Bioscience Methods 2024, Vol.15, No.3, 114-123 http://bioscipublisher.com/index.php/bm 116 biosynthesis and accumulation of these compounds (Singh et al., 2008; Wei et al., 2018). Understanding these factors is essential for optimizing the production and quality of tea to maximize its health benefits. 3 Metabolic Pathways in Tea Plants 3.1 Overview of metabolic pathways leading to bioactive compound synthesis Tea plants (Camellia sinensis) are renowned for their rich array of bioactive compounds, primarily catechins, theaflavins, and thearubigins, which are synthesized through complex metabolic pathways. The biosynthesis of these compounds predominantly follows the phenylpropanoid and flavonoid pathways. Catechins, for instance, are synthesized from phenylalanine through a series of enzymatic reactions involving phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate-CoA ligase (4CL), leading to the formation of flavan-3-ols such as (-)-epigallocatechin gallate (EGCG) and (-)-epicatechin gallate (ECG) (Punyasiri et al., 2004; Wang et al., 2018; Zhang et l., 2019). The transformation of catechins into theaflavins and thearubigins occurs during the fermentation process, which involves oxidative polymerization catalyzed by polyphenol oxidase (PPO) (Tanaka et al., 2004; Yu et al., 2020). The biosynthesis of other significant compounds like theanine and caffeine also involves specific pathways. Theanine is synthesized from glutamic acid and ethylamine, catalyzed by theanine synthetase, while caffeine biosynthesis involves the methylation of xanthosine (Wei et al., 2018; Zhang et al., 2021). These pathways are tightly regulated and influenced by various environmental factors, such as shading, which can alter the expression of key enzymes and subsequently the levels of bioactive compounds (Yu et al., 2020; Zeng et al., 2020). 3.2 Key Enzymes and genes involved Several key enzymes and genes play crucial roles in the metabolic pathways of tea plants. For catechin biosynthesis, enzymes such as anthocyanidin reductase (ANR) and leucoanthocyanidin reductase (LAR) are vital, converting anthocyanidins to catechins (Punyasiri et al., 2004; Wang et al., 2018). The gene CsPPO3, encoding polyphenol oxidase, is particularly important for the oxidation of catechins to theaflavins during tea processing. This gene is highly expressed under shading conditions, which enhances PPO activity and theaflavin content in preharvest tea leaves (Yu et al., 2020). In theanine biosynthesis, the gene encoding theanine synthetase (CsTSI) is regulated by the MYB transcription factor CsMYB6, which binds to the promoter region of CsTSI, enhancing its expression in the roots of tea plants (Zhang et al., 2021). For caffeine biosynthesis, genes involved in the methylation process, such as caffeine synthase, are critical. The draft genome sequence of Camellia sinensis has identified numerous gene families and their duplications, which are essential for the biosynthesis of these key metabolites (Wei et al., 2018). 3.3 Regulatory mechanisms of metabolic pathways The regulation of metabolic pathways in tea plants involves a complex network of genetic and environmental factors. Transcription factors such as MYB play a significant role in regulating the expression of genes involved in the biosynthesis of bioactive compounds. For instance, CsMYB6 regulates theanine synthesis by activating the theanine synthetase gene in the roots (Zhang et al., 2021). Additionally, environmental factors like shading can modulate the expression of genes such as CsPPO3, thereby influencing the levels of catechins and theaflavins in tea leaves (Yu et al., 2020). Post-transcriptional modifications, such as alternative splicing and the presence of long noncoding RNAs (lncRNAs), also contribute to the regulation of these pathways. These modifications can affect the stability and translation of mRNAs encoding key enzymes, thereby fine-tuning the biosynthesis of bioactive compounds (Zhang et al., 2021). Furthermore, feedback regulation mechanisms, where the accumulation of end products like catechins can down-regulate the expression of biosynthetic genes, ensure a balanced production of these compounds (Singh et al., 2008).
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