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Keywords:

  • metal tolerance;
  • ectomycorrhizal fungi;
  • intracellular chelation;
  • extracellular chelation;
  • transport system

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

Exposure to heavy metals, whether of natural origin, such as metalliferous rocks, or of anthropic activity origin such as pollutions, may be toxic for soil organisms. The degree of toxicity depends mainly on the metallic element and its bioavailability in the soil. Metal bioavailability is a function of abiotic factors such as metal concentration, humidity and soil pH value but also depends on biotic factors such as the presence of metal-liberating soil-bacteria. Various metals, e.g. Zn, Cu and Mn, are essential at low concentrations but become toxic at increasing concentrations, other metals have never been shown to be essential for the development of living organisms and are toxic even at very low concentrations e.g. Hg, Cd, Pb (Trevors et al., 1986; Hall, 2002). This latter dogma must be reconsidered, given the recent characterization of a protein that is a Cd-containing carbonic anhydrase from the marine diatom Thalassiosira weissflogii (Lane et al., 2005).

Mycorrhizal fungi participate in crucial symbiotic relationships with plants that grow on contaminated sites, and alleviate metal toxicity for their host plants (Godbold et al., 1998; Jentschke & Goldbold, 2000; Schützendübel & Polle, 2002). Previous reviews have summarized the available information on amelioration of metal toxicity by ectomycorrhizal (ECM) associations (Rapp & Jentschke, 1994; Leyval et al., 1997; Jentschke & Goldbold, 2000), and this will not be considered here. Instead, we will focus on the mechanisms involved in ECM fungal cells and possibly on their molecular basis. Various mechanisms potentially involved in metal tolerance have been characterized in ectomycorrhizal fungi and can be described as extracellular (chelation and cell-wall binding) or intracellular (binding to nonprotein thiols and transport into intracellular compartments) detoxification mechanisms. Extracellular mechanisms are mainly implied in avoidance of metal entry, whereas intracellular systems aim to reduce metal burden in the cytosol. Additional antioxidative detoxification systems, which allow the fungus to counteract the accumulation of reactive-oxygen species directly or indirectly, initiated by metals, may be part of tolerance mechanisms.

Extracellular chelation and cell-wall binding

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

Different organic molecules, and in particular di- and tricarboxylic acids that do not belong to the matrix of the cell wall, are excreted by fungal cells to chelate metal ions (Fig. 1), among other functions. In particular, citrate has been shown to be the most important Al3+ complex-former in soil solution from podzolized forest soils (Landeweert et al., 2001; Van Hees et al., 2001). The induction of oxalic acid efflux correlated closely with Cu tolerance in brown rot fungi (Green & Clausen, 2003), and overexcretion of oxalic acid probably contributed to the metal tolerance exhibited by Beauveria caledonica (Fomina et al., 2005a). Similarly, ectomycorrhizal fungi also often respond to metal exposure by increased oxalate exudation (Ahonen-Jonnarth et al., 2000; Cumming et al., 2001). Using 109Cd uptake experiments with Paxillus involutus, we found that oxalic acid reduced Cd uptake by more than 85% as shown in Fig. 2 (D. Blaudez, unpublished results). Therefore, an increased oxalate exudation inducing a decreased Cd availability would be an efficient mechanism to avoid Cd entry into living cells of ectomycorrhizal fungi.

image

Figure 1.  Schematic representation of cellular mechanisms potentially involved in metal tolerance in ectomycorrhizal fungi. M, metal-ion; 1, extracellular chelation by excreted ligands (L); 2, cell-wall binding; 3, enhanced efflux; 4, intracellular chelation by metallothionein (MT); 5, intracellular chelation by gluthathione (GSH); 6, subcellular compartmentation (vacuole or other internal compartments); 7, vacuolar compartmentation of GSH-M complex (i.e. ycf1).

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image

Figure 2.  Effect of organic acids on Cd accumulation by Paxillus involutus mycelia. Mycelium discs were exposed for 30 min to a solution containing 0.05 μM Cd2+ labelled with 109Cd (4.3 mCi mmol−1), 0.5 mM CaCl2 in 2 mM MES at pH 4.5, as described in Blaudez et al. (2000). Organic acids were added individually at a 2 mM concentration. Data are expressed as means±SE of four replicates. Asterisks indicate significant differences from the control treatment (ANOVA, P<0.05).

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Exudation of organic acids may provide a source of protons for metal solubilization from metal-containing minerals, often resulting in soil acidification (Devêvre et al., 1996; Fomina et al., 2005b). The recent finding that metal-tolerant ectomycorrhizal fungi grew and solubilized metal-containing minerals better than nontolerant species (Fomina et al., 2005b) confirm a possible relationship between tolerance to metals and extracellular chelation by extruded ligands. However, organic acid exudation should not be regarded as a general tolerance mechanism as it is both metal and species dependent (Meharg, 2003). For instance, strains of the ericoid mycorrhizal fungus Oidiodendron maius isolated from polluted soils showed little ability to solubilize Zn from both ZnO and Zn3(PO4)2, whereas strains from unpolluted soils showed a higher solubilization potential, which may reflect specific strategies to maintain homeostasis of essential metals under different soil conditions (Martino et al., 2003). These conflicting observations may be explained by the fact that ectomycorrhizal fungi do not adapt to metal toxicity through a downward adjustment of their organic acid capacity but use other mechanisms for tolerance. The metal-sensitive ectomycorrhizal isolates do not dissolve as much metal compounds as tolerant isolates because their growth and metabolism are more quickly affected because of metal toxicity. Interestingly, glomalin, a protein synthesized and excreted by arbuscular mycorrhizal fungi (Wright & Upadhyaya, 1998; González-Chávez et al., 2004) was shown to be able to sequester metal ions, especially Cu, Pb and Cd, found at high concentrations in polluted soils. There is an urgent need to search for similar proteins, which may be produced by ectomycorrhizal fungi.

However, molecular mechanisms involved in the synthesis and release of organic compounds, are generally still poorly understood, even in the well-studied model organisms Saccharomyces cerevisiae and Arabidopsis thaliana. There is no doubt that the understanding of extracellular complexation mechanisms will greatly benefit from advances in molecular studies in this area.

The contribution of cell-wall binding to metal tolerance in mycorrhizal fungi has been extensively reviewed recently (Meharg, 2003). The fungal cell wall is the first site of direct interaction (there could be excreted substances ahead) between the fungus and the metal. Its composition implies glucan-, chitin- and galactosamine-containing polymers, and a minor amount of proteins. Thus a large number of potential-binding sites are exhibited by free carboxyl, amino, hydroxyl, phosphate and mercapto groups (Strandberg et al., 1981). Binding to the wall, also called biosorption (Gadd, 1993), is a mechanism not depending on the metabolic activity of the fungus, whereas precipitation with excreted substances relies on the activity of the cells. Binding of Cd to cell walls was shown to represent a substantial fraction of the metal accumulated by Paxillus involutus and may also be part of the mechanisms by which mycorrhizal fungi tolerate high amounts of metals (Blaudez et al., 2000; Frey et al., 2000). Lanfranco et al. (2002) showed that changes in hyphal morphology occur when an ericoid mycorrhiza-forming ascomycete is treated with millimolar concentrations of Zn. This led to apical swellings and increased branching in the subapical parts as well as a significant increase in the amount of chitin in metal-treated hyphae. Bhanoori & Venkateswerlu (2000) have shown the formation of a complex between the Cd and chitin in Neurospora crassa cell walls and proposed a structure for the chitin–Cd complex based on the results of 13C-NMR spectroscopy, X-ray diffraction and infrared spectroscopy (Fig. 3).

image

Figure 3.  Hypothetical structure for chitin–cadmium complex. Oxygen molecule of C-3 hydroxyl and ring oxygen of N-acetyl glucosamine are participating in complexation (redrawn from Bhanoori & Venkateswerlu, 2000).

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The presence of melanins among the cell-wall components can further increase metal biosorption capacity and strength (Fogarty & Tobin, 1996). Recently, we found an induction of laccase activity and gene expression and production of polyphenolic compounds under Cd exposure, which may be an important determinant of the cellular response to excess metals in Paxillus involutus (Jacob et al., 2004). It has been previously shown that Cu can induce laccase isozymes in Pleurotus ostreatus (Palmieri et al., 2000), and metal-responsive elements in Pleurotus ostreatus laccase gene promoters have been recently found (Faraco et al., 2003). In the ericoid mycorrhizal fungus O. maius, the activity of polygalacturonase, an extracellular enzyme that hydrolyses the pectin component of the plant cell walls, increased under Cd or Zn exposure, a mechanism that may be considered as a preadaptive factor for the colonization of polluted soils by O. maius (Martino et al., 2000).

Intracellular complexation by peptides

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

Despite extracellular chelation and cell-wall binding capacities of ectomycorrhizal fungi described above, large amounts of metal may enter into the cells. Using a desorption method with 109Cd, Blaudez et al. (2000) have quantified the proportion of Cd in the cytosol and the vacuole of Paxillus involutus and estimated it to be 20% and 30%, respectively. This implies the presence of efficient detoxification systems within the cytosol of Cd-stressed cells.

Morselt et al. (1986) first observed that tolerance to metals in the ectomycorrhizal fungus Pisolithus tinctorius was based on the presence of ‘metallothionein (MT)-like’ peptides. Metallothioneins are a class of ubiquitously occurring low-molecular weight cysteine- and metal-rich proteins containing sulphur-based metal clusters. Crucial roles for this protein result in its involvement in homeostasis of essential trace metals, Zn and Cu, or sequestration of the environmentally toxic metals, Cd and Hg. However, experimental evidence currently available suggests that the proteins may play a role in multiple biological processes. They have been found in fungi and other kingdoms of life (Clemens & Simm, 2003). Other studies have observed Cu-binding proteins related to metallothioneins in various isolates of the ectomycorrhizal fungi Laccaria laccata and Paxillus involutus (Howe et al., 1997). More recently, glutathione was found to be increased under Cd exposure in Paxillus involutus (Ott et al., 2002; Courbot et al., 2004), as well as γ-glutamylcysteine and a compound mostly related to an metallothionein (Courbot et al., 2004). The finding of this metallothionein is supported by the presence of an metallothionein sequence (Table 1), homologous to a known metallothionein from Agaricus bisporus, in the cDNA array analysis of Paxillus involutus exposed to Cd (Jacob et al., 2004). The full-length sequence of the Paxillus involutus metallothionein expressed in yeast restored the growth of a yAP-1 mutant strain and the polypeptide gives the same eluting peak in high-performance liquid chromatography (Courbot et al., 2004). The expression of this metallothionein was studied at the transcriptional level in Paxillus involutus exposed to different metal stress, and the result indicated a correlation between metal exposure and expression level (M. Courbot and M. Chalot, unpublished results).

Table 1.   Putative proteins from ectomycorrhizal fungi similar to proteins belonging to yeast metal tolerance pathways
MechanismPathwayFunctionOrganismGenBank accession no
  1. Selected protein sequences identified in Saccharomyces cerevisiae being involved in metal tolerance pathway were used to search for expression sequence tags or open reading frames from ectomycorrhizal fungi encoding putative proteins similar to them. Searches were made by TBLASTn or BLASTp in the NCBI database (P value <2.e-05).

Transcription factorsYAP1-likeRegulation of genes involved in oxidative stress tolerance and metal resistanceTuber borchiiCN488390
ZAPI-likeRegulation of zinc transportersPaxillus involutusCN072154
Transport systems involved in metal tolerance and homeostasisMetal efflux into organellesCation diffusion facilitatorHebeloma cylindroposrumCK993155
 Cd-conjugate ABC transporterHebeloma cylindroposrumCK995083, CK992826
  Pisolithus microcarpusCB010722
 Metal-transporting ATPaseHebeloma cylindroposrumCK992318, CK994170
  Tuber borchiiAF487323
Metal influxManganese transporterHebeloma cylindroposrumCK995213, CK992334, CK995203
 Copper transporterTuber borchiiCN487781
 Iron transporterPaxillus involutusCD274893
Intracellular metal bindingMetal delivery to other proteinsMetallochaperonePaxillus involutusAAT91247, AAT31333, AAT91334
   AAT91335, AAT91336, CD273262
   CD273746, CD273829, CD275306
   CD274894
  Hebeloma cylindroposrumBU964154
Cu and Cd bindingMetallothioneinPaxillus involutusAAS19463
Glutathione synthesisγ-glutamylcysteine synthetaseHebeloma cylindroposrumCK995328
  Paxillus involutusCD273087
 Glutathione synthetasePaxillus involutusBG141319
Protection against metal-induced oxidative stressRegulation of cell redox homestasisThioredoxinPaxillus involutusAAS19462, CD275083, CD275423, CD276018
  Hebeloma cylindroposrumCK995145, CK995656
  Tuber borchiiBM26656, CN487764, CN487812
  Pisolithus microcarpusCB011224, BF942541
  Laccaria bicolorCB012066
 GlutaredoxinTuber borchiiBM266155
  Pisolithus microcarpusBF942586
  Laccaria bicolorCB010230, CB010243
Removal of reactive-oxygen speciesCatalaseLaccaria bicolorCB010617
 Superoxide dismutaseTuber borchiiBM266201
  Paxillus involutusAAD25353, AQ064502, AW064510
  Tuber borchiiBM266232
  Laccaria bicolorCB010250, CB010696
  Hebeloma cylindroposrumCK994166, CK991636, CK993733, CK992059, CK992841, CK994504, CK995143, CK991818, CK994684, CK994504, CK994795, CK991819

Conversely, these studies have highlighted the complete lack of phytochelatins (PCs) among the Cd-responsive thiols produced in Paxillus involutus (Courbot et al., 2004) and Suillus bovinus (J. V. Colpaert et al., personal communication) and seem to confirm the general lack of phytochelatins in fungi, except in rare cases such as Candida glabrata which produces phytochelatins in response to Cd (Zhou & Goldsbrough, 1995), Schizosaccharomyces pombe (Clemens & Simm, 2003) and in the aquatic hyphomycete Heliscus lugdunensis (Jaeckel et al., 2005). From this perspective, Paxillus involutus and probably most of the ectomycorrhizal fungi are closer to Saccharomyces cerevisiae than to Schizosaccharomyces pombe, with respect to their intracellular chelation mechanisms. A search for gene homologues in the recently sequenced genome of Laccaria bicolor confirms the lack of phytochelatin synthase gene, which explains the absence of phytochelatin synthase activity in this fungus. However, production of PC2-like compounds, via a phytochelatin synthase independent pathway, has been documented in Saccharomyces cerevisiae (Kneer et al., 1992) and in an arsenate-hypertolerant Aspergillus isolate (Canovas et al., 2004), that should prompt us to reconsider the role of phytochelatins in fungi.

A good correlation between nonprotein sulphydryl groups and the concentration of Hg and Cd has been found in the fruit bodies of various ectomycorrhizal species (Kojo & Lodenius, 1989). Under Cd exposure, an increase in sulphate assimilation and cysteine synthesis, and an increase of the nonprotein thiols glutathione and its precursor γ-glutamylcysteine were observed in L. laccata, although no metallothionein could be detected (Dameron et al., 1989; Galli et al., 1993). The role of glutathione as a metal chelator in fungi is now clearly established (Pocsi et al., 2004). Intracellular glutathione hinders the progression of heavy metal-initiated cell injuries by chelating and sequestering the metal ions themselves. Putative gene sequences encoding enzymes involved in the synthesis of glutathione and γ-glutamylcysteine have been identified in expression sequence tag (EST) databases obtained from the ectomycorrhizal fungi Hebeloma cylindroposrum and Paxillus involutus (Table 1).

Transport mechanisms involved in metal tolerance

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

There has been little recent work on the transport of metals in ectomycorrhizal fungi. Metal transport proteins may be involved in metal tolerance either by extruding toxic metal ions from the cytosol out of the cell or by allowing metal sequestration into intracellular compartments (Fig. 1) (Williams et al., 2000; Hall, 2002). Using radiotracer flux analyses, the significant accumulation of Cd found in the vacuolar compartment has been suggested as an essential Cd detoxification mechanism in the ectomycorrhizal fungus Paxillus involutus (Blaudez et al., 2000). A crucial step in Cd detoxification, certainly in fission yeasts and probably in higher plants, involves the accumulation of Cd-conjugated glutathione or Cd-conjugated phytochelatins in the vacuole. This process appears to be mediated by the ATP-binding cassette transporter Hmt1 located at the tonoplast (Ortiz et al., 1992), which would be of no significance in ectomycorrhizal fungi, given the lack of phytochelatin synthesis. The yeast cadmium factor (Ycf1) gene encodes a MgATP-energized glutathione S-conjugate transporter responsible for the vacuolar sequestration of bis(glutathionato) cadmium (Li et al., 1997) as well as bis(glutathionato) mercury (Gueldry et al., 2003). The presence of this specific permease in the tonoplast of Paxillus involutus could explain the high Cd content in the vacuole (Blaudez et al., 2000). This hypothesis was further supported by X-ray microanalysis, which revealed that the accumulation of Cd correlated tightly with the accumulation of sulphur in electron-dense bodies in the vacuolar compartment (Ott et al., 2002). However, the chemical nature of these sulphur components involved in Cd complexation was not confirmed in this study.

With a similar approach, it was recently found that an enhanced Zn efflux may act as a potential tolerance mechanism in the ectomycorrhizal fungus Suillus bovinus (Adriaensen, 2005). Alternatively, downregulation of transporter genes involved in the uptake of metal at the plasma membrane may also be part of tolerance mechanisms, as described in other fungi (Eide, 2003) and plants (Clemens, 2001; Hall, 2002).

Interestingly, an EST sequence showed a high similarity with the yeast transcription factor Zap1 (Table 1), involved in the regulation of numerous metal transporters in yeast (Zhao et al., 1998). Zap1 plays a direct role in controlling Zn-responsive gene expression in yeast by binding to Zn-responsive elements in the promoters of genes that it regulates. It thus constitutes an interesting target for Zn tolerance studies in ectomycorrhizal fungi.

However, it is clear that the molecular mechanisms underlying metal transfer in intracellular compartments are still ignored in ectomycorrhizal fungi and more generally, nothing has been published in relation to the genes encoding proteins mediating intracellular metal transport in ectomycorrhizal fungi. A search for EST sequences encoding metal transporters promisingly indicates the presence of potential genes belonging to the ATP-binding cassette (the Ycf1 Cd-conjugate ABC transporter), cation diffusion facilitator, natural resistance-associated macrophage protein (the Smf1 Mn transporter) or P-type ATPase families (Table 1). Members of these transporter families have been shown to actively participate in metal detoxification of cells in a broad range of organisms (Williams et al., 2000), and therefore they could also play a crucial role in metal protection in ectomycorrhizal fungi.

Antioxidative mechanisms

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

The formation of free radical species, which can be initiated directly or indirectly by metals, can cause severe damage to different cellular components. Formation of metal-induced reactive-oxygen species could occur via several mechanisms. The Fenton or Haber–Weiss reactions are catalysed by redox-active metals (e.g. Cu, Fe, Cr, V) and generate the highly reactive hydroxyl (OH) radical from H2O2 and superoxide (O2−•) substrates (Halliwell & Gutteridge, 1999). Redox-inactive metals such as Cd, Ni, Hg and Zn deplete glutathione and protein-bound sulphydryl groups, resulting in the production of reactive-oxygen species. Several indirect mechanisms are considered to account for the action of redox-inactive metals, for example, these metals might displace redox-active metals from cellular-binding sites (Avery, 2001). Evidence for a role of reactive-oxygen species in metal-induced damage to yeast includes increased metal tolerance during anaerobicity, protection exerted by certain free radical scavengers, and the many overlaps in the molecular mechanisms used by yeasts to cope with oxidative and metal stress (Avery, 2001).

In a previous paper, we hypothesized that Cd2+, although it is not a redox-active metal, induced an oxidative stress in Paxillus involutus (Jacob et al., 2001). It is possible that Cd2+ indirectly contributes to oxidative stress by affecting the cellular thiol redox balance. Indeed, we found that Cu2+ and Cd2+ markedly induced PiTrx1, a gene encoding a thioredoxin in Paxillus involutus (Table 1) (M. Courbot and M. Chalot, unpublished results). Thioredoxins are small heat-stable oxidoreductases, which contain two conserved cysteine residues in their active sites (Holmgren, 1989). Proposed roles include many cellular processes such as protein folding and regulation, reduction of dehydroascorbate, repair of oxidatively damaged proteins and sulphur metabolism. More recently it was also demonstrated that thioredoxins are required to maintain redox homeostasis in response to both oxidative and reductive stress conditions (Trotter & Grant, 2002). We suggest that upregulation of PiTrx1 expression is a rapid response determinant in the handling of Cu2+ or Cd2+, which might function as a first line of defence against intracellular metal ions. Similarly, thioredoxin was found to be induced upon exposure of yeast cells to Cd (Vido et al., 2001).

Ott et al. (2002), in a comprehensive study, analysed the antioxidative systems in the ectomycorrhizal fungus Paxillus involutus in response to Cd, which revealed the induction of superoxide dismutase (SOD) and the accumulation of glutathione, as well as the induction of glutathione-related systems at low Cd concentration (glutathione-dependent peroxidase, glutathione reductase) (Ott et al., 2002). Their study confirmed that Mn-dependent SOD activity was induced in response to exposure of Paxillus involutus to Cd (Jacob et al., 2001). In addition, SOD could promote Cd resistance through its capacity to bind and buffer cellular Cd as demonstrated for Cu and yeast (Culotta et al., 1995). The large number of ESTs found for SOD (Table 1) probably denotes a crucial function of these enzymes against oxidative stress in ectomycorrhizal fungi. It was found that a fast glutathione accumulation and maintenance of a relatively stable redox state prevented an accumulation of H2O2 in Paxillus involutus (Ott et al., 2002). The authors concluded that Paxillus involutus is able to detoxify high concentrations of Cd by a strong induction of glutathione synthesis accompanied by a rapid sulphur-dependent transport of Cd into the vacuole. This latter observation is in full agreement with our previous results (Blaudez et al., 2000), as discussed above. Interestingly, we found, under Cd exposure, a downregulation of hydrophobin genes, a family of small hydrophobic cysteine-rich proteins implicated in various developmental processes such as the emergence of aerial hyphae (Jacob et al., 2004). The synthesis of these cysteine-rich compounds may be efficiently reduced in Paxillus involutus, thus redirecting cysteine to the manufacture of cysteine-enriched compounds needed for the chelation of Cd. It was also suggested that metallothioneins have antioxidant activity in vivo, which could be involved in the cellular response to oxidative stress (Tamai et al., 1993). However this was not the case for the metallothionein detected in Paxillus involutus, which was unable to complement a gene-deficient yeast strain exposed to an oxidative stress (M. Courbot and M. Chalot, unpublished results).

Outlook

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References

This review has focused 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 other nonmycorrhizal fungal species are also used by the ectomycorrhizal fungi. These include mechanisms that reduce uptake into the cytosol by extracellular chelation or binding onto cell-wall components, intracellular chelation of metals in the cytosol by a range of ligands (glutathione, metallothioneins), or efflux from the cytosol into sequestering compartments. Specific features such as the lack of phytochelatin synthesis have also been described. However, most of the molecular mechanisms remain to be elucidated, among which transport mechanisms are of key interest. Furthermore, observations with a particular fungus exposed to a particular metal must be generalized with caution. It appears for instance that the major mechanism involved in Cd detoxification in Paxillus involutus consists in its compartmentation within the vacuole as sulphur-rich complexes, whereas Zn tolerance in Suillus bovinus could be primarily because of a reduced Zn accumulation within cells. Beyond these mechanisms, of course, is the problem of understanding tolerance in symbiosis, and this introduces a further level of complexity that is beyond the scope of this review.

With the completion of the L. bicolor genome sequencing project, together with genome sequences from other fungi, one can expect that the full range of genes that are potentially involved in metal tolerance and homeostasis will be revealed. One approach will be the use of full-genome gene arrays to study the potential interactions and synergies between different tolerance mechanisms in response to metal exposure. However, these data are limited to transcriptional-level responses. Proteomic approaches should provide the additional information that would be more closely related to cell function. Such studies on yeast have revealed that several proteins with antioxidant properties were induced when the cells were exposed to acute Cd exposure (Vido et al., 2001). In Schizosaccharomyces pombe, 27 proteins functionally classified as cell rescue and defence factors were upregulated for oxygen and radical detoxification (Bae & Chen, 2004). Further proteome analyses using mycorrhizal fungi will contribute to a more integrated understanding of the molecular events involved in metal stress alleviation in these fungi.

The ultimate demonstration that a particular gene product is involved in metal tolerance will require the development of more functional tools than are available now, while many research groups are actively working on transformation systems for ectomycorrhizal fungi that should provide for the efficient overexpression or disruption of target genes. Knowledge about metal tolerance mechanisms in ectomycorrhizal fungi will further lead to powerful applications in bioremediation, such as those suggested for Aspergillus (Vala et al., 2004).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Extracellular chelation and cell-wall binding
  5. Intracellular complexation by peptides
  6. Transport mechanisms involved in metal tolerance
  7. Antioxidative mechanisms
  8. Outlook
  9. References
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