Journal of Tea Science Research, 2024, Vol.14, No.6, 322-334 http://hortherbpublisher.com/index.php/jtsr 324 component analysis (PCA) and partial least squares discriminant analysis (PLS-DA), are helpful in distinguishing different aroma feature lineages and classifying tea varieties according to volatile characteristics (Chen et al., 2024; Wang et al., 2025). 3 Biosynthetic Pathways of Aroma Compounds in Tea 3.1 Biosynthesis of terpene-based aroma compounds Terpene aroma components, such as linalool, geraniol and nerolidol, are important sources of floral and fruity aromas in tea. In the tea plant (Camellia sinensis), these compounds are synthesized mainly through two metabolic pathways: the methylerythrose phosphate (MEP) pathway in the chloroplasts and the mevalonate (MVA) pathway in the cytoplasm (Liu et al., 2018). The key enzymes in the MEP pathway include 1-deoxy-D-xylulose-5-phosphate synthase (DXS) and geranyl diphosphate synthase (GGPPS), while the key enzymes in the MVA pathway are hydroxymethylglutaryl-CoA synthase (HMGS) and hydroxymethylglutaryl-CoA reductase (HMGR). Terpene synthases (TPSs) play a core catalytic role in the final stage of synthesis, determining the production of specific monoterpenes and sesquiterpenes. These related genes are often induced to express during stress or processing such as withering or mechanical damage, promoting the accumulation of aroma substances (Wang et al., 2021; Qiao et al., 2022). Differences in terpenoid biosynthesis ability are due to genetic variation among different tea varieties. For instance, in the high-flavor oolong variety "Huangdan", there is expansion and allelic variation in the TPS gene family, and these changes together shape its unique aroma characteristics (Wang et al., 2021; Gu et al., 2023). Quantitative trait loci (QTL) mapping and transcriptome analysis revealed candidate genes and regulatory modules associated with differential accumulation of terpenes in varieties such as "Huangdan", "Jinxuan", and "Jinguanyin" (Chen et al., 2023a; Gu et al., 2023; Gao et al., 2023; Wei et al., 2024). Transcription factors (e.g., MYB, bHLH, WRKY) and epigenetic modifications (such as DNA methylation and chromatin accessibility) also further regulate the expression of terpene synthesis genes, thereby forming variety-specific aroma expressions (Gao et al., 2023; Li et al., 2024b; Yue et al., 2025). 3.2 Phenylpropanoid and fatty acid-derived aroma compounds Phenylpropanoid aroma compounds, such as phenylethanol and benzaldehyde, are derived from the shikimate pathway, whose key precursor is L-phenylalanine. Fatty acid-derived volatiles, such as hexanal and (E)-2-hexenal, are converted from linolenic acid and linoleic acid through the lipoxygenase (LOX) pathway (Zeng et al., 2019; Wu et al., 2023). Linalool belongs to the terpenoid class of compounds, but is also affected by the above two pathways under the cross-regulation of metabolic networks. The biosynthesis of these compounds is finely regulated by structural gene expression and is highly sensitive to environmental signals and processing stress (Gao et al., 2023; Zhou et al., 2024). Because of differences in gene expression, allelic variation, and regulatory networks, the accumulation of phenylpropanoid and fatty acid-derived aroma components in different tea varieties varies significantly. For example, the expression of key biosynthetic genes (e.g., CsADH, CsLOX, and CsAOS) is regulated by transcription factors and stress signals (such as jasmonic acid JA and methyl jasmonate), which are often activated during postharvest processing (Wu et al., 2023; Li et al., 2024a; Zhou et al., 2024). In addition, epigenetic regulatory mechanisms such as DNA methylation and histone acetylation are also involved in the expression control of aroma-related genes, especially under adverse conditions (Yang et al., 2021; Li et al., 2024b). 3.3 Unique aroma compounds specific to tea plants Tea plants have unique genetic and metabolic networks that contribute to the formation of their unique aroma components. For instance, UDP-glycosyltransferases (UGTs) participate in the glycosylation of aroma precursors, storing volatile compounds in the form of glycosides, and releasing free aroma components through hydrolysis during processing (Zhou et al., 2017). In tea plants, the expansion and functional differentiation of UGTs and
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