Most fungi synthesize siderophores as chelating agents which form soluble complexes with Fe3+ with very high stability constants, thus solubilizing ferric Fe. Ericoid mycorrhizal fungi release ferricrocin or fusigen as the main siderophores. Ferricrocin was also shown to be produced by the ectomycorrhizal fungi, Cenococcum geophilum and Hebeloma crustuliniforme. Arbuscular mycorrhizal fungi are reported to enhance Fe-uptake rates of associated host plants which can be taken as an indication that mycorrhizal siderophores of a yet unknown structure may be involved. Mycorrhizal fungi of orchids were shown to produce as the main siderophores, both well known ferrichrome-type siderophores or the novel linear trishydroxamate basidiochrome.