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

Differential effects of tree species identity on rhizospheric bacterial and fungal community richness and composition across multiple trace element-contaminated sites

Soil microbial communities play a key role in plant nutrition and stress tolerance. This is particularly true in sites contaminated by trace metals, which often have low fertility and stressful conditions for woody plants in particular. However, we have limited knowledge of the abiotic and biotic factors affecting the richness and composition of microbial communities inhabiting the rhizosphere of plants in contaminated sites. Using high-throughput amplicon sequencing, we studied the rhizospheric bacterial and fungal community structures of 14 woody plant families planted in three contrasting sites contaminated by metals (Pb, Cd, Zn, Mn, Fe, S). The rhizospheric bacterial communities in the given sites showed no significant difference between the various woody species but did differ significantly between sites. The Proteobacteria phylum was dominant, accounting for over 25 % of the overall relative abundance, followed by Actinobacteria, Bacteroidetes and Gemmatimonadetes. Site was also the main driver of fungal community composition, yet unlike bacteria, tree species identity significantly affected fungal communities. The Betulaceae, Salicaceae and Fagaceae families had a high proportion of Basidiomycota, particularly ectomycorrhizal fungi, and the lowest diversity and richness. The other tree families and the unplanted soil harboured a greater abundance of Ascomycota and Mucoromycota. Consequently, for both bacteria and fungi, the site effect significantly impacted their community richness and composition, while the influence of plants on the richness and composition of rhizospheric microbial communities stayed consistent across sites and was dependent on the microbial kingdom. Finally, we highlighted the importance of considering this contrasting response of plant rhizospheric microbial communities in relation to their host identity, particularly to improve assisted revegetation efforts at contaminated sites.

Mitigation of heavy metal stress in the soil through optimized interaction between plants and microbes

Agricultural as well as industrial processes, such as mining and textile activities, are just a few examples of anthropogenic activities that have a long-term negative impact on the environment. Each of the aforementioned factors increases the concentration of heavy metals in soil. Heavy metal contamination in soil causes a wide range of environmental issues and is harmful to microbes, plants, and animals. Because of their non-biodegradability and toxic effects, preventing additional metal contamination and remediating the vast majority of contaminated sites around the world is critical. Hence, this review focuses on the effects of metal contamination on soil microbes, as well as plant-microbe interactions. Plant-associated probiotics reduce metal accumulation; the introduction of beneficial microbes is regarded as one of the most promising approaches to improving metal stress tolerance; thus, the study focuses on plant-microbe interactions as well as their actual implications via phytoremediation. Plant-microbe interaction can play an important role in acclimating vegetation (plants) to metalliferous conditions and should thus be studied to improve microbe-aided metal tolerance in plants. Plant-interacted microbes reduce metal accumulation in plant cells and metal bioaccumulation in the soil through a variety of processes. A novel phytobacterial approach, such as genetically modified microbes, is now being used to improve heavy metal cleanup as well as stress tolerance among plants. This review examines our current understanding of such negative consequences of heavy metal stresses, signaling responses, and the role of plant-associated microbiota in heavy metal stress tolerance and interaction.

Mechanisms of Mucor sp. CM3 isolated from the aquatic macrophyte Eichhornia crassipes (Mart.) Solms to increase cadmium bioremediation

Bioremediation of toxic metals is a feasible and low-cost remediation tool to reduce metal contamination. Plant-fungus interactions can improve this technique. Eichhornia crassipes (Mart.) Solms is a macrophyte reported to bioremediate contaminated water. Thus, the present study aimed to isolate endophytic fungi from E. crassipes, select a highly cadmium (Cd) tolerant isolate and evaluate its bioremediation potential. This was evaluated by (1) the fungus tolerance and capacity to accumulate Cd; (2) Cd effects on cell morphology (using SEM and TEM) and on the fungal antioxidant defense system, as well as (3) the effect on model plant Solanum lycopersicum L. cultivar Calabash Rouge, inoculated with the endophyte fungus and exposed to Cd. Our results selected the endophyte Mucor sp. CM3, which was able to tolerate up to 1000 g/L of Cd and to accumulate 900 mg of Cd/g of biomass. Significant changes in Mucor sp. CM3 morphology were observed when exposed to high Cd concentrations, retaining this metal both in its cytoplasm and in its cell wall, which may be linked to detoxification and metal sequestration mechanisms related to the formation of Cd-GSH complexes. In addition, Cd stress induced the activation of all tested antioxidant enzymes - superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR) - in this endophytic fungus. Moreover, when inoculated in tomato plants, this fungus promoted plant growth (in treatments without Cd) and induced an increased metal translocation to plant shoot, showing its potential to increase metal bioremediation. Therefore, this study indicates that the isolated endophyte Mucor sp. CM3 can be applied as a tool in different plant conditions, improving plant bioremediation and reducing the environmental damage caused by Cd, while also promoting plant growth in the absence of contaminants.

Effect of Arsenic Soil Contamination on Stress Response Metabolites, 5-Methylcytosine Level and CDC25 Expression in Spinach

Experimental spinach plants grown in soil with (5, 10 and 20 ppm) arsenic (As) contamination were sampled in 21 days after As(V) contamination. Levels of As in spinach samples (from 0.31 ± 0.06 µg g^-1 to 302.69 ± 11.83 µg g^-1) were higher in roots and lower in leaves, which indicates a low ability of spinach to translocate As into leaves. Species of arsenic, As(III) and As(V), were represented in favor of the As (III) specie in contaminated variants, suggesting enzymatic arsenate reduction. In relation to predominant As accumulation in roots, changes in malondialdehyde levels were observed mainly in roots, where they decreased significantly with growing As contamination (from 11.97 ± 0.54 µg g^-1 in control to 2.35 ± 0.43 µg g^-1 in 20 ppm As). Higher values in roots than in leaves were observed in the case of 5-methylcytosine (5-mC). Despite that, a change in 5-mC by As contamination was further deepened in leaves (from 0.20 to 14.10%). In roots of spinach, expression of the CDC25 gene increased by the highest As contamination compared to the control. In the case of total phenolic content, total flavonoid content, total phenolic acids content and total antioxidant capacity were higher levels in leaves in all values, unlike the roots.

S(-II) reactivates Cd2+-stressed Shewanella oneidensis via promoting low-molecular-weight thiols synthesis and activating antioxidant defense

Efficient remedies for living organisms including bacteria to counteract cadmium (Cd²⁺) toxicity are still highly needed. Plant toxicity studies have showed that exogenous S(-II) (including hydrogen sulfide and its ionic forms, i.e., H₂S, HS^-, and S^2-) application can effectively alleviate adverse effects of Cd stress, but whether S(-II) could mitigate bacterial Cd toxicity remains unclear. In this study, S(-II) was applied exogenously to Cd-stressed Shewanella oneidensis MR-1 and the results showed that S(-II) can significantly reactivate impaired physiological processes including growth arrest and enzymatic ferric (Fe(III) reduction inhibition. The efficacy of S(-II) treatment is negatively correlated with the concentration and time length of Cd exposure. Energy-dispersive X-ray (EDX) analysis suggested the presence of cadmium sulfide inside cells treated with S(-II). Both compared proteomic analysis and RT-qPCR showed that enzymes associated with sulfate transport, sulfur assimilation, methionine, and glutathione biosynthesis were up-regulated in both mRNA and protein levels after the treatment, indicating S(-II) may induce the biosynthesis of functional low-molecular-weight (LMW) thiols to counteract Cd toxicity. Meanwhile, the antioxidant enzymes were positively modulated by S(-II) and thus the activity of intracellular reactive oxygen species was attenuated. The study demonstrated that exogenous S(-II) can effectively alleviate Cd stress for S. oneidensis likely through inducing intracellular trapping mechanisms and modulating cellular redox status. It suggested that S(-II) may be a highly effective remedy for bacteria such as S. oneidensis under Cd-polluted environments.

Ectomycorrhizal fungi, two species of Laccaria, differentially block the migration and accumulation of cadmium and copper in Pinus densiflora

The root tips of host plant species can establish ectomycorrhizae with their fungal partners, thereby altering the responses of the host plants to heavy metal (HM) toxicity. Here, two species of Laccaria, L. bicolor and L. japonica, were investigated in symbiosis with Pinus densiflora to study their potential for promotion of phytoremediation of HM-contaminated soils in pot experiments. The results showed that L. japonica had significantly higher dry biomass than L. bicolor in mycelia grown on modified Melin-Norkrans medium containing elevated levels of cadmium (Cd) or copper (Cu). Meanwhile, the accumulations of Cd or Cu in L. bicolor mycelia were much higher than that in L. japonica at the same level of Cd or Cu. Therefore, L. japonica displayed a stronger tolerance to HM toxicity than L. bicolor in situ. Compared with non-mycorrhizal P. densiflora seedlings, inoculation with two Laccaria species significantly increased the growth of P. densiflora seedlings in absence or presence of HM. The mantle of host roots blocked the uptake and migration of HM, which led to the decrease of Cd and Cu accumulation in the P. densiflora shoots and roots, except for the root Cd accumulation of L. bicolor-mycorrhizal plants when 25 mg kg^-1 Cd exposure. Furthermore, HM distribution in mycelia showed Cd and Cu are mainly retained in the cell walls of mycelia. These results provide strong evidence that the two species of Laccaria in this system may have different strategies to assist host tree against HM toxicity.

Scandium, yttrium, and lanthanide occurrence in Cantharellus cibarius and C. minor mushrooms

There is a dearth of data on rare earth elements (REE), yttrium and scandium in foods which extends also to baseline datasets for edible wild mushrooms, though this has started to change in the last decade. Concentrations and shale normalized patterns of REE and Y (REY) were studied by using inductively coupled plasma-quadrupole mass spectrometer in 22 pools (2235 specimens) of Cantharellus cibarius (Golden Chanterelle) collected in Poland and also a pool of C. minor (Small Chanterelle) (153 specimens) from Yunnan (Chinese Province). The total REY plus Sc varied in C. cibarius from 10 to 593 µg kg-1 dw whereas that for the Yunnan's C. minor was 2072 µg kg-1 dw. C. minor from Yunnan has higher REY and Sc compared to the C. cibarius. Sc concentrations in twenty C. cibarius pools were below 1 µg kg-1 dw, but 17 and 27 µg kg-1 dw were detected at the other two sites and 66 µg kg-1 dw was detected in C. minor. The median Y content of C. cibarius and C. minor was 22 µg kg-1 dw and 200 µg kg-1 dw. The difference in REY and Sc concentrations and shale normalized patterns between mushrooms from Poland and Yunnan seems to reflect the regional difference in concentration and composition of these elements in the soil bedrock.

Cadmium in the hyperaccumulating mushroom Thelephora penicillata: Intracellular speciation and isotopic composition

Thelephora penicillata is an ectomycorrhizal mushroom that can accumulate extraordinarily high concentrations of Cd, As, Cu, and Zn in its fruit-bodies. To better understand its element accumulation ability, we compared the element concentrations in T. penicillata with 10 distinct ectomycorrhizal mushroom species growing at the same site (Karlina Pila, Czech Republic). On average, T. penicillata accumulated 330, 2130, 26, and 4 times more Cd, As, Cu, and Zn, respectively, than other mushrooms. Size-exclusion chromatography and an electrophoretic analysis of T. penicillata cell extracts indicate that intracellular Cd may be present mainly in >1 kDa, presumably compartmentalized, Cd species, and partially binding with 6-kDa cysteinyl-containing peptide(s) resembling metallothioneins. The cadmium isotopic composition of mushroom fruit-bodies, soil digests, and soil extracts was investigated by thermal ionization mass spectrometry (TIMS) with double spike correction. The isotopic composition (δ^114/110Cd) of ectomycorrhizal mushrooms from Karlina Pila varied in a wide range of -0.37 to +0.14 ‰. However, remarkably low δ^114/110Cd values were observed in the majority of the investigated mushrooms when compared to the relatively homogeneous Cd isotopic composition of bulk soil (δ^114/110Cd = +0.09 ‰) and the comparatively heavy isotopic composition of soil extracts (mean δ^114/110Cd values of +0.11 ± 0.01 ‰ and +0.22 ± 0.01 ‰, depending on the extraction method). The isotopic composition of Cd hyperaccumulated in T. penicillata essentially matched the mycoavailable soil Cd fraction. However, most isotopic data indicates isotopic fractionation at the soil/fruit-body interface, which could be of environmental significance.

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.

A new mushroom hyperaccumulator: Cadmium and arsenic in the ectomycorrhizal basidiomycete Thelephora penicillata

Fruit-bodies of six Thelephora species (Fungi, Basidiomycota, Thelephoraceae) were analyzed for their trace element concentrations. In Thelephora penicillata, extremely high concentrations of Cd and As were found, followed by highly elevated concentrations of Cu and Zn. The highest accumulation ability was found for Cd with a mean concentration of 1.17 ± 0.37 g kg^-1 (dry mass) in fruit-bodies collected from 20 unpolluted sites; the mean As concentration was 0.878 ± 0.242 g kg^-1. Furthermore, striking accumulation of Se (923 ± 28 mg kg^-1) was found in one sample of T. vialis and elevated concentrations of S were detected in T. palmata (19.6 ± 5.9 g kg^-1). The analyzed Thelephora species were sequenced and, based on the Maximum Likelihood phylogenetic analysis (ITS rDNA) of the genus, possible other Thelephora (hyper)accumulators were predicted on the basis of their phylogenetic relationship with the discovered (hyper)accumulators. The striking ability of T. penicillata to accumulate simultaneously Cd, As, Cu, and Zn has no parallel in the Fungal Kingdom and raises the question of a biological importance of metal(loid) hyperaccumulation in mushrooms.

A review on bioremediation approach for heavy metal detoxification and accumulation in plants

Nowadays, the accumulation of toxic heavy metals in soil and water streams is considered a serious environmental problem that causes various harmful effects on plants and animals. Phytoremediation is an effective, green, and economical bioremediation approach by which the harmful heavy metals in the contaminated ecosystem can be detoxified and accumulated in the plant. Hyperaccumulators exude molecules called transporters that carry and translocate the heavy metals present in the soil to different plant parts. The hyperaccumulator plant genes can confine higher concentrations of toxic heavy metals in their tissues. The efficiency of phytoremediation relies on various parameters such as soil properties (pH and soil type), organic matters in soil, heavy metal type, nature of rhizosphere, characteristics of rhizosphere microflora, etc. The present review comprehensively discusses the toxicity effect of heavy metals on the environment and different phytoremediation mechanisms for the transport and accumulation of heavy metals from polluted soil. This review gave comprehensive insights into plants tolerance for the higher heavy metal concentration their responses for heavy metal accumulation and the different mechanisms involved for heavy metal tolerance. The current status and the characteristic features that need to be improved in the phytoremediation process are also reviewed in detail.

Bioinformatic prediction of putative metallothioneins in non-ciliate protists

Intracellular ligands that bind heavy metals (HMs) and thereby minimize their detrimental effects to cellular metabolism are attracting great interest for a number of applications including bioremediation and development of HM-biosensors. Metallothioneins (MTs) are short, cysteine-rich, genetically encoded proteins involved in intracellular metal-binding and play a key role in detoxification of HMs. We searched approximately 700 genomes and transcriptomes of non-ciliate protists for novel putative MTs by similarity and structural analyses and found 21 unique proteins playing a potential role as MTs. Most putative MTs derive from heterokonts and dinoflagellates and share common features such as (i) a putative metal-binding domain in proximity of the N-terminus, (ii) two putative MT-specific domains near the C-terminus and (iii) one to three CTCGXXCXCGXXCXCXXC patterns. Although the biological function of these proteins has not been experimentally proven, knowledge of their genetic sequences adds useful information on proteins that are potentially involved in HM-binding and can contribute to the design of future biomolecular assays on HM-microbe interactions and MT-based biosensors.

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.

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.

Insights into nanomycoremediation: Secretomics and mycogenic biopolymer nanocomposites for heavy metal detoxification

Our environment thrives on the subtle balance achieved by the forever cyclical nature of building and rebuilding life through natural processes. Fungi, being the evident armor of bioremediation, is the indispensable element of the soil food web, contribute to be the nature's most dynamic arsenal with non-specific enzymes like peroxidase (POX), glutathione peroxidase (GPx), catalase (CAT), superoxide dismutase (SOD), non-enzymatic compounds like thiol (-SH) groups and non-protein compounds such as glutathione (GSH) and metallothionein (MT). Recently, the area of nanomycoremediation has been gaining momentum as a powerful tool for environmental clean-up strategies with its ability to detoxify heavy metals with its unique characteristics to adapt mechanisms such as biosorption, bioconversion, and biodegradation to harmless end products. The insight into the elaborate secretomic processes provides us with huge opportunities for creating a magnificent living bioremediation apparatus. This review discusses the scope and recent advances in the lesser understood area, nanomycoremediation, the state-of-the-art, innovative, cost-effective and promising tool for detoxification of heavy metal pollutants and focuses on the metabolic capabilities and secretomics with nanobiotechnological interventions.

Arsenic accumulation and speciation in two cultivars of Pteris cretica L. and characterization of arsenate reductase PcACR2 and arsenite transporter PcACR3 genes in the hyperaccumulating cv. Albo-lineata

Pollution and poisoning with carcinogenic arsenic (As) is of major concern globally. Interestingly, there are ferns that can naturally tolerate remarkably high As concentrations in soils while hyperaccumulating this metalloid in their fronds. Besides Pteris vittata in which As-related traits and molecular determinants have been studied in detail, the As hyperaccumulation status has been attributed also to Pteris cretica. We thus inspected two P. cretica cultivars, Parkerii and Albo-lineata, for As hyperaccumulation traits. The cultivars were grown in soils supplemented with 20, 100, and 250 mg kg-1 of inorganic arsenate (iAsV). Unlike Parkerii, Albo-lineata was confirmed to be As tolerant and hyperaccumulating, with up to 1.3 and 6.4 g As kg-1 dry weight in roots and fronds, respectively, from soils amended with 250 mg iAsV kg-1. As speciation analyses rejected that organoarsenical species and binding with phytochelatins and other proteinaceous ligands would play any significant role in the biology of As in either cultivar. While in Parkerii, the dominating As species, particularly in roots, occurred as iAsV, in Albo-lineata the majority of the root and frond As was apparently converted to iAsIII. Parkerii markedly accumulated iAsIII in its fronds when grown on As spiked soils. Considering the roles iAsV reductase ACR2 and iAsIII transporter ACR3 may have in the handling of iAs, we isolated Albo-lineata PcACR2 and PcACR3 genes closely related to P. vittata PvACR2 and PvACR3. The gene expression analysis in Albo-lineata fronds revealed that the transcription of PcACR2 and PcACR3 was clearly As responsive (up to 6.5- and 45-times increase in transcript levels compared to control soil conditions, respectively). The tolerance and uptake assays in yeasts showed that PcACRs can complement corresponding As-sensitive mutations, indicating that PcACR2 and PcACR3 encode functional proteins that can perform, respectively, iAsV reduction and membrane iAsIII transport tasks in As-hyperaccumulating Albo-lineata.

Fungal-Metal Interactions: A Review of Toxicity and Homeostasis

Metal nanoparticles used as antifungals have increased the occurrence of fungal-metal interactions. However, there is a lack of knowledge about how these interactions cause genomic and physiological changes, which can produce fungal superbugs. Despite interest in these interactions, there is limited understanding of resistance mechanisms in most fungi studied until now. We highlight the current knowledge of fungal homeostasis of zinc, copper, iron, manganese, and silver to comprehensively examine associated mechanisms of resistance. Such mechanisms have been widely studied in Saccharomyces cerevisiae, but limited reports exist in filamentous fungi, though they are frequently the subject of nanoparticle biosynthesis and targets of antifungal metals. In most cases, microarray analyses uncovered resistance mechanisms as a response to metal exposure. In yeast, metal resistance is mainly due to the down-regulation of metal ion importers, utilization of metallothionein and metallothionein-like structures, and ion sequestration to the vacuole. In contrast, metal resistance in filamentous fungi heavily relies upon cellular ion export. However, there are instances of resistance that utilized vacuole sequestration, ion metallothionein, and chelator binding, deleting a metal ion importer, and ion storage in hyphal cell walls. In general, resistance to zinc, copper, iron, and manganese is extensively reported in yeast and partially known in filamentous fungi; and silver resistance lacks comprehensive understanding in both.

Bibliometric analysis of European publications between 2001 and 2016 on concentrations of selected elements in mushrooms

This article presents a bibliometric study of 200 European publications released between 2001 and 2016, about the contamination of mushrooms by selected elements. The analysis includes figures on the type of analyte, its concentration, the species of fungi, and its country of origin. In the literature review, 492 species of mushrooms (wild-growing and cultured) found in 26 European countries and their concentration of 74 associated elements were analysed. The papers, which dealt mainly with the heavy metal (Cd, Cu, Fe, Pb, and Zn) concentrations of mushrooms, primarily came from Turkey, Poland, Spain, and the Czech Republic. More than 50% of the publications provided data about edible mushrooms. The results of the bibliometric analysis showed that over the 16 years, European research on fungal contamination by selected analytes has not lessened in popularity and is ongoing. Many of the studies underlined the need to assess the risk to human health arising from the consumption of contaminated mushrooms taken from various habitats. These results were the effect of, among other things, the strong interest in studies carried out on edible species, in which concentrations of mainly heavy metals that are dangerous to health and are marked were indicated (Cd, Pb, and Hg).

Increased Cd2+ biosorption capability of Aspergillus nidulans elicited by crpA deletion

The P-type ATPase CrpA is an important Cu²⁺ /Cd²⁺ pump in the Aspergilli, significantly contributing to the heavy metal stress tolerance of these ascomycetous fungi. As expected, the deletion of crpA resulted in Cu²⁺ /Cd²⁺ -sensitive phenotypes in Aspergillus nidulans on stress agar plates inoculated with conidia. Nevertheless, paradoxical growth stimulations were observed with the ΔcrpA strain in both standard Cu²⁺ stress agar plate experiments and cellophane colony harvest (CCH) cultures, when exposed to Cd²⁺ . These observations reflect efficient compensatory mechanisms for the loss of CrpA operating under these experimental conditions. It is remarkable that the ΔcrpA strain showed a 2.7 times higher Cd biosorption capacity in CCH cultures, which may facilitate the development of new, fungal biomass-based bioremediation technologies to extract harmful Cd²⁺ ions from the environment. The nullification of crpA also significantly changed the spatial distribution of Cu and Cd in CCH cultures, as demonstrated by the combined particle-induced X-ray emission and scanning transmission ion microscopy technique. Most important, the centers of gravity for Cu and Cd accumulations of the ΔcrpA colonies shifted toward the older regions as compared with wild-type surface cultures.

Zn overaccumulating Russula species clade together and use the same mechanism for the detoxification of excess Zn

It has been firmly established that macrofungi can accumulate large amounts of heavy metals in their sporocarps. However, the mechanisms of the accumulation and storage are being uncovered only recently. We have previously documented that Russula bresadolae can accumulate over 1 g Zn kg^-1 dry weight and that sequestration of a substantial proportion of overaccumulated Zn involves binding with peptides, RaZBPs, seen so far only in this species. In this work we examined Zn contents of 360 sporocarp collections from unpolluted environments covering 114 species of the genus Russula. Whilst the concentrations of Zn in most analysed species were in the range of 50-150 mg kg^-1, the species of subgenera Brevipes and Compactae accumulate very low Zn (< 50 mg kg^-1). We further identified five new Zn-overaccumulating species of subgenus Russula, which form with R. bresadolae a separate phylogenetic subclade in which the sporocarp Zn concentrations ranged from 326 to 845 mg kg^-1. We demonstrate that R. pumila and R. ochroleuca express at least one ZBP gene and when expressed in metal-sensitive S. cerevisiae, all ZBPs protected the yeasts against Zn (and Cd) toxicity equally well. The respective ZBPs were confirmed in the native Zn-complexes of R. pumila and R. ochroleuca, which represented 80% of Zn extracted from their sporocarps. This study is the first extensive genus-wide report of metal accumulation in macrofungi, which further demonstrates that the Zn binding with cytosolic ZBP peptides is not a trait restricted only to R. bresadolae.

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.

Role of the Fusarium oxysporum metallothionein Mt1 in resistance to metal toxicity and virulence

Soil organisms exhibit high tolerance to heavy metals, probably acquired through evolutionary adaptation to contaminated environments.

Understanding the phyto-interaction of heavy metal oxide bulk and nanoparticles: evaluation of seed germination, growth, bioaccumulation, and metallothionein production

The fast-growing use of nano-based products without proper care has led to a major public health concern. Nanomaterials contaminating the environment pose serious threat to the productivity of plants and via food chain to human health. Realizing these, four vegetable crops, radish, cucumber, tomato, and alfalfa, were exposed to varying concentrations of heavy metal oxide (TiO₂, ZnO, Al₂O₃ and CuO) submicron or bulk (BPs) and nanoparticles (NPs) to assess their impact on relative seed germination (RSG), seed surface adsorption, root/shoot tolerance index (RTI/STI), bioaccumulation, and metallothioneins (MTs) production. The results revealed a clear inhibition of RSG, RTI, and STI, which, however, varied between species of metal-specific nanoparticles and plants. SEM and EDX analyses showed significant adsorption of MONP agglomerates on seed surfaces. The concentration of metals detected by EDX differed among vegetables. Among the metals, Al, Cu, Ti, and Zn were found maximum in alfalfa (12.46%), tomato (23.2%), cucumber (6.32%) and radish (21.74%). Of the four metal oxides, ZnO was found most inhibitory to all vegetables and was followed by CuO. The absorption/accumulation of undesirable levels of MONPs in seeds and seedlings differed with variation in dose rates, and was found to be maximum (1748–2254 μg g⁻¹ dry weight) in ZnO-NPs application. Among MONPs, the uptake of TiO₂ was minimum (2 to 140 μg g⁻¹) in radish seedlings. The concentration of MTs induced by ZnO-NPs, ZnO-BPs, and CuO-NPs ranged between 52 and 136 μ mol MTs g⁻¹ FW in vegetal organs. Conclusively, the present findings indicated that both the nanosize and chemical composition of MONPs are equally dangerous for vegetable production. Hence, the accumulation of MONPs, specifically ZnO and CuO, in edible plant organs in reasonable amounts poses a potential environmental risk which, however, requires urgent attention to circumvent such toxic problems.

Phyto-interaction of heavy metal oxide nano and bulk particles with agriculturally important crops.

Metal release and sequestration from black slate mediated by a laccase of Schizophyllum commune

Schizophyllum commune is a filamentous basidiomycete which can degrade complex organic macromolecules like lignin by the secretion of a large repertoire of enzymes. One of these white rot enzymes, laccase, exhibits a broad substrate specificity and is able to oxidize a variety of substances including carbonaceous rocks. To investigate the role of laccase in bioweathering, laccase gene lcc2 was overexpressed, and the influence on weathering of black slate, originating from a former alum mine in Schmiedefeld, Germany, was examined. The metal release from the rock material was enhanced, associated with a partial metal accumulation into the mycelium. A sequestration of metals could be shown with fluorescent staining methods, and an accumulation of Zn, Cd, and Pb was visualized in different cell organelles. Additionally, we could show an increased metal resistance of the laccase overexpressing strain.

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.

Responses of Plant Proteins to Heavy Metal Stress-A Review

Plants respond to environmental pollutants such as heavy metal(s) by triggering the expression of genes that encode proteins involved in stress response. Toxic metal ions profoundly affect the cellular protein homeostasis by interfering with the folding process and aggregation of nascent or non-native proteins leading to decreased cell viability. However, plants possess a range of ubiquitous cellular surveillance systems that enable them to efficiently detoxify heavy metals toward enhanced tolerance to metal stress. As proteins constitute the major workhorses of living cells, the chelation of metal ions in cytosol with phytochelatins and metallothioneins followed by compartmentalization of metals in the vacuoles as well as the repair of stress-damaged proteins or removal and degradation of proteins that fail to achieve their native conformations are critical for plant tolerance to heavy metal stress. In this review, we provide a broad overview of recent advances in cellular protein research with regards to heavy metal tolerance in plants. We also discuss how plants maintain functional and healthy proteomes for survival under such capricious surroundings.

Lead, cadmium and mercury contents and bioaccumulation potential of wild edible saprophytic and ectomycorrhizal mushrooms, Croatia

Lead (Pb), cadmium (Cd), and mercury (Hg) contents in ten species of edible mushrooms in Trakošćan, Croatia were determined. In addition, the similarity between the studied species was determined by cluster analysis. The caps and stipes of the fruiting bodies were analysed separately. The analyses were carried out by inductively coupled plasma - optical emission spectrometry (ICP-OES). The greatest mean lead concentrations of 1.91 and 1.60 mg kg ^-1 were determined in caps and stipes of Macrolepiota procera. The greatest mean concentrations of cadmium (3.23 and 2.24 mg kg^-1) were determined in caps and stipes of Agaricus campestris and of mercury (2.56 and 2.35 mg kg^-1) in Boletus edulis. In terms of the anatomical parts of the fruiting body (cap-stipe), a considerably greater concentration of the analysed elements was found in the cap for all mushroom species. According to calculated bio-concentration factors, all the examined species were found to be bio-accumulators of Cd and Hg. On the basis of the accumulation of the studied metals, great similarity of mushroom species belonging to the same genus and partial similarity of species of the same ecological affiliation was obtained by cluster analysis.

Study of silver, selenium and arsenic concentration in wild edible mushroom Macrolepiota procera, health benefit and risk

The content and bioaccumulation of trace (Ag, Se, As) and major elements (Ca, Mg, Na and K) in wild edible mushroom Macrolepiota procera and its corresponding soil substrates, collected from five sites in the Rasina region in central Serbia, were investigated. The content of Ag, As and Se was determined by inductively coupled plasma mass spectrometer (ICP-MS) while the amount of Ca, Mg, Na and K was determined by inductively coupled plasma optical emission spectrometer (ICP-OES). The concentrations of major elements in the mushrooms were at typical levels. As far as trace elements are concerned, M. procera bioaccumulates silver although all samples were collected from unpolluted sites. It was found that the content of Ag depended on the geographical origin and the density of fruiting body on the certain site. Principal component analysis distinguished the mushroom samples from different geographical areas and revealed the influence of soil composition on metal content in fruiting bodies. Also, a linear regression correlation test was performed to investigate correlations between Ag, Cd, Se, Pb and As in caps and stipes at different geographic sites separately. In addition, our results indicated that M. procera could serve as a good dietary source of Mg, K and Se. The content of Ag and As was low, so it could not pose a health risk for consumers.

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.

Functional analysis of two genes coding for distinct cation diffusion facilitators of the ectomycorrhizal Zn-accumulating fungus Russula atropurpurea

Russula atropurpurea can accumulate remarkably high concentrations of Zn in its sporocarps. We have previously demonstrated that 40 % of the intracellular Zn in this species is sequestered by MT-like RaZBP peptides. To see what other mechanisms for the handling of the accumulated Zn are available to R. atropurpurea, we searched its transcriptome for cDNAs coding for transporters of the cation diffusion facilitator (CDF) family. The transcriptome search enabled us to identify RaCDF1 and RaCDF2, which were further subjected to functional studies in metal sensitive Saccharomyces cerevisiae. The expression of RaCDF1 and its translational fusion with green fluorescent protein (GFP) protected the yeasts against Zn and Co, but not Cd or Mn, toxicity and led to increased Zn accumulation in the cells. The GFP fluorescence, observed in the RaCDF1::GFP-expressing yeasts on tonoplasts, indicated that the RaCDF1-mediated Zn and Co tolerance was a result of vacuolar sequestration of the metals. The expression of RaCDF2 supported Zn, but not Mn, tolerance in the yeasts and reduced the cellular uptake of Zn, which is congruent with the proposed idea of the Zn-efflux function of RaCDF2, supported by the localization of GFP-derived fluorescence on the plasma membrane of the yeasts expressing functional RaCDF2::GFP. Contrarily, RaCDF2 increased the sensitivity to Co and Cd in the yeasts and significantly promoted Cd uptake, which suggested that it can act as a bidirectional metal transporter. The notion that RaCDF1 and RaCDF2 are genuine CDF transporters in R. atropurputrea was further reinforced by the fact that the RaCDF-associated metal tolerance and uptake phenotypes were lost upon the replacement of histidyl (in RaCDF1) and aspartyl (in RaCDF2), which are highly conserved in the second transmembrane domain and known to be essential for the function of CDF proteins.

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.

On the possible role of macrofungi in the biogeochemical fate of uranium in polluted forest soils

Interactions of macrofungi with U, Th, Pb and Ag were investigated in the former ore mining district of Příbram, Czech Republic. Samples of saprotrophic (34 samples, 24 species) and ectomycorrhizal (38 samples, 26 species) macrofungi were collected from a U-polluted Norway spruce plantation and tailings and analyzed for metal content. In contrast to Ag, which was highly accumulated in fruit-bodies, concentrations of U generally did not exceed 3mg/kg which indicates a very low uptake rate and efficient exclusion of U from macrofungi. In ectomycorrhizal tips (mostly determined to species level by DNA sequencing), U contents were practically identical with those of the non-mycorrhizal fine spruce roots. These findings suggest a very limited role of macrofungi in uptake and biotransformation of U in polluted forest soils. Furthermore, accumulation of U, Th, Pb and Ag in macrofungal fruit-bodies apparently does not depend on total content and chemical fractionation of these metals in soils (tested by the BCR sequential extraction in this study).

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.

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.