Transcriptome Analysis Reveals the Molecular Mechanism of GABA Accumulation during Quinoa (Chenopodium quinoa Willd.) Germination

Quinoa
(Chenopodium quinoa Willd.)
with a history of 5000 years as food is extremely rich in nutrients
and bioactive compounds, including γ-aminobutyric acid (GABA),
a natural four-carbon non-protein amino acid with great benefits to
human health. In quinoa, GABA generally increases with the germination
time, but the underlying molecular mechanism is unclear. Here, we
found that the GABA content in quinoa varied significantly among 25
varieties using an automatic amino acid analyzer. Next, six varieties
(three low-GABA and three high-GABA varieties) were used for further
analyses. The content of GABA in six varieties all showed an increasing
trend after germination. In addition, Pearson’s correlation
analysis showed that the changes in GABA content were closely related
to the transcript level or enzyme activity of three key enzymes including
glutamate decarboxylase (GAD), GABA transaminase (GABA-T), and succinate-semialdehyde
dehydrogenase (SSADH) in the GABA shunt, especially GAD. Based on
RNA-sequencing analysis, eight GAD genes, two GABA-T genes, one SSADH gene, nine polyamine oxidase (PAO) genes, five
diamine oxidase (DAO) genes, four 4-aminobutyraldehyde
dehydrogenase (BADH) genes, and three thermospermine
synthase ACAULIS5 (ACL5) genes were identified. Among
these, CqGAD8 and CqGABA-T2 may make a greater contribution to GABA accumulation
during quinoa germination.