Cadmium Transport Across Tonoplast of Vesicles from Oat Roots. Evidence for a Cd2+/H+ Antiport Activity

Cadmium accumulates in the vacuole of plant cells, but the mechanism driving its transport across the vacuole membrane is not understood. Here we present evidence for Cd2+ transport via a Cd2+/H+ antiport activity into tonoplast-enriched vesicles isolated from oat roots. Experimentally, accumulation of Cd2+ into vesicles could be driven by delta pH generated by either V-type ATPase or artificially using nigericin to exchange K+ and H+ in K(+)-loaded vesicles. When tonoplast-enriched vesicles were separated on a linear sucrose gradient, NO3(-)-sensitive ATPase, total MgATPase, and delta pH-dependent Cd2+ transport equilibrated at 1.11 g/ml, the density of tonoplast membrane. Cd2+ accumulation in vesicles was accompanied by efflux of protons in a Cd2+ concentration-dependent manner characteristic of an antiport activity. The delta pH-dependent Cd2+ accumulation process showed saturation kinetics with a Km(app) of 5.5 microM. Thus the process is a candidate for transport of Cd2+ from the cytoplasm to the vacuolar sap under conditions of low as well as high Cd2+ exposure.