Metal induction of a Paxillus involutus metallothionein and its heterologous expression in Hebeloma cylindrosporum

Physiological response mechanism of heavy metal-resistant endophytic fungi isolated from the roots of Polygonatum kingianum

This study aims to evaluate the tolerance of endophytic fungi isolated from the fibrous roots of Polygonatum kingianum to arsenic (As) and cadmium (Cd) and their physiological response mechanisms. Five isolated strains were obtained with EC50 values for As(V) ranging from 421 to 1281 mg/L, while the other three strains tolerated Cd(II) with an EC50 range of 407-1112 mg/L. Morphological and molecular identification indicated that these eight strains were Cladosporium spp. belonging to dark septate endophytes (DSEs). The contents of metal ions in mycelium sharply increased, reaching 38.87 mg/kg for strain MZ-11 under As(V) stress and 0.33 mg/kg for fungus PR-2 under Cd(II). The physiological response revealed that the biomass decreased with increasing concentrations of As(V) or Cd(II), and the activity of superoxide dismutase significantly improved under the corresponding EC50 -concentration As/Cd of the strains, as well as the contents of antioxidant substances, including metallothionein, glutathione, malondialdehyde, melanin, and proline. Taken together, the filamentous fungi of Cladosporium spp. accounted for a high proportion of fungi isolated from the fibrous roots of P. kingianum and had a strong capacity to tolerate As(V) or Cd(II) stress by improving antioxidase activities and the content of antioxidant substances, and immobilization of metal ions in hyphae.

Changes in physio-biochemical parameters and expression of metallothioneins in Avena sativa L. in response to drought

Drought is one of the major threats to food security. Among several mechanisms involved in plant stress tolerance, one protein family-the plant metallothioneins (MTs)-shows great promise for enhancing drought resistance. Plant metallothioneins in oat (Avena sativa L.) have not yet been deeply analysed, and the literature lacks a comprehensive study of the whole family of plant MTs in response to drought. In this study, we showed that the number and nature of cis-elements linked with stress response in promoters of AsMTs1-3 differed depending on the MT type. Drought stress in oat plants caused an increase in the expression of AsMT2 and AsMT3 and a decrease in the expression of AsMT1 compared to well-watered plants. Moreover, the low values of relative water content, water use efficiency, net photosynthesis (P_N), transpiration (E), stomatal conductance (g_s), chlorophyll a, and carotenoid were accompanied by high levels of electrolyte leakage, internal CO₂ concentration (C_i) and abscisic acid content, and high activity of antioxidants enzymes in plants under drought stress. The present study puts forward the idea that AsMTs are crucial for oat response to drought stress not only by regulating antioxidant activity but also by changing the plant water regime and photosynthesis. Our results support the hypothesis that structural differences among types of plant MTs reflect their diversified physiological roles.

Intracellular sequestration of cadmium and zinc in ectomycorrhizal fungus Amanita muscaria (Agaricales, Amanitaceae) and characterization of its metallothionein gene

Amanita muscaria is an ectomycorrhizal mushroom that commonly grows at metal-polluted sites. Sporocarps from the lead smelter-polluted area near Příbram (Central Bohemia, Czech Republic) showed elevated concentrations of Cd and Zn. Size exclusion chromatography of the cell extracts of the sporocarps from both polluted and unpolluted sites indicated that substantial part of intracellular Cd and Zn was sequestered in 6-kDa complexes, presumably with metallothionein(s) (MT). When the cultured mycelial isolates were compared, those from Příbram were more Cd-tolerant and accumulated slightly less Cd and Zn than those from the unpolluted site. The analysis of the available A.muscaria sequence data returned a 67-amino acid (AA) MT encoded by the AmMT1 gene. Weak Cd and Zn responsiveness of AmMT1 in the mycelia suggested its metal homeostasis function in A.muscaria, rather than a major role in detoxification. The AmMT1 belongs to a ubiquitous peptide group in the Agaricomycetes consisting of 60-70-AA MTs containing seven cysteinyl domains and a conserved histidyl, features observed also in a newly predicted, atypical 45-AA RaMT1 of the Zn-accumulator Russula bresadolae in which the C-terminal cysteinyl domains VI and VII are missing. Heterologous expression in metal-sensitive yeast mutants indicated that AmMT1 and RaMT1 encode functional peptides that can protect cells against Cd, Zn, and Cu toxicity. The metal protection phenotype observed in yeasts with mutant variants of AmMT1 and RaMT1 further indicated that the conserved histidyl seems to play a structural, not metal binding role, and the cysteinyls of the C-terminal domains VI and VII are important for Cu binding. The data provide an important insight into the metal handling of site-associated ectomycorrhizal species disturbed by excess metals and the properties of MTs common in Agaricomycetes.

Identification and characterization of eight metallothionein genes involved in heavy metal tolerance from the ectomycorrhizal fungus Laccaria bicolor

Metallothioneins (MTs) are small, cysteine-rich, heavy metal-binding proteins involved in metal homeostasis and detoxification. The increasing numbers of available genomic sequences of ectomycorrhizal (ECM) fungi enable deeper insights into the characteristics of MT genes in these fungi that form the most important symbiosis with the host trees in forest ecosystems. The aim of this study was to establish a comprehensive, genome-wide inventory of MT genes from the ECM fungus Laccaria bicolor. Eight MT genes in L. bicolor were cloned, and the expression patterns of their transcripts at various developmental stages based on expressed sequence tag (EST) counts were analyzed. The expression levels of four MTs were significantly increased during symbiosis stages. Quantitative real-time PCR (qRT-PCR) analysis revealed that transcripts of LbMT1 were dominant in free-living mycelia and strongly induced by excessive copper (Cu), cadmium (Cd), and hydrogen peroxide (H2O2). To determine whether these eight MTs functioned as metal chelators, we expressed them in the Cu- and Cd-sensitive yeast mutants, cup1∆ and yap1∆, respectively. All LbMT proteins provided similar levels of Cu(II) or Cd(II) tolerance, but did not affect by H2O2. Our findings provide novel data on the evolution and diversification of fungal MT gene duplicates, a valuable resource for understanding the vast array of biological processes in which these proteins are involved.

Metallothionein expression in Aspergillus exposed to environmentally relevant concentrations of heavy metals at different pH levels

Heavy metal pollution represents a health threat. Many fungal species are capable of tolerating various heavy metals, especially if they are isolated from a contaminated watercourse. One of the mechanisms by which fungi can sequester certain heavy metals is synthesizing stress proteins. The aim of this study is to investigate the production of metallothioneins in Aspergillus oryzae and Aspergillus clavatus exposed to environmentally relevant concentrations of Cd, Cu, Fe, and Zn at neutral, alkaline, and acidic pH conditions within 10 days. We determined the concentrations of these heavy metals in certain watercourses representing Behira and Giza governorates; also, we identified the most prevalent fungal species. We carried out a statistical correlation between different heavy metals and the isolated fungi. Then, in the laboratory, we exposed two of the most prevalent fungal species to the environmentally detected concentrations of the heavy metals and their doubles. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that in A. oryzae, the metallothionein bands appeared in neutral medium containing Cd and Cu and in alkaline medium containing Cd and Zn, while in A. clavatus, no metallothionein bands appeared at all. In conclusion, metallothionein is a good indicator of pollution with Cd, Cu, and Zn in Aspergillus oryzae, and pH plays a central role in metallothionein production.

Role of Exopolygalacturonase-Related Genes in Potato-Verticillium dahliae Interaction

Verticillium dahliae is a hemibiotrophic pathogen responsible for great losses in dicot crop production. An ExoPG gene (VDAG_03463,) identified using subtractive hybridization/cDNA-AFLP, showed higher expression levels in highly aggressive than in weakly aggressive V. dahliae isolates. We used a vector-free split-marker recombination method with PEG-mediated protoplast to delete the ExoPG gene in V. dahliae. This is the first instance of using this method for V. dahliae transformation. Only two PCR steps and one transformation step were required, markedly reducing the necessary time for gene deletion. Six mutants were identified. ExoPG expressed more in the highly aggressive than in the weakly aggressive isolate in response to potato leaf and stem extracts. Its expression increased in both isolates during infection, with higher levels in the highly aggressive isolate at early infection stages. The disruption of ExoPG did not influence virulence, nor did it affect total exopolygalacturonase activity in V. dahliae. Full genome analysis showed 8 more genes related to polygalacturonase/pectinase activity in V. dahliae. Transcripts of PGA increased in the △exopg mutant in response to potato leaf extracts, compared to the wild type. The expression pattern of those eight genes showed similar trends in the △exopg mutant and in the weakly aggressive isolate in response to potato extracts, but without the increase of PGA in the weakly aggressive isolate to leaf extracts. This indicated that the △exopg mutant of V. dahliae compensated by the suppression of ExoPG by activating other related gene.

Cadmium hyperaccumulating mushroom Cystoderma carcharias has two metallothionein isoforms usable for cadmium and copper storage

Cystoderma carcharias is one of the few macrofungal species that can hyperaccumulate Cd. As we have previously documented in C. carcharias collected from a smelter-polluted area, it stores 40% of Cd and nearly 90% of Cu in sporocarps in complex(es) of identical size. In this paper we examined whether metallothionein (MT) peptides that bind Cd and Cu through cysteinyl-thiolate bonds were associated with the metals in these complexes. Screening of a sporocarp cDNA expression library in yeasts allowed the identification of two transcripts, CcMT1 and CcMT2, encoding functional 34-amino acid (AA) MTs sharing 56% identity and appearing to be encoded by duplicate genes. CcMT1 conferred reasonable tolerance to Cu and a substantially higher tolerance to Cd than CcMT2, while CcMT2 clearly protected the yeasts better against Cu toxicity. While size-exclusion chromatography revealed that CcMT1 was contained in all Cd/Cu complexes isolated from wild grown sporocarps, CcMT2 was detected in a much narrower subset of the fractions. The striking difference between the CcMTs is that CcMT1 lacks the third metal-biding cysteinyl (C) within an otherwise highly conserved-in-agaricomycetes-MTs C-AA₄-C-AA-C-AA₃-C-AA-C-AA₄-C-AA-C motif. The elimination of the corresponding cysteinyl in CcMT2 only reduced the Cu-tolerant phenotype in yeasts to the levels observed with CcMT1. Altogether, these results indicate that CcMT2 is rather adjusted to perform Cu-related tasks and point to CcMT1 as the ligand for the storage of both Cd and Cu in C.carcharias, which is the first macrofungal species in which the potential of MT in Cd handling can be seen.

Characterizing the genetic basis of copper toxicity in Drosophila reveals a complex pattern of allelic, regulatory, and behavioral variation

A range of heavy metals are required for normal cell function and homeostasis. However, the anthropogenic release of metal compounds into soil and water sources presents a pervasive health threat. Copper is one of many heavy metals that negatively impacts diverse organisms at a global scale. Using a combination of quantitative trait locus (QTL) mapping and RNA sequencing in the Drosophila Synthetic Population Resource, we demonstrate that resistance to the toxic effects of ingested copper in D. melanogaster is genetically complex and influenced by allelic and expression variation at multiple loci. QTL mapping identified several QTL that account for a substantial fraction of heritability. Additionally, we find that copper resistance is impacted by variation in behavioral avoidance of copper and may be subject to life-stage specific regulation. Gene expression analysis further demonstrated that resistant and sensitive strains are characterized by unique expression patterns. Several of the candidate genes identified via QTL mapping and RNAseq have known copper-specific functions (e.g., Ccs, Sod3, CG11825), and others are involved in the regulation of other heavy metals (e.g., Catsup, whd). We validated several of these candidate genes with RNAi suggesting they contribute to variation in adult copper resistance. Our study illuminates the interconnected roles that allelic and expression variation, organism life stage, and behavior play in copper resistance, allowing a deeper understanding of the diverse mechanisms through which metal pollution can negatively impact organisms.

Transcriptomic Analysis of Two Lentinula edodes Genotypes With Different Cadmium Accumulation Ability

Lentinula edodes, also known as Xiang'gu, is commonly eaten in cultures around the world. However, L. edodes is particularly susceptible to enrichment with heavy metals, particularly cadmium (Cd), which is toxic to human health. Understanding the molecular mechanism and mining key genes involved in Cd enrichment will facilitate genetic modification of L. edodes strains. Two L. edodes genotypes, Le4625 (with higher Cd enrichment capability) and Le4606 (with lower Cd enrichment capability) were used in this study. The Cd concentrations in the mycelia of the tested genotypes differed significantly after Cd (0.1 mg/L) exposure; and the Cd content in Le4625 (1.390 ± 0.098 mg/kg) was approximately three-fold that in Le4606 (0.440 ± 0.038 mg/kg) after 7 h of Cd exposure. A total of 24,592 transcripts were assessed by RNA-Seq to explore variance in Cd accumulation. Firstly, differentially expressed genes (DEGs) were analyzed separately following Cd exposure. In comparison with Ld4625, Ld4606 showed a greater number of Cd-induced changes in transcription. In Ld4606, DEGs following Cd exposure were associated with transmembrane transport, glutathione transfer and cytochrome P450, indicating that these genes could be involved in Cd resistance in L. edodes. Next, Le4606 and Le4625 were exposed to Cd, after which DEGs were identified to explore genetic factors affecting Cd accumulation. After Cd exposure, DEGs between Le4606 and Le4625 encoded proteins involved in multiple biological pathways, including transporters on the membrane, cell wall modification, oxidative stress response, translation, degradation, and signaling pathways. Cadmium enrichment in cells may activate MAPK signaling and the anti-oxidative stress response, which can subsequently alter signal transduction and the intracellular oxidation/reduction balance. Furthermore, several possible candidate genes involved in the Cd accumulation were identified, including the major facilitator superfamily genes, heat shock proteins, and laccase 11, a multicopper oxidase. This comparison of the transcriptomes of two L. edodes strains with different capacities for Cd accumulation provides valuable insight into the cultivation of mushrooms with less Cd enrichment and also serves as a reference for the construction of engineered strains for environmental pollution control.

Necrotrophic fungus Macrophomina phaseolina tolerates chromium stress through regulating antioxidant enzymes and genes expression (MSN1 and MT)

Cr(VI) tolerance level of phytopathogenic fungus viz., Macrophomina phaseolina (Tassi) Goid was assessed through growth, morphological, physiological, and metal accumulation assays. Initially, the fungus growth assays indicated that the fungus can grow over concentration range of 20-3000 ppm and exhibited high tolerance index (0.88-1.00) and minimum inhibitory concentration at 3500 ppm of Cr. Observations under compound and scanning electron microscope un-revealed the structural features of hyphae under Cr stress as thick-walled, aggregated, branched, short and broken, along with attachment of irregular objects on them. Metal accumulation analysis revealed reduction in Cr(VI) accumulation by the fungus with increase in metal concentration in the growth medium (500-3000 ppm). Cr stress induced upregulation of antioxidant enzyme activities (catalase, peroxidase and polyphenol oxidase), expression of genes (MSN1 and metallothionein) and appearnace of new protein bands suggesting the possible role in protection and survival of M. phaseolina against Cr(VI)-induced oxidative stress. This study concludes that interference of Cr with growth and physiological process of M. phaseolina could affect its infection level on its host plant, therefore, synergistic action of two factors needs to be addressed, which may aid to guide future research efforts in understanding impact of plant-pathogen-heavy metal interaction.

Cadmium and arsenic responses in the ectomycorrhizal fungus Laccaria bicolor: glutathione metabolism and its role in metal(loid) homeostasis

Ectomycorrhizal fungi play an important role in protecting their host plant from metal(loid) stresses by synthesizing various thiol rich compounds like metallothioneins and glutathione. We investigated the effect of cadmium (Cd) and arsenic (As) stress with a specific interest on glutathione (GSH) in the ectomycorrhizal fungus Laccaria bicolor. The total GSH levels inside the cell were significantly increased with increase in external metal(loid) stress. An analysis of the transcript levels of genes responsible for GSH synthesis, γ-glutamylcysteine synthetase (Lbγ-GCS) and glutathione synthetase (LbGS), using qPCR revealed that expression of both genes increased as a function of external metal(loid) concentration. The enzyme activity of both Lbγ-GCS and LbGS were increased with increase in external Cd and As concentration. Further, the functional role of Lbγ-GCS and LbGS genes in response to Cd and As stress was studied using their respective yeast mutant strains gsh1 ^Δ and gsh2 ^Δ . The mutant strains successfully expressed the two genes resulting in wild-type phenotype restoration of Cd and As tolerance. From these results, it was concluded that GSH act as a core component in the mycorrhizal defence system under Cd and As stress for metal(loid) homeostasis and detoxification.

Purification and Characterization of a Cadmium-Binding Protein from Lentinula edodes

Many organisms possess the ability to produce metal-binding proteins to absorb cadmium. Metallothioneins, an important family of cysteine-rich metal-binding proteins, have been isolated and well characterized. However, Lentinula edodes may have a different type of cadmium-binding protein that contains fewer cysteine residues. In the present study, we purified a cadmium-binding protein from L. edodes (LECBP) by gel filtration and anion exchange chromatography and then identified LECBP by LC-MS/MS. We found LECBP to be a novel cadmium-binding protein, which contained 220 amino acid residues but no cysteine residue. LECBP had a high binding affinity for Cd(II) with a K_d value of 97.3 μM. The percentages of α-helix, β-sheet, β-turn, and random coil in LECBP were 15.7%, 39.4%, 8.0%, and 37.1%, respectively. In addition, high temperatures and an acidic environment influenced the conformation of LECBP. Our results will thus provide a new perspective to understand the mechanism of cadmium accumulation in L. edodes.

Speciation analysis of elements accumulated in Cystoderma carcharias from clean and smelter-polluted sites

Collections of Cystoderma carcharias sporocarps were sampled from clean and smelter-polluted sites and analyzed for Ag, As, Cd, Cu, Pb, Se, and Zn contents. Concentrations of all elements were significantly higher in samples from the smelter-polluted area. Except for As and Pb, all elements were effectively accumulated in the sporocarps at both clean and polluted sites. With the highest concentration of 604 mg Cd kg^-1, C. carcharias can be considered as Cd hyperaccumulator. As revealed by HPLC-ICPQQQMS analysis, the As species in sporocarps from clean and polluted areas involved besides the major arsenobetaine a variety of known and unknown arsenicals; the occurrence of dimethylarsinoylacetate and trimethylarsoniopropionate is reported for the first time for gilled fungi (Agaricales). Size-exclusion chromatography of C. carcharias extracts supported by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and RP-HPLC data indicated that detoxification of intracellular Cd and Cu may largely rely on metallothioneins (MT) or MT-like peptides, not phytochelatins.

Isolation of multi-metal tolerant ubiquitin fusion protein from metal polluted soil by metatranscriptomic approach

Release of heavy metals into the soil pose a significant threat to the environment and public health because of their toxicity accumulation in the food chain and persistence in nature. The potential of soil microbial diversity of cadmium (Cd) contaminated site was exploited through functional metatranscriptomics by construction of cDNA libraries A (0.1-0.5 kb), B (0.5-1.0 kb), and C (1-4 kb) of variable size, from the eukaryotic mRNA. The cDNA library B was further screened for cadmium tolerant transcripts through yeast complementation system. We are reporting one of the transformants ycf1^ΔPLBe1 capable of tolerating high concentrations of Cd (40 μM - 80 μM). Sequence analysis revealed that PLBe1 cDNA showed homology with ubiquitin domain containing protein fused with AN1 type zinc finger protein of Acanthameoba castellani. Further, this cDNA was tested for its tolerance towards other heavy metals such as copper (Cu), zinc (Zn) and cobalt (Co). Functional complementation assay of cDNA PLBe1 showed a range of tolerance towards copper (150 μM - 300 μM), zinc (10 mM - 12 mM) and cobalt (2 mM - 4 mM). This study promulgates PLBe1 as credible member of multi-metal tolerant gene in the eukaryotic soil microbial community and can be used as potential member to revitalise the heavy metal contaminated sites or can be used as a biomarker to detect heavy metal contamination in the soil environment.

Metal induction of two metallothionein genes in the ectomycorrhizal fungus Suillus himalayensis and their role in metal tolerance

Metallothioneins (MTs) are small proteins with highly conserved cysteine residues and are involved in metal homeostasis and metal detoxification. Two metallothionein genes ShMT1 and ShMT2 from the ectomycorrhizal fungus Suillus himalayensis were characterised for their potential role in heavy metal detoxification. The response of these MTs to the exogenous concentrations of copper and cadmium was studied by qPCR analysis. The exogenous copper but not the cadmium at the tested concentrations induced the expression of the MT genes. The functional role of ShMTs was validated by expressing the two genes through functional complementation in yeast mutant strain cup1^Δ (copper-sensitive), ycf1^Δ (cadmium- sensitive) and zrc1^Δ (zinc-sensitive). The mutant strain successfully expressed the two genes resulting in wild-type phenotype restoration of copper, cadmium and zinc tolerance. The present study shows that the ectomycorrhizal fungus S. himalayensis encodes two metallothionein genes (ShMT1 and ShMT2) which are more inducible by copper than cadmium and could play an important role in their detoxification.

Two P1B-1-ATPases of Amanita strobiliformis With Distinct Properties in Cu/Ag Transport

As we have shown previously, the Cu and Ag concentrations in the sporocarps of Ag-hyperaccumulating Amanita strobiliformis are correlated, and both metals share the same uptake system and are sequestered by the same metallothioneins intracellularly. To further improve our knowledge of the Cu and Ag handling in A. strobiliformis cells, we searched its transcriptome for the P_1B-1-ATPases, recognizing Cu⁺ and Ag⁺ for transport. We identified transcripts encoding 1097-amino acid (AA) AsCRD1 and 978-AA AsCCC2, which were further subjected to functional studies in metal sensitive Saccharomyces cerevisiae. The expression of AsCRD1 conferred highly increased Cu and Ag tolerance to metal sensitive yeasts in which the functional AsCRD1:GFP (green fluorescent protein) fusion localized exclusively to the tonoplast, indicating that the AsCRD1-mediated Cu and Ag tolerance was a result of vacuolar sequestration of the metals. Increased accumulation of AsCRD1 transcripts observed in A. strobiliformis mycelium upon the treatments with Cu and Ag (8.7- and 4.5-fold in the presence of 5 μM metal, respectively) supported the notion that AsCRD1 can be involved in protection of the A. strobiliformis cells against the toxicity of both metals. Neither Cu nor Ag affected the levels of AsCCC2 transcripts. Heterologous expression of AsCCC2 in mutant yeasts did not contribute to Cu tolerance, but complemented the mutant genotype of the S. cerevisiae ccc2Δ strain. Consistent with the role of the yeast Ccc2 in the trafficking of Cu from cytoplasm to nascent proteins via post-Golgi, the GFP fluorescence in AsCCC2-expressing ccc2Δ yeasts localized among Golgi-like punctate foci within the cells. The AsCRD1- and AsCCC2-associated phenotypes were lost in yeasts expressing mutant transporter variants in which a conserved phosphorylation/dephosphorylation site was altered. Altogether, the data support the roles of AsCRD1 and AsCCC2 as genuine P_1B-1-ATPases, and indicate their important functions in the removal of toxic excess of Cu and Ag from the cytoplasm and charging the endomembrane system with Cu, respectively.

Bioaccumulation of heavy metals, metalloids, and chlorine in ectomycorrhizae from smelter-polluted area

Ectomycorrhizal (ECM) fungi contribute to the survival of host trees on metal-rich soils by reducing the transfer of toxic metals into roots. However, little is known about the ability of ECM fungi to accumulate elements in ectomycorrhizae (ECMs). Here we report Ag, As, Cd, Cl, Cu, Sb, V, and Zn contents in wild-grown Norway spruce ECMs collected in a smelter-polluted area at Lhota near Příbram, Czech Republic. The ECMs data were compared with the element concentrations determined in the corresponding non-mycorrhizal fine roots, soils, and soil extracts. Bioaccumulation factors were calculated to differentiate the element accumulation ability of ECMs inhabited by different mycobionts, which were identified by ITS rDNA sequencing. Among the target elements, the highest contents were observed for Ag, Cl, Cd, and Zn; Imleria badia ECMs showed the highest capability to accumulate these elements. ECMs of Amanita muscaria, but not of other species, accumulated V. The analysis of the proportions of I. badia and A. muscaria mycelia in ECMs by using species-specific quantitative real-time PCR revealed variable extent of the colonization of roots, with median values close to 5% (w/w). Calculated Ag, Cd, Zn and Cl concentrations in the mycelium of I. badia ECMs were 1 680, 1 510, 2 670, and 37,100 mg kg^-1 dry weight, respectively, indicating substantial element accumulation capacity of hyphae of this species in ECMs. Our data strengthen the idea of an active role of ECM fungi in soil-fungal-plant interactions in polluted environments.

Heavy Metal Tolerance in Plants: Role of Transcriptomics, Proteomics, Metabolomics, and Ionomics

Heavy metal contamination of soil and water causing toxicity/stress has become one important constraint to crop productivity and quality. This situation has further worsened by the increasing population growth and inherent food demand. It has been reported in several studies that counterbalancing toxicity due to heavy metal requires complex mechanisms at molecular, biochemical, physiological, cellular, tissue, and whole plant level, which might manifest in terms of improved crop productivity. Recent advances in various disciplines of biological sciences such as metabolomics, transcriptomics, proteomics, etc., have assisted in the characterization of metabolites, transcription factors, and stress-inducible proteins involved in heavy metal tolerance, which in turn can be utilized for generating heavy metal-tolerant crops. This review summarizes various tolerance strategies of plants under heavy metal toxicity covering the role of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), various stress-inducible proteins (proteomics) as well as the role of plant hormones. We also provide a glance of some strategies adopted by metal-accumulating plants, also known as "metallophytes."

Metal induction of a Pisolithus albus metallothionein and its potential involvement in heavy metal tolerance during mycorrhizal symbiosis

Metallothioneins (MTs) are small, cysteine-rich peptides involved in intracellular sequestration of heavy metals in eukaryotes. We examined the role in metal homeostasis and detoxification of an MT from the ectomycorrhizal fungus Pisolithus albus (PaMT1). PaMT1 encodes a 35 amino acid-long polypeptide, with 7 cysteine residues; most of them part of a C-x-C motif found in other known basidiomycete MTs. The expression levels of PaMT1 increased as a function of increased external Cu and Cd concentrations and were higher with Cu than with Cd. Heterologous complementation assays in metal-sensitive yeast mutants indicated that PaMT1 encodes a polypeptide capable of conferring higher tolerance to both Cu and Cd. Eucalyptus tereticornis plantlets colonized with P. albus grown in the presence of Cu and Cd showed better growth compared with those with non-mycorrhizal plants. Higher PaMT1 expression levels were recorded in mycorrhizal plants grown in the presence of Cu and Cd compared with those in control mycorrhizal plants not exposed to heavy metals. These data provide the first evidence to our knowledge that fungal MTs could protect ectomycorrhizal fungi from heavy metal stress and in turn help the plants to establish in metal-contaminated sites.

Detoxification strategies and regulation of oxygen production and flowering of Platanus acerifolia under lead (Pb) stress by transcriptome analysis

Toxic metal pollution is a major environmental problem that has received wide attention. Platanus acerifolia (London plane tree) is an important greening tree species that can adapt to environmental pollution. The genetic basis and molecular mechanisms associated with the ability of P. acerifolia to respond lead (Pb) stress have not been reported so far. In this study, 16,246 unigenes differentially expressed unigenes that were obtained from P. acerifolia under Pb stress using next-generation sequencing. Essential pathways such as photosynthesis, and gibberellins and glutathione metabolism were enriched among the differentially expressed unigenes. Furthermore, many important unigenes, including antioxidant enzymes, plants chelate compounds, and metal transporters involved in defense and detoxification mechanisms, were differentially expressed in response to Pb stress. The unigenes encoding the oxygen-evolving enhancer Psb and OEE protein families were downregulated in Pb-stressed plants, implying that oxygen production might decrease in plants under Pb stress. The relationship between gibberellin and P. acerifolia flowering is also discussed. The information and new insights obtained in this study will contribute to further investigations into the molecular regulation mechanisms of Pb accumulation and tolerance in greening tree species.

Genetic transformation of extremophilic fungi Acidea extrema and Acidothrix acidophila

Intact, growing cells of strongly acidophilic fungi Acidea extrema and Acidothrix acidophila have been successfully transformed by introduction of heterologous DNA fragment (composed of the glyceraldehyde-phosphate-dehydrogenase gene promoter from Emericella nidulans, a metallothionein-coding gene AsMt1 from Amanita strobiliformis and glyceraldehyde-phosphate-dehydrogenase gene terminator from Colletotrichum gloeosporioides) with the length of 1690 bp. The transformation procedure was based on the DNA transfer mediated by Agrobacterium tumefaciens bearing disarmed helper plasmid pMP90 and binary vector pCambia1300 with inserted DNA fragment of interest. The transformants proved to be mitotically stable, and the introduced gene was expressed at least at the level of transcription. Our work confirms that metabolic adaptations of strongly acidophilic fungi do not represent an obstacle for genetic transformation using conventional methods and can be potentially used for production of heterologous proteins. A promising role of the fast growing A. acidophila as active biomass in biotechnological processes is suggested not only by the low susceptibility of the culture grown at low pH to contaminations but also by reduced risk of accidental leaks of genetically modified microorganisms into the environment because highly specialized extremophilic fungi can poorly compete with common microflora under moderate conditions.

Common and metal-specific proteomic responses to cadmium and zinc in the metal tolerant ericoid mycorrhizal fungus Oidiodendron maius Zn

Although adaptive metal tolerance may arise in fungal populations in polluted soils, the mechanisms underlying metal-specific tolerance are poorly understood. Comparative proteomics is a powerful tool to identify variation in protein profiles caused by changing environmental conditions, and was used to investigate protein accumulation in a metal tolerant isolate of the ericoid mycorrhizal fungus Oidiodendron maius exposed to zinc and cadmium. Two-dimensional gel electrophoresis and shotgun proteomics followed by mass spectrometry lead to the identification of common and metal-specific proteins and pathways. Proteins selectively induced by cadmium exposure were molecular chaperons of the Hsp90 family, cytoskeletal proteins and components of the translation machinery. Zinc significantly up-regulated metabolic pathways related to energy production and carbohydrates metabolism, likely mirroring zinc adaptation of this fungal isolate. Common proteins induced by the two metal ions were the antioxidant enzyme Cu/Zn superoxide dismutase and ubiquitin. In mycelia exposed to zinc and cadmium, both proteomic techniques also identified agmatinase, an enzyme involved in polyamine biosynthesis. This novel finding suggests that, like plants, polyamines may have important functions in response to abiotic environmental stress in fungi. Genetic evidence also suggests that the biosynthesis of polyamines via an alternative metabolic pathway may be widespread in fungi.

Differential expression of metallothioneins in response to heavy metals and their involvement in metal tolerance in the symbiotic basidiomycete Laccaria bicolor

Cysteine-rich peptides such as metallothioneins (MTs) are involved in metal homeostasis and detoxification in many eukaryotes. We report the characterization and expression of two MT genes, LbMT1 and LbMT2 from the ectomycorrhizal fungus Laccaria bicolor under metal stress conditions. LbMT1 and LbMT2 differ with respect to the length of the encoded peptides (58 versus 37 aa, respectively) and also by their expression patterns in response to metals. The expression levels of both LbMT1 and LbMT2 increased as a function of increased external Cu concentration, the expression levels for LbMT2 were always significantly higher compared with those of LbMT1. Only LbMT1, but not LbMT2, responded to Cd supply in the range of 25-100 µM while Zn did not affect the transcription of either LbMT1 or LbMT2. Both genes also responded to oxidative stress, but to a lesser extent compared to their responses to either Cu or Cd stress. Heterologous complementation assays in metal-sensitive yeast mutants indicated that both LbMT1 and LbMT2 encode peptides capable of conferring higher tolerance to both Cu and Cd. The present study identified LbMTs as potential determinants of the response of this mycorrhizal fungus to Cu and Cd stress.

Intracellular sequestration of zinc, cadmium and silver in Hebeloma mesophaeum and characterization of its metallothionein genes

Sequestration of intracellular heavy metals in eukaryotes involves compartmentalization and binding with cytosolic, cysteine-rich metallothionein (MT) peptides. We examined the roles of these processes in handling of zinc (Zn), cadmium (Cd) and silver (Ag) in sporocarps and a metal-exposed extraradical mycelium of Hebeloma mesophaeum, the Zn-accumulating ectomycorrhizal (EM) species frequently associated with metal disturbed sites. Size exclusion chromatography revealed that the majority of Zn and Cd in the sporocarps and mycelium was contained in a low molecular mass fraction attributable to compartmentalized metal. The staining of hyphal cells with the Zn-specific Zinquin and Cd-specific Leadmium fluorescent tracers labeled Zn and Cd in small, punctuated vesicles and vacuoles, respectively. By contrast, the sporocarp and mycelium Ag was associated with cysteine-rich, 5-kDa peptides. The peptides of the same size were also identified in minor Zn and Cd complexes from the metal-exposed mycelium. We have further isolated and characterized HmMT1, HmMT2 and HmMT3 genes coding for different 5-kDa MTs of H. mesophaeum collected at a lead smelter site. Heterologous complementation assays in metal-sensitive yeast mutants indicated that HmMTs encode functional, metal-specific peptides: only HmMT1 was able to complement sensitivity to Zn; HmMT1 conferred higher tolerance to Cd and Cu than HmMT2 or HmMT3; and both HmMT2 and HmMT3, but not HmMT1, conferred increased tolerance to Ag. The presence of HmMT1 and HmMT3, but not HmMT2, was also confirmed in a H. mesophaeum isolate from an unpolluted site. Gene expression analysis in the extraradical mycelium of this isolate revealed that the transcription of HmMT1 was preferentially induced in the presence of Zn and Cd, while Ag was a stronger inducer of HmMT3. Altogether, these results improve our understanding of the handling of intracellular Zn, Cd and Ag in Hebeloma and represent the first evidence suggesting involvement of MTs in sequestration of Zn in EM fungi.

Zinc export results in adaptive zinc tolerance in the ectomycorrhizal basidiomycete Suillus bovinus

On Zn-polluted soils, populations of the ectomycorrhizal basidiomycete Suillus bovinus exhibit an elevated Zn tolerance when compared to populations on non-polluted sites. To elucidate the mechanism of Zn tolerance, the time-course of Zn uptake was studied in isolates with contrasting Zn tolerance. Unidirectional fluxes and subcellular compartmentation of Zn were investigated through radiotracer flux analyses. Fluorescence imaging was used to support the subcellular Zn compartmentation. After 2 h of exposure to 200 μM Zn, significantly more Zn was accumulated in Zn-sensitive isolates compared to tolerant isolates, despite similar short-term uptake kinetics and similar extracellular Zn sequestration in cell walls. In Zn-sensitive isolates twice as much Zn accumulated in the cytoplasm and 12 times more Zn in the vacuole. (65)Zn efflux analyses revealed a considerably faster Zn export in the Zn-tolerant isolate. The adaptive Zn tolerance in S. bovinus is therefore achieved by a preferential removal of Zn out of the cytoplasm, back into the apoplast, instead of the usual transfer of Zn into the vacuole. Zn exclusion in the fungal symbiont eventually contributes to a lower Zn influx in host plants.

Potassium nutrition of ectomycorrhizal Pinus pinaster: overexpression of the Hebeloma cylindrosporum HcTrk1 transporter affects the translocation of both K(+) and phosphorus in the host plant

Mycorrhizal associations are known to improve the hydro-mineral nutrition of their host plants. However, the importance of mycorrhizal symbiosis for plant potassium nutrition has so far been poorly studied. We therefore investigated the impact of the ectomycorrhizal fungus Hebeloma cylindrosporum on the potassium nutrition of Pinus pinaster and examined the involvement of the fungal potassium transporter HcTrk1. HcTrk1 transcripts and proteins were localized in ectomycorrhizas using in situ hybridization and EGFP translational fusion constructs. Importantly, an overexpression strategy was performed on a H. cylindrosporum endogenous gene in order to dissect the role of this transporter. The potassium nutrition of mycorrhizal pine plants was significantly improved under potassium-limiting conditions. Fungal strains overexpressing HcTrk1 reduced the translocation of potassium and phosphorus from the roots to the shoots of inoculated plants in mycorrhizal experiments. Furthermore, expression of HcTrk1 and the phosphate transporter HcPT1.1 were reciprocally linked to the external inorganic phosphate and potassium availability. The development of these approaches provides a deeper insight into the role of ectomycorrhizal symbiosis on host plant K(+) nutrition and in particular, the K(+) transporter HcTrk1. The work augments our knowledge of the link between potassium and phosphorus nutrition via the mycorrhizal pathway.

Metallothionein-like peptides involved in sequestration of Zn in the Zn-accumulating ectomycorrhizal fungus Russula atropurpurea

The first evidence of the existence of gene-encoded Zn-binding peptides that sequester a substantial portion of intracellular Zn in ectomycorrhizal fungi under natural conditions.

Role of pCeMT, a putative metallothionein from Colocasia esculenta, in response to metal stress

Metallothioneins (MTs) play a major role in metal homeostasis and/or detoxification in plants. In this study, a novel gene, pCeMT, was isolated from Colocasia esculenta and characterized. Our results indicate that Escherichia coli cells expressing pCeMT exhibited enhanced Cd, Cu, and Zn tolerance and accumulation compared with control cells. Furthermore, pCeMT-overexpressing tobacco seedlings displayed better growth under Cd, Cu, and Zn stresses and accumulated more Cd and Zn compared with the wild type. Interestingly, transgenic tobacco displayed markedly decreased hydrogen peroxide (H(2)O(2)) and lipid peroxidation levels under Cd, Cu, and Zn treatments. These results suggest that pCeMT could play an important role in the protection of plant cells from oxidative stress by reactive oxygen species (ROS) scavenging and in the detoxification of free metals by metal binding, leading to improved plant metal tolerance.

The trace element content of top-soil and wild edible mushroom samples collected in Tuscany, Italy

The amount of the trace elements As, Ba, Cd, Cr, Cu, Hg, Li, Mn, Ni, Pb, Rb, Se, Sr, and Zn was measured in top soils and edible mushrooms, Boletus edulis, Macrolepiota procera, collected at five distinct green microhabitats inside the Lucca province, North-Central Italy (years 2008-2009). Results showed a top soil element content within the Italian statutory limits. Concerning the amount of mushroom elements, we observed significant species-differences obtaining higher levels of Ni, Rb, and Se in B. edulis or As, Pb, Cu in M. procera. Bioaccumulation factors (BCFs: element in mushroom/element in soil) resulted species-dependent and element-selective: in particular, B. edulis preferentially accumulated Se (BCFs varying from 14 to 153), while M. procera mainly concentrated Cu (BCFs varying from 5 to 15). As well, both species displayed between-site BCF differences. By a multivariate principal component approach, cluster analysis (CA), we could resolve two main clusters of soil element composition, corresponding to the most ecologically divergent sites. Besides, CA showed no cluster relating to element contents of B. edulis at the different collection sites, while a separation in groups was found for M. procera composition with respect to harvesting locations, suggesting uptake systems, in this saprotrophic species, sensitive to microhabitat. Regarding consumer safety, Cd, Hg, Pb levels resulted sometime relevant in present samples, never reaching values from current literature on mushrooms collected in urban-polluted areas. Our findings encourage a deeper assessment of the molecular mechanisms of metal intake by edible mushrooms, encompassing genetic biochemical and geo-ecological variables, with particular awareness to element bioavailability in soils and fungi.

Modulation of ethanol stress tolerance by aldehyde dehydrogenase in the mycorrhizal fungus Tricholoma vaccinum

We report the first mycorrhizal fungal aldehyde dehydrogenase gene, ald1, which was isolated from the basidiomycete Tricholoma vaccinum. The gene, encoding a protein Ald1 of 502 amino acids, is up-regulated in ectomycorrhiza. Phylogenetic analyses using 53 specific fungal aldehyde dehydrogenases from all major phyla in the kingdom of fungi including Ald1 and two partial sequences of T. vaccinum were performed to get an insight in the evolution of the aldehyde dehydrogenase family. By using competitive and real-time RT-PCR, ald1 is up-regulated in response to alcohol and aldehyde-related stress. Furthermore, heterologous expression of ald1 in Escherichia coli and subsequent in vitro enzyme activity assay demonstrated the oxidation of propionaldehyde and butyraldehyde with different kinetics using either NAD(+) or NADP(+) as cofactors. In addition, overexpression of ald1 in T. vaccinum after Agrobacterium tumefaciens-mediated transformation increased ethanol stress tolerance. These results demonstrate the ability of Ald1 to circumvent ethanol stress, a critical function in mycorrhizal habitats.

Tolerance to and accumulation of cadmium by the mycelium of the fungi Scleroderma citrinum and Pisolithus tinctorius

The behavior of ectomycorrhizal (ECM) fungi on exposure to cadmium dependent upon isolation remains a poorly understood phenomenon. The in vitro growth, tolerance, and accumulation of Cd were studied in three strains of ECM fungi exposed to six Cd concentrations (0-10 mg L(-1)). The fungi studied were a strain of Scleroderma citrinum Persoon (Sc) isolated from a tailings heap containing 5 mg kg(-1) available Cd, and two strains of Pisolithus tinctorius (Pers.) Coker and Couch from unpolluted sites (Pt1 and Pt2), both common ECM fungi used for remediation. The growth kinetic (36 days) of Sc was not affected by Cd concentration. By contrast, the ED(50) in Pt1 and Pt2 occurred at 4.8 and 6.9 mg L(-1) of Cd, respectively. The biomass of the three fungi exposed to the highest Cd concentration (10 mg L(-1)) was significantly different. Sc presented the highest biomass, while this was strongly reduced for Pt1 and Pt2. The tolerance index for Sc ranged from 78% to 95% at all Cd concentrations tested, while for Pt1 it was 49% and 31%, and for Pt2 it was 62% and 35% at 5 and 10 mg of Cd L(-1), respectively. The mycelium of both Pt strains accumulated more Cd than the Sc mycelium. At the highest Cd concentration, Pt1 and Pt2 accumulated 1.9 and 1.7 times more Cd than Sc. This study suggests that regardless of the differences in tolerance to Cd by the three ECM fungi, they could have biotechnological applications for soil remediation. However, Sc has greater possibilities of being used successfully when high concentrations of Cd prevail in the environment.

Three metallothionein isoforms and sequestration of intracellular silver in the hyperaccumulator Amanita strobiliformis

Metallothioneins (MTs) are cysteine-rich peptides involved in heavy metal tolerance of many eukaryotes. Here, we examined their involvement in intracellular binding of silver (Ag) in the ectomycorrhizal fungus Amanita strobiliformis. The Ag complexes and their peptide ligands were characterized using chromatography and mass spectrometry. The full-length coding sequences obtained from a cDNA library were used for complementation assays in yeast mutant strains. Abundance of respective transcripts in A. strobiliformis was measured by quantitative real-time reverse-transcribed polymerase chain reaction (qRT-PCR). Ag-speciation analyses showed that intracellular Ag was in wild-grown fruit bodies and cultured extraradical mycelia of A. strobiliformis sequestered by metallothioneins. The determined sequence of the peptide facilitated isolation of three cDNA clones, AsMT1a, AsMT1b and AsMT1c. These encode isomorphic MTs consisting of 34 amino acid residues and sharing 82% identity. In mycelia the expression of AsMT1s is induced by Ag. All AsMT1s expressed in yeasts complemented hypersensitivity of mutants to cadmium (Cd) and copper (Cu) and formed Ag complexes. Only the Ag-AsMT1a complex was detected in the A. strobiliformis fruit body in which AsMT1a was the prevailing transcript. The present study identified the existence of metallothionein isoforms in ectomycorrhizal fungi. We demonstrated that intracellular sequestration of Ag in fruit bodies and mycelia of hyperaccumulating A. strobiliformis is dominated by metallothioneins.

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

The ectomycorrhizal basidiomycete Suillus luteus (L.:Fr.), a typical pioneer species which associates with young pine trees colonizing disturbed sites, is a common root symbiont found at heavy metal contaminated sites. Three Cd-sensitive and three Cd-tolerant isolates of S. luteus, isolated respectively from non-polluted and a heavy metal-polluted site in Limburg (Belgium), were used for a transcriptomic analysis. We identified differentially expressed genes by cDNA-AFLP analysis. The possible roles of some of the encoded proteins in heavy metal (Cd) accumulation and tolerance are discussed. Despite the high conservation of coding sequences in S. luteus, a large intraspecific variation in the transcript profiles was observed. This variation was as large in Cd-tolerant as in sensitive isolates and may help this pioneer species to adapt to novel environments.

Bioaccumulation of silver in ectomycorrhizal and saprobic macrofungi from pristine and polluted areas

Macrofungi are effective accumulators of Ag. This study provides a comprehensive review of this phenomenon supported by original data on the Ag concentrations of macrofungi from pristine and Ag-polluted areas. In pristine areas, the median Ag concentrations of ectomycorrhizal (ECM) and saprobic (SAP) macrofungi were 0.79 and 2.94 mg kg(-1), respectively. In these areas, hyperaccumulation thresholds for Ag in ECM and SAP macrofungi are proposed as 100 and 300 mg kg(-1), respectively. In a Ag-polluted area, the Ag concentrations in macrofungi (ECM and SAP) were significantly elevated with the median value of 24.7 mg kg(-1) and the highest concentrations in Amanita spp. of the section Vaginatae (304-692 mg kg(-1)). The intracellular speciation of Ag in fruit-bodies of the Ag-accumulator Amanita submembranacea was inspected by size exclusion chromatography followed by sulfhydryl-specific fluorimetric assays of ligands using reverse phase high-performance liquid chromatography and improved polyacrylamide gel electrophoresis. Virtually all Ag was found to be intracellular and sequestered in the major 7 kDa and minor 3.3 kDa complexes. The lack of glutathione and phytochelatins and the presence of a single 3 kDa sulfhydryl-containing peptide in the isolated Ag-complexes suggest that detoxification of Ag in A. submembranacea may rely on metallothionein. Vertical distribution of Ag in a polluted forest soil profile has shown substantial enrichment in organic horizons; in polluted technosol, the highest Ag concentrations were found in surface layers. Standardized EDTA extraction of Ag in both the investigated soil profiles showed relatively low Ag extractibility, generally within the range of 2.2-7.7% of total Ag content.

Identification of heavy metal regulated genes from the root associated ascomycete Cadophora finlandica using a genomic microarray

The ascomycete Cadophora finlandica, which can form mycorrhizas with ectomycorrhizal and ericoid hosts, is commonly found in heavy metal polluted soils. To understand the selective advantage of this organism at contaminated sites heavy metal regulated genes from C. finlandica were investigated. For gene identification a strategy based on a genomic microarray was chosen, which allows a rapid, genome-wide screening in genetically poorly characterized organisms. In a preliminary screen eleven plasmids covering eight distinct genomic regions and encoding a total of ten Cd-regulated genes were identified. Northern analyses with RNA from C. finlandica grown in the presence of either Cd, Pb or Zn revealed different transcription patterns in response to the heavy metals present in the growth medium. The Cd-regulated genes are predicted to encode several extracellular proteins with unknown functions, transporters, a centaurin-type regulator of intracellular membrane trafficking, a GNAT-family acetyltransferase and a B-type cyclin.