Dear Editor,

Theanine, one of the most important tea quality components, confers the ‘umami’ taste and anti-stress benefits of tea infusion [1]. Theanine is a unique non-proteinaceous amino acid solely accumulating to a high level in tea plants, and is primarily synthesized in the roots from glutamate (Glu) and ethylamine (EA) by theanine synthetase (CsTSI) [1, 2]. The high EA availability was suggested to be why only tea plants can synthesize large amounts of theanine [3]. Our recent study further indicated that EA contents in the roots of various tea plant cultivars were highly correlated with the theanine contents in the roots [4]. EA is synthesized from alanine by alanine decarboxylase CsAlaDC [4, 5]. Further experiments suggested that the CsAlaDC and CsTSI work in tandem to determine theanine biosynthesis in tea plants [4]. Interestingly, the expression of CsTSI in the roots of different tea plant cultivars was not obviously correlated with the theanine contents; while the expression of CsAlaDC was shown to be highly correlated with both EA contents and theanine contents in the roots [4]. These results indicated that the regulation of CsAlaDC expression is critical for adjusting the activity of theanine synthesis in tea plants. However, the mechanism underlying the regulation of CsAlaDC expression is largely unknown in tea plants. Uncovering the mechanism will be critical for improving theanine biosynthesis in tea plants.

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