Genetic mechanisms of the biosynthesis of catechins, caffeine and L-theanine in the tea plant Camellia sinensis (L.) Kuntze (review)

Catechins, caffeine and L-theanine are the main secondary metabolites of the tea plant Camellia sinensis (L.) Kuntze. They play a key role in shaping the taste, nutritional and medicinal value of tea (W.J.M. Lorenzo et al., 2016; Z. Yan et al., 2020). In addition, they are involved in the regulation of plant life, in particular, in the processes of adaptation to extreme environmental conditions (Y.S. Wang et al., 2012; L.G. Xiong et al., 2013; G.J. Hong et al., 2014). The above determines the interest in the physiological, biochemical and molecular mechanisms of the production of catechins, caffeine and L-theanine, to increase their accumulation in the plant (R. Fang et al., 2017; W. Kong et al., 2022), as well as to studying their participation in plant response to stress (P.O. Owuor et al., 2010). In the recent 5 years, a lot of new knowledge has been gained on the genes for the biosynthesis of catechins, L-theanine and caffeine, but there are no new reviews that generalize these new data and connect them with new data on the regulation of stress responses in tea. The purpose of this review is to analyze and summarize current data on the genetic mechanisms of the biosynthesis of catechins, L-theanine and caffeine in tea plant tissues, as well as their relationship with genes that regulate abiotic stress responses. The biosynthesis of catechins is carried out along the phenylpropanoid and flavonoid pathways (A. Laura et al., 2019; S. Alseekh et al., 2020) with the participation of the chalcone synthase ( CHS ,), anthocyanidin synthetase ( ANS ), anthocyanidin reductase ( ANR ,) and leucoanthocyanidin reductase genes ( LAR ) (J. Bogs et al., 2005). The accumulation of catechins in the tea plant involves transcription regulation factors of the MYB family, which regulate the expression of the PAL , F3′H , and FLS genes (C.-F. Li et al., 2015). Caffeine formation occurs mainly in tea leaves during purine modification (H. Ashihara, 2015) involving the IMPDH (Inosine monophosphate dehydrogenase), SAMS (Synthetase gene family), MXMT (7-methylxanthine methyltransferase), and TCS (tea caffeine synthase) genes. There are already 132 known transcription factors belonging to 30 families (including those encoded by genes of the bZIP , bHLH and MYB families), which are associated with the expression of caffeine biosynthesis genes (C.-F. Li et al., 2015). In C. sinensis, the biosynthesis of L-theanine from glutamate with the participation of pyruvate is controlled by a cascade of genes, the main of which are GS (glutamine synthetase), GOGAT (glutamate synthase), GDH (glutamate dehydrogenase), ALT (alanine transaminase), ADC (arginine decarboxylase), and TS (theanine synthetase) (C.Y. Shi et al., 2011; Y. Li et al., 2019). The regulation of these genes is conducted by more than 90 transcription factors - members of the AP2-EREBP , bHLH , C2H2 and WRKY , bZIP , C3H , and REM families (C.-F. Li et al., 2015). The influence of stress conditions (drought, cold, salinity, nutrient deficiency) on accumulation of these biologically active substances is discussed. Nevertheless, the relationships between the expression of the metabolism genes of the studied compounds and transcription factors remain insufficiently studied; as well as changes in regulatory networks for the biosynthesis of valuable metabolites of tea plants under various environmental stresses.