The literature cited in this review ranged from 1904 to 2021. However, the majority of the articles targeted were from journal articles, book chapters, and books published between 2011 and 2021. The relevant literature surveyed were studied employing Google, Google Scholar, Web of Science, Research Gate, and Scopus using various keywords such as phytoremediation, arbuscular mycorrhizal fungi, metal contaminated soils, metal transporters, genes. Further, especially focused journals were Annual review of plant biology, Frontiers in Microbiology, Current Opinion in Toxicology, Journal of Plant Physiology, Plant Physiology, etc., were browsed for digging deeper into the relevant literature until 2021. Subsequently, we have examined the publication individually and eliminated the quotative and duplicate papers. Out of the total literature documents yielded, we have selected and referred to 168 articles. Out of which, total journal articles were 161 followed by six book chapters and one book. Around 51% of the cited documents were of the years 2011–2021. To the best of our knowledge, this article is an updated review article that focused and covered all dimensions of plant-mycorrhizal fungi interactions in metal phytoremediation.

The reactive oxygen species (ROS) formation accelerates under biotic and abiotic stresses (Hasanuzzaman et al., 2013; Bauddh and Singh, 2015; Jajic et al., 2015; Sachdev et al., 2021). ROS generation in plants has been reported as long-distance signals in response to various stresses (Mittler, 2017). To minimize the toxic effects of ROS, plants possess effective ROS-scavenging systems involving both enzymatic (e.g., ascorbate peroxidase and superoxide dismutase) and non-enzymatic (e.g., glutathione and ascorbic acid) ROS actions (Hasanuzzaman et al., 2013; Sachdev et al., 2021). Few researchers have reported that ROS generation occurs during early symbiotic interactions between AMF and host plant roots (Fester and Hause, 2005; Kiirika et al., 2014). To mitigate its potentially toxic effects, there must be a balance between ROS generation and scavenging. In plants, redox homeostasis, antioxidant signaling, and continuous production or removal of ROS at the cellular level are considered as stress signals (Jajic et al., 2015).

The plant-microbe symbiotic associations play a crucial role in alleviating biotic and abiotic stresses such as heat, salinity, drought, metals, and extreme temperatures (Goh et al., 2013; Schouteden et al., 2015). Studies on AMF mediated stress tolerance and increased growth of host plants have been the pivotal research on plant stress responses (Tahat and Sijam, 2012). Plant-AMF interactions can improve plant growth and health by controlling the generation and scavenging of ROS (e.g., H₂O₂, superoxide radicals, alkoxy radicals, singlet oxygen, perhydroxyl radicals, etc.) under biotic and abiotic stresses (Goh et al., 2013; Nath et al., 2016). For instance, a significantly higher amount of ROS is generated due to GC stress, therefore causing oxidative damage to the cellular structures of plants (Yang et al., 2015b). In response to such oxidative stress, plant-AMF associations can activate numerous antioxidant enzymes (e.g., thioredoxin, superoxide dismutase, glutathione peroxidases, and catalase) for scavenging the generated ROS to protect against cell damage (Hashem et al., 2018).

AMF are considered natural biofertilizers, because they help the host plants to develop their root system for absorption of water and essential nutrients in exchange for photosynthetic products and to protect plants against harmful pathogens (Berruti et al., 2016; Gao et al., 2020; Basiru et al., 2021). It is well documented that, the AMF-pant association has countless benefits in terms of healthy soil conditions and increased crop productivity (Berruti et al., 2016). Therefore, AMF are considered as the most important biotic soil components, the impoverished or missing AMF can lead to a less efficient ecosystem functioning (Berruti et al., 2016). The roles of AMF as biofertilizer in several biochemical and physiological processes are as follows:

Accumulation of GCs in the plants is a critical problem in the environment, high mobility of GCs has made them an extended component of food chain that affects the health of humans. Vesicles present in mycorrhizal fungi are comparable to fungal vacuoles and they accumulate huge amount of GC in them (Dhalaria et al., 2020). Immobilization of GC occurs in the fungal hyphae residing in symbiotic association with plants that decrease their availability to plants by retaining the GCs in the cytoplasm or vacuole, cell wall by chelation, thereby reducing metal toxicity in the plants (Punamiya et al., 2010). Metal detoxification induced by AMF has been considered as the key mechanism to help plants to alleviate metal toxicity (Table 1). By using scanning electron microscope equipped with energy dispersive spectroscopy (SEM-EDS), extended X-ray absorption fine structure (EXAFS), linear combination fitting results of X-ray absorption near-edge spectroscopy (LC-XANES), it has been demonstrated that Cr may be immobilized by AMF via reduction of Cr (VI) to Cr (III), forming analogues of Cr (III)-phosphate, probably on the surface of fungi. Apart from this, it has also been unraveled that extra radical mycelium may take up Cr actively and transport it to mycorrhizal roots, but the majority of Cr is immobilized in fungal structures (Wu et al., 2015). Ultra-structural changes were observed in roots and leaves of Leucaena leucocephala through a scanning electron microscope (SEM), transmission electron microscope (TEM), and light microscopy (LS). Results revealed that plant tissues were colonized by AMF and damage was observed in all treatments of As (Schneider et al., 2013b).

GCs may be immobilized in the fungal hyphae (Ouziad et al., 2004) that can fix GCs in the cell wall and store them in the vacuole or make a complex with other substances like glycoprotein-metal complex (Dhalaria et al., 2020) in the cytoplasm to decrease plants metal toxicity (Punamiya et al., 2010). AMF can enhance plant biomass by changing plant physiological and morphological properties (e.g., enhanced secondary metabolite levels, increased leaf area, increased seedling weight, etc.) under severely stressful conditions and uptake of important immovable nutrients (such as phosphorus, copper, and zinc; Miransari, 2017).

Increased plant biomass in rhizosphere soil is the primary cause of metal dilution in plant tissues (Audet, 2014). AMF may restrict Zn and Cd uptake in the cell wall of mental hyphae and cortical cells, thereby improving plant yield and health (Garg and Chandel, 2012). Metal detoxification mediated by Rhizophagus phaseolina in Glycine max was studied by Spagnoletti et al. (2016), where AMF boosted a defensive response by decreasing oxidative damage even in the presence of M. phaseolina and As.

Mycorrhizae may influence plant metal uptake from the rhizosphere and their translocation from the root zone to aerial parts (Li et al., 2015). The mycelium of several AMF has a high cation exchange capacity, and it helps in metal uptake (Takács and Vörös, 2003). For instance, Hammer et al. (2011) found an increase in the uptake of silicon in the hyphae and spores of Rhizophagus irregularis and its subsequent translocation to host roots. Cd toxicity and mobility can also be alleviated through AMF by enhancing the soil pH (Shen et al., 2006). AMF can restore Cd in the extraradical mycelium and bind Cd to glomalin (Janouškova and Pavlíková, 2010).

AMF colonization has been shown to reduce metal stress in a convincing way (Hall, 2002). For instance, AMF colonization considerably increased the glutamine synthetase activity, therefore enhancing Ni tolerance in Berkheya coddii (Sessitsch et al., 2013). To reduce metal toxicity, AMF resort to processes such as adsorption of GCs to the cell wall, immobilization of metallic compounds, chelation of GCs inside fungus, and precipitation of polyphosphate granules in soils (Meier et al., 2012; Wu et al., 2015). Janousková et al. (2006) reported that inoculation of Glomus intraradices with Nicotiana tabacum cultivated in Cd contaminated soil decreased Cd toxicity to the plants due to Cd immobilization in soil. A study conducted by Wu et al. (2015) investigated the mechanisms involved in Cr immobilization in Daucus carota inoculated with AMF and found that AMF can immobilize Cr via reduction of Cr(VI) to Cr(III) by forming Cr(III)–phosphate analogs.

Molecular regulation of genes plays a crucial role in accumulating GCs and fungal cell detoxification, subsequently leading to the prevention of translocation of these GCs toward the host plant (Emamverdian et al., 2015). Efflux of GCs is a strategy used by AMF to protect plants from metal toxicity (Latef et al., 2016; Shi et al., 2019). Several transcriptional genes take part in the efflux of GCs and the involved genes get activated by metal exposure (Dhalaria et al., 2020). To provide plant tolerance against Cd and Cu, GmarMT1 that is a cDNA-encoding metallothionein-like functional polypeptide has been discovered from germinated Gigaspora margarita spores (Lanfranco et al., 2002). Also, GC exposure upregulates GmarMT1 expression in the symbiotic mycelium (Lanfranco et al., 2002). GintABC1 identified as a putative ATP-binding cassette (ABC) transporter from extra radical mycelium of Glomus intraradices is believed to be involved in Cd and Cu mitigation (González-Guerrero et al., 2010). A number of genes are responsible for maintaining cellular homeostasis against GCs, such as GintABC1, GmarMT1, RintZn, and GrosMT1 (Azcón et al., 2013). To maintain the redox potential and safeguard the fungus from oxidative stress, GmarMT1 codes for metallothioneins have been found in G. margarita BEG 34 (González-Guerrero et al., 2007). GintABC1 assists in detoxifying Zn and Cu (González-Guerrero et al., 2007; Table 2). RintZnT1 isolated from Rhizophagus intraradices, helps in vacuolar sequestration of Zn (Gonzalez-Guerrero et al., 2005). GintGRX1, the first characterized glomeromycotan glutaredoxin, is a multifunctional enzyme that expresses in response to oxidative stress (Benabdellah et al., 2009).

AMF resorts to several molecular mechanisms to protect them from GC stress. One of mechanisms is the upregulation of several transcriptional factors that activate glutathione-S-transferase and Zn transporter in intra- and extra- mycelia of AMF Glomus intraradices against metal stress (Hildebrandt et al., 2007). GC stress also leads to expression of numerous genes. These genes encoding proteins are involved in detoxification/tolerance against GCs (Rivera-Becerril et al., 2005).

Based on molecular understanding, scientists have reported an upregulation in metallothionein gene expression of Gigaspora margarita BEG34 in the symbiotic mycelia due to Cu (Lanfranco et al., 2002) and also an enhanced level of transcription of a putative transporter gene for Zn (GintZnT1) that belongs to cation diffusion facilitator family. These genes have been found in the G. intraradices mycelium under short and long-term Zn exposure indicating that this enzyme protects plants against Zn stress (Gonzalez-Guerrero et al., 2005). The role of some AMF in phytoremediation of some GCs (Bundschuh et al., 2017) has been discussed in Table 3.

Metal-binding proteins called metallothioneins are generated in a diverse range of organisms when they are exposed to toxic metals (e.g., Cd, Zn, and Cu). Cu predominantly induces the production of metallothionein in non-AMF (Kumar et al., 2005). Cu-induced stress distinctly upregulates the metallothionein gene BI451899 in extraradical mycelium of G. intraradices. However, a certain concentration of Zn can also upregulate the metallothionein gene, but such a response is not observed due to Cd. This upregulation establishes and supports the primary function of fungal metallothioneins in detoxifying Cu (Lanfranco et al., 2002).

Strong binding of metals to soil particles or precipitation results in insolubilization of the significant fraction of metals in soils ultimately causing their unavailability for plant uptake. Metal solubilization and mobility have been considered as critical factors that affect phytoextraction efficiency (Ma et al., 2013). In this regard, microbes that can mobilize metals may be used to amend the habitat of rhizosphere in soils affecting metal element speciation as well as mobility inside soil by way of biogeochemical cycling processes of GCs that primarily include protonation, chelation, and acidification (Ma et al., 2011; Rajkumar et al., 2012; Sessitsch et al., 2013).

AMF may also acidify their environment through exporting protons to replace GC cations at the site of sorption (Rajkumar et al., 2012). Extensive studies have been performed to characterize them using attenuated total reflection-Fourier transforms infrared (ATR-FTIR) spectroscopy and thereafter to understand the interaction between fungal cells, protons, and metal ions. Results suggest that the carboxylate moieties present on the bacterial surface play a vital role in the extracellular biosorption of Ni²⁺, which establishes a comparatively weaker bond with the metal ion.

Natural organic chelators are metal-binding compounds that comprise siderophores, organic acid anions, metallophores, and biosurfactants (Sessitsch et al., 2013). Both fungi and plants can release these compounds that scavenge metal ions from sorption sites (Gadd, 2004) and ROS (Leitenmaier and Küpper, 2013). Metal chelation through metal-binding peptides such as metallothioneins and phytochelatins (PC) may eliminate the harmful effect of free metallic ions, thereby facilitating metal uptake and their sequestration, followed by compartmentation, loading in xylem tissues, and finally their transport (Cai and Ma, 2002). Phytochelatins are the GC binding peptides that are produced by tripeptide glutathione and/or by an enzyme-catalyzed reaction through PC synthase (Solanki and Dhankhar, 2011). Metallothioneins may also be found in AMF and genes that encode numerous enzymes for PC synthesis may be activated in the root of mycorrhiza. These enzymes assist in enhancing photosynthesis in mycorrhizal plants subjected to stress caused by metals (Rivera-Becerril et al., 2005).

Soil pH is one of the most important factors that affect metal content and its bioavailability. For several metals (e.g., Cu and Zn), a rise in soil pH caused a fall in their mobility (Richards et al., 2000). Generally, soil pH is affected by the action of both microorganisms and plants. Rhizosphere gets acidified due to H⁺ ions excreted by roots that may displace GC cations adsorbed on soil particles. Root exudates may decrease the pH of the rhizosphere (Sheoran et al., 2011), causing increased metal mobility and bioavailability in soil solution (Kim et al., 2010).

Phytostabilization is GC immobilization in the plant root system by precipitation, reduction, and absorption without its accumulation in the shoot (Radziemska et al., 2017). There is an extensive root system for immobilizing metals in hyperaccumulators (Mendez and Maier, 2008). In addition to some common mechanisms of tolerance, increase biosynthesis of the cell wall, metal inactivation in the rhizosphere and its accumulation in roots are very specific to phytostabilizers (Janeeshma and Puthur, 2020). An association with AMF increases the properties of metal stabilization of plants (Zhang et al., 2019). For instance, the association of Trifolium pratense with mycorrhizae enhanced Zn retention in the roots, thereby preventing its translocation in the aerial plant parts (Chen et al., 2003).

The glomalins released by AMF enhance toxic metal immobilization. Metallothionein protein, released by some AMF, is also known to reduce the toxicity caused by GCs. Besides, the synthesis of a 90 kD heat shock protein and glutathione-S-transferase as a response to GC stress suggest that these proteins are involved in immobilizing GCs in the rhizosphere of Lycopersicon esculentum plant (Bano and Ashfaq, 2013). Glomalins are known to sequester several metals such as Zn, Pb, Fe, Cd, Cr, and Cu and decrease their bioavailability (Gil-Cardeza et al., 2014). Glomalins may extract Pb, Cd, and Cu from polluted soil.

Several GCs get immobilized because of the binding capacity of fungal hyphae to metals. As a result of this binding capacity, there is a decreased translocation of GCs to plant tissues (Wasserman et al., 1987). A slight increase in the mycorrhizosphere pH may also cause immobilization of some GCs (e.g., Zn) due to mycorrhizal association (Bano and Ashfaq, 2013). Inoculation of Glomus species resulted in reduced mobility of metals in Zea mays (Kaldorf et al., 1999). Other studies demonstrated a notable absorption of Zn in the mycelium of AMF by using different glomus species in association with Lolium perenne or Trifolium sp. (Joner et al., 2000).

Metal immobilized in fungal hyphae that are symbiotically associated with the plants decreases their availability to host plants by holding the metals in the cytoplasm, vacuole, or cell wall, thereby reducing metal toxicity in plants (Punamiya et al., 2010). They also immobilize the GCs in the root cortical region by binding with them and prevent the translocation of metals to shoot, thus preventing leaf tissue damage (Schubler, 2001). Some AMF may decrease plant metal uptake or its translocation factor by reducing metal bioavailability in soils through several processes such as alkalinization, precipitation, and complexation (Ma et al., 2016).

A few AMF exhibit the capability to adsorb metals by substratum alkalinization activity, hence affecting the stability of metals in soils (Büdel et al., 2004). The effect of alkalinization induced by AMF via release of OH⁻, may result in active uptake of nitrate by microbes and reduction in metal bioavailability in the rhizosphere by secreting glomalin (Giasson et al., 2008). AMF may act as a sink of metals to reduce the available and mobile metal cations in soils, resulting in the creation of a more conducive environment for plants growing in metal contaminated soils (Gohre and Paszkowski, 2006). Inoculation of G. mosseae and G. caledonium with Lolium perenne and Sedum alfredi notably reduced soil DTPA-extractable Cd by 21%–38% through the alkalinization process, hence facilitating in stabilization and extraction of Cd in-situ from Cd infected soils (Hu et al., 2013).

Some plant-associated microorganisms can promote enzyme-catalyzed precipitation of toxic metals [e.g., chromium (Cr) and selenium] and radionuclides (e.g., technetium and uranium) via microbial reduction process, which is promising for phytoremediation of metal-polluted soils (Payne and DiChristina, 2006). Some studies suggest that fungi can protect the host plant from the inhibitory effects of an excess concentration of Cr⁶⁺ by reducing toxic and mobile Cr⁶⁺ to immobile and non-toxic Cr³⁺ in soils. Besides, some insoluble forms of minerals, metals, and radionuclides may also be immobilized either indirectly through bacterial oxidation of Fe (Zhou et al., 2013) or directly via enzymatic actions (such as microbial reduction process; Pagnanelli et al., 2010).

Extracellular polymeric substances (EPS) excreted by AMF are of immense importance, making a protective hindrance against the adverse effects of metal biosorption (Hou et al., 2013). The mechanisms involved in metal biosorption onto EPS include the complexation with negatively charged functional groups, precipitation, metal ion exchange, and adsorption (Zhang et al., 2006). In this regard, AMF may produce insoluble metal-absorbing glycoprotein named glomalin that decreases the metal mobility or sequesters them, which may be taken into account for metal stabilization in soils (Vodnik et al., 2008). In an in-situ field experiment, the glomalin-related soil protein was used to estimate AMF derived from glomalin in soils in sequestrating Pb and Cd (Wu et al., 2014).