L-alanine in tea tree is converted to L-theanine by L-glutamic acid instead of ethylamine

L-Theanine contributes to the function and taste of tea. Ethylamine is the limiting factor that distinguishes L-theanine accumulation between tea and other plants. Ethylamine has long been thought to be derived from L-alanine in tea. In this study, L-alanine in tea root cells was found mainly in the vacuole and mitochondria using non-aqueous separation techniques, while alanine decarboxylase (CsADC) in tea was located in the cytoplasm. Although CsADC is able to catalyze the decarboxylation of L-alanine to ethylamine in vitro, it may not provide the same enzymatic activity in tea tree. Tracing of stable isotope-labeled precursors in tea plants found that L-alanine was not a direct precursor of ethylamine, but a precursor of L-glutamic acid, which was associated with L-theanine in tea related to biosynthesis. The epidermal cortex from root tissue is the main distribution site for ethylamine. In conclusion, in the tea plant, L-alanine is converted to L-theanine by L-glutamic acid rather than ethylamine.