Journal of Tea Science Research, 2024, Vol.14, No.3, 169-181 http://hortherbpublisher.com/index.php/jtsr 173 4.3 Genetic and environmental factors affecting theanine levels Theanine levels in tea plants are influenced by a combination of genetic and environmental factors. Transcription factors such as MYB have been identified as critical regulators of theanine biosynthesis. For instance, CsMYB9 and CsMYB49 are involved in the regulation of theanine biosynthesis, while CsMYB73 acts as a repressor (Wen et al., 2020; Li et al., 2022). Environmental factors such as nitrogen availability also play a significant role. Nitrogen promotes theanine biosynthesis while repressing flavonoid biosynthesis in tea plant roots (Wang et al., 2021). Additionally, temperature variations can affect theanine levels through hormone signal transduction pathways involving jasmonic acid and its derivatives (Zhu et al., 2023). These findings highlight the complex interplay between genetic regulation and environmental conditions in determining theanine content in tea plants. 5 Biosynthesis Pathways of Alkaloids Alkaloids are a significant class of secondary metabolites in tea, influencing both the flavor and aroma of tea, as well as having notable physiological activities. The primary alkaloids in tea include caffeine, theobromine, and theanine. The biosynthesis pathways of these compounds involve a series of complex enzymatic reactions and are regulated by genetic factors and environmental conditions. 5.1 Caffeine in tea Caffeine is one of the most well-known alkaloids in tea, widely recognized for its stimulating effects and ability to enhance alertness. Researchers have identified 13 N-methyltransferase (NMT) genes responsible for the three key methylation steps in caffeine biosynthesis (Figure 2). Comparative analysis indicates that tea plants have fewer NMT genes than cocoa and coffee, and the biosynthesis of caffeine begins with the synthesis of purine alkaloid compounds (Xia et al., 2017). Initially, theanine produced during the theanine synthesis process is converted into the precursor of caffeine, xanthine nucleotide. Subsequently, xanthine nucleotide is gradually converted into caffeine through a series of enzymatic actions, including xanthine nucleotide synthase and xanthine nucleotide methyltransferase (Ashihara and Crozier, 2001). In this process, S-adenosylmethionine (SAM) serves as a methyl donor to participate in the methylation reactions, ultimately leading to the production of caffeine. Studies have shown that the key enzyme for caffeine synthesis, xanthine nucleotide methyltransferase (XMT), exhibits different expression levels among various tea cultivars, resulting in differences in caffeine content across different types of tea. Figure 2 Evolution of Caffeine Biosynthesis (Adopted from Xia et al., 2017)
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