The medium for P.digitatum growth and acid production was PDA.The temperature was 20 °C,the p H was 4,and 8,respectively,in the dark environment.2.ε-PL affects the pathogenicity of P.expansum through organic acid metabolism.When the concentration of ε-PL was 50 mg/L,the growth and acid production of P.expansum were inhibited,and when the concentration was 1000 mg/L or 2000 mg/L,the infection of P.expansum in fruits was effectively inhibited.After ε-PL treatment,the accumulation of gluconic acid was reduced,the activity of glucose oxidase was inhibited,and the rate of p H drop in the local environment was slowed down in apple fruit.In addition,cellulase,pectinase,and polygalacturonase activities in different regions of the extended P.expansum-apple interaction were also affected by p H changes after ε-PL treatment.The results showed that ε-PL inhibited the pathogenicity of P.expansum by affecting the accumulation of gluconic acid in fruit tissue and slowing the p H drop.3.ε-PL affects the pathogenicity of P.digitatum through organic acid metabolism.When the concentration of ε-PL was 50 mg/L,the growth and acid production of P.digitatum were inhibited,and when the concentration was 2500 mg/L,and 5000 mg/L,it could effectively inhibit the growth of P.digitatum on citrus fruits.of infection.After ε-PL treatment,the secretion of citric acid by P.digitatum in citrus fruits was reduced,the rate of local p H drop was slowed down,decreased activities of TCA-related enzymes citrate synthase,α-ketoglutarate dehydrogenase and isocitrate dehydrogenase.In addition,the activities of cellulase,polygalacturonase,and pectinase in different regions of interaction were also inhibited after ε-PL treatment.The results showed that ε-PL inhibited the pathogenicity of P.digitatum by affecting the accumulation of citric acid in fruit tissues and slowing the p H drop.4.Proteomic analysis reveals ε-PL inhibited the protein expression of two kinds of Penicillium.Through analysis,it is found that ε-PL inhibits the pathogenicity of P.expansum for the following reasons: ε-PL may hinder the normal transcription and translation process of P.expansum;After ε-PL treatment,the protease related to glycolysis was down regulated,which may affect the normal energy flow supply by inhibiting the normal process of glycolysis;The down-regulation of transmembrane transport related proteins may be detrimental to the formation of Penicillium resistance;After ε-PL treatment,the pathogenic proteins in P.expansum were down regulated,which may inhibit the pathogenicity of P.expansum to some extent.The reasons why ε-PL inhibits the pathogenicity of P.digitatum are as follows: ε-PL treatment interfered with the DNA repair pathway in P.digitatum,which may hinder the normal physiological process of cells and even lead to their death;The down-regulation of proteins related to oxidative stress may lead to the accumulation of ROS in cells,which may be toxic to itself;ε-PL treatment may lead to the decline of mitochondrial function,the imbalance of energy metabolism and the imbalance of normal physiological metabolism in P.digitatum.At the same time,the down-regulation of the expression of TCA related enzyme citrate synthase may lead to the decline of citric acid secretion,affect the normal metabolic pathway of organic acids,and lead to the decline of the pathogenicity of P.digitatum.