In the mining process, not only plants are removed, but also upon soil replacement changes in texture and deposits of salts and heavy metals often result (Allen 1989). The toxicity of metals not only depends on their concentration in the soil, but also their availability and transfer to plants. By providing a direct link between soil and plants, mycorrhizal function could be of great importance in heavy metal polluted soils (Leyval et al. 1997). As such, there is increasing interest in the potential for mycorrhizal fungi to be used as bioremediation agents or as bioindicators of heavy metal pollution.
It is generally believed that pollution inhibits the formation of mycorrhizal associations. However, little is known about the viability and activity of AM and EM fungi in soils with different heavy metal concentrations. Riihling and Soderstrom (1990) reported that the number of fruiting bodies and species decreased with increasing pollution along a heavy metal pollution gradient in Sweden. Isolates of P. tinctorius collected from old mining sites expressed increased aluminum tolerance and high mycelial growth when compared to isolates from rehabilitated and forested sites (Egerton-Warburton and Griffin 1995). For AM fungi, there is evidence to suggesting that at least some AM fungi are relatively resistant to high metal concentration. Gildon and Tinker (1981) reported that plant roots growing naturally on zinc- and cadmium-contaminated soils had significant AM colonization. Davies et al. (2001) reported high rates of AM colonization even at the most toxic levels of chromium in soils. And Rao and Tak (2001) found a significant improvement in root colonization and spore density of AM fungi isolated from gypsum mine spoils when used to inoculate five tree species growing in gypsum mine soils.
The mechanism that confers heavy metal tolerance in mycorrhizal fungi is largely unknown. The survival of AM and EM fungi in polluted soil may depend heavily on the density of the external hyphae. The absorption of heavy metals to the hyphal surface could reduce soil concentrations and thus accumulation of fungal and plant tissue (Denny and Wilkins 1987; Marschner and Dell 1994). Components of the fungal cell wall, such as chitin and melanin, can bind heavy metals to the extraradical mycelium (Denny and Wilkins 1987; Tam 1995). Turnau et al. (1996) found that the EM fungal mantle contained the highest levels of heavy metals while the Hartig net contained the lowest levels. Glomalin, the glycoprotein that coats AM fungal hyphae, could play an equally important role in protecting AM fungi and host plants from toxic metal concentrations in soils, although this has not yet been investigated. Other possible mechanisms conferring heavy metal tolerance in fungi may include intracellular chelation (Martin et al. 1994) and the sequestration of metals within mycorrhizal sheaths (Egerton-Warburton et al. 1993). The stability of metal tolerance in both AM and EM fungi remains to be examined (Leyval et al. 1997). Despite these uncertainties, isolation of tolerant fungal ecotypes that occur on polluted soils could have important application to the revegetation or inoculation of barren polluted sites (Gildon and Tinker 1981; Rao and Tak 2001).
The benefit of heavy metal tolerance in mycorrhizal fungi could have direct effects on host plant response to metal concentrations in soil (Meharg and Cairney 2000). Several studies have reported the beneficial role of EM associations in reducing metal concentrations in plant tissues. (Colpaert and Van Assche 1992;1993; Dixon and Buschena 1988). Ectomycorrhizal fungi can differ in their ability to reduce translocation from root to shoot, so the presence of the appropriate EM fungi may be critical (Denny and Wilkins 1987). The role of AM fungi in heavy metal uptake is harder to elucidate because of the obligate nature of the fungal symbiont. Regardless of their ability to grow in polluted soils, the extent to which AM fungi confer metal tolerance in their host plants, or accumulate heavy metals in roots preventing translocation to shoots is not fully understood (Leyval et al. 1997). Species of both AM and EM fungi differ in hyphal productivity and in their ability to take up and transfer metals. The turnover of fungal tissue could be an important factor in the ability of mycorrhizal fungi to protect host plants against prolonged elevated metal concentrations (Colpaert and Van Assche 1993).
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