The ectomycorrhizal activity (expressed as the number of mycorrhizas per plant) of indole-3-acetic acid (IAA) overproducer mutants of Hebeloma cylindruspurum Rumagnesi and of their mono- and dikaryotic progenies has been studied in an attempt to clarify the role of fungal IAA in the establishment of ectomycorrhizal symbiosis, The original mutants obtained from the hi monokaryon produced a higher number of mycorrhizas than did the wild type. The study of the mycorrhizal activity of synthesized dikaryons heterozygous for the FIR (fluoroindoleresistant) mutations, which are responsible for IAA overproduction, showed that these mutations are recessive.
The vitro fruiting of a dikaryon heterozygous for a FIR mutation allowed the isolation of IAA-overproducing sib-monokaryons. Their average mycorrhizal activity was higher than that of non-overproducers. indicating that, although the mycorrhizal activity of sib-monokaryons is under polygenic control, high IAA production gives an advantage in mycorrhiza formation. This was confirmed by the higher number of mycorrhizas formed by synthesized dikaryons homozygous for a FIR mutation than by wild type mycelia.
The mycorrhizas formed by the mono- or dikaryotic wild types were characterized by a uniseriate Hartig net, whereas it was pluriseriate in the case of mycorrhizas formed by IAA overproducer mutants. The mutants stimulated the growth of the host plants to the same extent as did the wild types, indicating that growth stimulation is not a direct consequence of fungal auxin production. The usefulness of these mutants as a model to study the role of fungal IAA in mycorrhiza formation is discussed.