Arbuscular mycorrhizal fungi and heavy metal tolerance in plants: An insight into physiological and molecular mechanisms

Abstract

Arbuscular mycorrhiza (AM) are the obligate symbiotic fungi which are integral part of plant roots systems and have been documented to ameliorate heavy metal (HM) stress. AM have the ability to impart tolerance by modulating various physiological, biochemical and molecular mechanisms in the root rhizosphere as well as within the plants. At physiological levels, AM immobilizes HMs in soil through binding of toxic ions to the cell wall components, secretes numerous organic acids in the rhizosphere such as citric, oxalic acid and a glycoprotein, glomalin, which act as a chelating agent and reduce metal uptake in the plants. At biochemical level, mycorrhizal hyphae enhance the uptake of nutrients which are otherwise unavailable to the plants and this AM induced improved nutrient acquisition is reported to enhance plant growth and reduce metal concentrations in the plant tissues, as a result of dilution effect. In addition to increased nutrient acquisition, AM also enhance the antioxidant defense responses to counteract HM induced oxidative stress. Furthermore, at molecular level, AM upregulates the genes responsible for nutrient uptake and stimulation of metallothioneins and phytochelatins synthesis resulting in HM sequestration in the host plant as well as in mycorrhizal structures. On the basis of available literature, this article summarizes various mechanisms modulated by AM fungi in imparting HM tolerance to the plants.