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 DELTApH 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 DELTApH-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 DELTApH-dependent Cd2+ accumulation process showed saturation kinetics with a K(m(app)) of 5.5 muM. 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.