| Peach is not resistant to storage,and it is prone to chilling injury at low temperatures,which could seriously reduce the edible and commercial value of the fruit.Analysing the mechanism of chilling injury of postharvest peach fruit and relieving its occurrence is the key to the research of postharvest conservation of peach fruit.Peach fruit is a typical climacteric fruit and is extremely sensitive to ethylene,ethylene treatment can promote its aging.1-Methylcyclopropene(1-MCP)is a new type of ethylene receptor inhibitor,which can delay post-harvest maturation and senescence,reduce chilling injury and increase disease resistance.In this paper,fresh peaches were used as materials and its post-harvest chilling injury were affected through 1-MCP and ethylene treatment.Compared with the control,the mechanism of chilling injury and the preservative mechanism of 1-MCP were analyzed.Firstly,the transcriptome analysis was performed on the peaches before and after the chilling injury to reveal the extremely significant metabolic pathways,and then the physiological changes of the metabolic pathways were analyzed.The main findings are as follows:(1)Transcriptome analysis of the process of chilling injury in peach fruitHigh-throughput sequencing technology was used to perform transcriptome sequencing of pre-chilled and post-chilled controls,post-chilled 1-MCP and ethylene-treated peach fruit,and a cDNA library with information about peach response to chilling injury were constructed to obtain differences in transcriptional level of different material.The functional of differentially expressed genes were also conducted annotation and enrichment analysis in seven databases of GO,COG,KEGG,NR,KOG,Pfam,and Swiss-Prot.Through transcriptome analysis,it was found that three metabolic pathways of active oxygen metabolism,cell wall metabolism,and arginine and proline metabolism were changed drastically.(2)Effects of 1-MCP and ethylene treatments on active oxygen metabolism in the process of peach fruit chilling injuryDuring cold storage,the MDA content and relative electrical conductivity of the peach fruit increased rapidly,and the related antioxidant enzyme activity was inhibited.High level of reduced glutathione(GSH)content and a low content of ascorbic acid(ASA)were maintained in peach fruit during cold storage.1-MCP treatment significantly(P<0.05)inhibited the increase of MDA content and relative electrical conductivity of peach fruit during cold storage,while ethylene treatment also played a certain inhibitory effect.Peach fruit slices were observed by peroxide staining and found that 1-MCP and ethylene both had a scavenging effect on O2-,and H2O2,1-MCP was far more effective than ethylene.1-MCP treatment significantly(P<0.05)increased the activity of superoxide dismutase(SOD),catalase(CAT),ascorbate peroxidase(APX),monodehydroascorbate reductase(MDHAR),dehydroascorbate reductase(DHAR)and glutathione reductase(GR)in peach fruit during chilling injury.The ethylene treatment also increased the activity of related antioxidant enzymes to a certain extent,but the effect was much less effective than that of 1-MCP.1-MCP treatment maintained high levels of ASA in peach fruit during cold storage by consuming large amounts of GSH,while ethylene treatment was not effective.In summary,both 1-MCP and ethylene can increase the antioxidant capacity of peach fruit during cold storage through antioxidant enzymes and ASA-GSH circulatory system,inhibit membrane lipid peroxidation,thereby improving the cold tolerance of peach fruit,but the effect of ethylene is much weaker.It may because that the ethylene treatment at room temperature stimulates the fruit to increase its antioxidant activity,this cross-resistance maintains its high antioxidant capacity at low temperature storage.(3)Effects of 1-MCP and ethylene treatments on cell wall metabolism in the process of peach fruit chilling injuryThe content of pectin,cellulose and hemicellulose of peach fruit decreased rapidly during cold storage,and the enzyme activity associated with cell wall metabolism increased dramatically,the enzyme activity associated with disease resistance was inhibited.1-MCP treatment significantly(P<0.05)inhibited the degradation of cell wall protopectin,cellulose and hemicellulose during cold storage,and its content was 1.38,1.37,and 1.41 times that of the control at 21 days,and ethylene treatment promoted cell wall degradation.Scanning electron microscopy revealed that 1-MCP treatment maintained the integrity of the cell wall during cold storage,while ethylene treatment caused cell wall structure collapse and break.1-MCP treatment significantly(P<0.05)inhibited the activity of polygalacturonase(PG),pectin esterase(PME),cellulase(Cx),and β-glucosidase during the cold storage.Ethylene treatment induced the increase of PG and PME activity,but to some extent it inhibited Cx and β-glucosidase.Both 1-MCP and ethylene could increase the activity of chitinase and β-1,3-glucanase in peach during storage and increase the disease resistance of fruits.The 1-MCP effect on β-1,3-glucanase and the effect is more pronounced,ethylene is more biased towards chitinase.In summary,1-MCP can maintain the stability of cell wall structure of peach fruit during cold storage by inhibiting the increase of activity of cell wall related enzymes,while ethylene increases the activity of some cell wall metabolic enzymes,destroy the metabolic balance of cell wall components,and leads to fruit texture abnormal change.Both 1-MCP and ethylene treatment can increase the activity of disease resistance related enzymes and increase the resistance of fruit to pathogen infection.(4)Effects of 1-MCP and ethylene treatments on the metabolism of arginine and proline in the process of peach fruit chilling injuryCold storage inhibited the synthesis of proline and polyamines and the activity of related key enzyme in peach fruit,increased the activity of nitric oxide synthase(NOS)and promoted the synthesis of nitric oxide(NO).1-MCP treatment could promote the synthesis of proline in peach during cold storage,while ethylene inhibited this process.On the 14th day,the proline content in 1-MCP treated fruit was 1.21 and 1.48 times that of that in control and ethylene groups,respectively.1-MCP can increase the activity of △-1-pyrroline-5-carboxylate synthetase(P5CS),a key enzyme in glutamate pathway in proline synthesis,in peach during cold storage,and inhibit the activity of proline dehydrogenase(PRODH).The effect of 1-MCP on the ornithine pathway in proline synthesis was not significant(P>0.05).1-MCP treatment promoted the synthesis of polyamines in peach fruit during cold storage and significantly(P<0.05)increased the level of putrescine(Put),spermidine(Spd)and spermine(Spm),while the synthesis of NO,which competing with polyamines for common substrate Spm,were restrained.The ethylene treatment inhibits the synthesis of both polyamines and NO.1-MCP treatment increased arginine decarboxylase(ADC)activity during cold storage and enhanced synthesis accumulation of polyamines in this pathway,whereas the increase in arginase and ornithine decarboxylase(ODC)activity only occurred during early stage,there was no significant difference in the change of enzyme activity at the late stage of cold storage(P>0.05).Ethylene treatment inhibited the activity of ADC and increased the activity of arginase and ODC at the early stage of cold storage.Both 1-MCP and ethylene treatment inhibited NO production during cold storage by restraining the NOS activity,and ethylene was more effective than 1-MCP.In summary,cold storage inhibited the synthesis of free proline and polyamines in peach fruit.1-MCP treatment promoted the accumulation of free proline and polyamines in peach during storage,and improved fruit cold tolerance,while ethylene had the opposite effect.(5)Effect of Put treatment on the metabolism of arginine and proline in the process of peach fruit chilling injuryThrough the observation of peroxide staining of peach slices,we found that Put treatment can effectively remove H2O2 and improve cell membrane stability.Through the observation of the cell wall by electron microscopy,we found that Put treatment can maintains the cell wall stability of peach fruit after cold storage.Put treatment promoted the synthesis of proline,increased the activity of P5CS,a key enzyme for proline synthesis,and ornithine transferase(δ-OAT)during early cold storage,and inhibited the PRODH activity.Put treatment promoted the polyamine synthesis and inhibited the NO accumulation in peach fruit during cold storage.Put treatment could increase the activity of ADC,a key enzyme of polyamine synthesis,and inhibit the activity of NOS,key enzyme of NO synthesis,in peach fruit during cold storage.Put treatment only enhanced arginase and ODC activity in the early stage of cold storage,and had no significant effect at the late stage of cold storage(P>0.05).In summary,Put treatment can clear a large amount of H2O2 in peach fruit tissue after cold storage,maintain the stability of the cell wall structure,promote the synthesis and accumulation of free proline and polyamine during the cold storage of peach fruit,so Put treatment can increase the fruit cold tolerance. |
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