Physiological and biochemical characterization of metal-phytosiderophore transport in graminaceous species

Physiological studies on the uptake of metal-phytosiderophores in strategy II plants indicated that besides iron-phytosiderophores also other phytosiderophore-chelated metals can be taken up. To verify whether this reflects a property of the recently isolated Fe(III)-phytosiderophore transporter ZmYS1, a biochemical study was undertaken using growth complementation of yeast mutants and two-electrode voltage clamp in Xenopus oocytes. These approaches allowed to show that ZmYS1 indeed transports other phytosiderophore-chelated metals, such as Cu, Zn, Co, or Ni, as well as nicotianamine-complexes with Fe(II), Fe(III), and Ni(II). Moreover, it was shown that Fe(III)-DMA is cotransported with protons by ZmYS1, thus allowing substrate transport to be driven by the negative membrane potential. Despite the transport of a broad range of phytosiderophore-chelated heavy metals, neither Zn nor Mn or Cu deficiency led to an upregulation of ZmYS1 transcript levels in leaves or roots of maize. It is therefore concluded that the transporter ZmYS1 itself does not represent a component of the stress response to Zn-, Mn-, or Cu-limiting growth conditions, but that transport of these metal-phytosiderophores represents rather a side activity of ZmYS1.