Effect Of Acid Stress On Biocontrol Efficacy Of Marine Antagonistic Yeast And Its Mechanism

The pathogenic microorganisms that cause the rot of harvested vegetables mainly include acidic and alkaline pathogenic fungi.Among them,acid pathogenic fungi occupies the main body of fungal diseases after harvest,causing the most serious losses.Acidic pathogenic fungi can secrete organic acids during their growth and infection process,acidify the environment and host tissues,activate their toxic factors,and inhibit the defense response of peel tissues.At present,among many researched methods that may replace chemical fungicides,the use of antagonistic microorganisms,especially antagonistic yeasts,is one of the most concerned new postharvest disease control methods at home and abroad.Antagonistic yeasts show good tolerance to environmental conditions such as low temperature,high temperature,dryness,low oxygen levels,pH fluctuations and ultraviolet radiation.However,when the organic acid reaches a certain concentration,it is proven to still cause stress to the growth of yeasts,inhibit their growth,and cause the lag period to extend.This thesis intends to study the growth,death and metabolism of antagonistic yeast under acid stress and its related mechanisms,in order to discover the stress and resistance mechanism of yeast under adversity.The main results are as follows:（1）The postharvest diseases of citrus fruit were mainly caused by P.digitatum,and the pH value of the fruit tissue infection point decreased from 4.25 to 3.98 before and after the onset.Among them,gluconic acid,malic acid and other pathogenic toxic factors were the main causes of pH changes,and the above organic acids were all effective carbon sources for the growth and reproduction of P.digitatum.At the same time,P.digitatum exhibits resistance to glucose and D-glucose.The preference for acid could grow into colonies with larger diameters.（2）Six organic acids,including ascorbic acid,oxalic acid,malic acid,gluconic acid,fumaric acid and citric acid,significantly inhibited the growth of the antagonistic yeast Rhodosporidium paludigenum at a concentration greater than 200 m M;D-gluconic acid and malic acid are reduced to 200 After m M or less,the reverse promotion effect could be achieved.Studies had found that the antagonistic yeast R.paludigenum had a better effect on the utilization of glucose and fructose.When the concentration was greater than 1×10⁶cells m L^-1,the antagonistic yeast could effectively control the occurrence of citrus green mold.During the fermentation process,adding 500 m M gluconic acid could not significantly affect the growth of the antagonistic yeast R.paludigenum Therefore,it could effectively enhance its ability to control citrus fruit green mold disease,reduced the incidence rate of fruit green mold disease,and reduce the occurrence of disease.（3）Research results showed that yeast co-cultured（domesticated）with 500 m M gluconic acid could effectively adapt to different acidic environments（D-gluconic acid,malic acid,citric acid,ascorbic acid,and oxalic acid）,and its survival rate was significantly improved.Compared with the control treatment,it increased by 21.2%,33.6%,19.4%,33.3%and 39.4%,respectively.The results of propidium iodide staining showed that the apoptotic rate of acid-acclimated yeast was significantly lower than that of normal yeast.Among them,the mechanism of domestication and acid tolerance was related to the reduction of intracellular mitochondrial damage and the suppression of reactive oxygen bursts in acid domesticated yeast.（4）In addition,different concentrations of pectin and cellulose could be used as protective agents against yeast to increase its survival rate on the surface of the fruit.When the concentration of pectin and cellulose was 0.5 g/100 m L,the antagonistic yeast R.paludigenum on the surface of citrus Best growth.Comparing the growth dynamics of red winter yeast pretreated with D-gluconic acid and untreated yeast on the surface of citrus fruits,it was found that there was no significant difference between the two,and pectin and cellulose were still better protective agents.