Plant growth promotion induced by the antagonistic fungus, Pythiumoligandrum, is the result of a complex interaction which includes an indirect effect through control of pathogens in the rhizosphere and/or a direct one mediated by plant-induced resistance. The present study shows an increased plant growth associated with direct interaction between P. oligandrum and roots, which is mediated by a fungus-produced auxin compound, tryptamine (TNH2). In vitro experiments provided evidence that P. oligandrum metabolised specifically indole derivatives, such as tryptophan and indole-3-acetaldehyde, to produce THN2 through the tryptamine pathway. When P. oligandrum grew in sterile root exudates, it also produced an auxin-like compound. Additional experiments on P. oligandrum– root interaction showed that, in amended nutrient solution of plants, the antagonist metabolised Trp into TNH2 and that root absorption of this newly formed auxin-compound in appropriate concentrations was associated with enhancement of plant growth. This phenomenon was observed only when nutrient solution was amended with low tryptophan (Trp) concentrations, i.e. 0.05 and 0.1 mM; higher concentration (0.5 and 1 mM Trp) induced abnormal root development. Similar experiments were performed with Pythium group F, a minor pathogen known for its ability to produce auxin-compounds through the tryptamine pathway. In this case, irregular root development was always noticed with all Trp concentrations added to the nutrient solution of plants. Moreover, Pythium group F colonization of roots was associated with leakage of auxin-compounds in the nutrient solution. Our results, therefore, highlight that the production of similar auxin-compounds by two Pythium species has contrary effects on plant development.