The properties of the Mn, Ni and Pb transport operating at plasma membranes of cucumber roots

To avoid metal toxicity, plants have developed mechanisms including efflux of metal ions from cells and their sequestration into cellular compartments. In this report, we present evidence for the role of plasma membrane efflux systems in metal tolerance of cucumber roots. We have identified the plasma membrane‐transport system participating in Cd, Pb, Mn and Ni efflux from the cytosol. Kinetic characterization of this proton‐coupled transport system revealed that it is saturable and has a different affinity for each of the metal ions used (with Km 5, 7.5 and 0.1 mM for Mn, Ni and Pb, respectively). Treatment of cucumber roots with 100 μM Cd prior to the transport measurements caused a great increase (over 250%) in Cd antiport activity in plasmalemma vesicles. After decreasing the metal concentration to 50 μM we still observed a large increase (over 150%) of this activity in comparison with the control. Moreover, the addition of 50 μM Cd to the external solution stimulated not only Cd antiport in the plasmalemma vesicles but also the antiport of other metals used in the experiments. Treatment of cucumber roots with 50 μM Ni revealed a similar effect: the antiport activity of Cd, Mn, Ni and Pb was stimulated, although to a lesser extent in comparison with stimulation by Cd. The data indicate that the root plasma membrane antiporter system is stimulated by the exogenous presence of heavy metals.