Dark Septate Endophytes (DSE) in Polluted Areas 137ROLE IN POLLUTED AREAS
Despite apparent toxicity, DSE survive, grow and flourish in metal-
polluted locations and a variety of mechanisms, both active and incidental,
contribute to their tolerance. All metals exert toxicity when present above
certain threshold concentrations in bio-available forms (Gadd, 1993).
Mechanisms of metal tolerance in fungi include reduction of metal uptake
and/or increased efflux, metal immobilization by cell-wall adsorption or
extracellular binding by polysaccharides, and intracellular sequestration by
metallothioneins and phytochelatins or vacuolar localization (Collin-Hansen et
al., 2003, 2007; Gadd, 2000, 2007). Besides, the establishment of a particular
organism may directly and/or indirectly rely on several survival strategies and
in the case of DSE, Zhang et al., (2011) indicated that the mechanism of heavy
metal tolerance of strains isolated from metal soil would be a complex process.
In the case of DSE one of these mechanisms appears to be the presence of
melanins in the hyphae. Melanins develop in large quantities in organisms that
live in unfavourable environments (Bell & Wheeler, 1986). The melanins are
known to provide rigidity to the cell wall, resistance to microbial grazing and
protection from desiccation and radiation damage (Kuo & Alexander, 1967;
Bell & Wheeler, 1986; Griffith, 1994). A variety of heavy metals might induce
or accelerate the production of melanin pigmentation in certain fungi (Zhan et
al., 2011; Gadd, 1984) like showing the work of Ban et al., (2012) where they
found that melanin content in Gaeumannomyces cylindrosporus increased
when it was exposed to 0.2 and 0.3 mg/ml Pb(II) and decreased slightly at
higher concentrations.
Redman et al., (2002) suggested that the fungal melanin could play a role
in heat dissipation or form complexes with oxygen radicals generated during
stress. If this is true, then the DSE that produce highly melanised hyphae and
microsclerotia, could perform similar functions, which may be essential to
plant survival and growth in those stressing environments. Martino et al.,
(2000) reported that a pigmented ericoid mycorrhizal fungus to tolerate heavy
metals on is deposition of melanin on the cell wall and secretion to the media.
Moreover, Vrålstad et al., (2002b) suggest that the melanized hyphae of the
outer mantle layer and extramatrical mycelium of DSE may function as
protective barriers between the external environment and the active fungus-
plant symbiosis inside the root been a key-factor to their apparent successful
colonization of burnt and metal polluted habitats.
Antioxidant enzymes such as glutathione, superoxide dismutase and
catalase are other important heavy metal tolerance agents. Glutathione is the