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Scientists have unlocked the genome of the tea tree, which may help explain why tea leaves are rich in antioxidants and caffeine, and how they produce so many flavours. The most popular varieties of tea - including black tea, green tea, Oolong tea, white tea, and chai - all come from the leaves of the evergreen shrub Camellia sinensis, otherwise known as the tea tree. Despite tea's immense cultural and economic significance, relatively little is known about the shrub behind the tea leaves. Now, researchers from Kunming Institute of Botany in China have studied how the tea tree genetically differs from its close relatives. The genus Camellia contains over 100 species including several popular decorative garden plants and C oleifera, which produces "tea tree" oil-but only two major varieties (C sinensis var assamica and C sinensis var sinensis) are grown commercially for making tea. "There are many diverse flavours, but the mystery is what determines or what is the genetic basis of tea flavours?" said Lizhi Gao, plant geneticist at the Kunming Institute of Botany in China. Previous studies have suggested that tea owes much of its flavour to a group of antioxidants called flavonoids, molecules that are thought to help plants survive in their environments. A bitter-tasting flavonoid called catechin is particularly associated with tea flavour.
Levels of catechin and other flavonoids vary among Camellia species, as does caffeine.Gao and his colleagues found that C sinensis leaves not only contain high levels of catechins, caffeine, and flavonoids, but also have multiple copies of the genes that produce caffeine and flavonoids. Caffeine and flavonoids such as catechins are not proteins (and therefore not encoded in the genome directly), but genetically encoded proteins in the tea leaves manufacture them. All Camellia species have genes for the caffeine - and flavonoid-producing pathways, but each species expresses those genes at different levels. That variation may explain why C sinensis leaves are suitable for making tea, while other Camellia species' leaves are not, researchers said. They estimated that more than half of the base pairs (67 per cent) in the tea tree genome are part of retrotransposon sequences, or "jumping genes", which have copied-and-pasted themselves into different spots in the genome numerous times. The large number of retrotransposons resulted in a dramatic expansion in genome size of tea tree and possibly many duplicates of certain genes, including the disease- resistant ones. The researchers think that these "expanded" gene families must have helped tea trees adapt to different climates and environmental stresses, as tea trees grow well on several continents in a wide range of climate conditions. The study was published in the journal Molecular Plant.