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 expla in enhanced zinc tolerance in higher plants. In order to determine whe ther the vacuolar membrane is involved in differential zinc tolerance, tonoplast vesicles were isolated from roots of zinc tolerant and sens itive Silene vulgaris. In vitro transport studies with K+/nigericin en ergized tonoplast vesicles showed that zinc (supplied as zinc citrate) was transported across the tonoplast only in the presence of MgATP. Z inc transport was about 2.5 times higher in tonoplast vesicles derived from zinc tolerant Silene vulgaris compared with those derived from s ensitive Silene vulgaris at high zinc concentration (0.8 mmol/L). At l ow zinc concentrations (0.1-0.3 mmol/l) there was no significant diffe rence in zinc transport between both types of plants. The difference i n zinc transport at high external zinc concentrations could be due to an additional transport system with lower zinc affinity that may opera te at higher zinc concentrations. Overall, these data strongly suggest that the tonoplast plays an important role in naturally selected zinc tolerance.