Accumulation of γ-aminobutyric acid due to adding ammonium or glutamine to cultured rice cells

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Abstract
γ-Aminobutyric acid (GABA) was accumulated in rice cell cultures when ammonium was added to the medium. After the addition of ammonium, a temporary increase in the glutamine (Gln) pool was observed before the accumulation of GABA. GABA also was markedly accumulated when Gln was added to the medium in place of ammonium. When glutamic acid (Glu) was added without ammonium, no accumulation of GABA occurred. When L-methionine-DL-sulfoximine (MSO), an inhibitor of glutamine synthetase, was added to the nitrate medium, the ammonium pool increased with no accumulation of Gln and GABA. Even when ammonium was supplied to the medium, no GABA accumulated in the presence of MSO. When azaserine (AZ), which inhibits the transamidation of Gln, was added with Gln, no GABA was accumulated, although the Gln pool in the cell cultures increased significantly. The accumulation of GABA in cultured rice cells produced by ammonium as the nitrogen source probably is related directly to the Gln pool size, which is increased when ammonium is supplied to the medium.