It is clear that in spite of a growing public and commercial interest and the success of several pilot studies and field scale applications more fundamental research still is needed to better exploit the metabolic diversity of the plants themselves, but also to better understand the complex interactions between contaminants, soil, plant roots, and microorganisms (bacteria and mycorrhiza) in the rhizosphere. Further, more data are still needed to quantify the underlying economics, as a support for public acceptance and last but not least to convince policy makers and stakeholders (who are not very familiar with such techniques).

& Introduction Heavy metal pollution is a strong driver of above-and belowground communities and triggers evolutionary adaptation in organisms. This review provides an overview of our knowledge on the effects of toxic concentrations of metals on ectomycorrhizal populations and communities. & Discussion Selection and adaptations within particular ectomycorrhizal species that colonise host plants in harsh environments is discussed. Among these adaptations, we focus on the metal exclusion strategy that is discovered in metal-tolerant ecotypes of Suillus species that thrive on metalliferous soils. Metal efflux in metaltolerant ecotypes prevents metal overloading of cytoplasm and vacuole. At the same time, this metal-specific efflux system in Suillus seems to reduce the transfer of large quantities of metals towards the plant-fungus interface, without hampering normal nutrient transfer to the host plant. & Conclusion The evolutionary adaptation in Suillus species contributes to the survival of host trees on metalliferous soils and might be exploited in phytostabilisation strategies for heavy metal-contaminated soils.

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Summary• Zn tolerance was investigated in populations of four ectomycorrhizal fungi: Suillus luteus , Suillus bovinus , Rhizopogon luteolus and Paxillus involutus. The fungi were collected in pioneer pine forests at 14 different locations, situated along a Zn pollution gradient. Genetic adaptation to Zn toxicity was previously presumed in a population of S. luteus .• Mycelial biomass production was assessed for 235 isolates exposed to increasing Zn 2+ stress. EC 50 concentrations were determined.• Adaptive Zn tolerance was found in the three species from the Suilloid clade and not in P. involutus . The Suilloid fungi collected within 5 km from a Zn smelter were highly Zn-tolerant, in contrast to isolates collected at least 15 km away from a pollution source. Mixed populations with tolerant and sensitive S. luteus isolates were found in a transition zone, between 5 and 15 km from the Zn smelters.• The severe Zn pollution in the surroundings of the Zn smelters has clearly triggered the evolution of an increased Zn tolerance in the pioneer Suilloid fungi. With increasing distances from the Zn smelters, the frequency of Zn-tolerant genotypes decreases.

Twenty-one isolates of the ectomycorrhizal fungus Suillus luteus were screened for their tolerance to the heavy metals Zn, Cd, Cu and Ni, measured as inhibition of radial growth and biomass production. Two populations from even-aged pine stands were investigated : 10 isolates were obtained from an area polluted with high levels of Zn, Cd and Cu, and 11 isolates were obtained from a control population located in a nearby unpolluted area. RFLP patterns of the internal transcribed spacer region of the isolates confirmed the morphological identification of the carpophores. All isolates were maintained on basic medium without elevated metals to avoid phenotypically acquired metal tolerance. The in vitro Zn and Cd tolerance of the S. luteus isolates from the polluted habitat were significantly higher than the tolerances measured in the isolates from the nonpolluted site. This observation suggests that the elevated soil metal concentrations might be responsible for the evolution of adaptive Zn and Cd tolerance. Tolerance was maintained in an isolate not exposed to elevated metals for 3 yr. The two S. luteus populations did not differ in tolerance to Cu and Ni. The mechanisms for the adaptive Zn and Cd tolerance are not identical as there was no correlation between response to the two metals ; the most Zn-tolerant isolate was the most sensitive for Cd in the metal-tolerant population. Zinc did not accumulate in basidiocarp tissue, whereas Cd levels in basidiocarps were significantly higher in the population on the polluted site. Inter-simple sequence-repeat fingerprints showed that 90% of the isolates were from different individuals. The genetic variation in the population from the unpolluted site was considerably larger than that observed at the polluted site.