Receptor modifiers indicate that 4-aminobutyric acid (GABA) is a potential modulator of ion transport in plants

GABA (4-aminobutyric acid) is a ubiquitous non-protein amino acid that accumulates rapidly in plants in response to stress. GABA was first identified in plants (potato tubers) and animals (brain tissue) 50 years ago. Although GABA is now recognized as the most important inhibitory neurotransmitter in the mammalian central nervous system (CNS), the role of GABA in plants remains unclear. Studies were performed using Lemna to investigate the possibility that GABA elicits a response in plants that may be related to that of a signaling molecule as described for GABA effects on the CNS. Lemna growth was increased 2 to 3-fold by 5 mM GABA, but growth was strongly inhibited by 0.5 mM of the isomers 3-aminobutyric acid and 2-aminobutyric acid. Growth promotion by GABA was rapidly terminated by addition of 2-aminobutyric acid to the culture medium, but inhibitory effects of 2-aminobutyric acid were not reversed by GABA regardless of amounts added. Promotion of Lemna growth by GABA was associated with an increase in mineral content of treated plants in a dose dependent manner. Results support the hypothesis that GABA activity in plants involves an effect on ion transport and an interaction with a receptor. Evidence for GABA receptors in Lemna was obtained from experiments with pharmacological agents that have been used to identify GABA receptors in animals. GABA mediated promotion of Lemna growth was inhibited by bicuculline and picrotoxin, which are respectively competitive and non-competitive antagonists of GABA receptors in the CNS. Growth inhibition by bicuculline was not relieved by increasing the amounts of GABA in the medium, indicating that the alkaloid is not acting, as in the CNS, by competitive antagonism of GABA at GABA receptor sites. Baclofen, a GABA agonist that promotes GABA activity in animals significantly increased GABA mediated promotion of Lemna growth. These findings and the known action of GABA in regulating ion channels in animals suggests a way that GABA could amplify the stress response in plants.