Adaptive heavy metal tolerance in the ectomycorrhizal fungi Suillus bovinus and Suillus luteus

Abstract

In the vicinity of non-ferrous industrial plants, the soil is often moderately to heavily contaminated with heavy metals (Zn, Cd, Cu, Pb). This historical pollution has resulted in the development of large unvegetated areas that are now slowly recolonised by pioneer vegetations dominated by typical metallophytes (e.g. Viola calaminaria, Thlaspi caerulescens) and metal tolerant ecotypes of some grass species (e.g. Agrostis capillaris). These grasslands are evolving towards open pioneer forests with birches, pines and willows. The absorption roots of these trees are colonised by ectomycorrhizal (ECM) fungi. In these low-fertility ecosystems, pines and most other tree species depend strongly on their association with ECM fungi for mineral nutrition. Their nutrient absorbing roots are ensheated by a mantle of fungal hyphae forming ectomycorrhizas. An extensive mycelial network extends from these symbiotic roots into the soil, forming the main interface between soil solution and mycorrhizal plant. Consequently, ECM fungi largely control the uptake of nutrients, whether in excess or at trace level, from the soil to the host plant. Recently, adaptive Zn tolerance was found in three ECM species from the Suilloid clade. The interpopulation differences in the response of these Suilloid fungi to Zn• were clearly related to the Zn pollution of their natural environment. In addition to the Zn tolerance found in the Suilloid -fungi, Suillus luteus, S. bovinus and Rhizopogon luteolus, we also found Cu tolerance in a population of Suillus luteus that colonized a toxic Cu-mine spoi l in Norway (Chapter 5). Apparently 5. luteus can deve lop tolerances to different metals, a phenomenon not described in ECM fungi before. This proves that Suilloid fungi have a large genetic potential for the developm ent of ecotypes with an increased metal tolerance. The existence of tolerances to different metals in ECM fungi opens opportunities to breed isolates with combined metal tolerances. ... (Excerpt from introduction)