Acquisition of iron by Stemphylium botryosum under iron-replete conditions

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Abstract
The assimilation of iron by Stemphylium botryosum under iron-replete conditions involved mainly three steps: (i) binding of iron to the cell surface, (ii) reduction of Fe3+ into Fe2+, and (iii) transport of the ferrous iron across the cell membrane. However, the possibility that the fungus is capable of transporting ferric iron also has not been excluded. Adsorption was responsible for up to 75% of the iron accumulated by the cells. Mycelial cells exhibited a high capacity for Fe3+ reduction. Approximately 70% of the reductive activity was assigned to cell surface bound reducing compounds which were also released to the medium. Iron transport by S. botryosum was an energy-dependent saturable process that required sulfhydryl groups. Uptake was significantly inhibited by Cu2+ but not by various other cations. On the basis of inhibition studies with chelates specific for Fe3+ or Fe2+ and on the high capacity of the mycelium to reduce external ferric iron, it was suggested that uptake of iron into the cell occurred predominately in the ferrous form. The phytotoxic iron chelate stemphyloxin I induced iron adsorption on the mycelial surface but did not appear to exert a direct effect on the membrane-mediated transport of iron.
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