Extracellular and cellular mechanisms sustaining metal tolerance in ectomycorrhizal fungi

Abstract

This review focuses on recent evidence that identifies potential extracellular and cellular mechanisms that may be involved in the tolerance of ectomycorrhizal fungi to excess metals in their environment. It appears likely that mechanisms described in the nonmycorrhizal fungal species are used in the ectomycorrhizal fungi as well. These include mechanisms that reduce uptake of metals into the cytosol by extracellular chelation through extruded ligands and binding onto cell-wall components. Intracellular chelation of metals in the cytosol by a range of ligands (glutathione, metallothioneins), or increased efflux from the cytosol out of the cell or into sequestering compartments are also key mechanisms conferring tolerance. Free-radical scavenging capacities through the activity of superoxide dismutase or production of glutathione add another line of defence against the toxic effect of metals.

Transcriptome analysis by cDNA-AFLP of Suillus luteus Cd-tolerant and Cd-sensitive isolates

“…Although the exact mechanisms of the adaptive Cd tolerance remain unclear, evolution towards higher metal tolerance, both in plants and fungi, are believed to be due to modifications of existing genetic networks controlling metal homeostasis and detoxification (Bellion et al 2006, Clemens 2006). S. luteus populations from contaminated soils displayed a higher level of tolerance to those metals that were enriched in the soil of origin when compared with populations from non-contaminated soils Colpaert et al 2004;Muller et al 2004).…”

“…S. luteus populations from contaminated soils displayed a higher level of tolerance to those metals that were enriched in the soil of origin when compared with populations from non-contaminated soils Colpaert et al 2004;Muller et al 2004). Tolerance mechanisms to heavy metals in ectomycorrhizal fungi seem to include reduced uptake of metals into the cytosol by extracellular chelation through extruded ligands and binding onto cell-wall components, enhanced intracellular chelation of metals in the cytosol, increased efflux from the cytosol out of the cell or into sequestering compartments and enhanced free-radical scavenging capacities (Blaudez et al 2000;Courbot et al 2004;Jacob et al 2004;Bellion et al 2006Bellion et al , 2007. Tolerance mechanisms are little-known in S. luteus but enhanced Zn tolerance seems to rely, at least partially, on enhanced Zn exclusion .…”

Metallophilic fungi research: an alternative for its use in the bioremediation of hexavalent chromium

“…Many mechanisms of interaction of fungi and Cr(VI) have been characterised as mechanisms of extracellular (chelation and linkage to the cell wall) or intracellular detoxification (linked to non-proteic thiols and transport to intracellular compartments). The extracellular mechanisms are mainly involved in preventing the entry of Cr(VI) into the cell, while the intracellular systems aim at reducing chromate in the cytosol (Bellion et al 2006). Such mechanisms include (1) chemical transformation (intracellular or extracellular reduction) by reductive organic biomolecules (indirect mechanism); (2) biosorption (anionic coupled to the reduction and anionic/cationic); (3) transport and intracellular bioaccumulation (chelation, precipitation, compartmentalisation) (Ross 1975;Gadd 1993bGadd , 2000Saha and Orvig 2010).…”

“…In the intracellular mechanism, transport proteins of Cr(VI) could be involved in the tolerance or expulsion of toxic Cr(VI) from the cytosol, or they could allow Cr(VI) sequestration in the vacuolar compartment (Bellion et al 2006). Thiol compounds, including glutathione (GSH), are often considered to be antioxidant agents (Halliwell and Gutteridge 2007); however, intracellular chelates could generate harmful free radicals for biological membranes (Pócsi et al 2004).…”

“…The sequestration of Cr(VI) by different components of the cell wall mainly relates to polysaccharides (galactosamine, chitin and glycan), and proteins, lipids and melanin have minor contribution. Therefore, the fungi cell wall is considered to be a mosaic of functional groups that includes carboxyl (-COOH), phosphate (PO 4 3-), amine (-NH 2 ), thiol (-SH) and hydroxide (-OH) groups (Bellion et al 2006), which act as interaction sites between Cr(VI) and fungi, where ionic coordination and/or ion exchange complexes can be formed with Cr(VI) anion species. Ramrakhiani et al (2011) conducted cell wall surface characterisation of Termitomyces clypeatus to determine the biosorption mechanism by the inactive fungal biomass.…”