Roots of Iron-Efficient Maize also Absorb Phytosiderophore-Chelated Zinc

To investigate the recognition of Zn-phytosiderophores by the putative Fe-phytosiderophore transporter in maize (Zea mays L.) roots, short-term uptake of 65Zn-labeled phytosiderophores was compared in the Fe-efficient maize cultivar Alice and the maize mutant ys1 carrying a defect in Fe-phytosiderophore uptake. In ys1, uptake and translocation rates of Zn from Zn-phytosiderophores were one-half of those in Alice, but no genotypical difference was found in Zn uptake and translocation from other Zn-binding forms. In ys1 and in tendency also in Alice, Zn uptake decreased with increasing stability constant of the chelate in the order: ZnSO4 [greater than or equal to] Zn-desferrioxamine > Zn-phytosiderophores > Zn-EDTA. Adding a 500-fold excess of free phytosiderophores over Zn to the uptake solution depressed Zn uptake in ys1 almost completely. In uptake studies with double-labeled 65Zn-14C-phytosiderophores, ys1 absorbed the phytosiderophore at similar rates when supplied as a Zn-chelate or the free ligand. By contrast, in Alice 14C-phytosiderophore uptake from the Zn-chelate was 2.8-fold higher than from the free ligand, suggesting that Alice absorbed the complete Zn-phytosiderophore complex via the putative plasma membrane transporter for Fe-phytosiderophores. We propose two pathways for the uptake of Zn from Zn-phytosiderophores in grasses, one via the transport of the free Zn cation and the other via the uptake of nondissociated Zn-phytosiderophores.