Light-promoted diffusional amino acid efflux from Commelina leaf disks : Indirect control by proton pump activities

Abstract

The release (=the measured loss) of amino acids was studied in Commelina benghalensis leaf disks. The release is assumed to be the result of influx and efflux, therefore, both movements were investigated.The uptake of (14)C-labeled valine exhibited a biphasic isotherm. The uptake was pH-dependent, especially at low substrate concentrations (pH optimum 4.8). Signals for amino acid/proton co-transport were observed: stimulation of the uptake by fusicoccin (FC), inhibition by diethylstilbestrol (DES) or by high K(+) concentrations. In the light, the ATP level of the disks was maintained during the uptake period (2 h), in darkness the ATP content decreased from 87 to 24 nmol g(-1) fr. wt. However, light-promoted uptake, which is explained in the proton pump concept by an intensified proton extrusion as the result of high ATP production, was lacking.The release of amino acids was increased by washing with p-chloromercuriphenyl sulphonic acid (PCMBS), nystatin, 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), or KCN. The release (Q10 about 1.5) was independent of the external pH and was linearly related to the intracellular amino acid concentration. Light enhanced the rate of release to the same extent at all intracellular concentrations. The present results suggest that the release is balanced by a, at least partially, proton-driven influx and a diffusional ligh-promoted efflux. A provisional model shows how the diffusional effulx can be indirectly controlled by a counter-flow fueled by the metabolism.