138 Elena Fernández-Miranda Cagigal
most abundant cellular thiol-rich heavy metal binding peptide in fungi (Singh
et al., 1997) and some works tend to consider the soluble tripeptide as the first
line of defence against heavy metal cytotoxicity (Viarengo & Nott, 1993).
Superoxide dismutase and catalase are crucial for cellular detoxification,
controlling the levels of superoxide anion radical and hydrogen peroxide
(Pócsi et al., 2004; Bai et al., 2003). Zhan et al., (2011) found that superoxide
dismutase and catalase activities in the hyphae of Exophiala pisciphila had
positive correlations with Pb(II) and Cd(II) concentrations, and Ban et al.,
(2012) confirmed these results observing that these enzymes play an effective
role in protecting G. cylindrosporus against oxidative stress induced by Pb(II).
Moreover superoxide dismutase appears to act as the primary defence against
acute Pb(II) stress.
The presence of melanins and antioxidant enzymes are not the only
mechanisms that these fungi possess. It has been demonstrated, the
relationship between toxic metal mineral solubilisation and metal in Cu-
tolerant isolates of P. ericae demonstrated a much higher ability to solubilize
Cd, Cu and Zn phosphates than isolates from non-polluted areas (Fomina &
Gadd, 2007). Bartholdy et al., (2001) revealed that P. fortinii excreted
hydroxamate siderophore for Fe(III) mobilization (ferricrocin, ferrirubin and
ferrichrome C). All these results, further and more in-depth knowledge relating
to these metal tolerance mechanisms in fungal endophytes is necessary and
their potential positive effects on the survival of their plant hosts in metal-
enriched soils will ultimately lead to powerful applications in bioremediation.
CONCLUSION
Based on the studies available, we can conclude that DSE fungi are
prevalent in various habitats and colonise a high rage of plant species. This
group of fungi cannot be overlooked while assessing the fungal communities
of any ecosystem. Even in the absence of clear consensus about positive
impacts on host fitness, growth or performance, DSE can be said to be likely
to perform functions similar to those attributed to mycorrhizal fungi. Although
experimental evidence is limited and experimental results may conflict, DSE
are seems to be involved in host nutrient uptake. And finally, the presence of
melanins may indicate a function of altering environmental tolerances.
Further and more in-depth studies on the metal tolerance mechanisms in
DSE and their effects on host plant survival in metal-enriched soils, but the
possibility of pre-inoculation of seedling with DSE as a strategy for achieving