Role of Arbuscular Mycorrhizal Fungi in Heavy Metals Homoeostasis in Plants

Arbuscular Mycorrhizal (AM) fungi have substantial involvement in the existence of plants under heavy metal (HM)-stressed conditions. An overwhelming number of studies are there which advocate for the AM fungi as a future tool for remediation and revegetation of HM-polluted soils. One of the major complications associated with AM fungi facilitated phytoremediation is that the AM association is very much host as well as HM specific. Diverse strains of AM fungi behave differently with diverse hosts and HMs. AM fungi in association with host plants enhance the tolerance of HMs to the host. It enhances the absorption of nutrients in deficient soil, whereas it decreases the accumulation of HMs in polluted soils. AM fungi enhance the biomass production and, thus, dilute the HM concentration in plants. The association of AM fungi also protects the host roots from exposure of HMs by trapping them in the extracellular polymeric substances and the glomalin secreted by the AM fungal hyphae. The hyphal components also contain hydroxyl and carboxyl ligands that can bind the positively charged HMs and immobilize them outside the soil. In hyperaccumulator plants, AM fungi contribute differently as the transporters of HMs increase the uptake of HMs from the substrates. In this way, it enhances the accumulation of HMs beyond the permissible level. Inside the AM fungi as well as in the host cell, these HMs are either converted to the less toxic forms or conjugated with Metallothionein (MT), Glutathione (GSH), and Phytochelatin (PC) and safely stored in the vacuole. All these functions are specifically controlled by a number of transporters of HMs localized in the AM fungal hyphae, inside the AM fungi and inside the host cell. GintABC1 is one of the most studied transporters in AM fungi regulating the Zn. In this review, a deeper insight into the all-possible mechanisms of AM fungi facilitated HM stress alleviation in plants is summarized and discussed.