IAA production by the ectomycorrhizal fungus Paxillus
involutus grown in pure culture was investigated by testing
IAA accumulation or IAA-synthesizing activity. All the Paxillus
involutus strains studied released indole
compounds either when incubated with 0·5 mm tryptophan
or in its absence. HPLC analyses of indole compounds
all yielded two main peaks identified by reference to controls as
being IAA and indole-3-carboxylic acid and a
small peak of indole-3-aldehyde. Mass spectrometry unequivocally
confirmed IAA production in the culture
filtrates and in the incubation solution used to study in vitro
IAA-synthesizing activity. The IAA-synthesizing
activity was found to vary widely, from 7·6 to 137·4 nmol
IAA h−1 g−1
f. wt, between strains. The mycorrhizal
activity of these strains with seedlings of Pinus sylvestris
cultured under aseptic conditions was tested. Fungal
strains that were characterized in pure culture conditions by high
IAA-synthesizing activity induced more fine
roots and significantly higher numbers of mycorrhizas than strains which
showed
a low activity of IAA
synthesizing enzymes. The increased ectomycorrhiza formation was generally
accompanied by an increased growth of seedlings.