Evidence for an important role of the tonoplast in the mechanism of naturally selected zinc tolerance in Silene vulgaris

Compartmentalization of zinc in the vacuole has been proposed to explain enhanced zinc tolerance in higher plants. In order to determine whether the vacuolar membrane is involved in differential zinc tolerance, tonoplast vesicles were isolated from roots of zinc tolerant and sensitive Silene vulgaris. In vitro transport studies with K+/nigericin energized tonoplast vesicles showed that zinc (supplied as zinc citrate) was transported across the tonoplast only in the presence of MgATP. Zinc transport was about 2.5 times higher in tonoplast vesicles derived from zinc tolerant Silene vulgaris compared with those derived from sensitive Silene vulgaris at high zinc concentration (0.8 mmol/L). At low zinc concentrations (0.1-0.3 mmol/l) there was no significant difference in zinc transport between both types of plants. The difference in zinc transport at high external zinc concentrations could be due to an additional transport system with lower zinc affinity that may operate at higher zinc concentrations. Overall, these data strongly suggest that the tonoplast plays an important role in naturally selected zinc tolerance.