Production of Hydroxamate Siderophore Iron Chelators by Ectomycorrhizal Fungi

SUMMARY

Numerous ectomycorrhizal fungi were surveyed for their ability to produce iron-chelating, hydroxamate siderophores (HS). Production of HS was determined by a bioassay based on the stimulation of the siderophore auxotrophic bacterium Arthrobacter flavescens JG-9. Fourteen strains, distributed among three basidiomycetous families and one form-group, and representing nine species in five genera, produced significant quantities of HS. Experiments with one species, Boletus edulis, demonstrated that the overriding factor regulating production was iron concentration. The pattern of production suggested that concentrations of HS increased to highest levels in media with the lowest concentrations of iron. Although calcium carbonate was the only other factor which significantly enhanced production of HS, the effect was not directly related to Ca⁺² ion. Column and thin-layer chromatography showed that the siderophores produced by B. edulis represented a mixture of compounds. It is suggested that the ectomycorrhizal fungi associated with plant roots may provide iron to plants by liberation of extracellular HS into the rhizosphere.