Exogenous Methyl Jasmonate Inhibits Chilling Injury Of Peach Fruit With Different Flesh Types During Postharvest

Peach fruit(Prunus persica L.)is favored by consumers for its high nutritional value and sweet taste.However,peach fruit is not suitable for storage at room temperature because of its rapid softening and decay.At the same time,chilling injury(CI)is easy to occur in peach fruit at low temperatures.Therefore,finding an effective postharvest treatment to reduce the chilling injury of peach fruit and studying its mechanism will provid an important guide in prolonging the storage time at low temperature and breeding new varieties with a high cold tolerance.Methyl jasmonate(Me JA)is a natural plant growth regulator that has been studied to slow the ripening process and reduce the chilling injury in fruit and vegetables.However,the efficiency and optimal concentration of Me JA are different depended on peach varieties,and its mechanism is still unclear,which needs to be further clarified.In this research,transcriptome analysis in peach fruit during cold storage was conducted to determine the central metabolic pathways associated with CI formation.The effect of 10 μmol·L-1 Me JA and 100 μmol·L-1 Me JA on CI in peach fruit with different fleshes was studied.The stony hard peach "Yingshuanghong" and melting flesh peach "Yuhualu" were used as experimental materials.The regulation of exogenous Me JA treatment on endogenous α-linolenic acid metabolism and jasmonic acid(JA)signaling pathway in peach fruit and the relationship between Me JA treatment and CI formation were explored.The main results are as follows:(1)Transcriptome sequencing analysis was conducted at five-time points(0 d,7 d,14 d,21 d and 21+3 d)during storage based on the data of CI index,firmness and decay rate.32577 differential expressed genes(DEGs)were identified.After the functional annotation and KEGG enrichment analysis for the identified DEGs,six metabolic pathways were considered to be associated with the development of CI in peach fruit,including glutathione metabolism,pyruvate metabolism,starch and sucrose metabolism,pentose and glucuronate interconversions pathway,α-linolenic acid metabolism and jasmonate acid signaling pathway.After further analysis,the results indicated that the development of CI in peach fruit accompanied by the changes in the conversion of glutathione and oxidative glutathione,the activation of pyruvate synthesis,the suppression of Acetyl-Co A synthesis,the activation of sucrose synthesis and degradation,the suppression of starch synthesis,the activation of pectin degradation,and the suppression of jasmonate acid synthesis and signaling.The changes in the above metabolic pathways might respond to the cold stress in peach fruit.(2)It was found that different concentrations(10,100 μmol·L-1)of Me JA alleviated CI symptoms,maintained fruit quality and increased antioxidant activity in peach fruit during storage were studied.Among them,compared with CK(contrast),Me JA treatments effectively decreased peach softening and the CI index in "Yingshuanghong" stony hard peach and "Yuhualu" melting flesh peach during storage.10 μmol·L-1 Me JA had better effect on inhibiting the CI symptoms of peach fruit.It was also found that CI symptoms were different between the two peach cultivars."Yingshuanghong" stony hard peach mainly showed browning and melting phenomenon,while "Yuhualu" melting flesh peach mainly showed reddening fruit flesh.This might be related to the different fruit varieties.Me JA significantly decreased the respiratory rate and increased ethylene production during the shelf life at room temperature.In addition,10 μmol·L-1 Me JA could delay the peak of total phenols and flavonoids in "Yingshuanghong" stony hard peach and increase the concentrations of total phenols and flavonoids in "Yuhualu" melting flesh peach during the late stage of storage.Moreover,vitamin C and glutathione concentrations were maintained at a high level during different stages of storage,thus delaying fruit ripening.Finally,these indicators showed that compared with 100 μmol·L-1 Me JA treatment,10 μmol·L-1 Me JA was selected as the optimal treatment concentration for the two peach cultivars,which was used to explore its regulation mechanism in the subsequent experiments.(3)The results showed that 10 μmol·L-1 Me JA treatment regulated the composition of membrane lipid and maintained the integrity of cell membrane during storage of peach fruits with different flesh types.Among them,10 μmol·L-1 Me JA could inhibit the increase of membrane permeability and malondialdehyde(MDA)of "Yingshuanghong" stony hard peach and "Yuhualu" melting flesh peach,which was helpful to maintain the integrity of the membrane and alleviate the oxidative damage of peach fruit during storage.18 fatty acids were identified in "Yingshuanghong" stony hard peach and "Yuhualu" melting flesh peach.Linoleic acid and α-linolenic acid were the most abundant in unsaturated fatty acids,while palmitic acid and stearic acid are the most abundant in saturated fatty acids.Me JA treatment delayed the decrease of unsaturated fatty acids(USFAs)and the increase of saturated fatty acids(SFAs),suggesting that Me JA treatment may regulate membrane lipid composition and maintain a high degree of unsaturation,thus contributing to the maintenance of membrane integrity and alleviating the occurrence of CI symptoms.It was found that USFAs concentration of "Yingshuanghong" stony hard peach and "Yuhualu" melting flesh peach were similar,but SFAs concentration of "Yingshuanghong" stony hard peach was higher and USFAs/SFAs ratio was lower,which indicated that the saturation in stony hard peach fruit might be higher than that in melting flesh peach fruit.(4)The results showed that 10 μmol·L-1 Me JA activated α-linolenic acid metabolism and JA signaling pathway in two flesh types peach fruits.Among them,Treatment with 10 μmol·L-1 Me JA had a high concentrations of the α-linolenic acid and JA during storage.Me JA treatment significantly increased the activities of lipoxygenase(LOX),allene oxide cyclase(AOC)and allene oxide synthase(AOS),thereby initiating the first three steps of α-linolenic acid conversion to JA in both cultivars.In addition,Me JA treatment up-regulated the expressions of Pp LOX,Pp AOS,Pp AOC,Pp ACOX and Pp Fad A in the two peach cultivars during shelf life at room temperature.These results suggest that Me JA treatment could activate α-linolenic acid metabolism and promote endogenous JA biosynthesis.It was also found that Me JA treatment significantly increased the concenrations of JA,jasmonic acid-isoleucine(JA-Ile)and Me JA in peach fruit when the "Yingshuanghong" stony hard peach was transferred to the room temperature compared with the CK.In contrast,Me JA treatment mainly maintained high JA,JA-Ile and Me JA concentrations in "Yuhualu" peach fruit during low-temperature storage.Me JA treatment significantly up-regulated the expressions of Pp COI1,Pp JAZ and Pp MYC2 in the two peach cultivars during storage.These results further suggested that Me JA treatment activates JA signaling pathway,which may promote the expression of down-stream JA responsive genes,thus improving the cold tolerance of peach fruits.In conclusion,the results of this study suggest that Me JA treatment can maintain cell membrane integrity,improve antioxidant content,regulate membrane lipid composition,and activate α-linolenic acid metabolism and jasmonic acid signaling pathways in both solute peach fruits,thus reduce the CI symptoms of peach fruit and maintain the quality of peach fruit.