Characterization of glutamate decarboxylase mediating γ-amino butyric acid increase in the early germination stage of soybean (Glycine max [L.] Merr)

Glutamate decarboxylase (GAD) is an enzyme that catalyzes the production of γ-amino butyric acid (GABA) from glutamate through a decarboxylation reaction. A full-length cDNA encoding glutamate decarboxylase (GmGAD1) was isolated from germinating soybean seeds (Glycine max [L.] Merr.). The GmGAD1 gene had a 1512-bp open reading frame, which encodes 503 amino acids. According to its sequence similarity with other GAD genes, GmGAD1 was classified into GAD1 in the plant GAD family. Recombinant GmGAD1 protein expressed in E. coli catalyzed α-decarboxylation of glutamic acid. The levels of GABA were rapidly increased in soybean seeds during the early imbibition period (6 h) of germination or during the soaking treatment, whereas mRNA of GmGAD1 gene was not detected in these materials. The GmGAD1 protein was observed in seeds of various states such as developing, matured and soaking. These data suggest that the increased levels of GABA during the early stage of germination or soaking treatment were mediated by GmGAD1 protein synthesized in developing soybean seeds.