The Regulatory Mechanism Of Physiology,Biochemistry And Disease Resistance Mediated By Methyl Jasmonate In Postharvest Blueberries

Blueberry fruit is sweet and sour and has a delicate texture.In addition to being rich in nutrients,such as protein,dietary fiber,vitamins,and minerals,blueberry also contains a large number of biologically active substances,such as phenolic,flavonoids,anthocyanins,which are good for human health.Therefore,blueberries are favored by many consumers in our country.However,blueberries are mostly picked in the hot and rainy summer,coupled with the strong metabolism and rapid aging after picking,greatly reducing the nutritional value and economic value of blueberries.In addition,due to the lack of protection from the hard exocarp,blueberries are extremely susceptible to mechanical damage and pathogen infection during the picking,storage and transportation stages,leading to their decay and deterioration.Botrytis cinerea,the causal agent of gray mold,has been considered as one of the main factors leading to blueberry decay.Methyl jasmonate(MeJA),a naturally occurring compound,can mediate diverse developmental processes and defense responses against biotic and abiotic stresses.In this study,postharvest blueberry fruits were used as experimental materials.Firstly,the influence of exogenous MeJA treatment on the preservation effect of postharvest blueberry fruits was explored by analyzing nutrient content,antioxidant system,cell wall degradation,and energy metabolism.Secondly,the effects of MeJA treatment on the control of gray mold in blueberry fruit were evaluated from the level of physiological,biochemical metabolism,enzyme,gene,signal transduction and metabonomics.Our study aimed to provide some theoretical reference for the application of MeJA in the preservation of blueberry.The main results are shown as follows:1.The effects of postharvest MeJA treatment on the quality of blueberry fruit were investigated.Results indicated that MeJA treatment could delay the decrease of soluble solids,titratable acid and L*value in blueberry fruits,inhibit the increase of weight loss rate,and maintain the quality of blueberry after harvest.MeJA significantly increased the content of,phenolics,flavonoids,anthocyanin,ascorbic acid,reduced glutathione,and the activity of superoxide dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX).This enhances the scavenging ability of blueberry fruits to DPPH free radicals during storage,inhibits the accumulation of hydrogen peroxide(H₂O₂)in the late storage period,reduces the degree of membrane lipid peroxidation,and inhibits the increase of malondialdehyde(MDA)content.These results indicate that MeJA maintains the antioxidant capacity of blueberry fruits during storage,reduces oxidative damage and delays senescence,thereby maintaining fruit quality.2.The effects of MeJA treatment on the softening of postharvest blueberry fruits were studied.The results showed that 50?m MeJA could maintain the firmness of postharvest blueberry fruit.MeJA treatment inhibits the activity and transcription abundance of cellulase,polygalacturonase,pectin methylesterase and?-galactosidase,inhibits the increase of water-soluble pectin,and delay the decline of CDTA-soluble pectin and sodium carbonate-soluble pectin,hemicellulose and cellulose,thereby delaying the degradation of the blueberry fruit cell wall during storage.In addition,MeJA also induces the activities and gene expression levels of energy metabolism-related enzymes,such as H⁺-ATPase,Ca²⁺-ATPase,cytopigment C oxidase and succinate dehydrogenase,which promotes the accumulation of ATP and ADP in fruits and provides sufficient energy supply for cell wall repair and reinforcement.These results suggest that MeJA delays the softening of postharvest blueberry by restraining the degradation of cell wall materials and maintaining energy metabolism.3.The effects of MeJA treatment on the control of gray mold in blueberry fruit were evaluated.After MeJA treatment,the incidence index of blueberry fruits was significantly decreased,indicating that MeJA could enhance the defense response of blueberry fruits to Botrytis cinerea.No direct chemical effect of MeJA on antifungal activity against mycelium growth and spore germination of B.cinerea was observed.MeJA treatment increased the synthesis of NO and H₂O₂in the blueberry fruits inoculated with B.cinerea at the early stage of storage,but inhibited the accumulation of H₂O₂at the later stage of storage.MeJA also induced the activities of SOD,CAT,APX,CHI,Gl U,PAL,C4H and 4CL,increased the contents of phenolic substances and flavonoids,inhibited the increase of MDA content,and thus reduced the sensitivity of blueberry fruits to B.cinerea.These results suggested that MeJA could inhibit postharvest gray mold by inducing disease resistance of blueberry fruits.4.The effects of MeJA on the metabolites of blueberry fruits infected by B.cinerea were studied.The results of principal component analysis showed that MeJA treatment significantly changed the metabolic profile of blueberry fruits.On day 3 of storage,60 and 25 different metabolites were identified under POS and NEG mode,respectively.On day 7 of storage,43 and13 different metabolites were identified under POS and NEG mode,respectively.Cis-aconitate was significantly up-regulated during the whole storage process.KEGG enrichment analysis showed that the pathway to which the differential metabolites were enriched changed with storage time.At the third day,the five pathways with the most significant enrichment were glyoxylate and dicarboxylate metabolism,2-oxocarboxylic acid metabolism,biosynthesis of unsaturated fatty acids,citrate cycle and C5-Branched dibasic acid metabolism.At the seventh day,the five pathways with the most significant enrichment were citrate cycle,2-oxocarboxylic acid metabolism,glyoxylate and dicarboxylate metabolism,microbial metabolism in diverse environments and biosynthesis of amino acids.These results indicate that the emphasis of MeJA on blueberry disease resistance regulation will change over time.5.The involvement and the relationship between nitric oxide(NO)and hydrogen peroxide(H₂O₂)in MeJA-induced immune responses in blueberries against B.cinerea was explored.c PTIO treatment not only abolished the induction effect of MeJA on NO and H₂O₂,but also significantly inhibited the activities of NADPH oxidase,SOD,CAT,APX,CHI,GLU,PAL,C4H and 4CL,and down-regulating the expression levels of key genes VaPAL,VaCHS,VaCHI,VaF3H,VaFLS and VaDFR in phenylpropane pathway,resulting in a significant increase in the sensitivity of blueberry fruits to B.cinerea.DPI treatment can weaken the promotion of MeJA on the defense-related enzymes and phenylpropane pathways in blueberry fruits,and reduce the disease resistance of blueberries,but it has no significant effect on the activity of NOS and early NO accumulation induced by MeJA.These results indicated that NO and H₂O₂were involved in the MeJA-induced resistance in blueberries,and NO worked upstream of H₂O₂in this process.