IAA biosynthesis by the ectomycorrhizal fungus Hebeloma hiemale as affected by different precursors

Abstract

The effect of different precursors, aromatic amino acids, or intermediates of the shikimate pathway (pathway for aromatic amino acids biosynthesis), on indole-3-acetic acid (IAA) synthesis by the ectomycorrhizal fungus Hebeloma hiemale was studied. This fungus did not release detectable amounts of IAA when cultivated on a medium containing no precursor or supplemented with 1 mM phenylalanine, 1 mM tyrosine or 1 mM shikimic acid. IAA accumulation in culture filtrates was low (0.5 μmol per flask) when the medium was supplemented with 1 mM anthranilic acid. The fungus released 1.6 μmol of IAA when cultivated on a medium containing 1 mM indole and 6.9 μmol in the presence of 1 mM tryptophan. These results were confirmed by studying the ability of crude enzyme extracts to convert these precursors to IAA. Specific IAA synthesizing activity was of the same order when indole or tryptophan were used as precursors. The comparison of in vivo and in vitro activity of IAA synthesizing enzymes demonstrated that a need for tryptophan concentrations higher than 0.1 mM to obtain detectable IAA synthesis is due to the low ratio of tryptophan breakdown into IAA. The inability of H. hiemale to synthesize IAA in the absence of precursors or in the presence of shikimic acid may be ascribed to a very poor endogenous tryptophan accumulation in the hyphae due to feed back inhibition of the anthranilate synthetase by tryptophan. These results indicate that precursor availability in root exudates is probably one of the main limiting factors for IAA release by ectomycorrhizal fungi under symbiotic association. Key words: ectomycorrhizal fungus, Hebeloma, indole-3-acetic acid, tryptophan, indole, shikimate pathway.