| High-aroma tea varieties have been deeply concerned by breeders and growers,and the processed tea are favored by consumers.’Chungui’is a new type of tea plant with high aroma that is naturally hybrid with’Huangdan’.The oolong tea made from it has mellow and refreshing taste,with special aroma similar to jasmine flowers.At present,research on Chungui oolong tea mainly focuses on biochemical composition or low-flux aroma detection.However,there are less reports on the mechanism of flavor quality formation during Chungui oolong tea processing by combining high-flux metabolomics with molecular level analysis.In this study,we taking Chungui oolong tea processing samples as test materials,systematically analyzed the dynamic changes of metabolites during the processing period of Chungui oolong tea using metabolomics technology,then combined with transcriptome,constructed co-expression networks between taste,aroma,and genes,excavated key regulatory genes related to flavor quality as well as explored the formation and transcriptional regulatory of characteristic flavor substances.This study would provide a reference for optimizing the processing technology and promoting the cultivation of new varieties besides Chungui.The main results are as follows:1.Research on the dynamic changes of taste substances during the processing of Chungui oolong teaUsing extensive targeted metabolomics techniques,a total of 1060 metabolites were detected from 31 classes in the processing samples of Chungui Minbei oolong tea,with flavonoids,phenolic acids,amino acids and their derivatives,and lipids being the most important metabolite types.By combining multiple statistical analysis and multiple difference analysis,660 differential metabolites were selected from the processing samples of Chungui oolong tea.Functional enrichment analysis showed that amino acid biosynthesis,flavonoid biosynthesis,flavonoid and flavonol biosynthesis,and phenylpropanoid biosynthesis pathways related to the formation of tea taste quality were significantly enriched multiple times.The analysis of dynamic changes in differential metabolites showed that during the processing of Chungui oolong tea,the amino acid content showed an overall upward trend,and the amino acid content of L-histidine,L-asparagine,L-glutamine,L-lysine,L-leucine,and other amino acids significantly increased and reached their peaks during the turning-over.The content of flavonoids such as naringin,eriodictyol,and apigenin increased during the processing,while the content of catechin monomers such as EC,C,EGC,GC,and EGCG decreased.The content of organic acids,lipids,nucleotides,and theaflavins significantly increases during the processing.The content of phenolic acids such as salicylic acid and syringic acid,as well as sugars such as D-sucrose and xylitol,reaches their peaks during the drying process.2.Study on dynamic changes of aroma substances during the processing of Chungui oolong teaUsing volatile metabolomics technology to determine aroma substances in the processing of Chungui Minbei oolong tea.A total of 220 aroma substances from 10 categories were identified,mainly alcohols,esters,and alkenes.The total aroma content reached the highest level at the end of the fixation.The 25 key aroma substances selected from the processing samples showed three changing trends: the content of nerolidol,indole,geraniol and other substances with floral and fruity aroma reached their peaks during the withering or turning-over stage of oolong tea;the content of leaf acetate,3-hexene-1-alcohol and other substances with grass gas decreased with the processing;the contents of benzyl alcohol,phenylethyl alcohol,and γ-caprolactone fluctuated during processing and reached their peaks after drying.According to the relative odor activity value(ROAV)and aroma characteristic influence value(ACI),linalool,geraniol,indole,jasmone,γ-caprolactone,and α-farnesene were selected as the key aroma substances to constitute the "variety fragrance" of Chungui oolong tea.Among them,indole(ROAV=186.54,ACI=3.07%)and jasmone(ROAV=142.83,ACI=2.35%)with jasmine fragrance are the key components to engender the special jasmine fragrance of Chungui oolong tea.Besides,withering and turning-over are the key processes to promote the accumulation of these two contents.3.Analysis of differential genes expression patterns during the processing of Chungui oolong teaTranscriptome sequencing was performed on the samples of the enzymatic oxidation stage in the processing of Chungui Minbei oolong tea.17256 differently expressed genes were identified.GO annotation indicated that a large number of differential genes were annotated into biological processes of metabolism and stimulus response,as well as molecular functional items such as catalytic activity and binding.KEGG enrichment analysis showed that differential genes were mainly concentrated in phenylpropane biosynthesis,flavonoid biosynthesis,and terpenoid synthesis pathway directly related to aroma synthesis.The results of transcription factor analysis showed that MYB,ERF,WRKY,b HLH,NAC,and other transcription factor families had potential regulatory effects on the formation of flavor substances during the processing of Chungui oolong tea.The transcriptome of fresh leaves between Chungui and Huangdan varieties were compared.The results showed that the expression levels of key genes such as ASA1,PAT1,TSB2,DXS,DXR,NES,TPS,which were related to the formation of indole and terpenoid aroma,were significantly higher in Chungui than that in Huangdan.The expression level of ANR,ANS,LAR,DFR,FLS,C4 H,CHI,CHS,F3 H,and F3’H,related to the formation of flavor substances like flavonoids were significantly lower than that of Huangdan.4.Transcription regulation analysis of characteristic flavor substances formation during the processing of Chungui oolong teaPathway analysis showed that the upstream key genes PAL and C4 H in the flavonoid synthesis pathway were significantly up-regulated in the middle and late stages of the turning-over process,regulating the content of L-phenylalanine and cinnamic acid to reach their peaks in this stage.The key genes encoding the biosynthesis of flavonols and catechins,such as CHS,CHI,FLS,DFR,LAR,and ANR,were most significantly down-regulated in the middle and late stages of withering and turning-over,leading to a decrease trend of the content of flavonoids such as catechins.The key genes for terpenoid aroma synthesis,such as NES,LIS,and GES,were significantly up-regulated after the withering of Chungui,and the corresponding aroma substances nerolidol and geraniol also increased simultaneously.The tryptophan synthase genes TSB1 and TSB2 related to indole synthesis were highly expressed during the withering process,and were significantly reduced in the later stage of turning-over,but still maintained extremely high expression levels.Jasmonic acid signal transduction pathway genes(JAR,COI1,MYC2)were significantly up-regulated in the withering process,and significantly down-regulated in the turning-over process.MYC2 transcription factors had similar expression patterns with TSB1 and TSB2.A weighted gene co-expression network analysis(WGCNA)was used to construct co-expression networks between taste,aroma,and genes in Chungui oolong tea processing.A total of 17 transcription factors including 2 WRKY,3 MYB,3 ERF,3 NAC and ARF,DBB,GRAS,GRF,MYB＿related,SBP,and 5 structural genes including GGPS,NES,TSB2,JMT,and CHS were obtained.These key hub genes play a potential regulatory role in the biosynthesis of terpenoid volatiles,indole,jasmone,flavonoids,and other flavor substances,thus affecting the flavor characteristics of Chungui oolong tea with fresh,mellow taste and formation of jasmine fragrance. |
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