Tim Wallace
Few plants can claim such a significant place in human history as Camellia sinensis, better known as the tea tree, whose leaves are used to produce the huge number of varieties of tea – black, green, white or oolong – routinely consumed by 3 billion people around the world.
Tea is intrinsic to national cultures, intertwined with the rise and fall of empires and integral to social and economic life – not least because the habitual act of preparing tea spared generations from the pathogens lurking in unboiled water.
So what makes this particular species – one of more than 100 within the Camellia genus – so special, cultivated for three millennia and now grown across the globe in more than 100 countries?
The first high-quality mapping of the plant’s genome, by a team of mostly Chinese scientists, gives insights into the unique biology behind the qualities praised by philosophers, physicians and poets alike. The research reveals the tea tree’s past and illuminates its potential future, opening up the possibility of cultivating “super tea” varieties rich in flavour and health effects.
This “first draft” sequencing of the genome, published in the journal Molecular Plant, took more than five years to complete.
“Our lab has successfully sequenced and assembled more than twenty plant genomes,” said Li-Zhi Gao, of the Kunming Institute of Botany in China. “But this genome, the tea tree genome, was tough.”
The complexity of the task was due to the tea tree’s unusually long genome – four times longer than that of coffee and eclipsing the other highly cultivated asterids whose genomes have been mapped – potato, tomato and pepper.
The genome’s size is inflated by its high proportion of repetitive DNA, with multiple copies of duplicated sequences. The researchers suggest this high degree of repetition is due to both the tea tree’s long evolutionary history (50 million years) and a paucity of the efficient DNA-removal mechanisms identified in other flowering plants.
Yet it is this very “inefficiency” that seems to have contributed to the tea tree’s evolutionary success, facilitating the expansion of those gene families associated with the plant’s environmental adaptability and resistance to disease as well as qualities valued by humans: its flavour and medicinal benefits.
Tea gets its flavour and nutritional and medicinal properties from three major metabolites: catechin, theanine and caffeine. Catechin, a flavonoid and natural antioxidant, has been linked to health benefits, particularly for cardiovascular function. Theanine, an amino acid, is associated with inducing feelings of physical and mental calmness. It works in concert with caffeine, a central nervous system stimulant, to combat fatigue and improve alertness.
The genome mapping identifies 24 important genes in the tea tree that encode enzymes involved in the biosynthesis of catechins, theanine, and caffeine. Of those genes, 14 were involved in the last few steps of producing catechins, six with theanine, and four with caffeine. The researchers say their analysis suggests “the three characteristic metabolic pathways” have remained well-conserved in the tea tree’s lineage for more than 6 million years.
In addition, the researchers have identified that the tea genome is also significantly enriched in functions related to the synthesis of terpenoids, natural compounds that are major components of essential oils and aromas, and which may therefore associated with tea’s enticing aromas.
While tea’s flavour may be affected by terpenoids as well as still-unrecognised secondary metabolic compounds, the researchers say it is the tea tree’s high content of catechins and caffeine that constitutes the fundamental basis of tea’s flavour, and which determine a tea variety’s processing suitability and ultimate quality.
The future development of “super teas” to satisfy and attract more tea drinkers globally, the research therefore suggests, will come from developing tea tree varieties with different combinations of those secondary metabolite characteristics.
“Our findings imply that wild relatives of cultivated tea tree present a huge reservoir for novel gene discovery toward the improvement of tea quality-related traits,” the researchers write.
“Considering the rapid extinction and severely endangered status of natural wild tea tree populations due to leaf over-harvesting, creating tea products at high market prices, the genome assembly of the tea tree and transcriptomic variation data presented here will offer valuable information to aid the global conservation of these precious wild tea tree species.”
