Cloning and characterization of a cDNA encoding calcium/calmodulin-dependent glutamate decarboxylase from Scutellaria baicalensis.

ABSTRACT

Gamma-aminobutyric acid (GABA), synthesized by glutamate decarboxylase (GAD), plays an important role in plants. To study the molecular mechanism of GAD regulation and to examine the levels of GABA in Scutellaria baicalensis, we isolated cDNA clones (SbGAD1 and 2) encoding GAD from S. baicalensis. The open reading frames of SbGADI and 2 were 1,503 and 1,494 bp long and had 450 and 497 amino acid residues, respectively. Quantitative real-time RT-PCR analysis was performed to show the variation of transcript levels among different organs of S. baicalensis. Transcript levels of SbGAD1 and 2 were highest in the root and flower, respectively. The GABA content of different parts (ranked in descending order) was as follows leaf > flower > stem > root. We concluded that the expression pattern of SbGAD1 and 2 did not match the accumulation pattern of GABA in different organs. We presume that GABA biosynthesis might be more controlled by SbGAD2 than SbGADI. These data will aid in future studies that seek to understand the mechanisms underlying GABA biosynthesis, an important amino acid that is synthesized by the GAD enzyme. To explain adequately the GABA biosynthesis mechanisms in S. baicalensis, the enzyme activities of SbGAD1 and 2 should be determined in the near future.