Effect Of Substances Of Biocontrol Yeast Cell Wall On Induced Resistance Of Fruit

Fungal disease is a main factor leading to postharvest losses of fruits.The antagonistic mechanisms of biocontrol yeast,an important antagonist of postharvest biological control,comprise four processes of:nutrient and space competition,secretion of extracellular substances,hyperparasitism and induced resistance.However,the material bases for induced resistance are yet to be reported.This paper used Cryptococcus laurentii and Rhodosporidium paludigenum as research targets and Saccharomyces cerevisiae as control,aiming to investigate the effects of inactivated yeasts and yeast cell walls on induction of disease resistance of fruits and the main influence factors;analyze the effects of inactivated yeasts and yeast cell walls on pathogen,resistance-related physiological and biochemical indexes and gene expressions of fruit to determine the possible mechanisms involved;preliminarily reveal the material bases for induced resistance by comparing the efficacy of substances in yeast cell wall on inducing resistance.The main research results are as follows:(1)S.cerevisiae,C.laurentii and R.paludigenum could directly inhibit blue mold caused by Penicillium expansum in pear fruit.After inactivating,the yeasts could not directly suppress the disease,but the yeast inactivated by autoclaving could induce resistance of pear fruit against blue mold.And the effect of inactivated R.paludigenum was the best,and the effect was related to its concentration,the induction time and the concentration of pathogen spores.Inactivated R.paludagenum at a certain concentration(1×106-1×107 cells/mL)could induce resistance of cherry tomato against Alternaria alternata.The results of antifungal tests showed that inactivated R.paludigenum at 1×10-5-1×108 cells/mL could not inhibit the growth and spore germination of P.expansum in vitro,whilst it could inhibit the spore germination of P.expansum in the pear fruit wounds at 1×107 cells/mL when the spores were inoculated 24 h after inactivated yeast treatment.(2)The determination of resistance-related physiological and biochemical indexes in pear fruit showed that inactivated R.paludigenum could significantly stimulate the activities of superoxide dismutase(SOD),catalase(CAT),peroxidase(POD)and phenylalanine ammonia lyase(PAL)and reduce the content of malondialdehyde(MDA).These results indicated that the physiological mechanisms of induced resistance by inactivated yeasts were related to the clear of excessive reactive oxygen species(ROS)to alleviate the intracellular oxidative damage and the metabolism of phenylpropane pathway.However,the activation of the ?-1,3-glucanase(GLU)and chitinase(CHI)by inactivated R.paludigenum was not obvious(3)The effects of yeast cell walls on induction of resistance of pear fruit against blue mold were better than inactivated yeasts.There was no significant difference in the induction effect between different yeast cell walls,but the cell wall of C.laurentii and R.paludigenum was the best.Cell wall proteins had no significant effect on the induction effect and cell wall polysaccharide played a major role.The determination of resistance-related enzymes in pear fruit showed that the co-treatment with cell wall of R.paludigenum and P.expansum quickly activated the activities of GLU and CHI,which was significantly higher than those in control and treatment with cell wall or with P.expansum alone.The results showed that there was Priming mechanism in the resistance induced by yeast cell wall.The expression of PR gene in pear fruit showed that the cell wall of R.paludigenum stimulated the PR1 gene up-regulated expression,and it was more significant when pathogen was inoculated,indicating that cell wall treatment could induce systemic acquired resistance(SAR);The GLU gene and CHI4 gene were also involved in the resistance reaction and also showed Priming effect at the gene level,but CHI3 gene might not participate in the resistance reaction.(4)S.cerevisiae,C.laurentii and R.paludigenum showed significant differences in cell surface characteristics and cell wall structure.The folds and fibrous extensions on the surface of biocontrol yeast cells might be related to the antagonistic activities.Autoclaving destroyed the interior of yeast cells but had no effect on the cell structure,and the cell wall remained intact.The compositions of cell wall from different species of yeast were different,and the chitin in cell wall might play an important role in the induction of fruit resistance.Induced resistance by yeast cell wall was not only a material effect,but also might be related to the structure and connection of polysaccharides.