CHLORATE TRANSPORT IN ISOLATED TONOPLAST VESICLES FROM RED BEET (BETA-VULGARIS L) STORAGE TISSUE

There is strong evidence that 36ClO3- can serve as a suitable radiotracer for nitrate in transport experiments with plant tissue. In this study, 36ClO3- was used to investigate the mechanism of nitrate influx and efflux in sealed tonoplast membrane vesicles isolated from red beet (Beta vulgaris L. cv. Detroit Dark Red) storage tissue. Uptake of 36ClO3- into red beet tonoplast vesicles was ATP-dependent, inhibited by carbonylcyanide m-chlorophenylhydrazone and demonstrated saturation kinetics with a Km of about 5 mM. While ATP-driven 36ClO3- uptake was inhibited by the collapse of the membrane electrical potential (ΔΨ) and an artificially imposed pH gradient could not drive radiolabel uptake, radiolabel uptake could be driven by an artificially imposed, positive-interior ΔΨ. These results suggest that ATP-driven 36ClO3- influx at the tonoplast may be mediated by a uniport system which responds to the positive interior membrane potential. When 36ClO3- was pre-loaded into the vesicles and radiolabel efflux was examined, no evidence could be found for the existence of an ATP-driven H+/36ClO3- symport for efflux. Overall, this study demonstrates the feasibility of using 36ClO3- as a radiotracer with isolated membrane vesicles and sets the stage for further research on the mechanism of NO3- transport systems. © 1990.