Journal of Tea Science Research, 2024, Vol.14, No.3, 169-181 http://hortherbpublisher.com/index.php/jtsr 172 the uniqueness of population 1 in metabolite accumulation. These findings reveal significant differences in the metabolite profiles of different tea plant populations, potentially related to the upregulation of specific genetic pathways, highlighting the complex relationship between genes and metabolites. 3 Biosynthesis Pathways of Phenolic Compounds 3.1 Flavonoids in tea Flavonoids are a major class of secondary metabolites in tea plants (Camellia sinensis) that significantly contribute to the quality and flavor of tea. The biosynthesis of flavonoids in tea involves a series of enzymatic reactions that convert phenylalanine into various flavonoid compounds. Key enzymes in this pathway include phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), and flavanone 3-hydroxylase (F3H) (Xia et al, 2017; Huang et al., 2018; Zhu et al., 2020). The expression of these enzymes is regulated by various transcription factors, including MYB and bHLH, which modulate the biosynthesis of flavonoids during different growth stages and environmental conditions (Zhu et al., 2020; Zhao et al., 2022). 3.2 Catechins in tea Catechins are a specific type of flavonoid that are abundant in tea leaves and are known for their health-promoting properties. The biosynthesis of catechins involves the conversion of flavan-3-ols, such as epicatechin and epigallocatechin, through the action of enzymes like anthocyanidin reductase and dihydroflavonol 4-reductase. These enzymes catalyze the reduction of anthocyanidins to produce catechins, which are then accumulated in the tea leaves (Fang et al., 2021). The levels of catechins in tea can be influenced by various factors, including nitrogen availability, which affects the expression of key biosynthetic genes (Huang et al., 2018). 3.3 Theaflavins and thearubigins in tea Theaflavins and thearubigins are polyphenolic compounds that are formed during the oxidation of catechins in the process of black tea production. Theaflavins are formed through the enzymatic oxidation of catechins, catalyzed by polyphenol oxidase, resulting in the formation of dimeric structures. Thearubigins, on the other hand, are higher molecular weight polymers that contribute to the color and astringency of black tea. The specific pathways and regulatory mechanisms involved in the formation of these compounds are complex and involve multiple enzymatic steps and environmental factors (Zhao et al., 2022). 4 Biosynthesis Pathways of Theanine in Tea 4.1 Theanine as a key amino acid contributing to tea flavor Theanine, a unique non-proteinogenic amino acid found abundantly in tea plants (Camellia sinensis), is a major contributor to the umami taste and relaxation effects of tea infusions. Its presence is directly correlated with the quality and price of green tea (Zhu et al., 2021; Lin et al., 2022). Theanine is synthesized primarily in the roots and then transported to the shoots, where it accumulates in the leaves (Lin et al., 2022). This amino acid not only enhances the flavor profile of tea but also offers various health benefits, making it a critical component in tea production (Lin et al., 2022; She et al., 2022). 4.2 Enzymatic steps involved in theanine biosynthesis The biosynthesis of theanine involves several key enzymatic steps. Theanine is synthesized from ethylamine and glutamate by the enzyme Theanine Synthetase I (CsTSI) (Zhu et al., 2021; She et al., 2022). Ethylamine itself is produced from alanine through the action of alanine decarboxylase (CsAlaDC) (Zhu et al., 2021). The expression of CsTSI is closely correlated with theanine and glutamine levels in various tissues of the tea plant, and its activity is influenced by nitrogen supply (She et al., 2022). Additionally, glutamine synthetase genes (CsGSs) have been shown to play a dual role in the synthesis of both glutamine and theanine, particularly in the tender shoots of certain tea cultivars (Yu et al., 2021). Research indicates that genetic differences among tea plant varieties and growth conditions significantly affect the theanine content (Yang et al., 2021). For example, shading cultivation can significantly increase theanine content in tea leaves due to the upregulation of theanine synthetase gene expression under low light conditions.
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