Reductive Solubilization of Fe ( III ) by Certain Products of Plant and Microbial Metabolism as a Possible Alternative to Siderophore Secretion

Fe(III) oxides hydrated to various extents are the most abundant iron species in soil. Their extremely low solubility makes them almost unavailable to living organisms. To cope with iron deficiency, many microorganisms synthesize and secrete siderophores [1–5], which are low-molecular-weight compounds that specifically chelate Fe(III) to form stable complexes, into the soil (association constants of 10 29 –10 32 [4]). Plants either secrete similar chelating compounds, namely, phytosiderophores, or use exogenous siderophores produced by microorganisms. The ligand groups of siderophores are of two major types: (1) hydroxamic groups characteristic of most soil microorganisms and (2) catechol groups Ph(OH) 2 containing hydroxyl groups in the orthoposition. However , other chelating structures, including α-hydroxy-acids and 2-(2-hydroxyphenyl)-oxazolin, are also possible [4]. In some microorganisms, organic acids with a relatively low Fe(III) affinity are used to chelate iron. In microorganisms with the citric acid cycle, the mechanisms of iron transport across the cell membrane and assimilation of ferric ion, an inorganic component of the transported complex, are similar to those in microorganisms using siderophores [2, 5]. Less common pathways of iron assimilation have also been described [5]. Cells can assimilate only iron released from its complex. Several mechanisms of Fe(III) release are known: (1) an internal membrane-associated chelator substitutes for the external transporting ligand at the cell surface and further transports the iron ion into the cell (the taxicab mechanism); (2) the chelating complex reduces iron at the cell surface and releases it intracellularly (the reductive taxi mechanism); or (3) the chelating complex is transported into the cell, where iron is then reductively released with the involvement of intracellu-lar enzymes (ferric siderophore reductases) and physiological reducing agents [2]. The organic component of the complex can be used as a carbon source or released extracellularly in its initial form to chelate a new ferric ion (the shuttle mechanism). Thus, at normal physiological pH values, Fe(III) is either bound to a stable complex or hydrolyzed completely. In any case, if it is not included in magnetite or ferritin, which are intracellular iron stores [5–7], ferric iron undergoes reduction and is assimilated from its soluble complexes. The in situ chemical reduction of ferric iron is a fundamentally different way to increase its bioavailability. The only Fe(III)-solubilizing agent that the aerobic soil bacterium Rhizobium leguminosarum secretes under iron-deficient conditions was identified as anthranilic acid [3]. It was suggested that anthranilic acid reduces Fe(III). Anthranilic acid is the key …