RESUMEN
The four carbon non-proteinogenic amino acid γ-aminobutyric acid (GABA) accumulates to high levels in plants in response to various abiotic and biotic stress stimuli, and plays a role in C:N balance, signaling and as a transport regulator. Expression in Xenopus oocytes and voltage-clamping allowed characterizing Arabidopsis GAT2 (At5g41800) as low affinity GABA transporter with a K0.5GABA~8 mM. L-alanine and butylamine represented additional substrates. GABA-induced currents were strongly dependent on the membrane potential, reaching highest affinity and highest transport rates at strongly negative membrane potentials. Mutation of Ser17, previously reported to be phosphorylated in planta, did not result in altered affinity. In short term stress experiment, AtGAT2 mRNA levels were upregulated at low water potential and under osmotic stress (polyethylene glycol, mannitol). Furthermore, AtGAT2 promoter activity was detected in vascular tissues, in maturating pollen, and the phloem unloading region of young seeds. Even though this suggested a role of AtGAT2 in long distance transport and loading of sink organs, under the conditions tested neither AtGAT2 overexpressing plants nor atgat2 or atgat1 T-DNA insertion lines, or atgat1 atgat2 double knockout mutants differed from wild type plants in growth on GABA, in amino acid levels or resistance to salt and osmotic stress.
