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

Abstract

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 a-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 NaN3, 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.The cells of many organisms, from bacteria to mammals, accumulate high concentrations of amino acids and carbohydrates from surroundings in which these compounds are dilute (8,11,19,21). Many workers propose that these energy-requiring transport processes are mediated by membrane carrier mechanisms that use the inwardly directed free energy (chemical potential) gradients of either sodium ions in animal cells (2,12,19) or hydrogen ions in bacteria, fungi, algae, and higher plants (6, 7, 8, 14, 22). This process, called co-transport (21) or symport (14), is essentially a means for coupling the active transport of one substrate to the passive movement of another substance-both moving in the same direction.Theory. Figure I shows one of several possible models for cotransport (21). The model assumes a membrane carrier that combines with a substrate (an amino acid in our case) and a proton on one side of the membrane forming a ternary positively charged carrier-substrate-proton complex that moves to the inner membrane surface; at that point the substrate and proton are released and the carrier is allowed to recycle. The direction of the net transport of substrate and its final internal to external concentration ratio depend upon the cytoplasmic and medium substrate and hydrogen ion concentrations, as well as on the potential difference across the membrane. Presumably, the proton concentration on either side of the membrane would affect the rates of proton association or dissociation with the carrier. The membrane potential would act electrophoretically upon the charged membrane species.In the model (Fig. l) (b) membrane depolarizations should be related to substrate transport rates; (c) substrate transport rates and substrate-induced depolarizations should be directly related to proton chemical potential differences caused either by changes in membrane potentials, changes in external to internal proton concentration ratios, or combinations of both; and (d) transport of substrate into the cell with a proton should leave the external solution more alkaline. In the present study, the first th...

Glutamate Receptor Subtypes Evidenced by Differences in Desensitization and Dependence on theGLR3.3andGLR3.4Genes

“…Using hindsight, evidence of their existence can be seen in early plant electrophysiological studies. Etherton and colleagues demonstrated that large, transient membrane depolarizations were triggered by some amino acids and that the response displayed desensitization (Etherton and Rubinstein, 1978;Novacky et al, 1978). Models based on electrogenic amino acid transport were constructed to explain the results Etherton, 1980, 1982), but the possibility that plants possess amino acid-gated ion channels was not yet raised.…”

Electrical Evidence for Turgor Inhibition of Proton Extrusion in Sugar Beet Taproot

“…After incubation times of one to several hours the tissue-containing holders were inserted into Lucite chambers fitted with inlet and outlet ports for the perfusion solutions. These chambers were used in an assembly for implanting glass microelectrodes used for the measurement of Em (3,7).…”

“…We conclude from these and previous findings that high turgor inhibits proton extrusion in the sugar beet, but that proton extrusion can be activated in fully turgid tissue by acidification of the cytoplasm. A possible function of this turgor effect may be the control of turgor itself.sliced and pushed into Lucite tissue holders.Tissue-containing holders were incubated in a solution (referred to as IX here and in previous publications [3,7]) composed of 1 mM NaCl, 1 mm Ca(NO3)2, 1 mm KH2PO4, and 0.25 mM MgSO4. This solution was variously supplemented with mannitol, acetic acid, and NaCl as described in the individual experiments.…”

“…Tissue-containing holders were incubated in a solution (referred to as IX here and in previous publications [3,7]) composed of 1 mM NaCl, 1 mm Ca(NO3)2, 1 mm KH2PO4, and 0.25 mM MgSO4. This solution was variously supplemented with mannitol, acetic acid, and NaCl as described in the individual experiments.…”

Primary Action of Indole-3-acetic Acid in Crown Gall Tumors

“…However, it may be assumed that in tumor tissue LAA causes an increased electrochemical proton gradient across the plasmalemma in a similar way as it does in the classical coleoptile system (7) and thus stimulates the symport of organic solutes with protonsintothe cells (8,10,12,14,15,17,21). An enhanced uptake of organic nutrients certainly is a prerequisite for the establishment ofan efficient metabolic sink ifthe tumor develops on a host plant.…”