Biosynthesis and Application of Catechins and Their Derivatives in <i>Camellia sinensis</i>

Tea is one of the world's most widely consumed non-alcoholic beverages and is a rich source of bioactive compounds. The synthesis and accumulation of catechins, central flavonols in the secondary metabolism of the tea plant, not only define the bitterness and astringency of the tea infusion but also form the chemical cornerstone of its remarkable antioxidant activity, playing a key physiological role in the plant's abiotic stress tolerance. Catechin biosynthesis involves a complex enzymatic network in tea plants (<i>Camellia sinensis</i>). With the development of molecular biotechnology, the biosynthetic and chemical pathways have been progressively clarified, and the roles of key enzymes have been increasingly understood. Moreover, accumulating evidence shows that catechin distribution displays tissue-specific patterns, providing important references for genetic improvement and utilization of tea resources. This review summarizes current knowledge on catechins and their derivatives, including biosynthesis and applications. It also highlights the specific accumulation of monomeric and polymeric catechins in different tissues and germplasm and discusses their prospective applications in functional foods, pharmaceuticals, and other health-related industries. This work synthesizes the biosynthesis, tissue accumulation, and multifaceted health benefits of catechins, offering a theoretical basis for optimizing tea germplasm and deep-processing. It underscores the nutrient and medicinal potential of catechins, thereby informing the development of functional foods and pharmaceuticals and enhancing the value chain of the tea industry.