Study On The Mechanism And Regulation Of Aroma Weakening Of Refrigerated 'Nanguo' Pears Based On Fatty Acid Metabolism

'Nanguo' pears is mainly produced in Anshan,Liaoning Province,which belongs to the Pyrus ussuriensis Maxim.It usually harvest in mid-September,after harvest it can only be released at room temperature for about half a month.Production is often stored in cold storage to prolong its preservation period.However,�Nanguo' pears will have a phenomenon that the flavor is significantly reduced after cold storage,aroma is a very important quality indicator of �Nanguo' pears.Therefore,the problem of aroma reduction has already affected the commercial value of �Nanguo' pears.This problem has become a technical problem that �Nanguo' pears industrial development urgently needs to solve.Unsaturated fatty acids are important precursors for the synthesis of esters aroma.In this paper,unsaturated fatty acid synthesis and its effect on the synthesis of esters was studied in order to study the mechanism of aroma reduction of �Nanguo' pears.The changes in the content of unsaturated fatty acids and the expression of PuFADs,which is the key enzymes in the synthesis of unsaturated fatty acids,were studied.Changes in activity of key enzyme and related gene expression in ester synthesis using unsaturated fatty acids as substrates were also studied.Based on this,the effects of Low temperature conditioning(LTC)and Intermittent warming(IW)on the aroma of �Nanguo' pears during cold storage were explored.The main results are as follows:1.The original sequence was obtained by �Nanguo' pears transcriptome sequencing.Sequence comparisons of NCBI nBLAST showed that the fatty acid desaturase gene family members PuFAD2,PuFAD3,PuFAD6 and PuFAD7 were selected,and the lipoxygenase gene family members PuLOX1,PuLOX6 and PuLOX13 were selected.Phylogenetic tree analysis showed that PuFADs were divided into two types: ?-6 and ?-3.PuFAD2 and PuFAD3 were located in the endoplasmic reticulum,and PuFAD6 and PuFAD7 were located in the chloroplast.2.Compared with unrefrigerated fruit,the refrigerated �Nanguo' pears has reduced the type and content of esters in the optimal taste period,and this phenomenon is even more pronounced with the prolonged refrigerating time.The contents of oleic acid,linoleic acid and linolenic acid in �Nanguo' pears were lower than those of unrefrigerated fruits during the cold storage and subsequent shelf life at room temperature.As the refrigerated time increases,the content becomes lower.The expression of PuFAD2,PuFAD3 and PuFAD7 at room temperature was significantly lower than that of unrefrigerated fruit at room temperature.The activity of LOX,AAT,ADH and the expression of PuLOX6 were significantly decreased during the cold storage,and the longer the refrigerated time,the greater the decline in activity and expression.At the same time,the activities of LOX and AAT and the expression levels of PuLOX6,PuLOX13,PuADH3,and PuAAT1 were significantly lower than those of unrefrigerated fruit during shelf life at room temperature,and their activity and expression levels were significantly lower at 150 days.The correlation analysis showed that the total ester content was positively correlated with the content of unsaturated fatty acids and the expression levels of PuFAD2,PuLOX1,and PuLOX13,indicating that unsaturated fatty acids and PuFAD2,PuLOX1,and PuLOX13 play an important role in the synthesis of esters.As the linoleic acid content accounts for the highest proportion of unsaturated fatty acids and PuFAD2 directly participates in the synthesis of linoleic acid,it is speculated that the linoleic acid catalyzed by PuFAD2 is the main precursor for the synthesis of esters of �Nanguo' pears.The second chapter found that the expression level of PuLOX1 in refrigerated fruit was higher than that of unrefrigerated fruit during the cold storage and subsequent shelf life at room temperature,indicating that the low temperature had little effect on the expression of PuLOX1.It was hypothesized that the decreased expression of PuLOX13 was the main reason for the decrease of aroma esters of �Nanguo' pears..3.Both LTC and IW treatments fruit were significantly increased in the type and content of esters at day 0 and the optimal taste period.and LTC treatment was more effective than IW treatment.The expression levels of PuFAD2,PuFAD6,and PuFAD7 in fruit treated with LTC were significantly higher than those of IW-treated fruit and refrigerated fruit on the day of delivery,The expression of PuFAD3 and PuFAD7 in IW-treated fruit was also significantly higher than that of refrigerated fruit on the day of delivery.The expression level of PuFADs in poikilothermic treated fruit was significantly higher than that of refrigerated fruit after cold storage at room temperature,and the expression levels of PuFAD6,PuFAD3,and PuFAD7 were significantly higher during the whole shelf life.The content of oleic acid and linoleic acid in LTC treatment increased significantly at day 0,and the content of linolenic acid in IW treatment also increased significantly at day 0.The content of unsaturated fatty acids in poikilothermic treated fruit of both group was significantly higher than that of refrigerated fruit during the shelf life after refrigeration,and the oleic acid content of LTC-treated fruit remained significantly higher throughout the shelf life.The activities and expression levels of LOX,ADH and AAT in fruit with poikilothermic treatment was significantly higher than those of refrigerated fruit after cold storage at day 0 and subsequent shelf life at room temperature.Among them,the AAT activity and expression level of IW-treated fruit was significantly higher,and the LOX and ADH activity and expression levels of LTC-treated fruit was significantly higher.Experiments show that the poikilothermic treatment can effectively alleviate the phenomenon of aroma weakening in refrigerated �Nanguo' pears.It is speculated that poikilothermic treatment can reduce the decrease of unsaturated fatty acid content by effectively suppressing the decrease of PuFADs expressions,inhibited the decrease in the activity and expression of LOX,ADH,AAT and effectively alleviated the aroma reduction of �Nanguo' pears.