Tea Tree Genome Sequenced

A multi-institutional team of researchers from China and the United States has sequenced the genome of the cultivated tea tree (Camellia sinensis).

A tea tree plantation in Kerala, India. Image credit: Rajib Ghosh.

Socially and habitually consumed by more than 3 billion people across 160 countries, tea is the world’s oldest (since 3000 BC) and most popular caffeine-containing beverage with immense economic, medicinal, and cultural importance.

Camellia sinensis is the source of commercially grown tea and a member of the genus Camellia (family Theaceae), which contains over 100 species — including several other economically important species, e.g. well-known camellias with their attractive flowers (C. japonica, C. reticulata, and C. sasanqua) and the traditional oil tree C. oleifera.

The research team, headed by Dr. Lizhi Gao, a plant geneticist at the Kunming Institute of Botany, China, sequenced the genome of Yunkang 10, a diploid elite cultivar of C. sinensis var. assamica widely grown in Southwestern China.

“We have successfully sequenced and assembled more than twenty plant genomes. But this genome, the tea tree genome, was tough,” said Dr. Gao, who is the corresponding author of a paper about the research that was published in the journal Molecular Plant on May 1, 2017.

The tea tree genome turned out to be much larger than initially expected: at 3.02 billion base pairs in length, it is more than four times the size of the coffee plant genome and much larger than most sequenced plant species.

Further complicating the picture is the fact that many of those genes are duplicates or near-duplicates.

Dr. Gao and co-authors estimate that more than half of the base pairs (67%) 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, 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.

Since much of the retrotransposon copying and pasting seems to have happened relatively recently in the tea tree’s evolutionary history, the authors theorize that at least some of the duplications are responses to cultivation.

This diagram shows the shared and unique gene families among Camellia sinensis and seven other plant species; each number in parentheses represents the number of genes within corresponding families (without parentheses). Image credit: Xia et al, doi: 10.1016/j.molp.2017.04.002.

Previous studies have suggested that tea owes much of its flavor to a group of antioxidants called flavonoids, molecules that are thought to help plants survive in their environments.

One, a bitter-tasting flavonoid called catechin, is particularly associated with tea flavor. Levels of catechin and other flavonoids vary among Camellia species, as does caffeine.

Dr. 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 aren’t.

“A comparative study among 25 Camellia species revealed that higher expression levels of most flavonoid- and caffeine- but not theanine-related genes contribute to the increased production of catechins and caffeine and thus enhance tea-processing suitability and tea quality,” the scientists said.

“These novel findings pave the way for further metabolomic and functional genomic refinement of characteristic biosynthesis pathways and will help develop a more diversified set of tea flavors that would eventually satisfy and attract more tea drinkers worldwide.”

_____

En-Hua Xia et al. The Tea Tree Genome Provides Insights into Tea Flavor and Independent Evolution of Caffeine Biosynthesis. Molecular Plant, published online May 1, 2017; doi: 10.1016/j.molp.2017.04.002