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 K_m 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.