Journal of Tea Science Research, 2024, Vol.14, No.6, 313-321 http://hortherbpublisher.com/index.php/jtsr 315 3.4 Amino acid metabolism and theanine biosynthetic mechanism Theanine is a unique amino acid in tea plants, which is primarily produced in the roots as an enzyme-catalyzed reaction between glutamic acid and ethylamine. It is catalyzed by theanine synthetase and is transported to the aerial parts of the plant produced as theanine. Both its biosynthesis and accumulation are controlled by environmental and transcriptional regulation, and a few MYB transcription factors participate in the process of regulation (Zhao et al., 2020). Similarly, catechins, being major secondary metabolites of tea plants, are biosynthesized through coordinated regulation of some significant structural genes and transcription factors. Experiments have shown that the catechin biosynthetic pathway is linked to gene expression and catalytic activity of genes such as CHS, CHI, F3H, F3′H, F3′5′H, DFR, ANS, LAR, ANR, and SCPL(Wei et al., 2018). These genes have tissue-specific expression modes, with higher expression levels in apical buds and young leaves, which are highly correlated with catechin accumulation. At the transcriptional level, co-expression network analysis has also demonstrated that various transcription factors (e.g., MYB and bHLH) are co-expressed with catechin biosynthetic genes, regulating their expression and metabolic processes in different tissues (Wei et al., 2018) (Figure 1). These findings reveal both the shared transcriptional processes and complex regulatory networks involved in the biosynthesis of secondary metabolites such as catechins and theanine in tea plants. Figure 1 Evolution and expression of key genes involved in catechins biosynthesis. (A) Biosynthetic pathway of the principal catechins. CHS, CHI, F3H, F3′H, F3′ 5′H, DFR, ANS, LAR, ANR, and SCPL represent genes encoding chalcone synthase, chalcone isomerase, flavanone 3-hydroxylase, flavonoid 3′-hydroxylase, flavonoid 3′,5′-hydroxylase, dihydroflavonol 4-reductase, anthocyanidin synthase, leucoanthocyanidin reductase, anthocyanidin reductase, and type 1A serine carboxypeptidase-like acyltransferases, respectively. (B) Expression profiles of key genes in different tissues of the tea plant in relation to their contents of different catechins. (B, Left) Expression levels of key genes associated with catechins biosynthesis in eight tea plant tissues: apical buds, young leaves, mature leaves, old leaves, young stems, flowers, young fruits, and tender roots. Expression data are plotted as log10 values. The horizontal axis of the boxplot (Right) shows statistics of catechins contents from different tissues, and the vertical axis exhibits different forms of catechins. “Cis” represents the contents of cis-flavan-3-ols, and “trans” represents the contents of trans-flavan-3-ols. The significant correlations of gene expression with the contents of ECG, EGCG, and cis-flavan-3-ols are indicated by black lines (Pearson’s correlation test, P< 0.05). The error bar represents the maximum and minimum catechins content in eight different tea plant tissues. (C) Transcriptional regulation of catechins biosynthetic genes. A coexpression network connecting structural genes in cat- echins biosynthesis with transcription factors represents the regulation of catechins biosynthetic genes. The color-filled hexagons represent the structural genes associated with catechins biosynthesis that was highly (green) or lowly (red) expressed in bud and leaf. Expression correlations between TFs (colored solid circles) and catechins-related genes (colored solid hexagons) are shown with colored lines (Pearson’s correlation test, P≤ 1e-6) (Adopted from Wei et al., 2018)
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