Quality Deterioration Mechanism Of Fresh-cut Melon During Storage Based On Metabolomics And Transcriptomics

Fresh-cut melon is often eaten as snack foods for traveling or as after-dinner fruits.The cutting process inevitably induces a number of physiological disorders including an increase in respiration rate,tissue softening,decreasing nutrient levels and so on,and resulting in lower product quality and a shorter shelf life.This process involves a series of changes in physiological and biochemical at the molecular level in postharvest fruits,as well as regulation.The research about the metabolism of sucrose and organic acids and the molecular mechanism underlying the changes of sucrose and organic acids in melon fruit in response to cut-wounding stress are scarce.Due to the important roles of sucrose and organic acids as a response signal molecule of biotic and abiotic stress in the respiratory pathway,and the roles of energy and ROS in the quality deterioration of postharvest fruits and vegetables,the objective of this work was to investigate the effect of cutting on the molecular metabolism of sucrose and organic acids,and to explore the relationship among ROS accumulation,energy change and quality deterioration in fresh-cut melon.In this study,‘Xizhoumi’ and ’Yugu’ melon were used to explore the changes of quality,physiology and biochemistry,enzyme activity,metabolism and gene expression during fresh-cut melon storage based on routine analysis,metabolomics and transcriptomics.According to revealing the molecular regulation mechanism of fresh-cut melon during storage,this useful molecular information will be helpful to provide a guideline for improving the technology of storage and logistics,delaying the quality deterioration and controlling postharvest losses in fresh-cut melon.The main results are as follows:1.The molecular changes underlying the fluctuations in the content of soluble sugars and organic acids were investigated in whole and fresh-cut melon fruit(cv.’Xizhoumi’ and’Yugu’)during storage.Cutting greatly accelerated quality deterioration of melon fruit(cv.’Xizhoumi’ and ’Yugu’)when stored at 15℃,as indicated by lower firmness,TSS and pH at the later stage of storage.However,fresh-cut melon stored at 15℃ maintained its good quality at the earlier stage of storage,accompanied by higher hexose and organic acids content.This was confirmed by the results that AI,NI and SS-c were the main regulators of sucrose metabolism in fresh-cut melon while organic acids metabolism was regulated by the combined function of PEPC,CS,ACO,IDH,MDH and ME.The changes of the content of soluble sugars and organic acids in fresh-cut‘Xizhoumi’ melon were due to the up-regulation of CmAI1/2,CmNI3,CmPEPC3,CmCS1,CmACO1/2,CmMDH1/3 and CmME3/4 and down-regulation of CmIDH1.Different from the ’Xizhoumi’ melon,the expression of CmIDH1,CmNIl and CmPEPCl in ’Yugu’ melon was up-regulated by cutting and the changes of soluble sugars and organic acids content in fresh-cut ’Yugu’melon resulted from the up-regulation of CmAIl/2,CmNI1/3,CmPEPC1/3,CmCS1,CmACO1/2,CmIDH1,CmMDH1/3,and CmME3/4.Storage at 5℃ significantly extended the shelf life of fresh-cut melon and maintained higher content of sucrose and organic acids,which resulted from the up-regulation of CmPEPC2/3 and the down-regulation of CmAI1/2,CmNI3,CmCS1,CmACO1/2 and CmME3/4 during storage of fresh-cut ’Xizhoumi’ melon,and these findings were due to the up-regulation of CmPEPC2/3 and the down-regulation of CmAI1/2,CmNI1/3,CmCS1,CmACO1/2,CmIDH1 and CmME3/4.Low temperature can significantly decrease the expressions of CmIDH1 and CmMDH1/3 in fresh-cut ’Yugu’melon compared with ’Xizhoumi’ melon.2.’Xizhoumi’ and ’Yugu’ melon were used in the experiment to explore the relationship among ROS accumulation,energy change and quality deterioration in fresh-cut melon during storage.It is clear that cutting changed the respiratory metabolism pathway in melon during storage,enhanced the levels of EMP,PPP and TCA pathways,decreased the level of CCP pathway,and maintained higher activities of H+-ATPase and Ca2+-ATPase during storage.This led to a large production of ATP and ADP in fresh-cut melon.Simultaneously,cutting enhanced the NOX activity,which resulted in a strong increase of ROS level in fresh-cut melon.Compared with whole melon,both POD and GPX in fresh-cut ’Xizhoumi’ melon were temporarily maintained at higher activities at the early stage of storage,while AsA,GSH,APX,and GR retained lower level.Similar to’Xizhoumi’ melon,only POD at the early stage of storage of fresh-cut ’Xizhoumi’ melon was temporarily maintained at higher activities in comparison with whole melon while AsA,GSH,APX,SOD and GR retained lower level.Low temperature significantly inhibited the increase of NOX activity in fresh-cut melon,enhanced the antioxidant system activity,decreased the ROS level,and maintained the good quality of fresh-cut melon.The H+-ATPase activity was increased in fresh-cut melon during storage at 5℃ in comparison with samples stored at 15℃,accompanied by a lower content of ATP in fresh-cut’Xizhoumi’ melon at the early stage of storage.However,lower ATP content and higher H+-ATPase activity were only observed in fresh-cut ’ Yugu’ melon during the early stage of storage at 5℃,compared to samples stored at 15℃.These results show that cutting accelerated the quality deterioration of melon,and induced a higher energy status in fresh-cut melon,which was accompanied by higher ROS accumulation.3.Comparative metabolomics profiling was conducted to explore the total molecular changes of quality deterioration in whole and fresh-cut melon(cv.Xizhoumi-17)during storage.A total of 99 differential metabolites were found during the storage of whole and fresh-cut melon,among which 19 of the differential metabolites were only found during the storage of fresh-cut melon compared with whole melon.In comparison with fresh-cut samples stored at 15℃,10 of the differential metabolites were only found during fresh-cut melon storage at 5℃.MEG and pyranocyanin A of flavonoids were accumulated during the storage period of melon;in comparison,cutting did not increase the content of MEG and pyranocyanin A during the storage of melon,low temperature only decreased the MEG content of fresh-cut melon.Moreover,the composition of cell membrane in melon changed with storage period,as indicated by the increasing content of PA,PI,PG-(P-16:0/14:1),PGP-(16:1/16:1)and MGDG and the decreasing content of LPA,PG-(P-16:0/0:0),LPI,LPC and LPE during melon storage.Compared to whole melon,cutting accelerated these changes of cell membrane composition and induced higher SAG content in fresh-cut melon to resist cut-wounding stress.In comparison with 15℃,low temperature inhibited the increase of PA,PG-(P-16:0/14:1),PGP-(16:1/16:1),MGDG and SAG content during fresh-cut melon storage,and also inhibited the decrease of PG-(P-16:0/0:0),LPI,LPC and LPE content.However,storage at 5℃ did not significantly inhibit the increase of PI content of fresh-cut melon compared to the samples stored at 15℃.In summary,cutting accelerated the quality deterioration of melon,and induced the changes of cell membrane composition;however,fresh-cut melon can enhance its defense system activity by increasing the content of PA and SAG in response to cut-wounding;PI,PA and SAG can be used as marker metabolites for the quality deterioration during fresh-cut melon storage.4.The molecular basis of quality changes in whole melon and fresh-cut melon(cv.Xizhoumi-17)during storage was investigated based on transcriptomics.Transcriptome analyses revealed that cutting significantly increased the transcription level of NOX,which was consistent with the higher ROS levels during the storage of fresh-cut melon at 15℃compared with whole melon.In comparison with whole melon,cutting of melon enhanced the transcription level of glutathione S-transferase in Glutathione metabolism,increased the transcription levels of 3-ketoacyl Co A thiolase,acyl-desaturase,fatty acid desaturase,very-long-chain(3R)-3-hydroxyacyl-CoA dehydratase,and acyl-coenzyme A oxidase in Biosynthesis of unsaturated fatty acids,and up-regualted the transcription levels of cinnamate 4-hydroxylase and caffeoyl-CoA O-methyltransferase in Flavonoid biosynthesis.These changed genes of these pathways in fresh-cut melon will be beneficial for exploring the mechanism of quality deterioration in fresh-cut melon or understanding the detailed mechanism of how melon fruit activates its defense system activity to resist the cut-wounding stress.Low temperature storage down-regulated the transcription levels of the above pathways in fresh-cut melon.Moreover,SA synthesis in fresh-cut melon is mainly mediated through the PAL pathway.Combined with previous results,the transcriptomics confirmed that cutting of melon increased the expression level of NOX,which triggered the production of ROS,accelerating the quality deterioration of fresh-cut melon.Fresh-cut melon up-regulated the gene expression of AI,NI,PEPC,PAL,PLC and PLD,accelerating sucrose decomposition,leading to higher levels of PA and SAG in response to cut-wounding stress.