Effect And Mechanism Of Abscisic Acid On Postharvest Softening Of Blueberry Fruit

Blueberry fruit(Vaccinium corymbosum L.Cv Bluecrop)is widely recognized as one of the most productive and high commercial fruits because it full of phenolic acids which are alleged to be beneficial for the maintenance of human health.However,marketing can be a challenge as blueberry fruit are easily injured,subjected to excessive softening and had a very short postharvest storage time due to its particular texture.So efforts are continuously devoted to identify the mechanism of softening and developing effective technologies for blueberry postharvest preservation in order to extend their shelf-life while retain their quality.Based on this,this experiment used Bluecrop blueberry,which is mainly grown breeds in Liaoning province and is considered to be the soft resistant breeds,as the test material.Firstly,the climacteric types of blueberries were identified,and the physiological changes such as morphology and hardness during postharvest storage at room temperature was explored,and the pattern of fruit softening was determined.Then the changes of the content of five plant hormones,such as abscisic acid(ABA),in the process of softening,were investigated.Subsequently,the key genes and key metabolic pathways that produced differences during postharvest softening of blueberry fruits were identified by transcriptome sequencing.The significant pathways related to ABA synthesis,signal transduction and cell wall metabolism were identified.The differential genes on the pathway were screened and full-length sequences of key differential genes were obtained.The treatment of exogenous ABA and ABA inhibitors was used to clarify the ABA effects on blueberry effect and molecular mechanism of solid softening while exploring the regulation of blueberry fruit softening by ABA inhibitors.The main findings are as follows:1.There was no significant peak in ethylene production during postharvest storage period,no significant peak in respiration intensity during postharvest storage period and no peak in propylene treatment.Meanwhile,we detected five important plant hormones in the postharvest storage of blueberry fruits,with ABA,auxin(IAA),salicylic acid(SA),jasmonic acid(JA)and gibberellin(GA3)in order of their contents from high to low.The highest content was ABA,which reached 697 at peak ng g-1,At this time,the content was 63 times of IAA content,116 times of SA,348.5 times of JA and 1482 times of GA3.Furthermore,statistical analysis showed that the content of ABA showed a significant negative correlation with the decrease of fruit firmness.2.The firmness of blueberry fruit decreases rapidly between 2 d and 4 d to 8 d under room temperature during storage after harvest.At the same time during the softening process,the enzyme activity related to cell wall was significantly higher than that of 0 d after harvest whereas the polygalacturonase(PG),pectin methyl esterase(PE)and galactosidase(β-Gal)all peaked on day 6 during postharvest storage,while cellulase(Cx)peaked on day 4 during storage.And along with the increase of color angle h°,the nutrient like reducing sugar,total phenol and ascorbic acid content increased,the acid content decreased and the solid andstarch content floated.The genes of cell wall degradation enzymes were up-regulated with those encoding plant hormones ABA synthetases.At the same time,we obtained the gene sequences of 4 Vc PGs,3 Vc Gal,1 Vc NCEDs and 1 ABA2 gene.The further correlation analysis showed that Vc PGs,Vc Gal,Vc Cx,Vc NCEDs,Vc ZEP,Vc AAO3,Vc ABA2 was significantly positively correlated with ABA synthesis,Vc PGs,Vc Gal,Vc Cx was significantly positively correlated with cell wall degradation,and both negatively correlated with blueberry fruit softening.3.In order to prove the effect of ABA in postharvest blueberry fruit on fruit softening,we applied two kinds of exogenous treatments to blueberry fruit,and found that the firmness of blueberry fruit treated with exogenous ABA was significantly lower than that of control blueberry fruit during storage period,and the enzyme activities associated with cell wall degradation were all regulated by exogenous ABA.All of the PG、PE、Gal and Cx activity and the expression of genes encoding them were upregulated in the fruit treated with exogenous ABA.Meanwhile ABA treated fruits contained higher water-soluble pectin(WSP)content,lower EDTA soluble pectin(ESP),NA2CO3-soluble pectin(NSP)content,and lower cellulose and hemicellulose content.Nutrients such as sugar,ascorbic acid and total phenol in blueberry fruits are positively regulated by ABA,and endogenous ABA levels are self-accumulated by external ABA.Meanwhile,the application of ABA synthetic inhibitors NDGA can reduce the ABA content by suppressing the gene expression of Vc NCEDs,Vc ZEP2,and Vc AAO3,while delaying the decrease of the firmness of blueberry fruit and maintaining the postharvest quality of blueberry fruit.