& 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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SummaryAdaptive Zn and Cd tolerance have evolved in populations of the ectomycorrhizal fungus Suillus luteus. When exposed to high concentrations of both metals in vitro, a one-sided antagonism was apparent in the Zn-and Cd-tolerant isolates. Addition of high Zn concentrations restored growth of Cd-stressed isolates, but not vice versa. The antagonistic effect was not detected in a S. luteus isolate from non-contaminated land and in Paxillus involutus. The fungi were inoculated on pine seedlings and subsequently exposed to ecologically relevant Zn and Cd concentrations in single and mixed treatments. The applied doses severely reduced nutrient acquisition of nonmycorrhizal pines and pines inoculated with metalsensitive S. luteus. Highest translocation of Zn and Cd to shoots occurred in the same plants. Seedlings inoculated with fungi collected from the polluted site reduced metal transfer to their host and maintained nutrient acquisition under high metal exposure. The isolate showing highest tolerance in vitro also offered best protection in symbiosis. The antagonistic effect of high Zn on Cd toxicity was confirmed in the plant experiment. The results indicate that a Zn-and Cd-polluted soil has selected ectomycorrhizal fungi that are able to survive and protect their phytobiont from nutrient starvation and excessive metal uptake.

The use of ectomycorrhizal (ECM) fungi for afforestation, bioremediation, and timber production requires their maintenance over long periods under conditions that preserve their genetic, phenotypic, and physiological stability. Cryopreservation is nowadays considered as the most suitable method to maintain the phenotypic and genetic stability of a large number of filamentous fungi including the ECM fungi. Here, we compared the ability of eight ECM fungal isolates to colonize Pinus sylvestris roots and to transport inorganic phosphate (Pi) and NH4 (+) from the substrate to the plant after cryopreservation for 6 months at -130 °C or after storage at 4 °C. Overall, the mode of preservation had no significant effect on the colonization rates of P. sylvestris, the concentrations of ergosterol in the roots and substrate, and the uptake of Pi and NH4 (+). Comparing the isolates, differences were sometimes observed with one or the other method of preservation. Suillus bovinus exhibited a reduced ability to form mycorrhizas and to take up Pi following cryopreservation, while one Suillus luteus isolate exhibited a decreased ability to take up NH4 (+). Conversely, Hebeloma crustuliniforme, Laccaria bicolor, Paxillus involutus, and Pisolithus tinctorius exhibited a reduced ability to form mycorrhizas after storage at 4 °C, although this did not result in a reduced uptake of Pi and NH4 (+). Cryopreservation appeared as a reliable method to maintain important phenotypic characteristics (i.e., root colonization and nutrient acquisition) of most of the ECM fungal isolates studied. For 50 % of the ECM fungal isolates, the colonization rate was even higher with the cultures cryopreserved at -130 °C as compared to those stored at 4 °C.