Journal of Tea Science Research, 2024, Vol.14, No.3, 169-181 http://hortherbpublisher.com/index.php/jtsr 174 Figure 2 in Xia et al. (2017) illustrates the caffeine biosynthesis pathway and the expression distribution of NMT genes in different tissues. Figure A shows the three key methylation steps catalyzed by NMTthat convert xanthine into caffeine. Figure B compares the number of NMT genes in tea, coffee, and cocoa, highlighting that tea plants have the fewest. Figure C displays the expression levels of NMT genes across various tissues, indicating higher expression in leaves and flowers. This figure underscores the independent evolution of caffeine biosynthesis in tea plants and the tissue-specific expression of key biosynthetic genes, providing a comprehensive overview of caffeine synthesis and accumulation in tea plants. 5.2 Theobromine in Tea Theobromine is another important alkaloid in tea, structurally similar to caffeine but with different physiological effects. Theobromine has diuretic, smooth muscle relaxant, and cardiac stimulant properties. The biosynthesis of theobromine is similar to that of caffeine, starting from the metabolism of theanine. Theanine undergoes a series of reactions to produce xanthosine nucleotide, which is then partially converted into theobromine instead of caffeine under the catalysis of xanthosine nucleotide methyltransferase (Ashihara and Crozier, 2001). Studies have found that environmental conditions, such as light intensity and temperature, can significantly affect theobromine content in tea, indicating the influence of environmental factors on its biosynthesis. The biosynthesis pathways of alkaloids in tea are diverse and complex, involving various enzymes and metabolic steps. The synthesis of caffeine and theobromine depends not only on the genetic background of the tea plant but is also significantly influenced by environmental conditions. By deeply studying these synthesis pathways and regulatory mechanisms, we can better understand and optimize the flavor and functional properties of tea, thereby improving its quality and market competitiveness. Moreover, utilizing modern biotechnological approaches such as gene editing and metabolic engineering can further enhance the synthesis capabilities of these secondary metabolites in tea plants, meeting the growing market demand. 6 Biosynthesis Pathways of Terpenoids Terpenoids are a diverse class of naturally occurring organic chemicals derived from five-carbon isoprene units. They play a significant role in the flavor profile of tea. This section delves into the biosynthesis pathways of three key terpenoids: linalool, geraniol, and nerolidol, which contribute to the unique aroma and taste of tea. 6.1 Linalool in tea Linalool is a monoterpenoid alcohol that imparts a floral scent to tea. The biosynthesis of linalool in tea involves the mevalonate (MVA) pathway and the methylerythritol phosphate (MEP) pathway. Studies have shown that the levels of linalool and its oxides are significantly higher in certain tea cultivars, such as 'Bai-Sang Cha' (BAS), compared to others like 'Fuding-Dabai Cha' (FUD) (Han et al., 2016). The enhanced transcription of terpenoid biosynthetic genes in 'BAS' suggests a genetic basis for the increased production of linalool. Additionally, the withering process during tea manufacturing has been found to enhance the metabolism of terpenoids, including linalool, thereby increasing its content in the final product (Wang et al., 2019). 6.2 Geraniol in tea Geraniol is another monoterpenoid alcohol that contributes to the floral aroma of tea. Similar to linalool, geraniol is synthesized via the MVA and MEP pathways. The levels of geraniol in tea can be influenced by both genetic factors and manufacturing processes. For instance, 'Bai-Sang Cha' (BAS) contains higher levels of geraniol compared to 'Fuding-Dabai Cha' (FUD), which can be attributed to the enhanced transcription of terpenoid biosynthetic genes in 'BAS' (Han et al., 2016). However, the levels of geraniol tend to decrease during certain manufacturing processes, such as pan-firing, which reduces the floral scent of the tea (Han et al., 2016). Moreover, the expression of specific microRNAs (miRNAs) has been found to be positively correlated with the content of geraniol, indicating a regulatory role of miRNAs in its biosynthesis (Li et al., 2021).
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