Use of arbuscular mycorrhizal fungi in phytoremediation of heavy metal contaminated soils

Soils globally are contaminated with significant quantities of heavy metals due to natural and anthropogenic activities. Phytoremediation, is an in situ phytorestoration technique that harness the capabilities of plant to take up and sequester heavy metals (HMs) or to immobilize HMs in soil. Arbuscular mycorrhizal (AM) fungi are an integral part of plant roots in most natural and agricultural systems. Mycorrhization alters several aspects of plant physiology, nutrient availability, microbial composition and certain soil properties. These effects modify plant growth patterns in heavy metal (HM) contaminated soils and determine the outcome of phytoremediation attempts. Though it is presumed that most of the metallophytes belong to non-mycorrhizal families, recent evidences do indicate that several plant species in mycorrhizal families also accumulate HMs. Arbuscular mycorrhizal fungi never disappear from HM contaminated soils, though their propagule numbers or diversity may be greatly reduced. In addition, AM fungi also directly help the plant to escape from the build-up of phytotoxic concentrations of certain HMs by secreting specific detoxifying or binding compounds (e.g. organic acids, glomalin). They bind the HMs into fungal tissues associated with the roots and thus create a physical barrier against their translocation to the shoots of the host plant. This article reviews the influence of heavy metals on AM fungi and the role of AM fungi in plants' tolerance to HM in soil and possible mechanisms involved in it.