Theanine (N-ethyl-γ-l-glutamine) is a special nonprotein
amino acid that contributes to the umami taste and health function
of tea. Although recent studies on tea breeding have focused on albino
tea because of its umami taste, a factor of higher theanine concentration,
the mechanism of biosynthesis of l-theanine is still unclear.
In this study, four glutamine synthetase genes (CsGSs) were obtained and functionally characterized by overexpressing
them in Arabidopsis. The enzyme activities
of the purified CsGS proteins from Escherichia coli were detected. The results showed that CsGSs have a dual function
in the synthesis of glutamine and theanine in vivo and in vitro. Interestingly, l-theanine was abundantly synthesized in the tender shoots of
“Huabai 1”. In the white tender shoots, the cytosol CsGS1.2 might exhibit increased expression to compensate
for decreasing levels of chloroplast CsGS2, which
plays a vital role in high accumulation of theanine in “Huabai
1”. In addition, CsGS2 was most likely the
key l-theanine synthases in green tissues of tea. The present
findings will provide basis for and considerably broaden the scope
of understanding the function of CsGSs and the mechanism
of l-theanine accumulation in the tender shoots of “Huabai
1”, and will be useful for breeding and screening tea with
high l-theanine content.
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