The role of ectomycorrhiza in heavy metals bioremediation in soil

Abstract

The increase of natural and human activity has caused the migration of contaminants, such as heavy metals, to non contaminated areas. In general, their high availability causes serious problems to ecosystems, because when present in the form of soluble ions are absorbed by plants inhibiting a series of biological and physiological factors. High levels of zinc, cadmium and copper are responsible for areas degradation and difficulty of revegetation, and Cd offers greater environmental risk due to its high toxicity. To minimize such effects, studies are being conducted using mycorrhizal associations, which consist of a symbiotic association between soil fungi (Basidiomycetes and Ascomycota) and roots of vascular plants, whereas the fungus provides water and nutrients to the plant and the vegetable provides carbohydrates for the fungus produced from photosynthesis. They are classified into different types, depending on the plant, the fungus and the colonization characteristic. The ectomycorrhiza represent a type of mycorrhizal association and are characterized by intercellular growth, forming a mantle of hyphae around the root. Its widespread occurrence in forests of host plants and the effects on these, make these associations important compounds of natural ecosystems and planted forests. Several studies have shown that ectomycorrhiza protect plants against typical soil pollutants such as heavy metals, including Cd, Pb, Cu, Zn, Ni, etc., removing them from the soil and preventing them from being absorbed by the plants roots. This ability is due to the fact that mycorrhizal fungi possess various mechanisms that prevent the translocation of metals to the plant. Among them are the binding of metals to cell wall components such as chitin, cellulose, cellulose derivatives and melanin, certain proteins such as metallothionein, glutathione and polyphosphate granules. The mycelium retention capacity and the extrametrical mycelium density can act as a physical barrier against metals, and the intracellular absorption and subsequent detoxification in the fungal vacuoles may reduce the metal absorption in the host plant. Another process involved in tolerance of ectomychorryzal to heavy metals is the transport of ions and other metabolites mediated by membrane proteins. Among them is the plasma membrane H+- ATPases, responsible for generating an electrochemical gradient of H+ responsible for regulate the transport of secondary substances such as heavy metals. For the correct operation of this enzyme an exact regulation mechanism is required, in which several factors are involved mainly acidic pH and glucose. Several studies have shown an increased activity of H+-ATPase in the presence of ectomycorrhizal fungi. Thus, it is necessary to perform further studies to elucidate the mechanisms of bioremediation provided by ectomycorrhizal fungi, especially a better understanding of the regulation of proton pumps and transporters in the fungal membrane.