| Apple is one of the most popular and common fruits in the world.It is loved by consumers because of its rich vitamins and unique taste.Apples are often refrigerated to achieve annual supply,but while maintaining the external quality of the fruit,it will lead to a decrease in aroma quality,resulting in economic losses.Ester volatiles are one of the most important components that constitute the characteristic aroma of apple fruit.Alcohol acyltransferase AAT catalyzes the synthesis of esters,and carboxylesterase CXE catalyzes the hydrolysis of esters.It is of great significance to understand the formation of fruit flavor and improve flavor quality to identify the metabolism and regulation mechanism of esters.In this experiment,the apple variety‘Ruixue’ was used as the test material,and 1-methylcyclopropene(1-MCP)combined with cold storage treatment was carried out after harvest.The hardness of ‘Ruixue’fruit,the determination of aroma substance synthase activity,the determination and analysis of aroma substances,and the change mechanism of ester degradation were studied.It is of great significance to improve the aroma quality of fruit during storage.The main results are as follows:1.The inherent hardness of the fruit was maintained after 1-MCP treatment,and the fruit hardness and fruit brittleness of ‘Ruixue’ showed a gradual downward trend with the storage time.In addition,1-MCP treatment inhibited(LOX,HPL,ADH,PDC)enzyme activity.2.The volatile substances of ‘Ruixue’ during storage were determined by solid phase microextraction-gas chromatography-mass spectrometry(SPME-GC-MS).It was found that the content and type of volatile substances during storage showed a trend of increasing first and then decreasing.1-MCP treatment combined with cold storage had a significant effect on the aroma substances of ‘Ruixue’.With the extension of storage time,1-MCP had a more significant inhibitory effect on the aroma substances in the later stage of storage.3.1-MCP combined with cold storage treatment inhibited the formation of volatile aroma substances during storage,and the most obvious inhibition was esters.Esters are the most important volatile substances of apple aroma,and the main aroma substances of esters change most significantly during storage.Among them,butyrate and hexyl esters play a leading role,including hexyl hexanoate,butyl butyrate,hexyl butyrate,hexyl acetate,hexyl 2-methylbutyrate,hexyl thiocyanate,isoamyl2-methylbutyrate etc.,and the inhibition is most obvious after 1-MCP treatment.Ethyl butyrate,butyl propionate,and propyl hexanoate were not detected in the ‘Ruixue’variety before 30 days of storage,only detected in the refrigerated fruit after 30 days,and not detected in the 1-MCP treated fruit.4.MdCXE20 was involved in the hydrolysis and metabolism of volatile esters in apple fruits,and 42 family members were identified using the motif model of the conserved region of the CXE gene family.Through RT-qPCR gene expression determination and correlation analysis of ester metabolism,it was found that MdCXE20 gene expression was negatively correlated with ester content.The subcellular localization results showed that the MdCXE20 gene involved in the hydrolysis and metabolism of esters played a role on the cytoplasm.Combined with the results of apple homologous transient transformation overexpression and silencing of MdCXE20 gene,it was found that the esters were significantly reduced after overexpression of MdCXE20,and the opposite was true after silencing.At the same time,stable overexpression of ‘Orin’ callus also found the same results as above.The above results indicate that MdCXE20 regulates the decomposition of esters in apple fruits. |
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