Metal toxicity and ectomycorrhizas

Abstract

Metal toxicity (Al and heavy metals) is a major constraint affecting root growth in a number of natural or managed ecosystems. Fine roots of the majority of plant species are associated with mycorrhizal fungi, which may modify the sensitivity of roots to metal stress. In this review, we summarise the available evidence demonstrating beneficial effects of ectomycorrhizas in alleviation of metal toxicity in forest tree seedlings. We identify experimental shortcomings of past research (e.g. the use of shoot metal concentrations as a measure of metal uptake, use of microanalytical techniques biased by element redistribution) that may confound major conclusions drawn from these experiments. Although there is no doubt that in many cases ectomycorrhizal fungi indeed ameliorate metal stress in their host plants, the mechanism(s) involved remain(s) unclear. The role of metal sorption on fungal tissues thought to reduce metal exposure of the host plant is critically reviewed. As direct evidence (both under artificial and soil conditions) supporting a unique role of fungal immobilisation of metals is lacking so far, there is an urgent need to also test alternative tolerance mechanisms such as the release of metal chelating substances, or nutritional and hormonal effects mediated by mycorrhizal fungi.

Ectomycorrhizal communities of Quercus garryana are similar on serpentine and nonserpentine soils

“…Ectomycorrhizas were abundant at all sites; communities on serpentine soils were similar in morphotype richness to those on neighboring nonserpentine soils with no single fungal morphotype dominating on either soil type. These results were unexpected because fungi take up heavy metals and because some fungi have ecotypes or strains with tolerance to heavy metals (Gadd and deRome 1988;Gadd 1993;Galli et al 1994;Jentschke and Godbold 2000;Meharg and Cairney 2000;Panaccione et al 2001;Colpaert et al 2000). Possible explanations for the finding of similar ectomycorrhizal communities on serpentine and nonserpentine soils by Moser et al (2005) include too great a distance between sites, the weakly serpentinic composition of two of the sites, and lack of molecular confirmation of morphotype identity.…”

Effects of five years of frequent N additions, with or without acidity, on the growth and below-ground dynamics of a young Sitka spruce stand growing on an acid peat: implications for sustainability

“…NO 3-and SO 4 2-can both impoverish soil and enhance the solubility of toxic metal ions, i.e. Al 3+ (Jentschke and Godbold, 2000;deVries et al, 1995;NEGTAP 2001). It is predicted that, where the removal of base cations, both through leaching and N-led increases in the uptake of base cations by trees, exceeds the ability of the soil to replenish supplies, any benefits arising from eutrophication through N deposition will be reversed or worse (Sverdrup et al, 1994).…”

Cadmium-Responsive Thiols in the Ectomycorrhizal Fungus Paxillus involutus

“…An additional compound with a 3-kDa molecular mass, most probably related to a metallothionein, increased drastically in mycelia exposed to cadmium. The relative lack of phytochelatins and the presence of a putative metallothionein suggest that ectomycorrhizal fungi may use a different means to tolerate heavy metals, such as Cd, than do their plant hosts.The response of ectomycorrhizal fungi to toxic metals is important, since these organisms are present at polluted sites, participate in crucial symbiotic relationships with trees that grow at these sites, and alleviate metal toxicity in the host plants (8,13,26). Paxillus involutus is an ectomycorrhizal fungus with a high tolerance to cadmium, one of the most toxic heavy metals, that can form symbioses with a broad range of host species.…”