Journal of Tea Science Research, 2024, Vol.14, No.6, 313-321 http://hortherbpublisher.com/index.php/jtsr 316 3.5 Precursors and branched pathways of aroma compound biosynthesis Aroma compounds in tea are derived from multiple branched pathways, including those for terpenoids, phenylpropanoids, and amino acid derivatives. The expression of key biosynthetic genes and the interplay between different metabolic branches contribute to the diversity of volatile aromatic compounds in tea. Tissue-specific expression and developmental regulation further shape the aroma profile (Li et al., 2022). 4 Advances in Functional Genes and Key Enzymes 4.1 Cloning and expression regulation of structural genes Recent studies have succeeded in cloning and isolating genes of structural genes involved in secondary metabolism, such as genes encoding phenylalanine ammonia-lyase (PAL), flavonoid 3′-hydroxylase (F3′H), and some MYB transcription factors. These genes are regulated by sophisticated networks of regulators such as transcription factors MYB, bHLH, and WD40, which respond to environmental triggers and developmental cues (Jiao et al., 2023). The establishment of gene co-expression databases and integrative transcriptomic profiling has enabled the detection of conserved gene modules and regulatory elements controlling secondary metabolic pathways in tea plants (Zhang et al., 2020). 4.2 Functional identification of key metabolic enzymes Functional identification of key metabolic enzymes, such as those involved in proanthocyanidin and theanine biosynthesis, has been advanced through transient expression systems and recombinant protein assays. For example, subgroup 5 R2R3-MYB transcription factors have been shown to regulate proanthocyanidin biosynthesis, while specific MYB genes have been identified as regulators of theanine accumulation (Jiao et al., 2023). Transient transformation systems now allow for rapid gene function analysis and protein localization in tea leaves, accelerating the functional characterization of metabolic enzymes (Li et al., 2022). 4.3 Comparative analysis of varietal differences and expression patterns Comparative transcriptomic and metabolomic analyses across different tea cultivars have revealed significant varietal differences in gene expression and metabolite accumulation. Weighted gene co-expression network analysis (WGCNA) and multi-omics approaches have identified key drivers of flavonoid variation and stress response, as well as the impact of natural and artificial selection on gene family expansion and functional divergence, such as in glycosyltransferase (UGT) genes (Wang et al., 2024). These findings provide valuable resources for breeding programs aimed at improving tea quality and stress tolerance. 5 Regulatory Mechanisms at Transcriptional and Epigenetic Levels 5.1 Roles of transcription factors in regulation Transcription factors (TFs) like MYB, bHLH, WRKY, GRAS, and BZR1 families are key regulators of secondary metabolite biosynthesis in tea plants. MYB TFs, for instance, contribute to flavonoid, caffeine, theanine, and terpenoid biosynthesis, shoot development, and stress response (Li et al., 2022). BZR1 TFs are key factors in brassinosteroid signaling, integrating hormone and stress response, while WRKY and GRAS TFs play roles in abiotic stress resistance and developmental regulation. The TFs often act as components in complex regulatory networks, having responses to developmental and environmental cues in order to modulate the expression of metabolic genes. 5.2 Regulatory mechanisms involving non-coding RNAs Non-coding RNAs (ncRNAs), including microRNAs (miRNAs), long non-coding RNAs (lncRNAs), and circular RNAs (circRNAs), have emerged as important regulators of secondary metabolism in tea plants. They modulate the expression of key biosynthetic genes by acting as molecular sponges, forming competing endogenous RNA (ceRNA) networks, and targeting mRNAs for degradation or translational repression (Bordoloi et al., 2022). Recent studies have identified thousands of lncRNAs and miRNAs responsive to biotic and abiotic stresses, nitrogen application, and temperature, with many implicated in the regulation of catechin, theanine, and caffeine biosynthesis (Hu et al., 2023) (Figure 2).
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