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.

Cheng Deng,Limao Fan,Lin-Lin Cheng,Si-An Pan,Xiuping Ku,Jian Zhao,Wei-Wei Deng,Zheng-Zhu Zhang

doi:10.3389/fpls.2020.551288

Abstract

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.

Highlights

As one of the most popular non-alcoholic beverages in the world, tea is made from the leaves of tea plants [Camellia sinensis (L.) O
When hydroponically grown tea seedlings or detached tea plant shoot tips were fed with 15 mM caffeine and theophylline, a clear feedback inhibition on caffeine biosynthesis was observed in both roots and shoot tips, for these tea plant tissues fed with caffeine
The feedback inhibition was reflected by immediate decrease in theobromine precursor level in shoot tips after being fed with caffeine for 6 h, and the decreases were lasted till 72 h (Figures 1B, C)

Summary

Introduction

As one of the most popular non-alcoholic beverages in the world, tea is made from the leaves of tea plants [Camellia sinensis (L.) O. The tea flavors and health functions are mostly attributable to several characteristic secondary metabolites, including catechins, caffeine, theanine, and terpenoids in tea plant leaves (Chacko et al, 2010; Pinto, 2013; Yang and Zhang, 2019). Caffeine (1,3,7-trimethylxanthine) is one key bioactive component present in many popular drinks worldwide, including tea and coffee. It could stimulate central nervous system, accelerate metabolic rate and be diuretic (Higdon and Frei, 2006). Because high-dosage intake of caffeine can cause health problems such as in sleeping and learning, an increasing number of consumers pursue caffeine-free or lowcaffeine teas without changing their flavors. Breeding of caffeine-free or low-caffeine tea plant varieties becomes one of the targets in tea plant breeding (Ashihara and Crozier, 2001; Mohanpuria et al, 2011; Deng et al, 2015)

Methods

Results

Conclusion