Characterization of zinc transport by divalent metal transporters of the ZIP family from the model legume Medicago truncatula

To understand how plants from the Fabaceae family maintain zinc (Zn) homeostasis, we have characterized the kinetics of three Zn transporting proteins from the ZIP family of divalent metal transporters in the model legume Medicago truncatula. Of six ZIP's studied, MtZIP1, MtZIP5 and MtZIP6 were the only members from this family determined to transport Zn and were further characterized. MtZIP1 has a low affinity for Zn with a K-m of 1 mu M as compared to MtZIP5 and MtZIP6 that have a higher affinity for Zn with K-m of 0.4 mu M and 0.3 mu M, respectively. Zn transport by MtZIP1 was more sensitive to inhibition by copper (Cu) concentrations than MtZIP5 and MtZIP6, because 3 mu M Cu inhibited Zn transport by 80% in MtZIP1 while 5 mu M Cu was required to achieve the same inhibition of Zn transport in MtZIP5 and MtZIP6. Cadmium (Cd) had a greater effect on the ability of MtZIP1 to transport Zn than MtZIP5 and MtZIP6, because at a concentration of 3 mu M Cd, the Zn transport by MtZIP1 was inhibited 55% and the transport of Zn by MtZIP5 and MtZIP6 was inhibited by 20-30%. However, only MtZIP6 transported Cd at higher rates than those observed in the control plasmid pFL61, demonstrating a low affinity for Cd based on a K-m of 57 mu M. These results suggest that Medicago truncatula has both high and low affinity Zn transporters to maintain Zn homeostasis and that these transporters may function in different compartments within the plant.