Screening And Verification Of The Key Early Warning Genes For The Occurrence Of Superficial Scald In ’Granny Smith’ Apples

Apple superficial scald is a physiological disease that can easily to occur in the middle and later stages of cold storage.It does not affect the internal quality of the fruit,but with the development of the disease,it will seriously affect the appearance quality and commodity value of the fruit.Therefore,early warning of apple superficial scald and its early release from the warehouse before the occurrence of superficial scald can not only maintain fruit quality but also reduce economic loss,which is of great significance to the development of the apple industry.’Granny Smith’(Malus domestica Borkh.cv.’Granny Smith’)apple fruit is a famous green apple variety in the world.Because of their excellent quality and good storage,apples are loved by the majority of consumers.The fruit of this variety is not only edible directly but also the first choice for processing apple juice.However,this variety is susceptible to superficial scald,which will appear as typical symptoms of superficial scald in the later storage and shelf life stage.In this study,’Granny Smith’ apples were harvested during the commercial harvest period and stored for a long time at low temperatures(0 ± 0.5℃)to monitor the color and ethylene production,and the incidence of superficial scald during cold storage and shelf life after cold storage was counted and sampled and sequenced.These genes related to superficial scald were studied through GO functional enrichment analysis and literature analysis.It was revealed that they were associated with the occurrence of superficial scald of ’Granny Smith’ apples.The expression level of genes associated with superficial scald of ’Granny Smith’ apples was detected by real-time fluorescence quantitative PCR.The critical genes expressed before the occurrence of superficial scald of ’Granny Smith’ apples during storage were screened and verified from year to year,to further determine their repeatability and determine the early warning gene for the occurrence of superficial scald,which provides a theoretical basis for the early warning research of apple superficial scald.The main results are as follows:1.The superficial scald of ’Granny Smith’ apples mainly occurred in the middle and late periods of low temperature storage.That is,44～50 days of storage when the fruit was transferred to the shelf life,the fruit began to appear superficial scald,and the incidence and disease index of superficial scald increased sharply within 1～2 days of shelf life,and increased slowly in the later stage.With the extension of low temperature storage,the incidence and disease index of superficial scald increased sharply,and the fruit began to occur in cold storage when stored for 70 days.Therefore,based on the results of two years’ data,low temperature storage for 37～40 days is the critical period for early warning of superficial scald of ’Granny Smith’ apples.2.The ethylene release rate of ’Granny Smith’ apples changed steadily and increased slowly in the early stage of low temperature storage;with the extension of storage time,the ethylene release rate increased sharply and reached the peak of ethylene at 60 days of low temperature storage.During the period of low temperature storage,the L* value of ’Granny Smith’ apples showed a downward trend,the b* value showed an upward trend,and the change of the a* value was relatively stable.While during the period of shelf life after low temperature storage,the L* value,a* value and b* value increased slightly with the extension of shelf life;at the same time,during the whole shelf life,with the extension of low temperature storage time,the L* value showed a downward trend,while the a* value and b* value showed an upward trend.3.Based on transcriptome data and previous studies,10 genes related to superficial scald of ’Granny Smith’ apples were identified,including Md ACO,Md AFS,Md CAT1,Md CAT2,Md PAL1,Md PAL2,Md C3 H,Md PPO,Md POD1,Md POD2,and the real-time fluorescence quantitative PCR technique was used for screening and analysis,and year-to-year verification tests were carried out to determine the repeatability of the two years,that is,the expression of Md AFS,Md CAT1 and Md C3 H increased sharply before the occurrence of superficial scald,and then showed a downward trend,which determines that Md AFS,Md CAT1 and Md C3 H could be used as the critical early warning genes for the occurrence of superficial scald in ’Granny Smith’ apples during storage.To sum up,it is preliminarily defined that the threshold of relative daily expression of key early warning genes ≥ 0.113 can be used to predict the occurrence of superficial scald of ’Granny Smith’ apples.The predictive ability of Md CAT1 is better than that of Md AFS and Md C3H;and the results still need to be verified in other apple varieties to determine the applicability of the critical value.