Some heavy metal elements such as Cu, Fe, Mn, Ni and Zn are essential for normal growth and development of plants. These metals are required in numerous enzyme-catalyzed or redox reactions, in electron transfer, and have structural function in nucleic acid metabolism (Gohre and Paszkowski, 2006). In contrast, metals like Cd, Pb, Hg, and As are not essential (Mertz, 1981) and may be toxic to plants at very low concentrations in soils. Heavy metals occur in terrestrial and aquatic ecosystems from both natural and anthropogenic sources, and are also emitted into the atmosphere.
The roots of terrestrial plants are in immediate contact with soil metal ions. Essential heavy metals are transferred into the root by specific uptake systems, but at high concentrations they also enter the cell via nonspecific transporters. At high concentrations heavy metals interfere with essential enzymatic activities by modifying protein structure or by replacing an essential element, resulting in deficiency symptoms. As a consequence toxicity symptoms such as chlorosis, growth retardation, browning of roots, effects on both photosystems, cell cycle arrest, and others are observed.
Anthropogenic soil contamination resulting from mining activities, industrial processes, agriculture, and military activities have resulted in high localized concentrations of heavy metals. Conventional soil remediation practices in most countries rely primarily on the excavation of the contaminated soil. However, physical displacement, transport and storage, or alternatively soil washing are expensive procedures which leave a site devoid of soil microflora.
AM fungi are significant in the remediation of contaminated soil as accumulation (Jamal et al., 2002). The external mycelium of AM fungi allows for wider exploration of soil volumes by spreading beyond the root exploration zone (Khan et al., 2000), thus providing access to greater quantities of heavy metals present in the rhizosphere. Higher concentrations of metals are also stored in mycorrhizal structures in the root and in fungal spores. AM fungi can also increase plant establishment and growth despite high levels of soil heavy metals due to improved nutrition (Taylor and Harrier, 2001), water availability (Auge, 2001), and soil aggregation properties (Kabir and Koide, 2000) associated with this symbiosis.
AM fungi occur in the soil of most ecosystems, including polluted soils. By acquiring phosphate, micronutrients and water and delivering a proportion to their hosts they enhance the host nutritional status. Similarly, heavy metals are taken up via the fungal hyphae and can be transported to the plant. Thus, in some cases mycorrhizal plants experience enhanced heavy metal uptake and root-to-shoot transport while in other cases AM fungi contribute to heavy metal immobilization within the soil. The result of mycorrhizal colonization on remediation of contaminated soils depends on the plant-fungus-heavy metal combination and is influenced by soil chemical and physical conditions.
The significance of AM fungi in soil remediation has been recognized (Gaur and Adholeya, 2004; Khan, 2005). A vast amount of literature is available on the effects of mycorrhizal colonization on plants under heavy metal stress but contradictory observations and wide variations in results are reported (Khan, 2005). Enhanced understanding of heavy metal tolerance of plants and AM fungi has defined valuable parameters for improving phyto-remediation, i.e., the engineered use of green plants to remediate an affected site. The utility of AM fungi in soil remediation is also important for sustainable agriculture. Application of these fungi is generally useful to overcome heavy metal problems and to alleviate soil stress, and ultimately increases agricultural production.
In many cases AM fungi serve as a filtration barrier against transfer of heavy metal ions from roots to shoots. The protection and enhanced capability of mineral uptake result in greater biomass production, a prerequisite for successful remediation.
AM isolates existing naturally in heavy metal-polluted soils are more metal-tolerant than isolates from non-polluted soils, and are reported to efficiently colonize plant roots in heavy metal-stressed environments. Thus, it is important to screen indigenous and heavy metal-tolerant isolates in order to guarantee the effectiveness of AM symbiosis in restoration of contaminated soils. The potential of phytoremediation of contaminated soil can be enhanced by inoculating metal hyperaccumulating plants with mycorrhizal fungi at the contaminated site. However, there is a need to optimize the conditions to grow AM fungi in large quantities, and to characterize and screen a large number of AM fungal species for tolerance to metals.
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