Multi-omics Analysis Reveals A Comprechensive Mechanism Of Superficial Scald Of ’Granny Smith’ Apple Fruits

Superficial scald is a physiological disease that occurs very easily in apples in the middle and late stages of low-temperature storage,bringing serious economic losses to the industry.’Granny Smith’(Malus x domestica var ’Granny Smith’)is a world-renowned green apple cultivar with excellent quality and good storability,which can be eaten fresh and is the preferred cultivar for processing into apple juice.However,this cultivar can show typical superficial scald symptoms in late low temperature storage and become more evident during subsequent shelf life,which can seriously affect its commercial value.Therefore,in this study,we used ’Granny Smith’ apple fruit as the test material,and observed the changes of cellular tissue structure during the pathogenesis of superficial scald by paraffin section,transmission electron microscopy and scanning electron microscopy observation,and revealed the transcriptional and material metabolism differences of fruit peel during the pathogenesis of superficial scald by combined transcriptome and metabolome analysis,based on which the pathogenesis of superficial scald was revealed.The main findings of the study are as follows:1.The phenotype and cytoarchitecture of ’Granny Smith’ apples transferred to the shelf life after low-temperature stored showed that superficial scald started from the stomata at the calyx end of the yellowish side of the fruit,followed by a rapid expansion of the spot area and connected into a sheet,thereafter,with the increase of the scald level,the fruit peel depression deepened and the color of the subcutaneous cells gradually turned brown.In addition,the degree of cuticle depression,the degree of loose and disorderly arrangement of epidermal and subcutaneous cells,and the number of browning layers showed positive correlation with the degree of scald;at the same time,the waxy layer appeared lacerations,the morphology of chloroplasts gradually became swollen from regular long oval,and the stroma and grana lamellae gradually showed disorder and sparse.2.A total of 37 volatile compounds and 32 phenolic compounds were detected by GCMS and LC-MS at the metabolic level.Metabolites such as α-farnesene,conjugated trienol,methyl hexanoate,catechin,epicatechin,phlorizin,hyperoside,rutin and chlorogenic acid were found to be closely associated with the development of superficial scald by PCA and other analyses.3.A total of 15,364 differentially expressed genes were included in this study.PCA and HCA analyses showed that gene expression profiles were significantly different between harvest samples and post-harvest samples and between healthy and scalded fruit samples.In addition,the differential genes were mainly enriched in terpenoid and polyketide metabolic pathways(including terpenoid backbone biosynthesis and sesquiterpene and triterpene biosynthesis,etc.)and secondary metabolite biosynthesis(including flavonoid biosynthesis,flavonoid and flavonol biosynthesis,and phenylpropanoid biosynthesis)pathways.4.Analysis of metabolomic and transcriptomic data by weight gene co-expression network analysis(WGCNA)showed that the MEblue and MEyellow modules were positively correlated with the incidence of superficial scald and highly correlated with metabolites such as α-farnesene,CTols,MHO,catechins,epicatechin,chlorogenic acid,and hyperoside,and the genes of the modules were significantly enriched to phenol metabolism and terpenes biosynthetic pathways,where genes related to phenolics and volatiles synthesis such as PAL,C3 H,HCT and HMGR were differentially expressed in healthy(L80)and scalded tissues(LB80);in addition,gene expression of polyphenol oxidase(PPO)and laccase(LAC),which use phenolics as substrates,were differentially expressed in healthy tissues(H,L30,L50,L80)and scalded peel tissues(LB80).The degree of Md LAC gene is higher than that of other genes mentioned above,so it is presumed that this gene is highly related to the occurrence of apple superficial scald in ’Granny Smith’ apples.5.Overexpression of the Md LAC gene significantly reduced the content of rutin,hyperoside,and phlorizin.It was hypothesized that the Md LAC gene could make the incidence index of ’Granny Smith’ apple superficial scald higher than that of the control group by reducing the content of the above substances.