Production of Hydroxamate Siderophore Iron Chelators by Ectomycorrhizal Fungi

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Abstract
Numerous ectomycorrhizal fungi were surveyed for their ability to produce Fe-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 3 basidiomycetous families and 1 form-group, and representing 9 spp. in 5 genera [Amanita muscaria, Boletus edulis, Suillus lakei, S. brevipes, S. punctipes, S. granulatus, S. tomentosus, Pisolithus tinctorius, Cenococcum geophilum] produced significant quantities of HS. Experiments with 1 sp., Boletus edulis, demonstrated that the overriding factor regulating production was Fe concentration. The pattern of production suggested that concentrations of HS increased to highest levels in media with the lowest concentration of Fe. Although calcium carbonate was the only other factor which significantly enhanced production of HS, the effect was not directly related to Ca2+ ion. Column chromatography and TLC showed that the siderophores produced by B. edulis represented a mixture of compounds. The ectomycorrhizal fungi associated with plant roots may provide Fe to plants by liberation of extracellular HS into the rhizosphere.

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