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8321 Today’s knowledge on the molecular background of metal/metalloid toxicity in eukaryotes arises in great part from extensive genome, transcriptome, deletome, proteome, interactome and metabolome analyses having been done in baker’s yeast cultures. Saccharomyces cerevisiae is an excellent model organism to address important biological questions because the available molecular biological, genetic and bioinformatic tools are unprecedently sophisticated and versatile with this hemiascomycete. As shown by numerous examples, knowledge obtained from yeast-based models, e.g. on the mechanism of action of and the tolerance against toxic metals, can be transferred with high efficiency to higher eukaryotes including humans (Tamás et al. 2005; Wysocki and Tamás 2010).

It is the hope of the author that a biotechnology-oriented evaluation and systematization of our current understanding of fungal metal/metalloid tolerance would help us to develop new, environmentally friendly and economically feasible technologies in the remediation of heavy metal contaminated soil (Gadd 2000, 2010; Schützendübel and Polle 2002; Gaur and Adholeya 2004; Khan 2005; Lebeau et al. 2008; Marques et al. 2009; Bothe et al. 2010; Hong-Bo et al. 2010; Purakayastha and Chhonkar 2010; Wu et al. 2010) and water (Baldrian 2003; Agrawal et al. 2006; Swami and Buddhi 2006; Wang and Chen 2006; Pan et al. 2009, More et al. 2010; Sankaran et al. 2010), in bioleaching of heavy metals from preservative-treated wood (Mai et al. 2004; Sierra-Alvarez 2007, 2009), and in the mining and recovery of metals (Mulligan et al. 2004; Fujii and Fukunaga 2008; Kuroda and Ueda 2010; Simate et al. 2010). Fungal toxic metal/metalloid resistance genes heterologously expressed in plants (Mejáre and Bülow 2001; Song et al. 2003; Wawrzyński et al. 2006; Guo et al. 2008) may enhance the efficiency of the available phytoremediation or phytomining technologies (Lasat 2002; Chaney et al. 2007; Sheoran et al. 2009).

Considering the wealth of information accumulated on the toxic metal/metalloid tolerances of yeasts and filamentous fungi in the last decade, an applied microbiology/biotechnology-oriented researcher may feel a bit puzzled when she/he aims at the production of fungal strains with a significantly augmented toxic metal/ metalloid tolerance and/or with enhanced metal/metalloid

biosorption and bioaccumulation properties. For example, the overlap between the genome-wide screens for genes contributing to toxic metal/metalloid tolerance may be as low as 10–20% although the gene groups coming from different laboratories and surveys complement nicely each other in terms of physiological functions (Thorsen et al. 2009). Because excellent reviews are available in this field, which are summarizing today’s yeast-based knowledge on toxic metals and metalloids in an easy-to-read and easyto-understand way (Tamás et al. 2005; Wysocki and Tamás 2010), the focus of this review is placed on the possible future targets of the genetic engineering of toxic metal/metalloid tolerant fungi. Moreover, the chapter incorporates the most relevant physiological and genetic data gained in toxic metal/metalloid exposed filamentous fungi as well as in yeast and filamentous fungus cultures exposed to essential micronutrient metals like Fe, Cu or Zn.

It is the hope of the author that a biotechnology-oriented evaluation and systematization of our current understanding of fungal metal/metalloid tolerance would help us to develop new, environmentally friendly and economically feasible technologies in the remediation of heavy metal contaminated soil (Gadd 2000, 2010; Schützendübel and Polle 2002; Gaur and Adholeya 2004; Khan 2005; Lebeau et al. 2008; Marques et al. 2009; Bothe et al. 2010; Hong-Bo et al. 2010; Purakayastha and Chhonkar 2010; Wu et al. 2010) and water (Baldrian 2003; Agrawal et al. 2006; Swami and Buddhi 2006; Wang and Chen 2006; Pan et al. 2009, More et al. 2010; Sankaran et al. 2010), in bioleaching of heavy metals from preservative-treated wood (Mai et al. 2004; Sierra-Alvarez 2007, 2009), and in the mining and recovery of metals (Mulligan et al. 2004; Fujii and Fukunaga 2008; Kuroda and Ueda 2010; Simate et al. 2010). Fungal toxic metal/metalloid resistance genes heterologously expressed in plants (Mejáre and Bülow 2001; Song et al. 2003; Wawrzyński et al. 2006; Guo et al. 2008) may enhance the efficiency of the available phytoremediation or phytomining technologies (Lasat 2002; Chaney et al. 2007; Sheoran et al. 2009).

Considering the structure of this review, after the presentation and discussion of the major elements of fungal metal/metalloid stress defense systems, readers’ attention is called to some promising future targets and genetic tools to increase the metal/metalloid tolerance of fungi.

First Line of Defense: Extracellular Chelation and Binding to Cell Wall Constituents Metal chelation by small molecular mass metabolites, peptides and proteins is a crucially important element of almost all metal/metalloid detoxification processes (Tamás et al. 2005; González-Guerrero et al. 2009; Wysocki and Tamás 2010) and, hence, the significance of extracellular and cytosolic chelation reactions cannot be overestimated. Glutathione (GSH) secretion is a very important element of the GSH-homeostasis in yeast under different environmental conditions (Perrone et al. 2005), and it is sensible that yeast cells intensify GSHsecretion under As(III)exposures to relieve the intracellular detoxification pathways (Wysocki and Tamás 2010).

A wide rage of fungi has been reported to produce extracellular mucilaginous materials (ECMM or “emulsifier”) with excellent toxic metal binding capabilities. As demonstrated by Paraszkiewicz et al. (2007, 2009, 2010),

Oxalate secretion is well-documented in both brown-rot and white-rot fungi, and this process seems to be stimulated under Cu(II) and Cd(II) stress (Clausen and Green 2003; JaroszWilkołazka et al. 2006). The bulk formation of water-insoluble metal-oxalate crystals is undoubtedly an efficient way to prevent toxic metal ions entering fungal cells (Jarosz-Wilkołazka and Gadd 2003). In addition, oxalate is primarily important to maintain the lignolytic system of white rot basidimycetes (Schlosser and Höfer 2002).

Isolates of the arbuscular mycorrhizal fungi Glomus and Gigaspora species produce a soil glycoprotein called glomalin (Wright et al. 1996), which possesses a remarkable capability to sequester Cu(II) (González-Chávez et al. 2004; Cornejo et al. 2008; Ferrol et al. 2009). Glomalin is located mainly in the cell wall (Purin and Rilling 2008; Ferrol et al. 2009). Besides glomalin, other cell wall polymers like chitin and melanin can also take part in metal biosorption (González-Guerrero et al. 2009).

Although decreasing the bioavailability of the toxic metals/metalloids through extracellular complexation, precipitation, and binding to cell wall constituents represents a reasonable and straightforward strategy in strain developments the genetic tools to approach this aim are still in a premature stage. The lack of yeast systems suitably modeling extracellular toxic metal/metalloid detoxification pathways obviously represents a major handicap especially when it is compared to the abundance of models and information available on the organization and regulation of intracellular detoxification pathways.

Second Line of Defense: Transport, Intracellular Chelation and Compartmentalization

Heavy metals enter cells through channels and transporters, which normally facilitate the uptake of essential transition metal micronutrients like Fe, Mn and Zn, anions including phosphate and sulphate as well as sugars (glucose) and sugar derivatives (glycerol) (Tamás et al. 2005; Wysocki and Tamás 2010). In theory, one of the most simple and most effective way to keep off toxic metals/metalloids outside the cell is to eliminate the channel or transporter responsible for the uptake of a given toxic metal/metalloid ion. Unfortunately, these ions may be channeled through multiple transporters into the cytoplasm and, aggravating the situation, the absence of even one of these transport routes may disturb the normal metabolism of the cells. The elimination of the following plasma membrane channels and transporters has been demonstrated to confer metal tolerance to metal/metalloid exposed S. cerevisiae cells (Tamás et al. 2005; Wysocki and Tamás 2010):

Metallothioneins are low molecular mass metal chelator proteins with high affinity towards Cu(II), Zn(II) and Cd(II) (Ecker et al. 1986; Borrelly et al. 2002; Zhang et al. 2003; Kumar et al. 2005; Wysocki and Tamás 2010). S. cerevisiae may contain tandem repeats of the CUP1 metallothionein gene, and the number of the gene copies correlates with the Cu(II) and Zn(II) binding capacities and the Cu(II)tolerance of the yeast cells (Stroobants et al. 2009). Genetic engineers may take advantage of the Cu(II) {and Zn(II)?} binding potential of Cu,Znsuperoxide dismutases in the future when Cu(II) and Zn(II) tolerant fungal strains are required for different technological purposes like in water or soil bioremediation programs (Vallino et al. 2009; Villegas et al. 2009).

In filamentous fungi, intracellular siderophores like ferricrocin and hydroxyferricrocin have been reported to keep excess iron in a thermodynamically inert state (Eisendle et al. 2003, 2006; Schrettl et al. 2007, 2008; Johnson 2008).

These observations point to a novel role of fungal iron chelators, e.g. in the field of food biotechnology in the development of functional foods and food additives (Pócsi et al. 2008; Tóth et al. 2009).

Third Line of Defense: The Antioxidative Defense System

Fungi exposed to toxic metal/metalloid stress commonly face oxidative cell injuries caused by reactive oxygen species (Avery 2001). Fungal cells possess a wide array of antioxidants to cope with different kinds of oxidative stress. For example, GSHindependent and GSH-dependent enzyme activities are able to neutralize reactive oxygen species with remarkable efficiency (Pócsi et al. 2004).

Another option to gain metal/metalloid tolerant fungal strains is the overexpression of elements of the signal transduction and regulatory pathways, which are normally operating in

metal/metalloid stress exposed wild-type strains (Avery 2001; Haugen et al. 2004; Tamás et al. 2005; Rodrigues-Pousada et al. 2010; Wysocki and Tamás 2010).

Genome, transcriptome, deletome, proteome, metabolome and interactome analyses have been performed and are in progress to learn how different fungi respond to versatile toxic metal/metalloid exposures. To become familiar with the elements and the regulation of the metal/metalloid stress response networks may provide us with suitable tools to augment the metal/metalloid tolerance of selected fungi with potential applications in a wide spectrum of environmental technologies. This subchapter summarizes the most promising pieces of information extracted from the abundant experimental data having been generated in the last decade using robust “-omics” techniques. In their substantial work, Jin et al. (2008) Eeb 11 khan Bioremediation of such soils involves intentional release of microorganisms to the contaminated site for clearance of the pollutants. Fungi plays an important role in removing hazardous compounds from water and soil. Sediment particles contaminated with petroleum products from spills is one of the ecological niches for fungi, which use carbon from hydrocarbons in polluted sediment particles, leading to their biodegradation. Fungi have been found to be better degraders of petroleum than those used in traditional bioremediation techniques, such as bacteria (AlNasrawi 2012). The size of the microbal biomass is generally considered to be important in bioremediation. The microbial biomass itself represents a considerable pool of nutrients, which is continuously diverted into growth cycles of micro- and macrophytes Consequently, soils that maintain a high level of microbial biomass are capable of storing more nutrients, as well as cycling more nutrients through the ecosystem (Torstensson et al. 1998).

Identification of fungal isolates

Fungal genera were identified according to morphological characters and classified according to taxonomical keys published in the literature (Nelson-Smith 1973; Malloch 1997). The inoculated plates were identified on the basis of cultural (colour and colonial appearance of the fungal colony) and morphological characteristics

1 Ectomycorrhizal fungi on metalliferous soils Soils that contain highconcentrationsof heavy metals, whether from natural origin or from anthropogenic activity, may pose considerable challenge to exposed biota. Although some of these metals like Cu, Fe, Mn, and Zn are essential micronutrients required for a wide variety of cellular processes, they eventually become toxic to most biota including plants and their associated mycorrhizal fungi. Nevertheless, it has been suggested several times that microorganisms in general exhibit higher tolerance against metal toxicity than plants (Hartley et al. 1997). Jan Colpaert, A broad taxonomic range of ECM fungi seems to have the potential to successfully colonise tree roots under the extreme edaphic conditions of serpentine soils (Moser et al. 2009; Urban et al. 2008) and also on anthropogenically contaminated sites investigators do not find specific metaladapted taxa (Blaudez et al. 2000a; Krpata et al. 2008). Highly adapted ECM fungi with a distribution restricted to metalliferous soils have not been reported. Some authors suggested that there is little evolutionary adaptation towards elevated tolerance in ECM fungal communities as there might be sufficient ECM fungi with a high constitutive tolerance that are selected for and thus become dominant in metal-contaminated environments (Blaudez et al. 2000a; Meharg and Cairney 2000). Nevertheless, significant shifts in ECM communities in extreme environments have been observed (Colpaert 2008; Ruotsalainen et al. 2009; Staudenrausch et al. 2005)

Such evidence is only recently coming up for ECM fungi. Adaptive metal tolerance has been suggested for a few higher fungi, including Pisolithus tinctorius and Pisolithus albus (EgertonWarburton and Griffin 1995; Jourand et al. 2010), Suillus species ( Colpaert et al. 2000; 2004; Krznaric et al. 2009), Cenococcum geophilum (Goncalves et al. 2009) and some other

ascomycetes. Ecotypes are specifically adapted against Al, Ni, Zn, Cd or Cu. Similar to the situation in the plant world, evolution for metal-tolerant ecotypes shows up in a small number but ever recurring species. It is remarkable that some organisms seem to be predestined to evolve specific metal-tolerance mechanisms. Such particular species are well-known among prokaryotes (Mergeay et al. 2003; von Rozycki and Nies 2009) and plants (Ernst 1990; Schat et al. 2000). These organisms can acquire tolerances against many different metals. In bacteria, plasmids play a major role in this multiple heavy metal tolerance. In most eukaryotic organisms, true adaptation seems to be governed by a relatively small number of genetic determinants (Verbruggen et al. 2009; Willems et al. 2010). Nevertheless, such genetic modifications occur slowly, in particular in species with long reproductive cycles. Elevated metal tolerance is probably just a specific case of the general demand of every living cell for some metal homeostatic system. Metals, whether they are essential micronutrients or not, become toxic when their free ion concentration passes some threshold level in the cytoplasm. For the transition metals, Cu and Zn, the free concentration is estimated to be less than one free ion per cell (Krämer et al. 2007). To achieve this, the intracellular concentration of metal ions has to be tightly controlled, and organisms have a whole battery of molecules and regulatory mechanisms at their disposal to mobilise sufficient micronutrients, to build up a small storage pool and to avoid damage by excess metals. Therefore, metal uptake must be regulated and well coordinated with detoxification and storage mechanisms. Homeostasis of essential transition metals such as Cu and Zn requires balanced activities of transporters that mediate import into the cell, distribution to organelles and export from the cell (Pilon et al. 2009). Transcriptional control is important for the regulation of this cellular homeostasis. Nevertheless, when metals are present in very high concentrations in the environment, the regulatory mechanisms may fail and selection pressure for a more robust homeostasis will increase. Several specific adaptations have been described in a number of eukaryotic organisms, but only some of these were explored in ECM fungi (Bellion et al. 2006). ,mdiwmtux, Industrialization and extraction of natural resources have resulted in large scale environmental contamination and pollution. Contamination of soils, groundwater, sediments, surface water, and air with hazardous heavy metals and toxic chemicals is one of the major problems facing the

world today. The need to remediate these natural resources (soil, water and air) has led to the development of new technologies that emphasize the destruction of the pollutants rather than the conventional approach of disposal because of their potential to enter the food chain (Asha et al., 2013). Metals when present in our body are capable of causing serious health problems, by interfering with our normal functions. Some of these metals are useful to the body in low concentration like arsenic, copper, iron, nickel, etc. but are toxic at high concentration (Suranjana et al., 2009). Anthropogenic activities like metalliferous mining and smelting, agriculture, waste disposal or industry discharge a variety of metals such as Ag, As, Au, Cd, Co, Cr, Cu, Hg, Ni, Pb, Pd, Pt, Rd, Sn, Th, U and Zn, which can produce harmful effects on human health when they are taken up in amounts that cannot be processed by the organism. Physical and chemical methods have been proposed for the removal of these pollutants (Vargas et al.,).

Table 1.1: Microorganisms and Plants/Fungi that Utilize Heavy Metal Sl.

Microorganisms Element s

Plants/ Fungi

Element s

1

Bacillus Spp

Cu, Zn

Brassica juncea (L.) czern

Cd

2

Pseudomonas aeruginosa

U, Cu, Ni

B.juncea

Cr(IV)

B.juncea

Cu

B.juncea Zea mays L, B. campetris L,

Ni

3 4 5

6

Co, Ni, Cd Citrobacter spp. Cd, U, Pb Au, Cu, Chlorella Ni, U, Pb, vulgaris Hg, Zn Cd, Zn, Aspergillusniger Ag, Th, U Zooglea spp.

B.juncea B. chinesis L, B.juncea

Avenasativa, B. juncea,napusL.Hor d eumvulgare, B.rapa Viola, Baoshanensis, Sediumalfredii,Ru Rhizopusarrhizu Ag, Hg, P 8 m excrispus, s Helianthus, Annus, B.juncea

Pleurotusostreat 7 us

Cd, Cu, Zn

Pb

U

Zn

Cd, Pb, Zn, As

Study carried out by Irma et al., (2013) revealed that the Aspergillus fumigatus fungal isolated from contaminated site has good biosorption capacity towards selected heavy metals. Vargas et al., (2009) showed efficient detoxification of multi polluted heavy metals by fungi isolated from compost. According to Jin et al., (2011), the study relies on soil microorganisms bioavailability can enhance the bioremediation process and this can be done by addition on organic amendments such as biosolids, compost, MSW, compost to the soil. Hadis et al., (2011)

Thesis dr qadir Fungi. too, are able to remove heavy metal ions from contaminated sites, especially from industrial effluents, in substantial quantities (Siegel et al., 1990; Holan and Volesky, I995; Kapoor and Viriaghavan, 1995) Furthermore, in some instances, the removal of heavy metal ions by fungal biomass has...