Journal of Tea Science Research, 2024, Vol.14, No.6, 335-343 http://hortherbpublisher.com/index.php/jtsr 336 Such a knowledge gap needs to be filled for the genetic improvement of tea cultivars through molecular breeding (Wang et al., 2018). This study outlines the recent advances in the genetic and molecular control of flavonoid biosynthesis in Camellia sinensis. It includes the principal classes of tea flavonoids, the involved biosynthetic pathways, the functions of structural genes and transcriptional regulators, and epigenetic and non-coding RNA-mediated regulation. Besides, it highlights the application of multi-omics approaches in flavonoid network dissection and explores the prospects of these discoveries in molecular breeding and functional tea product development. By the integration of existing knowledge, this study is supposed to facilitate future research and innovation in tea quality and plant metabolic engineering. 2 Overview of Flavonoid Types and Biosynthetic Pathways in Tea Plants 2.1 Major types of flavonoids: catechins, anthocyanins, flavonols, etc. Tea plants synthesize a diverse array of flavonoids, with catechins (such as epigallocatechin gallate, EGCG), anthocyanins, and flavonols (like quercetin and kaempferol derivatives) being the most prominent. Catechins are especially abundant in young leaves and are key contributors to tea’s taste, color, and health benefits. Anthocyanins are responsible for the purple coloration in certain tea cultivars, while flavonols are widely distributed and contribute to both color and antioxidant properties. Flavonoid glycosides, such as 7-O-neohesperidoside, also play a role in bitterness and astringency (Song et al., 2022). 2.2 Core biosynthetic pathways: phenylpropanoid pathway and flavonoid-specific branches Flavonoid biosynthesis in tea plants is activated by the phenylpropanoid pathway, in which cinnamic acid is formed from phenylalanine by phenylalanine ammonia-lyase (PAL). This is succeeded by a series of enzyme-catalyzed reactions by chalcone synthase (CHS), chalcone isomerase (CHI), and flavanone 3-hydroxylase (F3H) that leads to the production of various flavonoid subclasses. Branch pathways produce particular metabolites such as catechins, anthocyanins, and flavonols each regulated by particular enzymes and transcription factors. Recent research emphasizes the function of gene duplication (e.g., CHS genes), protein complexes, and post-translational modifications in regulating pathway efficiency and diversity (Shen et al., 2022). 2.3 Tissue-specific and spatiotemporal expression patterns of flavonoid biosynthesis Flavonoid biosynthesis in tea plants is highly tissue- and stage-specific. Young leaves and buds accumulate the highest levels of catechins and flavan-3-ols, while anthocyanins and certain flavonols are enriched in purple leaves or at specific developmental stages. The expression of biosynthetic genes and regulatory factors varies across tissues and in response to environmental cues such as light, nitrogen, and sugar signals, reflecting complex spatiotemporal regulation. For example, UV-B light and sugar signals can upregulate key biosynthetic genes, while nitrogen deficiency can enhance flavonoid accumulation in roots (Wang et al., 2021; Song et al., 2022; Li et al., 2024) (Figure 1). 3 Key Structural Genes Involved in Flavonoid Biosynthesis 3.1 Genes encoding key enzymes in the phenylpropanoid pathway: PAL, C4H, 4CL Phenylpropanoid pathway is the pathway that regulates the initiation of flavonoid biosynthesis. The conversion of phenylalanine to cinnamic acid is catalyzed by phenylalanine ammonia-lyase (PAL), followed by cinnamate 4-hydroxylase (C4H) and 4-coumarate:CoA ligase (4CL) to form 4-coumaroyl-CoA, a major precursor in flavonoid biosynthesis. There are various PAL and 4CL genes in Camellia sinensis plants, such as Cs4CL1 and Cs4CL2, which have tissue-specific expression and varied responses to environmental stimuli such as UV-B and wounding (Li et al., 2022). 3.2 Structural genes in the flavonoid pathway: CHS, CHI, F3H, DFR, ANS Chalcone synthase (CHS) catalyzes the first committed step in flavonoid biosynthesis, producing chalcones from 4-coumaroyl-CoA. Chalcone isomerase (CHI) isomerizes chalcones to flavanones, which are hydroxylated by flavanone 3-hydroxylase (F3H) to dihydroflavonols. Dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS) act on these intermediates to form anthocyanidins and other flavonoids. Tea plants contain multiple CHS genes, wherein gene duplication incidents have influenced functional diversity and environmental
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