Element profiles and growth in Zn-sensitive and Zn-resistant Suilloid fungi

Zn pollution has triggered evolution for adaptive Zn tolerance in populations of Suilloid ectomycorrhizal fungi. The objectives of this study were to determine differential physiological responses that are linked to the Zn tolerance trait and to obtain more insight in the general mechanism responsible for the differential growth in Zn-enriched medium. Therefore, we identified intrinsic growth rates and element profiles in Zn-sensitive and Zn-tolerant genotypes. Isolates from Zn-polluted and unpolluted sites were exposed in vitro to increasing Zn2+ stress. The Zn concentration which inhibits growth by 50% (EC50) was determined, and element (Zn, Fe, Mn, Cu, Mg, Ca and P) profiles in the mycelia were analysed. The intraspecific variation in growth rate and nutrient content of the in vitro grown mycelia is great and was not reduced in Zn-tolerant populations. The Zn resistance was not correlated to the intrinsic mycelial growth rate of the isolates or to the concentrations of the elements analysed, except for Zn. At low external Zn, Zn-resistant genotypes had lower Zn concentrations than sensitive isolates. At high external Zn, the differential Zn accumulation pattern between resistant and sensitive isolates became very prominent. Zn-exclusion mechanisms are most likely involved in the naturally selected adaptive Zn resistance. Other mechanisms of Zn detoxification such as sequestration of Zn on cell wall compounds or intracellular chelation and/or compartmentation are probably active but cannot explain the differential Zn sensitivity of the isolates.