THE ROLE OF LIGAND-EXCHANGE IN THE UPTAKE OF IRON FROM MICROBIAL SIDEROPHORES BY GRAMINEOUS PLANTS

The siderophore rhizoferrin, produced by the fungus Rhizopus arrhizus, was previously found to be as an efficient Fe source as Fe-ethylenedi amine-di(o-hydroxphenylacetic acid) to strategy I plants. The role of this microbial siderophore in Fe uptake by strategy II plants is the f ocus of this research. Fe-rhizoferrin was found to be an efficient Fe source for barley (Hordeum vulgare L.) and corn (Zea mays L.). The mec hanisms by which these Gramineae utilize Fe from Fe-rhizoferrin and fr om other chelators were studied. Fe uptake from Fe-59-rhizoferrin, Fe- 59-ferrioxamine B, Fe-59-ethylenediaminetetraacetic acid, and Fe-59-2' -deoxymugineic acid by barley plants grown in nutrient solution at pH 6.0 was examined during periods of high (morning) and low (evening) ph ytosiderophore release. Uptake and translocation rates from Fe chelate s paralleled the diurnal rhythm of phytosiderophore release. In corn, however, similar uptake and translocation rates were observed both in the morning and in the evening. A constant rate of the phytosiderophor e's release during 14 h of light was found in the corn cv Alice. The r esults presented support the hypothesis that Fe from Fe-rhizoferrin is taken up by strategy II plants via an indirect mechanism that involve s ligand exchange between the ferrated microbial siderophore and phyto siderophores, which are then taken up by the plant. This hypothesis wa s verified by in vitro ligand-exchange experiments.