| Hami melon is a characteristic fruit of Xinjiang,which has developed into an important local cash crop and one of the pillar industries of special agriculture.Gold Queen is the dominant Hami melon cultivar in Xinjiang,with wide planting area and large yield,but it is sensitive to low temperature and prone to cold injury in long-term or unsuitable low temperature storage.In this paper,the Gold Queen Hami melon stored at 0.5℃,3℃ and 21℃ was selected as the research object.Firstly,the storage quality indexes of Hami melon at different temperatures were determined to clarify the influence of temperature on its storage performance;subsequently,transcriptomics techniques were used to screen key differentially expressed genes and metabolic pathways.By analyzing the expression changes of genes involved in starch and sucrose metabolism at different temperatures,the transcriptional regulation mechanism of Gold Queen Hami melon in response to cold stress was preliminately elucidated;based on transcriptome data,gene co-expression networks for storage quality and metabolic indexes of starch and sucrose were constructed,and core genes related to specific traits were mined;the effects of chitosan coating on starch and sucrose metabolism and storage quality of Gold Queen Hami melon were also studied.The main research and conclusions of this paper are as follows:(1)The results of phenotypic and quality indexes showed that 3℃ was more suitable for postharvest low temperature storage of Gold Queen Hami melon.3°C storage showed less chilling injury symptoms in the fruit,and compared with 0.5°C,3°C storage reduced postharvest weight and hardness loss,maintained soluble solids content,enhanced the antioxidant capacity of fruits by increasing the activities of antioxidant enzymes(catalase,superoxide dismutase,ascorbate peroxidase and peroxidase),and maintained antioxidant system homeostasis;it slowed down the color change of fruit skin and delays fruit browning by inhibiting the activity of polyphenol oxidase and enhancing the activity of phenylalanine deaminase.(2)The transcriptome sequencing results showed that storage temperature and storage time significantly affected gene expression in cantaloupe fruits.Compared with 0 d,the number of differentially expressed genes at 0.5℃,3 ℃ and 21℃ was 6,479,7,350 and 3,642 at 12 d,and 7,914,6,488 and 3,533 at24 d,respectively.At the same storage time point,the number of differentially expressed genes in the0.5°C vs.21°C group(12d: 8414;24d: 8967)was consistently higher than that of the 3°C vs.21°C group(12d: 6464;24d: 6578).suggesting that low temperature induced more differential expression of genes.(3)GO functional analysis showed that a high number of differentially expressed genes enriched in metabolic processes,cells,cell parts,membranes,and catalytic activity.KEGG classification and enrichment analysis showed that low temperature induced differential expression of genes related to starch and sucrose metabolism,energy metabolism and lipid metabolism pathways.Among them,β-amylase genes(103490827 and 103497884),β-glucosidase genes(103484429,103500655 and 103492048),sucrose synthase genes(103483781),alglucose-6-phosphate synthase,and alglucose-6-phosphate phosphatase genes(103495491 and 103484434)in the starch and sucrose metabolic pathways,H+-ATPase genes(103493966,103483162 and 103489484)and cytochrome C oxidase genes(107990889 and103494723)in the energy metabolism pathway,as well as phospholipase D genes(103497303,103485168 and 103502993)in the lipid metabolism pathway were closely associated with the enhancement of cold tolerance in Gold Queen Hami melon.(4)The changes of starch and sucrose metabolism in Hami melon fruit under different storage temperatures showed that: In the late storage period,low temperature treatment could regulate starch and sucrose metabolism of Hami melon of by maintaining or enhancing α-amylase,β-amylase andβ-glucosidase activities.At the end of storage,theα-amylase,β-amylase and β-glucosidase activities of fruits in 21℃ group were 18.34%,64.29% and 58.01% of those in 3℃ group,respectively,and 60.00%,59.71% and 44.58% of those in 0.5℃ group.At this time,sucrose,glucose and fructose contents of Hami melon fruits in 21℃ group were also significantly lower than those in low temperature group.(5)Storage temperature significantly affected the expression of genes involved in starch and sucrose metabolic pathways.62 candidate genes for chilling injury were identified by differentially expressed gene analysis,including 9 α-amylase genes,7 α-1,4-galacturontransferase genes,7 pectinase genes,4glucurontransferase genes and 4 glucan endo-1,3-β-glucosidase genes.Postharvest softening of cantaloupe fruit was delayed by up-regulating the expression of β-glucosidase genes(103484429,103484880,103494776 and 103494400)and pectinase genes(103483059,103482964 and 103499033)involved in the negative regulation of catalytic activity and pectin breakdown process,as well as down-regulating the expression of polygalacturonase genes under cold stress.In addition,the expression of α-amylase andβ-amylase genes could be regulated to promote starch degradation and resist physiological or cellular damage to the fruit by cold stress.(6)Based on the transcriptome data of Hami melon stored at 3℃,12 co-expression modules were constructed by weighted gene co-expression network analysis to characterize the association between genes and storage quality indexes,starch and sucrose metabolism indexes of Hami melon.The blue module was identified to be strongly correlated with chilling injury index,weight loss,hardness and SSC,and low temperature upregulated WRKY transcription factor(103484689)and ethylene response transcription factor(103499414),and down-regulated phospholipase D gene(103490965),chitinase gene(103482936 and 103485182)and glucan endo-1,3-β-glucosidase gene(103500207)to regulate the postharvest low temperature storage quality of Hami melon fruit.A total of six growth hormone response factors(brown: 103500611;red: 103503391 and 107990918;pink: 103497280 and 103502995;yellow:103489925)were screened in the core genes of the brown,red,pink and yellow modules,and low temperature positively or negatively regulated pectinase,α-amylase and β-amylase activities by regulating the expression of these genes.(7)Based on the previous results,the effects of chitosan coating treatment on storage quality,starch and sucrose metabolism of Hami melons stored at 3℃ were investigated.Chitosan treatment effectively reduced the degradation of covalently bound pectin and inhibit the increase of water-soluble pectin in Hami melons during chilling injury;meanwhile,it reduced the activity of pectinase and polygalacturonase and the expression of related genes,and delayed the softening of fruits.Chitosan treatment also reduced the chilling injury of Hami melon fruit by regulating starch and sucrose metabolism,and there was a strong correlation between fruit softening and starch and sucrose metabolism.The results tentatively indicate that chitosan can be used as a reliable and promising coating material for postharvest preservation and chilling injury control of Hami melon. |
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