Evidence for Amino Acid-H+ Co-Transport in Oat Coleoptiles

Microelectrode and tracer techniques were used to test for possible amino acid-H(+) co-transport in coleoptiles of Avena sativa L. cv. "Garry." The amino acid analogue alpha-aminoisobutyric acid (AIB) caused transient depolarization of the membrane potential. The absolute magnitude of the maximum depolarization was affected by the same factors that affected AIB transport. Both increased with higher concentrations of AIB, increased with higher acidities in the medium, and were enhanced by indoleacetic acid (which hyperpolarized the membrane potential). AIB transport was reduced as K(+) concentrations in the medium were increased and by the metabolic inhibitor NaN(3), both of which reduce membrane potentials. Our data fit an amino acid-H(+) co-transport model in which transport is controlled by both the membrane potential and proton concentration components of the chemical potential difference of protons across the coleoptile cell membrane.