Biotechnological Potential of ECM Fungi Producing Metal Chelating Agents

The biotechnological potential of ECM fungi producing metal-chelating agents is related to the isolated chelators with specific aims, or to the use of the fungi in the presence or absence of symbiotic association. Due to their extremely efficient metal-chelating properties, for many years the main application of siderophores has been in chelation therapy to treat Fe and Al overload in humans; its possible use in the decorporation of actinides is also being investigated. The main siderophore used for this purpose is desferrioxamine B, a hydroxamate obtained from Strepto-myces pilosus (Messenger and Ratledge 1985; Renshaw et al. 2002; Ansoborlo et al. 2007). However, siderophores can have other important applications related to the environment; in the uptake of metals from industrial waste, low-grade ores, serpentine soils, contaminated terrestrial and aquatic environments, tailings of abandoned mines, etc. The uptake of metals can serve to remediate an environment and/or recover metals for recycling. In addition, the capacity to chelate actinides (Pu, U, Np, and Th) has been demonstrated in siderophores and for this reason their application has been proposed for the remediation of radioactive waste and the reprocessing of nuclear fuel (Hernlem et al. 1999; Renshaw et al. 2002, 2003). The majority of these studies have been conducted with commercial hydroxamate siderophores like desferrioxamine B and some with siderophore-producing soil microbes (John et al. 2001; Keith-Roach et al 2005; Mullen et al 2007). Therefore, this is an area where other fungal hydroxamate siderophores, like those produced by ECM fungi, could have a great potential for application. Another important possible application of siderophores is in the treatment of asbestos, a carcinogenic fibrous mineral with varied industrial applications that is prohibited in many countries. Although the toxicity mechanisms are not well understood, it is believed that the Fe present on the surface of asbestos fibers cause cell damage by generating free radicals. It was recently demonstrated that hydroxamates and soil fungi-producing siderophores, among them ECM fungi, inactivate fibers by removing the Fe from the material surfaces (Martino et al. 2003, 2004; Daghino et al. 2008). These fungi represent a potential tool for the bioremediation of asbestos waste, contaminated waters and soils, and asbestos abandoned mines. It must be pointed out that due to their chelating properties, LMW organic acids could have most of the environmental applications found for siderophores, as previously described.

The identification and selection of new strains of ECM fungi, efficiently producing siderophores and/or LMW organic acids, is fundamental to the mycorrhization of plants used in programs to recover degraded or destabilized forest ecosystems, poor in mineral nutrients either from natural causes or through anthropogenic action. The production of these chelators by ECM roots must contribute to the solubilization and uptake of mineral nutrients into the rhizosphere, facilitating the establishment and development of plants in these ecosystems. In addition, the modifications in the speciation and bioavailability of metals and nonmetals as a result of the chelators released by the hyphae beyond the rhizosphere promotes the restoration in the mineral nutrient balance, favoring not only vegetal species, but also all microorganisms inhabiting these ecosystems (Gadd 2007; Van Scholl et al. 2008).

The revegetation of metal-contaminated soils is another important application of plants mycorrhized with ECM fungi producing a wide range of chelators: siderophores, LMW organic acids, MTs, PC, and GSH. Given that these chelators contribute to the detoxification of metals through intracellular or extracellular mechanisms, the fungal species that produce them must protect their host plants in contaminated environments. This protection consists of excluding metals or accumulating them in their fruiting bodies, using both ways to prevent their entry into the mycorrhized roots. The use of plants in the bioremediation of soils contaminated by metals, aided by synthetic chelators like EDTA and called assisted phytoremediation, has been described in the literature (Khan et al. 2000; Lasat 2002; Wenzel 2009). Plants inoculated with ECM fungi producing metal-chelating agents could be used in the same way to aid in the increased uptake of metals by the roots, and at the same time improve the nutritional status of the plants, and with it the production of biomass for phytoremediation.

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