IAA-OVERPRODUCER MUTANTS OF HEBELOMA-CYLINDROSPORUM ROMAGNESI MYCORRHIZAL WITH PINUS-PINASTER (AIT) SOL AND P-SYLVESTRIS L IN HYDROPONIC CULTURE

Indole-3-acetic acid (IAA) is thought to play a role in the regulation of ectomycorrhiza development, and vigorous mycorrhiza formers such a s Pisolithus and Laccaria have previously been shown to accumulate lar ge amounts of IAA in the culture medium in vitro, particularly in the presence of tryptophan. Recently, 5-fluoroindole-resistant and IAA-ove rproducing mutant strains of Hebeloma cylindrosporum Romagnesi have be en developed ana described by Durand et al. (1992). We have used some of these and corresponding wild-type strains as mycobionts on seedling s of Pinus pinaster (Ait.) Sol. and P. sylvestris L. in semi-hydroponi c culture in an attempt to study IAA effects independent of species-sp ecific differences. However, no significant differences between strain s were found in host growth rate, shoot carbohydrate concentration, ro ot morphology, root IAA concentration or mycorrhizal biomass. Since pr evious work showed a stimulation by these and other mutants and strain s on mycorrhiza formation in Petri dish and test tube cultures, we ass ume that a semi-hydroponic culture system prevents the build Up of try ptophan of fungal origin, which is most likely a precondition for enha nced IAA production.