For the perfect cup of tea, does one add milk to boiling water, or the reverse? Neither, if new research in the journal Current Biology is to be believed. What must be prepared first, instead, are the microbes.

Scientists are increasingly aware that the fungi and bacteria living around a plant’s roots can significantly affect its growth. Previous studies on Arabidopsis, a species of cress that is the lab rat of the botanical world, have shown that the presence of the right microbes can enhance the absorption of critical nutrients like iron, phosphorus and nitrogen.

The possibility of enhanced nitrogen absorption was particularly intriguing to tea experts Wei Xin and Wenxin Tang at Fujian Agriculture and Forestry University in China because tea plants rely upon nitrogen-rich ammonium in the soil around their roots to produce theanine, an amino acid that generates their distinctive savoury flavours. Collecting the roots and leaves of 17 different tea varietals at different times of year, Dr Xin and Dr Tang found that theanine production varied widely, even among similar teas. Some highly valued oolong teas (the term refers to those whose leaves are partially oxidised after harvesting), such as Rougui, produced a lot of the compound, whereas others, such as Maoxie, produced less.

After growing these two varietals under identical conditions, the researchers collected soil samples and used genetic analysis to identify which microbes were present around the roots. The microbiomes of the two were very different. Crucially, Rougui had more microbes associated with the metabolising of nitrogen than did Maoxie. Consequently, Rougui also showed a greater ability to absorb nitrogen from ammonium in the soil. These differences were particularly stark in the autumn, when the roots were accumulating theanine.

This provoked a natural question. If the microbial community found around Rougui were transplanted to other tea plants, might this increase their theanine production and, in turn, improve their flavour? To explore this, the researchers created a community of microbes made up of 21 bacterial strains that resembled the community found around autumnal Rougui roots and applied them to the roots of Maoxie plants. They report this week that this not only enhanced root growth in Maoxie seedlings by giving them a nitrogen boost, but also nearly tripled the theanine accumulation in the leaves. While the time-consuming process of transforming leaves into drinkable tea has yet to be completed, the researchers have little doubt that there will be big differences.

Flavours aside, the Rougui microbe sample that Dr Xin and Dr Tang worked with has the potential to make a wider environmental impact. Most farmers, not just tea growers, add some form of nitrogen to their soil to enhance growth. While some of this is absorbed by plants, most runs off during heavy rains and ends up polluting natural water bodies. When the researchers applied their microbial cocktail to soil around the roots of Arabidopsis plants, they found that, just as with Maoxie, the newcomers enhanced nitrogen uptake. This hints that these microbes have the potential to create new flavours while also reducing agricultural pollution. That should be everyone’s cup of tea. ■

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