Biocontrol Efficacy And Mechanism Of Two Antagonistical Yeasts Against Postharvest Brown Rot In Plum Fruit

‘Qingcui' plum with green skin is a commercially important cultivar in southwest China.The fruit is mainly harvested in summer(July to August each year),when the temperature is high and the air humidity is large.Plums are quite susceptible to postharvest diseases caused by a number of fungal pathogens,especially brown rot,which is caused by Monilinia spp.(syn: Monilia spp.).Current management of infectious disease still relies heavily on the use of synthetic chemical fungicides.However,the extensive application of these chemical fungicides result in negative effects on human safety and environment protection,as well as the increasing development of pathogen resistance to fungicides.Biocontrol methods involving microbial antagonists have potential as alternative measures to control postharvest diseases of fruits and vegetables.Among these antagonists,yeast that do not produce toxic secondary metabolites but have an inhibitory effect against pathogens is as one alternative to unpopular synthetic fungicides and have been pursued actively.In the present study,we evaluated the biocontrol activity of P.membranaefaciens and K.apiculata against the major postharvest pathogen(Monilinia fructicola)on plum fruits and studied the biocontrol mechanisms.Besides,we also screen the nutrients to combine with the yeasts in order to increase the biocontrol effect.The main research findings were summarized as follows:(1)Effect of P.membranaefaciens and K.apiculata on the control of brown in plum fruit of M.fructicola showed that: When inoculated in the same wounds with M.fructicola,5×108 cells/mL of P.membranaefaciens or K.apiculata resulted in a remarkably lower disease incidence brown rot compared with the control.At the temperature of 20 °C,P.membranaefaciens played better disease control efficacy than K.apiculata,but when the temperature was 0 °C,K.apiculata was more effective;When yeasts were inoculated in different wounds with M.fructicola,no matter at 0 °C or 20 °C,P.membranaefaciens played the highest disease control efficacy during the whole storage period.When M.fructicola was inoculated after the plum soaking in yeast suspension,P.membranaefaciens and K.apiculata attached to the surface of plum and used the nutrients on the fruit surface to grow and reproduce.That could reduce the occurrence of brown rot.(2)When M.fructicola was not inoculated,the yeast population rapidly increased and then remained relatively steady.At 20 °C,the number of P.membranaefaciens is more than that of K.apiculata,and at 0 °C,it showed the opposite result.When M.fructicola was inoculated,the presence of pathogen did not affect the growth of antagonistic yeast,but it had a catalytic effect.That indicated the antagonistic yeasts had competition with the pathogen on nutrition and space.(3)P.membranaefaciens and K.apiculata could inhibit the growth of M.fructicola.When cultivated together,P.membranaefaciens or K.apiculata significantly reduced the mycelial growth and spore germination of M.fructicola;P.membranaefaciens and K.apiculata could gather and touch the surface of M.fructicola and produced extracellular chitinase(CHI)and ?-1,3-glucanase(GLU)to damage the cytoderm of the pathogen.It indicated that P.membranaefaciens and K.apiculata had parasitic effects on M.fructicola;P.membranaefaciens and K.apiculata could also release the volatile organic compounds(VOCs)to significantly inhibite the mycelial growth and spore germination of M.fructicola in vitro and in vivo.(4)Pichia membranaefaciens and Kloeckera apiculata could activates the phenylpropanoid pathway to strengthen the cell wall and prevented pathogenic invasion.Both of the yeasts significantly increased the activities of the related enzymes such as phenylalanine ammonia lyase(PAL),cinnamic acid 4-hydroxylase(C4H),4-coumaroyl-CoA ligase(4CL)and peroxidase(POD).That enhanced the biosynthesis of active metabolites including lignin,flavonoid and total phenolics.(5)Exogenous carbohydrates and amino acids enhanced the control effect of P.membranaefaciens and K.apiculata on brown rot of plum.Besides,they promoted the growth dynamics of P.membranaefaciens and K.apiculata.(6)In vitro,P.membranaefaciens used sugar(carbon source): glucose,fructose and amino acid(nitrogen source): glutamic acid,leucine and aspartic acid in large number.The main nutrients used by K.apiculata were: sugar: glucose,fructose;amino acids: L-threonine,L-proline and L-aspartic acid.K.apiculata was more stringent for nitrogen sources.M.fructicola could use all sugars and amino acids involved in the experiment,but grew slowly in medium with L-histidine or L-aspartic acid as the sole nitrogen source.(7)Glucose and L-aspartic acid could enhance the efficacy of P.membranaefaciens in reducing the brown decay in plum fruit,besides,glucose had the same synergistic effect on K.apiculata.