Studies On The Mechanisms Of Bacillus Amyloliquefaciens TNX-1 Inhibiting Disease Development Of Harvested Litchi Fruit Infected By Peronophythora Litchii

Litchi(Litchi chinensis Sonn.)is a featured fruit in tropical and subtropical countries or areas.It has been favored by the consumers for it shigher nutritional value and good flavor,hence promising higher commercial potential.However,litchi fruit is harvested at the period with high temperature and humidity.This makes the fruit vulnerable to pathogen infection and consequent fruit rot.It is not conducive to maintaining the edibility and commodity value of harvested litchi fruit.(Peronophthora litchi,P.litchi)was found to be a key pathogen fungus leading to postharvest disease of litchi fruit.Bacillus amyloliquefaciens strain TNX1,was isolated as a biologic antagonist with strong antifungal activity by our research group.Therefore,this effort investigated the effects of B.amyloliquefaciens TNX-1 on the disease development of harvested litchi fruit caused by P.litchii,regarding the metabolisms of respiration,reactive oxygen species,membrane lipids,energy,and antifungal substance with’Wuye’ litchi fruit as matrial.The objectives of this paper were to put forward a possible mechanism for B.amyloliquefaciens TNX-1 inhibiting disease development of harvested litchi fruit infected by P.litchii,and to provide the scientific theoretical foundation and guidanceto control litchi fruit’s postharvest disease.The main results are listed as below:1.Effect of B.amyloliquefaciens TNX-1 on diseases of litchi fruit caused by P.litchii and its relationship to physiology,quality and storibility.Compared with P.litchii-inoculated fruit,B.amyloliquefaciens TNX-1 effectively retarded the rapid increase of respiration rate and cell membranepermeability in P.litchii-inoculated litchis,delayed the decrease of chlorophyll and carotenoid contents,andsubsequently maintained a better appearance and color quality.It also significantly kept higher contents of total soluble solids(TSS),titratable acid(TA),total soluble sugars and vitamin C in pulp of litchi fruit,and subsequently delayed pericarp browning,reduced the decay of litchi fruits and kept higher commercially acceptable fruit rate.2.Effect of B.amyloliquefaciens TNX-1 on diseases of litchi fruit caused by P.litchii and its relationship to respiratory metabolism.Compared with P.litchii-inoculated fruit,B.amyloliquefaciens TNX-1 effectively decreased the activities of pericarp respiratory terminal oxidase including cytochrome C oxidase(COX)and ascorbic acid oxidase(AAO),as well as activities of succinodehydrogenase(SDH)andphosphohexose isomerase(PGI),but maintained the total activity of 6-phosphaogluconate dehydrogenase(6-PGDH)and glucose-6-phosphate dehydrogenase(G-6PDH),as well as the activity of nicotinamide adenine dinucleotide kinase(NADK),restrained the level of oxidized form of nicotinamide adenine dinucleotide(NAD)and its reduced form(NADH),thus maintained the content of oxidized form of nicotinamide adenine dinucleotide phosphate(NADP)and its reduced form(NADPH),maintained a higher NAD/NADH ratio,and reinforced pentose phosphate pathway.3.Effect of B.awyloliquefaciens TNX-1 on postharvest diseases of litchi fruit caused by P.litchii and its relationship to reactive oxygen species metabolism.For the pericarp of P.litchii-inoculated fruits treated with B.amyloliquefaciens TNX-1,the O2-· production rate and malondialdehyde(MDA)content were effectively retarded,comparing with P.litchii-inoculated fruit in the later period of storage.Furthermore,the activities of reactive oxygen scavenging enzymes like superoxide dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX)were largely promoted.In addition,the reduce of reactive oxygen scavenging substances such as ascorbic acid(AsA)and glutathione(GSH)were further increased,and the 1,1-diphenyl-2-picrylhydrazyl(DPPH)scavenging radical ability and total reduction ability were further maintained.These maintained the balance between reactive oxygen species(ROS)production and scavenging,reduced the degree of membrane lipid peroxidation,maintained the structural integrity of the cell membrane and delayed the aggravation of the disease.4.Effect of B.amyloliquefaciens TNX-1 on diseases of litchi fruit caused by P.litchii and its relationship to membrane lipid metabolism.Compared with P.litchii-inoculated fruit,B.amyloliquefaciens TNX-1 treatment effectively retarded the rapid increase in the activities of fatty acid degrading enzymes including phospholipase D(PLD),lipase and lipoxygenase(LOX),alleviated the degradation of membrane phospholipids like phosphatidylcholine(PC)and phosphatidylinositol(PI),as well as the degradation of unsaturated fatty acids like oleic acid(C18:1),linoleic acid(C18:2)and linolenic acid(C18:3),delayed the increase of phosphatidic acid(PA),as well as the relative contents of saturated fatty acids like capric acid(C10:0),palmitic aicd(C16:0)and stearic acid(C18:0),consequently maintained higher index of unfatty acid(IUFA)and fatty acid(U/S),delayed the membrane structural damage and protected the membrane structure and function.5.Effect of B.amyloliquefaciens TNX-1 on postharvest diseases of litchi fruit caused by P.litchii and its relationship to energy metabolism.For the pericarp of P.litchii-inoculated fruits,B.amyloliquefaciens TNX1 treatment significantly kept higher level of ATP and ADP contents,as well as energy charge,kept the activities of Ca2+-ATPase,Mg2+-ATPase and H+-ATPase across the membrane of protoplasm,vacuole and mitochondria,leading to maintain the cellular Ca2+ and H+ concentrations balance,protected the membrane structure and delayed the rise of disease index.6.Effect of B.amyloliquefaciens TNX-1 on diseases of litchi fruit caused by P.litchii and its relationship to antifungal substance metabolism.Compared with P.litchii-inoculated fruit,B.amyloliquefaciens TNX-1 treatment effectively retarded the rapid decrease in the content of lignin in pericarp and the activities of anti-pathogen enzymes like phenylalanine ammonia-lyase(PAL),peroxidase(POD),polyphenol oxidase(PPO),chitinase(CHI)and β-1,3-glucanase(GLU)in the middle and late storage,leading to maintained disease resistance of P.litchii-inoculated fruits.