Analysis Of Mechanisms For Postharvest Chilling Injury And The Effect Of Ethylene On Alleviating Chilling Injury In Peach Fruit

Peach is sensitive to low temperature and can develop CI（chilling injury）at low temperature condtion.Ethylene play an important role in CI development process.However,mechanisms of CI affected by low temperuatre and ethylene remain unclear.In this study,‘Zhonghuashoutao’peach cultivar acted as experimental materials,stored at different temperatures（0,5,8,12 and 16℃）was used to reveal the epigenetic mechanisms of CI development through transcriptome and DNA methylome.Additionally,peach fruit treated with ethylene was used to test the effect and mechanism of ethylene on alleviation of CI via transcriptome and lipidomic.Major results are as follow:1.Low temperature treatments lead to the decreased endogenous ethylene production,abnormal softening,and IB in‘Zhonghuashoutao’peach fruit.IB was observed in fruit stored at 5,8 and 12℃,but did not observe in fruit stored at 0 and16℃for 45 days.Lower ethylene production level is associated with the increase of expression levels of ethylene biosynthesis genes（PpSAMS1,PpACS1,PpACO1 and PpACO2）.Abnormal softening is associated with the lower expression levels of cell wall related genes.Low temperature-leaded internal browning might be correlated with imbalance of reactive oxygen species that H₂O₂ content is lower and·OH content is higher.And,imbalance of reactive oxygen species is related with lower expression level of PpCATs and PpSODs.Meantime,higher expression levels of PpPPOs and PpPODs are also involved with IB.Flesh browning is correlated with inhibiton of genes related to fatty acid biosynthesis and desaturation,and is also associated with the decreased expression levels of genes related phospholipid（PpLPP1,PpPLD1 and PpLPAT1 etc.）and sphingolipid biosynthesis（PpKSRs,PpPpLCBs and PpLOH etc.）,as well as the up-regulation of expression levels of sphingolipid catabolism（PpSPP and PpSPL）.Coexpressed found that plant hormone signaling transduction related genes were highly correlated with CI symptom.2.DNA methylation was increased at low temperature,and showed the highest level in‘Zhonghuashoutao’peach fruit stored at 5℃where IB is the highest.The methylation level of differentially methylated regions was also showed elevated trends with the decrease of storage temperature.Hiher promoter methylation levels of differentially expressed lipid genes were found in CI fruit.The DNA methylation level was highly correlated with the expressions of the DNA methyltransferase genes and DNA demethylase gene.DNA methylation is involved in regulation of chilling injury development through affecting the transcript levels of genes related to ethylene biosynthesis,cell wall metabolism,browning and reactive oxygen species metabolism.3.Ethylene promoted endogenous ethylene production,normal softening,and alleviated IB in‘Zhonghuashoutao’peach fruit.Expression levels of genes related to ethylene biosynthesis（PpACS1 and PpACO2）,cell wall metabolism（PpPLY1/3,PpPME,PpEGase,PpXyl,PpExp1/3,PpGal and PpGS）and browning（PpPOD1 and PpLOX2）were significantly up-regulated by ethylene.Moreover,expression levels of ethylene downstream signaling transduction elements genes,PpETR2,PpEIN4 and PpCTR1 were higher,but PpEIN2 were lower in ethylene-treated fruit.4.Fatty acid biosynthesis metabolism was not affected by ethylene,but ethylene promoted the biosynthesis of phospholipids and glycerolipids and the catabolic reaction of ceramide in‘Zhonghuashoutao’peach fruit.Higher levels of phospholipids and glycerolipids were associated with down-regulation of PpLACSs,PpAAPT,PpPSS,PpPGPP,PpCLS,PpPI3P5K and PpDGD,as well as up-regulation of PpLPP1,PpLPCAT,PpPDAT and PpPI3K in ethylene-treated fruit.However,lower level of ceramide was correlated with up-regulation of PpCDase,PpFAH and PpGONST1 in ethylene-treated fruit.Lipidomic analysis showed different lipid component change at cold storage and shelf,which suggests that there are different mechanisms at these two phases involved in ethylene-induced alleviation of chilling injury in peach fruit.5.The expression levels of ERF family genes were affected by ethylene,and showed up-regulated and down-regulated expression patterns.Coexpression analysis showed that some ERFs were highly coexpressed with cell wall and lipid metabolism genes.Among all of ERFs,the transcript level of PpABR1 was significantly down-regulated by ethylene,being about 3.2-fold in comparison with control,and up-regulated by low temperature about 105-fold in comparison with samples at 0 day,as well as showed higher correlation with lipid genes（PpAAPT,PpFAH and PpGONST1）.Another ERF,PpESE3 was significantly up-regulated by ethylene（2-fold）and shelf（146-fold）in transcript levle,and showed the highly correlation with cell wall metabolism genes PpExp1,PpXyl and PpGal through coexpression and correlation analysis.Furthermore,the finds of ERF binding site in the promoter of lipid and cell wall metabolism genes suggest that PpABR1 and PpESE3 might regulate the expression levels of lipid and cell wall metabolism genes,respectively.