Triterpenoid saponins in tea (Camellia sinensis) plants: biosynthetic gene expression, content variations, chemical identification and cytotoxicity.

Very little is known about saponins in tea and their biosynthesis in tea plants despite of the importance. Here, we studied tea saponins and their biosynthesis genes. Saponins were promptly recovered in tea infusions. Cytotoxicity of tea saponin extracts on human tongue squamous and hepatocellular carcinoma lines showed respective IC50 values of 29.2 and 17.5 µg/mL, which may be attributable to over 40 saponins identified in green tea. Saponin contents varied in shoot tips of 42 tea plant varieties but did not change drastically during tea processing. Saponin biosynthetic gene expression was consistent with its contents in plant tissues. Thus, plant tips produce significant amounts of saponins, which are stable during tea processing, and ready to be recovered to tea infusions to provide potent health benefits to consumers. This study paves a road towards clarifying the biosynthesis and genetic improvement of saponins in tea plants.

Metabolite and Transcriptome Profiling on Xanthine Alkaloids-Fed Tea Plant (Camellia sinensis) Shoot Tips and Roots Reveal the Complex Metabolic Network for Caffeine Biosynthesis and Degradation.

While caffeine is one of the most important bioactive metabolites for tea as the most consumed non-alcohol beverage, its biosynthesis and catabolism in tea plants are still not fully understood. Here, we integrated purine alkaloid profiling and transcriptome analysis on shoot tips and roots fed with caffeine, theophylline, or theobromine to gain further understanding of caffeine biosynthesis and degradation. Shoot tips and roots easily took up and accumulated high concentrations of alkaloids, but roots showed much faster caffeine and theophylline degradation rates than shoot tips, which only degraded theophylline significantly but almost did not degrade caffeine. Clearly feedback inhibition on caffeine synthesis or inter-conversion between caffeine, theophylline, and theobromine, and 3-methylxanthine had been observed in alkaloids-fed shoot tips and roots, and these were also evidenced by significant repression of TCS and MXMT genes critical for caffeine biosynthesis. Among these responsively repressed genes, two highly expressed genes TCS-4 and TCS-8 were characterized for their enzyme activity. While we failed to detect TCS-4 activity, TCS-8 displayed N-methyltransferase activities towards multiple substrates, supporting the complex metabolic network in caffeine biosynthesis in tea plants since at least 13 TCS-like N-methyltransferase genes may function redundantly. This study provides new insight into complex metabolic networks of purine alkaloids in tea plants.