Although microbial siderophores are characterized by high affinity and selectivity for Fe3+, they are usually less efficient as Fe-carriers to plants than synthetic chelates. An exception to this is rhizoferrin, a fungal siderophore produced by Rhizopus arrhizus isolated and purified by our group. A ferric complex of rhizoferrin was used in this study as an Fe source for tomato and cucumber ("strategy I") and barley and corn ("strategy II") plants grown in nutrient solutions. The Fe-rhizoferrin complex was found to be an efficient carrier of Fe to these plants. The efficiency of this chelate is comparable with that of the commonly used ferric complexes of ethylenediaminetetraacetic acid (EDTA) and ethylenediamine-di(o-hydroxyphenyl)acetic acid (EDDHA). The application of Fe-rhizoferrin resulted in enhanced plant weight in barley and corn and higher leaf chlorophyll concentration in tomato, barley and corn. Iron uptake by Festressed cucumber plants from the 59Fe chelate of rhizoferrin and other chelators followed the order: EDDHA > rhizoferrin > EDTA ≥ desferrioxamine B (DFOB); 59Fe translocation from roots to shoots followed the order: rhizoferrin = EDDHA > EDTA ≥ DFOB. The high availability of Fe complexed by rhizoferrin for "strategy I" plants is related to its relatively high redox potential and low affinity to Fe2+. The high availability of Fe-rhizoferrin to "strategy II" plants is related to its relatively low apparent stability constant with Fe3+, which results in the ability of this compound to transfer Fe3+ via ligand exchange to the phytosiderophores which is specifically taken up by the roots.
Bibliographical noteFunding Information:
The authors wish to thank the US-Israel Bionational Agricultural Research and Development Fund (BARD) and the Baden-Württemberg - Hebrew University Research Fund for their financial support.