Metal ions at their higher concentration are toxic to plants, but they are necessary as trace elements. Many heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) are now hazardous to the environment globally and lead to a negative impact on human health. Due to their persistence in the environment for very long periods, such as for many hundreds to thousands of years, they negatively impact human and animal health (Alengebawy et al., 2021). Long-term exposure to heavy metals through the air, water, soil, and food causes various diseases like cancer, neurological effects, myocardial infarction, high blood pressure, skin lesions, organ system damage, urinary, reproductive, and respiratory systems (Rahimzadeh et al., 2017; Li et al., 2019). Lead (Pb) can persist in soil for more than 150–5,000 years and remains at high concentrations for up to 150 years after sludge application to the soil (Jabeen et al., 2009), whereas the biological half-life of cadmium (Cd) is approximately 10–30 years (Berglund et al., 2015). Removing heavy metals from the environment is very difficult because their degradation, like other pollutants, is not possible either biologically or chemically. Various technologies are adopted for ex situ and in situ heavy metal remediation of the contaminated soil. Some common technologies are chemical reduction, electrophoresis, excavation, pneumatic fracturing, soil washing, soil flushing, solidification, and nitrification (Dada et al., 2015). All of these traditional approaches are colloquially known as “pump and treat” and “dig and dump” techniques; however, these techniques are restricted to small areas and have limitations (Leung, 2004).

The conventional methods of removing pollutants from the environment are associated with numerous issues, such as partial removal, needing high energy, producing a significant amount of toxic sludge, being limited to a small area, and being costly (Li et al., 2019; Zamora-Ledezma et al., 2021). The economic burden of soil remediation by physical methods can be understood by the report of Salt et al. (1995). The phytoremediation of soil in 1 ac costs only approximately 60,000–1,000,000 US$, while physical remediation costs four-to-six times more to clean.

Over the last 10 years, a rapidly emerging, economically sound, and environmentally supportive alternative to traditional remediation practices has gained attention. This technique, known as “phytoremediation,” uses plants to clean up the environment as they can extract, accumulate, and depollute the substrate (soil, air, and water) from the contaminants through physical, chemical, or biological processes.

Several soil and plant factors influence phytoremediation efficiency, including chemical and physical soil properties, exudates from plants and microbes, metal bioavailability, and the plant ability to “uptake, accumulate, translocate, sequester, and detoxify metals” (Wang et al., 2020b). Bioremediation is economical as well as highly efficient; thus, these strategies have been proposed as an appealing alternative (Mejáre and Bülow, 2001). The application of plants and microorganisms either alone or in association to decontaminate heavy metal pollution has gained increasing attention. Many microorganisms, including fungi, mycorrhizal and non-mycorrhizal plants, and cultivated and wild plants, are tested in labs and the field for their ability to decontaminate metalliferous substrates in the environment (Tabrizi et al., 2015; Bahraminia et al., 2016; Yang et al., 2016; Yang et al., 2021; Antoniadis et al., 2021).

Understanding the mechanisms of how plants tolerate a specific metal is critical for increasing the number of plants that can be used for the phytoremediation of heavy metal–polluted sites. Various metal-binding proteins (MBPs) in the plants are involved in the absorption, accumulation, translocation, and detoxification of heavy metals and hence provide tolerance to the plants (Feki et al., 2021; Sharma et al., 2021). The MBPs include phytochelatins (PCs), metallothioneins (MTs), and transporter proteins [heavy-metal ATPase (HMA)] (Chaudhary et al., 2018; Mathur and Chauhan, 2020). This review focused on the different techniques used and the role of plant proteins to remediate soils contaminated with heavy metals. Biotechnological approaches or genetic engineering can also be used to tackle the problem of heavy metal contamination.

The selection criteria for the plants used in phytoremediation are that they should be highly metal tolerant and have a short life cycle, broad distribution, large biomass, and a translocation factor (TF) greater than 1 (Mazumdar and Das, 2015). Some plant species are more suitable for phytoremediation than others. Two main factors are commonly applied for the assessment of the phytoremediation potential of a plant: bioconcentration factor (BCF) and TF. The shoot-to-root ratio of heavy metal and the root-to-soil ratio of heavy metal are defined as the TF and BCF. Plants with more than one TF and BCF (TF > 1 and BCF > 1) are expected to be used in phytoextraction ( Table 1 ) (Li et al., 2022a).

The phytoremediation of heavy metal–contaminated soil includes any one mechanism or a combination of two or more phytoremediation mechanisms. The phytoremediation mechanisms mainly involved phytoextraction, phytostabilization, phytovolatilization, and rhizofiltration ( Figure 1 ).

Several MBPs have been reported in plants, which include MTs; PCs; metalloenzymes (MEs); metal-activated enzymes; and many metal storage proteins, carrier proteins, and channel proteins (Memon and Schröder, 2009). Additionally, PCs are low-molecular-weight peptides having a high affinity for transition metals, synthesized by glutathione-derived metal-binding peptides (Clemens, 2006). MBPs are compounds that bind to metals such as Fe, Cr, Zn, As, Cd, Ni, and Pb (Mejáre and Bülow, 2001). Cysteine residues are abundant in naturally occurring heavy MBPs such as PCs and MTs. The presence of an increased chelating molecule in plant cells, such as MTs, which are cysteine-rich proteins, and PCs, is thought to be responsible for hyperaccumulating plants’ greater tolerance or resistance (cysteine- and glutathione-rich compounds) (Sharma et al., 2021). Plants possess different types of cadmium-binding proteins that have fewer cysteine residues (Yu et al., 2018). MBPs have often been introduced and/or overexpressed to improve bacteria and plants’ metal-binding capacity, tolerance, or accumulation. Plant PC biosynthesis has recently been changed to improve metal accumulation, while various peptides containing metal-binding amino acids (mainly histidine and cysteine residues) have been investigated in bacteria for greater heavy metal accumulation (Mejáre and Bülow, 2001). Some plant proteins involved in the phytoremediation of heavy metals are listed in Table 2 .

Literature on the phytoremediation ability of cereal crops is sparingly available, although studies are mainly focused on the model plant A. thaliana and rice. In the present review, literature available on the other crops is also reviewed.

The mechanism of the phytoremediation technique needs to be improvised with time to enhance efficiency as well as cost-effectiveness. Different approaches have been implied for the enhancement, and researchers are continuing to investigate new techniques; these include genetic engineering and phytoremediation assisted with biochar, chemicals, and microorganisms (Sarwar et al., 2017).

Biochar is an economical carbon-based material having porous nature, and, because of this, it has outstanding potency and adaptability in a variety of contexts (Brewer et al., 2014). Since it has an inherent benefiting soil-conditioning ability, which improves water-holding capacity, fertility, pH, nutrients, carbon sequestration, the activity of microorganisms, and the remediation of pollution, in recent years, there has been an increase in biochar research as a soil-ameliorating agent (Ennis et al., 2012). This is because biochar improves soil fertility. In its natural state, biochar has a pH range of 8–11 and its cation exchange capacity (CEC) ranges from 25 to 485 cmol(+) kg¹. It has a large surface area (140–336 m² g¹), high porosity (0.0–1.32 cm³ g¹), and a specific surface area that ranges from 10 to 400 m² g¹. The COOH, –CO–, –OH, and ester groups are all found on the surface of biochar, and their presence boosts the CEC and adsorption while simultaneously lowering the leachability of nutrients (Ahmad et al., 2014; Ghosh and Maiti, 2021). Biochar’s high pH organic material has the potential to minimize the bioavailability of heavy metals, which is a benefit to the environment. Because biochar remediation is not harmful to the environment and is cost-effective, its use for the amendment of soil that is polluted with HM has become more common. After applying biochar to soil, the average concentrations of accessible Cd, Pb, Cu, and Zn were found to have decreased by 52%, 46%, 29%, and 36%, respectively (Chen et al., 2018). According to Lu et al. (2017), adding bamboo, rice straw, and biochar at a concentration of 5% (w/w) reduced the amount of extractable Cd, Cu, Pb, and Zn found in polluted soil. Gong et al. (2019) reported the use of tea waste–derived biochar that can enhance the phytoremediation capacity of the plants. Biochar alleviates the toxicity induced by the Cd and improves plant growth. Biochar also promotes the enzyme-producing microorganisms in the Cd-contaminated sediments. Immobilization occurs as a result of the surface functional groups’ contact with the HMs. This interaction is responsible for 38%–42% of the total Pb²⁺ that is adsorbed. However, it is possible that biochar on its own will not be able to clean up a very polluted mine soil that has been contaminated with HM. Because of this, the interaction between biochar and phytoremediation has been observed to be successful. Various chemical compounds like EDTA, EGTA, and SDS have been added to soil or water to stimulate plant growth and increase phytoextraction. The accumulation of metals in various plant sections may be enhanced by chemical amendments without negatively impacting plant development. This makes one wonder how much and how precisely chemical additions need to be made to the soil to ensure optimal plant growth and metal phytoremediation. The application of flavonoids (rutin) to Amaranthus hypochondriacus under different Cd stress conditions reduces the cell membrane damage and provides tolerance to Cd toxicity. The application of ‘rutin’ can immobilize the Cd in the cell wall, and a less amount of Cd is transported to the vacuole; it also enhances the synthesis of GSH and conversion of GSH to PCs. The rutin enhances the phytoextraction capacity of Cd in A. hypochondriacus (219%–260%) (Kang et al., 2022). The application of the metal-chelating agent tetrasodium glutamate diacetate (GLDA) in combination with Tagetes patula L. improves the plant biomass and accumulation of cadmium (Cd). Plants can withstand Cd stress as well as can remediate contaminated soil; T. patula L. removes 12.9% of Cd from the contaminated agricultural land when GLDA is applied to the soil (Li et al., 2022b).

Although phytoremediation seems like a good way to get rid of metals in polluted areas, most plants have a negligible capacity to absorb metals. Therefore, plants with enhanced metal accumulation efficiency have been developed via genetic engineering. Plant species can be developed by gene transfer and gene editing focusing on metal absorption and transport processes involving PC and MT proteins (Ozyigit et al., 2021).

Growing in heavy metal-contaminated soil is never easy for plants; the first organ of the plant is the root, which gets exposed to the contaminants and faces severe stress from the surrounding metals and needs immediate help to withstand stress. Under stress conditions, the plant roots evolved an adaptive strategy, ‘cry for help,’ which attracted the beneficial microorganisms to help in minimizing damage (Rizaludin et al., 2021; Rolli et al., 2021). Plants are synthesizing many thousands of primary and secondary metabolites under different conditions and needs (Hartmann, 2004). These metabolites include volatile and soluble compounds that play important roles in recruiting plant growth–promoting microorganisms (PGPMs), which alleviate metal toxicity and promote plant growth (Ma et al., 2016; Rolfe et al., 2019). Under stress conditions, plants communicate with the different PGPMs by employing the root exudates of different quantities and compositions depending upon the type of stress (Rizaludin et al., 2021). The root exudates are very useful energy-rich nutrients for the soil microorganisms and are enriched with amino acids and organic acids; they also contain PCs, which bind the heavy metals (Mishra et al., 2017).

The recruited microorganisms by the above-mentioned strategies together with the root exudates free the heavy metals bound to the soil particle and make them available to the plants for phytoremediation (Khanna et al., 2022). Plant-associated microorganisms transform the heavy metals from a non-bioavailable form to a bioavailable form by various mechanisms like methylation, changing soil pH, redox processes, the production and secretion of siderophores, organic acids, and biosurfactants (Shah and Daverey, 2020; Sharma et al., 2021). The plant–microbial interaction between plant growth–promoting rhizobacterium (PGPR) Variovorax paradoxus 5C-2 and plant Lotus edulis and L. ornithopodioides. Under heavy metal stress, bacteria produce enzyme 1-aminocyclopropane-1-carboxylate deaminase, which enhances the uptake of Cd and promotes plant growth (Safronova et al., 2012). Siderophore formation, ACC-deaminase activity, and IAA production by the PGPRs Pseudomonas reactans EDP28 and Chryseobacterium humi ECP37 in association with maize plants improve the Cd uptake from the contaminated soil (Moreira et al., 2016).

Different biological processes by the microorganisms such as the chelation, complexation, immobilization, precipitation, solubilization, transformation, translocation, and volatilization of heavy metals change the mobility of the heavy metals, which improves heavy metal uptake and facilitates phytoremediation (Rajkumar et al., 2012; Singh et al., 2016; Shah and Daverey, 2020; Azhar et al., 2022). Microorganisms synthesize MBPs under heavy metal stress to increase the tolerance and accumulation of metals (Jach et al., 2022). In the plant–microbe association, MBPs significantly increase the accumulation of heavy metals and provide tolerance or resistance. Under stress conditions, plants and microbes adopt various mechanisms such as compartmentalization, the formation of complexes, exclusion, and the synthetization and secretion of MBPs like PCs and MTs (Sharma et al., 2021). Various plants and their associated microorganisms (bacteria/fungi) are given in Table 3 .

Genetically modified Arabidopsis showed enhanced accumulation capacity for certain heavy metals like mercury, cadmium, and lead. Hsieh et al. (2009) created a transgenic Arabidopsis that expresses the mercuric ion-binding protein (MerP) of Bacillus megaterium and reported the excellent metal-accumulating capability of Arabidopsis. Genetically modified rice plants were reported to accumulate more Cd in their root than shoot under the overexpression of the rice V-PPase (Cao et al., 2020). The coexpression of the wheat gene for the NHX antiporter and V-PPase proton pump diminishes the toxicity of copper in transgenic tobacco (Gouiaa and Khoudi, 2019). An artificially synthesized PC gene ‘PPH6HIS’ was used to make the transgenic lines of the tobacco. The expression of the PPH6HIS gene in transgenic tobacco improves the accumulation capacity of Cd in the plant and provides resistance to the toxic effect of Cd on the plant (Vershinina et al., 2022).

The tolerance of heavy metal stress in plants is a defining characteristic of both their capacity to protect themselves and their efficient remediation systems. Utilizing model crops has provided several benefits in recent years for a better understanding of the biosynthesis, function, expression, and regulation of MT and PC. Even though several studies have shown that plant MTs play a significant role, we still have a long way to go before we can determine all of the tasks that MTs do. Their capacity to connect with a wide variety of metals, which results in a wide variety of functions, is shown by the enormous diversity that exists in the areas of metal binding in plant MTs compared to those in animal MTs. In Arabidopsis, MT-deficient mutants are not readily available, and it is probable that the members of the MT gene family have redundant functions. As a result, reliable information about the function of the gene is yet unavailable. Extensive research on “phytoremediation,” also known as the detoxification of contaminated surroundings, provides an overview of the potential of MT to combat stress tolerance. It is very necessary to exercise some level of control on the expression of MTs to boost the phytoremediation capabilities of plants. In addition, verifying their usefulness will need an understanding of how the accumulation and tolerance of metals are affected by the overexpression of these genes in certain organs. Microorganisms such as Pseudomonas sp., Bacillus sp., and Aspergillus and plants including Brassica juncea, Solanum nigrum, and Z. mays have a great ability to remediate an environment contaminated with heavy metals. Furthermore, several studies have revealed that plants and microorganisms work together to remove metal pollutants from the soil. The addition of plant growth-stimulating bacteria/fungi and metal-tolerant microorganisms improved the phytoremediation process considerably. As a result, selecting appropriate plant species and microorganisms can have a considerable impact on the outcome of phytoremediation. A comprehensive study of the root chemistry of metal-tolerant plants and microorganisms under stress conditions is much needed. How microorganisms and plants work together for the wellness of the metaorganism (resulting from the plant–microbe association) under particular stress conditions is required for further study for effective remediation of contaminated soil.

An emerging field of study and one with significant potential for commercial use is the creation of transgenic plants with an outstanding capacity to chelate certain metals and prevent the deleterious effects of these metals. In the future, the bioremediation of polluted places may benefit from the coordinated use of traditional breeding techniques in combination with molecular biology. Additionally, the identification of genes associated with metal tolerance via the use of genome sequencing might pave the way for the construction of transgenics with desirable characteristics that can be employed in phytoextraction technology. These findings, in association with considerable evolutionary research conducted across putative genes related to heavy metal tolerance, could provide some encouraging outcomes.

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.