Abstract

Several studies have shown that ecotypes of ectomycorrhizal fungi from metal-contaminated sites present a high metal tolerance under in vitro conditions, but it is not clear whether fungi from non-contaminated sites can also develop ecotypes with high metal tolerance. Tolerance to Cd2+, Cu2+ and Zn2+ ions, and production of chelating compounds as a detoxification mechanism were evaluated in ectomycorrhizal fungi collected from three uncontaminated sites (SIS, SRS and ESC). The fungi were grown in solid medium with 5 and 20 mu mol.l(-1) Cd, 0.1 and 1mmol.l(-1) Cu, and 1 and 10mmol.l(-1) Zn, and the tolerance index was determined. The metal-chelating compounds were determined by Chrome Azurol S (CAS) assay, and the chemical nature (hydroxamate or catecholate) of the compounds was analyzed. There was a clear inter- and intraspecific variation in the fungal responses at low and high-metal concentrations. Some ecotypes of Rhizopogon roseolus and Suillus luteus were the most tolerant at 1mM Cu and 10mM Zn. S. luteus SRS showed tolerance to the three metals. The addition of Cu and Cd stimulated CAS-detected metal-chelating compounds and dark pigmentation production in all isolates. In the presence of Cd the lowest pH values of the culture media were detected. Hydroxamates and catecholates were detected only in some isolates, and the catecholates were stimulated by Cd in S. luteus and S. bellinii. Among fungi collected from uncontaminated sites it was possible to found ecotypes tolerant to high metal concentrations and also producers of siderophore-type chelating compounds.