Phytoremediation of Cadmium: Physiological, Biochemical, and Molecular Mechanisms

Cadmium (Cd) is one of the most toxic metals in the environment, and has noxious effects on plant growth and production. Cd-accumulating plants showed reduced growth and productivity. Therefore, remediation of this non-essential and toxic pollutant is a prerequisite. Plant-based phytoremediation methodology is considered as one a secure, environmentally friendly, and cost-effective approach for toxic metal remediation. Phytoremediating plants transport and accumulate Cd inside their roots, shoots, leaves, and vacuoles. Phytoremediation of Cd-contaminated sites through hyperaccumulator plants proves a ground-breaking and profitable choice to combat the contaminants. Moreover, the efficiency of Cd phytoremediation and Cd bioavailability can be improved by using plant growth-promoting bacteria (PGPB). Emerging modern molecular technologies have augmented our insight into the metabolic processes involved in Cd tolerance in regular cultivated crops and hyperaccumulator plants. Plants’ development via genetic engineering tools, like enhanced metal uptake, metal transport, Cd accumulation, and the overall Cd tolerance, unlocks new directions for phytoremediation. In this review, we outline the physiological, biochemical, and molecular mechanisms involved in Cd phytoremediation. Further, a focus on the potential of omics and genetic engineering strategies has been documented for the efficient remediation of a Cd-contaminated environment.


Introduction
Cadmium (Cd) is a non-essential element for plants and humans but is present in many soils in excessive amounts [1,2]. When it enters into the food chain, it poses a major threat to the living biota. The control of Cd accumulation in plants is complicated by the fact that most of the essential nutrient transporters, such as copper (Cu), manganese (Mn), iron (Fe), and zinc (Zn), also facilitate lipid peroxidation, metal uptake, and translocation to rice shoots, and improved the chlorophyll biosynthesis [67].

Phytoremediation Processes and Their Salient Features
Phytoremediation refers to the biological cleaning of the environment (soil, water, and air) by plants. Plants make a symbiotic association with microorganisms, which helps in the remediation of the soil, particularly from heavy metals and organic pollutants. Phytoremediation is generally considered as a green technology because of its excellent decontamination ability of heavy metals with a minimum influx of secondary waste to the environment. Alternatively, phytoremediation is highly acceptable among the general public due to its ease of application, low cost, and environmentally friendly nature [1,2]. However, hampered growth activities, such as reduced biomass and increased sensitivity to Cd, were observed in the plants involved in phytoremediation processes [6].
Phytoremediation involves various processes, such as phytoextraction, phytoaccumulation, phytovolatilization, phytostabilization, and phytotransformation. The phytoextraction and phytoaccumulation processes work in association. For instance, during phytoextraction, plants uptake heavy metals, such as Cd, Zn, nickel (Ni), chromium (Cr), and other minerals and nutrients from the soil. After this, these elements accumulate in the shoots and leaves with the help of the phytoaccumulation mechanism [6]. Many plants species have been reported previously for their high accumulation capacity; these are potential candidates for phytoremediation.
In Cd phytoremediation, plants are often used to absorb or translocate Cd into harvestable plant parts. Plants have evolved many diverse adaptations to maintain normal growth even under high Cd-contaminated soils, which also includes detoxification mechanisms [68]. The Cd concentration in plant parts shows the following trend: root > stem > leaves [69]. Many techniques are being used to increase the efficiency of Cd phytoremediation (Table 1).

Phytoextraction
This technique is used to absorb inorganic and organic contaminants through the stem and roots. Plants that are already growing in the ecosystem should be chosen for this technique. After harvest, they are exposed to another method known as composition, or burned in an incinerator [70]. Hyperaccumulator families, such as Scrophulariaceae, Lamiaceae, Asteraceae, Euphorbiaceae, and Brassicaceae, are essential for this technique. Moreover, some particular plant species, like Celosia argentea [71], Salix mucronata [72], Cassia alata [73], Vigna unguiculata, Solanum melonaena, Momordica charantia [74], Nicotiana tabacum, Kummerowia striata [75], and Swietenia macrophylla [76], may be used as potential plant choices to increase the process of Cd phytoextraction. Moreover, a sub-division of phytoextraction, known as chelate-assisted phytoextraction, is also used as a possible solution for metals that have no hyperaccumulator species. Several amino polycarboxylic acid and chelating agents have been applied to soil to increase the solubility of trace elements. For instance, EDTA-assisted phytoextraction of Cd was preferred by Farid et al. [77]. Similarly, citric acid was used as a chelating agent to increase the Cd uptake ability of jute mallow (Corchorus olitorius) [78]. Phytoextraction helps to reduce metalloid toxicity by improving substrate geochemistry for future colonization of native plants [79]. It is an effective, affordable, environmentally friendly, and potentially cost-effective technique for remediating soils [80]. Despite the generally agreed advantages of phytoextraction, there are some disadvantages, such as the time required for the remediation of highly contaminated soils may be decades [81], and a limitation for mine waste applications [82]. Mostly hyperaccumulator plants have developed the capacity to accumulate only one metal and may be sensitive to the presence of other elements [81].

Phytostabilization
There has been a progressing shift from phytoextraction to phytostabilization. Phytostabilization is the ability of plants to store and immobilized heavy metals by binding with biomolecules; this process prevents metal transport, and converts them into less toxic substances [83]. Most of the plants growing on contaminated soils are not hyperaccumulators but work as excluders. An excluder transforms the metals and metalloids into a less toxic mobile form without extracting them from the soil and accumulates these compounds in roots by absorption or precipitation within the rhizosphere [84]. Recently, promising results of Virola surinamensis for Cd phytostabilization have been documented [85]. Likewise, Miscanthus × giganteus [86], and oats and white mustard [87] also have phytostabilization potential for Cd. In another example, the putative role of Fe-Si-Ca, organic fertilizers, and coconut shell biochar has been reported to enhance the phytostabilization ability of Boehmeria nivea L. for Cd [88].
Phytostabilization is one emerging ecofriendly phytotechnology, which immobilizes the environmental toxins [89]. Roots take part in phytostabilization, so the metal availability is reduced to the plants, thus reducing the exposure to the other tropic level of the environment [90]. At the same time, the major disadvantage is the fact that pollutant remains in the soil or in the root system, generally in the rhizosphere [91].

Phytofiltration
Phytofiltration is categorized as rhizofiltration that includes blastofiltration (use seedlings) and caulofilteration (use of excised plant) ( Table 1) [92]. Rhizofiltration is the remediation of water in which roots effectively absorb contaminates [93]. In rhizofiltration, contaminant clings or assimilates to the roots, and can be transported to the plants. This method is mostly used to sterilize underground wastes or polluted water. Mostly radioactive substances or metals are removed by this method. Abhilash et al. [94] used the phytofiltration technique to increase the Cd uptake from water by using Limnicharis flava as an experimental plant. Islam et al. [95] reported the phytofiltration capability of Micranthemum umbrosum to remove Cd and arsenic (As) from a hydroponic system. In another experiment, the rhizofiltration potential of Arunda donax for Zn and Cd removal, it and recommended the use of the rhizofilteration technique for Cd elimination [96].
It is a cost-effective technique, and plants act as solar-driven pumps to extract the contaminants from the environment [93,95]. However, any contaminant below the rooting depth is not extracted. It is a time-consuming technique and will not suffice for the extraction of both organic and metal contaminants [95,97].

Phtytostimulation
Phytostimulation is a technique used to boost the process of phytoremediation by stimulating the root-released compounds to enhance microbial activities. These exudates enhance microbial growth by fulfilling their nutrient requirements. This process is being used in rhizoremediation technologies. It is a low-cost technique for Cd removal and other organic compounds [98]. Another method is the addition of resistant microbial inoculants into the soil, which can cause the accumulation of heavy metals, including Cd [99].
It is a more effective technique for converting toxic contaminants into non-toxic chemicals. Both in situ and ex situ practices can be done with low-cost treatments [100]. Microbes are able to help limit the growth of plant pathogens and increase nitrogen (N) fixation [101]. However, it is a more time-consuming technique, and the use of volatile and biodegradable compounds ex situ is not an easy practice. The process is sensitive to the level of toxicity in soil, and in some cases, incomplete breakdown of the organic compounds is observed. Moreover, well-controlled monitoring is required for this technique [100].  [109,110]

Effect of Phytoremediation on Cd Removal from Soils
The phytoremediation of soil contaminated with Cd has been a serious issue worldwide. In phytoremediation, hyperaccumulator plants are of particular importance as they are mainly involved in the uptake of Cd from soil [111]. Different hyperaccumulators vary in their capacity for Cd extraction from soil. This is because of the low affinity of Cd and its mobile nature [111]. The mobility of Cd in soil makes it easily available for the plant to extract it from the soil, which is later transported from the root to the aerial parts [112]. Some of the factors that facilitate the remediation of Cd process are pH, temperature, and the presence of other heavy metals in the soil [113]. For example, Chromolaena odorata, Gynura pseudochina, Conyza sumatrensis, and Nicotiana tabacum were tested in field conditions. The field soil was heavily contaminated with Cd; however, all the tested hyperaccumulator plants significantly reduced the Cd concentration in soil [114]. Various mechanisms of Cd phytoremediation are discussed comprehensively below.

Long-Distance Cd Transport
Accumulation of Cd is regulated by several processes, including vacuolar sequestration, xylem loading, cytoplasm across the membrane, energy-driven transport, cell wall adsorption, and Cd apoplastic influx into root tissues (Figures 1 and 2) [115,116]. One of the proposed prerequisites for bioremediation is that heavy metals are transported to and sequestered in aerial parts. Long-distance transport contributes substantially to maintain a low Cd concentration in roots, and takes part in overflow protection machinery [117,118]. These processes are mediated by several families related to metal and metalloid transport, such as P1B-ATPase [(enzymes that catalyze the hydrolysis of a phosphate (P) bond in adenosine triphosphate (ATP) to form adenosine diphosphate (ADP)] [118,119]. When present in ionic form, Cd transport from root to other tissues is mediated by three major transport system [120][121][122], such as low-affinity calcium (Ca) transporter 1 (TaLCT1) ZIP [(Zn transporter proteins (ZRT)-and Fe-regulated transporter (IRT)-like protein)] transporters, TcZNT1/TcZIP4 and Zn/Fe-regulated transporter-like protein (AtRT), and natural resistance-associated macrophage protein (NRAMP) [123], which includes OsNARMP1, 5, and 6. Moreover, the transport system of Fe uptake is also involved in Cd uptake. Takahashi et al. [123] and Milner et al. [124] also observed that OsNramp1 enhanced Cd accumulation in the shoot. Furthermore, yellow strip-like 1 (YS1/YSL1) Fe transporters transport Fe in its chelating form. Murata et al. [125] also identified Fe phytosiderophore transporters (HvYS1) in barley, which showed strict specificity for both metals and ligands. In addition, Sasaki et al. [122] and Ishimaru et al. [126] deduced that OsNRAMP that plays a key role in Mn 2+ transport and also showed a major route for Cd transport in rice. From all the above-reported transporters, NRAMP may be involved in several functions, such as metal detoxification, uptake, intracellular transport, and translocation, in many plants [126][127][128][129]. Moreover, the Ca 2+ blocker also inhibits Cd transport in Suada salsa, suggesting their contribution to Cd transport [130]. Collectively, Cd is transported through Zn, Fe, and Ca transporters in plants that include LCT1 and ZIP family (Zn/Fe transporters), especially ZIP-IRT [124], and macrophage protein Nramp channels [42]. Therefore, Table 2 shows the summaries of Cd transporters, their function, and location in plants.
AtPDR8 is an ABC transporter that is involved in metal homeostasis and Cd tolerance, and is mainly localized on the epidermis and membrane of root hairs [131]. Once taken by the roots, Cd is then transported into the xylem and other shoot parts. The transport differences in Cd transport in the xylem and shoot are due to genetic variations [132]. Metal tolerance proteins (MTPs) and cation diffusion facilitators (CDFs) can be involved in this whole process [133,134]. Moreover, the P 18 -type metal transporter ATPase (HMAs) also takes part in Cd transport across the membrane, which is required for metal homeostasis. While working on an Arabidopsis hma2/hma4 mutant, it was observed that OsHMA (pericycle transporter) also transports Cd [135]. OsHMA2 has been described as a key transporter of vascular Cd [136]. The knockout mutant of OsHMA2 resulted in the reduction of Cd in the shoot and grain, which has been confirmed in several studies [118,119,137,138].
Furthermore, low-affinity cation transporters OsLCT1 load Cd metal into the phloem sap [120]. Glutathione (GSH) and its derivatives, phytochelatins (PCs), showed strong bounding with As, Hg, and Cd ( Figure 1) [139]. A Cd complex with PCs was also seen in rapeseed phloem sap. According to Mendoza-Cozatl et al. [139] and Kato et al. [140], GSH-Cd (reduced GSH-Cd complex) contributes to long-distance transport of Cd in the phloem. However, PCs appeared in phloem sap after the application of Cd, and they showed a strong affinity for Zn [141]. Mendoza-Cozatl et al. [139] observed thiol-conjugates in phloem that were transported in different sinks after uptake from ATP-binding cassette subfamily C proteins (ABCC1 and ABCC2). PCs-Cd conjugates were involved in root vacuole Cd sequestration, while the GSH-Cd complex was only detected in the seed source. Moreover, the LCT1 transporters also mediates phloem-based Cd distribution ( Figure 2) [120,142]. Depiction of major transporters present on the root, shoot, and leaves for Cd sequestration and storage (these processes are related to phytoremediation). Read text and Table 2 for more information. Depiction of major transporters present on the root, shoot, and leaves for Cd sequestration and storage (these processes are related to phytoremediation). Read text and Table 2 for more information. AtPDR8 is an ABC transporter that is involved in metal homeostasis and Cd tolerance, and is mainly localized on the epidermis and membrane of root hairs [131]. Once taken by the roots, Cd is then transported into the xylem and other shoot parts. The transport differences in Cd transport in the xylem and shoot are due to genetic variations [132]. Metal tolerance proteins (MTPs) and cation diffusion facilitators (CDFs) can be involved in this whole process [133,134]. Moreover, the P18-type metal transporter ATPase (HMAs) also takes part in Cd transport across the membrane, which is required for metal homeostasis. While working on an Arabidopsis hma2/hma4 mutant, it was observed that OsHMA (pericycle transporter) also transports Cd [135]. OsHMA2 has been described as a key transporter of vascular Cd [136]. The knockout mutant of OsHMA2 resulted in the reduction of Cd in the shoot and grain, which has been confirmed in several studies [118,119,137,138].
Furthermore, low-affinity cation transporters OsLCT1 load Cd metal into the phloem sap [120]. Glutathione (GSH) and its derivatives, phytochelatins (PCs), showed strong bounding with As, Hg, and Cd ( Figure 1) [139]. A Cd complex with PCs was also seen in rapeseed phloem sap. According to Mendoza-Cozatl et al. [139] and Kato et al. [140], GSH-Cd (reduced GSH-Cd complex) contributes to long-distance transport of Cd in the phloem. However, PCs appeared in phloem sap after the application of Cd, and they showed a strong affinity for Zn [141]. Mendoza-Cozatl et al. [139] observed thiol-conjugates in phloem that were transported in different sinks after uptake from ATP-binding cassette subfamily C proteins (ABCC1 and ABCC2). PCs-Cd conjugates were involved in root vacuole Cd sequestration, while the GSH-Cd complex was only detected in the seed source. Moreover, the LCT1 transporters also mediates phloem-based Cd distribution ( Figure 2) [120,142].

Vacuolar Storage and Sequestration
Several families of transporters, such as ABCCs, NRAMPs, Ca 2+ exchanger (CAXs), and HMAs, have been investigated in vacuolar sequestration of Cd [143,144]. Among these, ABCC transport PCs (PCs-Cd conjugated). Similarly, Park et al. [143] reported that ABCC1 and ABCC2 are important vacuolar transporters that confer tolerance to Cd, mainly AtAbCC3 plays a role in PC-mediated Cd tolerance [145]. The NRAMPs transport various divalent metals, such as Zn, Mn, Fe, and Cd. NRAMP3 and NRAMP4 are located on the tonoplast and play an important role in the remobilization of essential metals from the vacuole to the cytosol [146]. The CAXs are tonoplast-localized transporters that have specific transportability of Ca 2+ . However, Korenkov et al. [147] reported that AtCAX2 and AtCAX4 transporters are not only specific to Ca 2+ but also transport

Vacuolar Storage and Sequestration
Several families of transporters, such as ABCCs, NRAMPs, Ca 2+ exchanger (CAXs), and HMAs, have been investigated in vacuolar sequestration of Cd [143,144]. Among these, ABCC transport PCs (PCs-Cd conjugated). Similarly, Park et al. [143] reported that ABCC1 and ABCC2 are important vacuolar transporters that confer tolerance to Cd, mainly AtAbCC3 plays a role in PC-mediated Cd tolerance [145]. The NRAMPs transport various divalent metals, such as Zn, Mn, Fe, and Cd. NRAMP3 and NRAMP4 are located on the tonoplast and play an important role in the remobilization of essential metals from the vacuole to the cytosol [146]. The CAXs are tonoplast-localized transporters that have specific transportability of Ca 2+ . However, Korenkov et al. [147] reported that AtCAX2 and AtCAX4 transporters are not only specific to Ca 2+ but also transport other metals, including Cd. In this consistency, the Cd hyperaccumulator Arabidopsis halleri showed Cd tolerance with higher expression of AhCAXI [3]. In low-Cd-accumulating rice cultivars, OsHMA3 is functional and able to sequester Cd into vacuoles; however, in high-Cd-accumulating cultivars, it is present in an inactive form due to a single amino acid mutation (Table 2) [148,149]. Moreover, higher NcHMA3 expression also plays a role in Cd hyperaccumulation in Noccaea caerulescens [150]. Furthermore, the Cd hyperaccumulator Sedum alfredii also showed a higher expression of SaHMA3 [151]. Currently, Liu et al. [152] has discovered that SpHMA3 is critical for Cd detoxification and vacuolar sequestration in young leaf cells of the Sedum plumbizincicola plant. They found elevated expression of SpHMA3 in shoots, while Ueno et al. [150] observed the same expression level of HMA3 in both the shoot and roots. The CDF transporter family is also involved in vacuolar sequestration and storage and transport of metal ions [153].

Mechanism of Cd Crossing the Plasma Membrane of Root
At the root plasma membrane, H 2 CO 3 dissociates into HCO 3 and H + trough root respiration, so absorbed H + rapidly exchanges with Cd + , and then Cd absorbed on the surface of the root epidermis cells, and its exchange into root epidermis cells layers occurs trough the apoplastic pathway [154]. Roots hairs provide a large surface area for Cd absorption from the soil through diffusion [155]. Plant roots also secrete certain organic compounds, such as chelates, that complex with Cd ions to form ligands, allowing its entry into the root epidermis [156]. Moreover, Cd is also taken up by non-selective cation channels, Zn/Fe-regulated transporters [157], and MTPI [158]. Additionally, certain protein transports, such as NRAMPs [159], P-type ATPase (AtHMA4 and AtHMA9) [160,161], ABC transporters (OsPDR9 and AtPDR8 [131,162], and the CAX family (AtCAX2 and AtCAX5) [163,164], impart a critical role in Cd transport across the root plasma membrane. In general, after Cd uptake by plant roots, the maximum portion of Cd gets fractionated into the roots, and only a small portion gets fractionalized to the upper areal parts [160].

Antioxidant Defense: A Key Mechanism of Cadmium Tolerance and Phytoremediation
Cadmium inhibits the activity of various metabolic cycles and a non-redox active metal that induces many ROS, including H 2 O 2 , superoxide radicals (O 2 •− ), and hydroxyl radical (OH • ). Plants have an established mechanism to eradicate oxidative impairment through the protective antioxidant defense system that includes enzymatic antioxidants (SOD, CAT, APX, glutathione reductase (GR), glutathione peroxidase (GPX), glutathione S-transferase (GST)) and non-enzymatic antioxidants (GSH, carotenoids, ascorbic acids (AsA), and tocopherol) [179,180]. Antioxidants' response against Cd toxicity varies amongst different plant species and experimental conditions [180]. The modulation of antioxidant machinery during the phytoremediation process is important because it prevents the plant from Cd toxicity. For example, some hyperaccumulators have the tendency to uptake/remove an abundance of Cd from the soil, but their physiological and biochemical processes still function properly. This could be because of the high antioxidant enzyme activities, which keeps these hyperaccumulators alive even in the most unfriendly environment. Table 3 documents the experiments showing the potential of the antioxidant defense system as a key mechanism of Cd tolerance and phytoremediation.
In wheat, Cd tolerance is linked with high activity of antioxidant enzymes, photosynthetic rate, and hormone concentrations [181]. Cd stress increases the activity of SOD, CAT, POD, and MDA, and reduces the photosynthetic rate, transpiration rate, stomatal conductance, and auxin, gibberellin, and zeatin nucleoside concentrations in wheat leaves. Shah and Nahakpam [182] found six SOD isozymes in a Cd-tolerant rice cultivar (Bh-1) while only three were found in sensitive (DR-92) rice cultivars, suggesting the SOD improves the tolerance capability of rice against Cd stress. Different plant nutrients, such as silicon, sulfur, and iron, reduce the Cd toxicity in higher plants by affecting its accumulation. Glutamate (Glu; 3 mM) is also involved in the abiotic stress response and has been found to significantly elevate Cd accumulation in rice by up to 44% (root) and 66% (shoot) [169,183]. Macleaya cordata has shown a high Cd phytoremediation ability (393 µg plant −1 ) with increased biomass. The high Cd concentration, showing high SOD and MDA activity required, and increased capacity of ROS, proves M. cordata's efficiency in Cd phytoremediation [184]. Phosphorus with 100 µM Cd enhances the activity of SOD, POT, CAT, AsA, and α-tocopherol in wheat and decreases the Cd accumulation in shoots [185]. Sunflower seeds under Cd stress showed reduced biomass, carotenoid, and low chlorophyll concentration with an increase Cd in the shoot and root. Pan et al. [186] examined the genetic insight of high Cd-tolerant Kandelia obovata, a mangrove plant, and reported two genes KoFSD2 and KoCSD3 that showed greater SOD levels and differentially maintained the reactive oxygen mechanism when overexpressed in Nicotiana benthamiana under Cd toxicity.
An increased level of H 2 O 2 decreases the efficiency of the plant for Cd tolerance; therefore, its accumulation is avoided properly by the action of the oxidoreductase enzymes CAT and POD [187]. An increased content of CAT in the leaf of Cd-tolerant wheat lines reduced the translocation of Cd from the root to shoot whereas the 50 µM Cd enhanced the MDA and reduced the activity of CAT and SOD in leaf [188]; while in Glycine max, greater Cd accumulation was observed in root with an increased level of GR (up to 370%) followed by CAT (271%) and SOD (193%) in a tolerant cultivar [189].
Gratao et al. [190] applied the grafted technique in Micro-Tom to analyze the antioxidant response to Cd stress. The grafted plant showed a better signaling response to Cd stress from the root to shoot while the non-grafted plants showed decreased levels of CAT, APX, and GR. In rapeseed, high Cd stress decreased the activity of antioxidant enzymes, i.e., SOD, GR, APX, and CAT, while the lipid peroxidation level increased. Likewise, Brassica juncea exposed to Cd stress showed no change in antioxidant activity, with an increased level of NP-SH and PCs, that worked under metal stress [191], or increased GR activity up to 50 µM L −1 with more accumulated Cd in the leaf of the sensitive cultivar [192]. Enzyme activity increased with an increase in the Cd concentration (100 mg kg −1 ), such as SOD (81%), CAT (36%), and POX (57%), in Brassica juncea as compared to the control [66]. Table 3. Summary of experiments showing the potential of the antioxidant defense system as a key mechanism of cadmium tolerance and phytoremediation. Abbreviations are explained in the text.

Chelate-Assisted Cadmium Phytoremediation
Due to the limitations in the phytoextraction technique, the use of chelating agents is considered as a suitable alternative to other conventional methods to remediate contaminated soils [202]. In general, chelates are known as stimulating chelating agents in the release of divalent and trivalent cationic metals into soil water to enhance their absorption by the roots of plants. They are classified into three groups of (i) synthetic amino-polycarboxylic acids (APCAs), such as EDTA, ethylene-glycol-tetraacetic acid (EGTA) and sodium-dodecyl-sulfate (SDS); (ii) natural amino-polycarboxylic acids, including S,S-ethylene-diamine-disuccinic acid (EDDS) and nitrile-triacetic acid (NTA); and (iii) low molecular weight organic acids (LMWOAs) containing oxalic acid (OA), citric acid (CA), and tartaric acid (TA) [203][204][205].
Several investigations have been carried out on EDTA as a heavy metals chelator from polluted water and soils for effective phytoextraction [204,206,207]. In several studies, it has been reported that the application of EDTA in contaminated soils leads to elevated Cd accumulation in the aerial parts of plants [207,208]. As stable EDTA-metal complexes have a long-term persistence in soil water, concerns have been raised about the leaching of soil-cationic metal ions and contamination of groundwater and adverse impacts on rhizosphere microorganisms. Therefore, natural chelating agents, such as EDDS, were proposed as a biodegradable chelating agent, a substitute for EDTA [202]. Use of EDDS can decrease metal leaching compared to EDTA but not wholly prevent it [203]. For example, Evangelou et al. [209] described that the half-life of EDDS, depending on the dose of EDDS added to the soil, was between 4.2 and 6.6 days, which is less than EDTA; however, there was Cd and Cu leaching. APCA chelates help to increase the solubility of Cd in soils and efficient phytoextraction but have no role in its elimination. Adding low doses of LMWOAs to Cd contaminated-soil has been suggested as a suitable alternative to other chelants for phytoremediation by (i) the formation of soluble organic acid-Cd complexes, (ii) providing a carbon source for the microorganisms present in the rhizosphere and increasing their diversity; (iii) amending soil quality; (iv) a high degree of biodegradability and low leaching risk; (v) and also improving soil acidity, thus elevating the solubility of Cd ions in soil water [181,210]. In a study on biodegradable chelators, effective phytoextraction of Cd, As, and Pb by Pteris vittata in the presence of 1 mM kg −1 EDDS compared to OA and CA was identified. However, adding of 2.5 mM kg −1 OA improved the soil quality and diversity of soil microorganisms [211]. Moreover, the results of the experiments proved that usually LOWMs in low doses had the highest efficiency of phytoextraction, while higher doses of these chelates negatively affect the Cd uptake and plant biomass. For instance, a study using sunflower plants in Cd-polluted soil showed that Cd absorption at the plant's roots in the existence of higher levels of CA was much lower than in plants exposed to lower doses of CA [212]. A hypothetical model of phytoremediation by adding chelating agents to remediate Cd-contaminated soil is illustrated in Figure 3. Generally, the disadvantages of the application of chelating agents in soils can reduce the growth and biomass of the plant, as well as the adverse effects on the soil quality [213]. Moreover, some studies have reported that EDTA elevates Cd mobility in soils and its absorption by the roots, though EDTA is not able to overcome the restrictions of the translocation of Cd from the root to shoot [214,215]. Since the absorbed EDTA-Cd complex moves through the apoplastic pathway, the existence of the Casparian strip and suberin deposits disrupt the transfer of this complex from roots to aerial parts, so greater amounts of Cd are accumulated in roots than aerial parts [216,217]. Despite the enhanced levels of soluble Cd in the soil by chelators, regarding the existence of high amounts of Ca and Fe in the soil and their competition for binding to these chelators, higher amounts of these chelating agents must enter the soil to bind to Cd. On the other hand, plants able to absorb only a small portion of the soluble Cd, and a high amount of the soluble Cd-chelate complexes persist in the soil [217]. Notably, the Cd-chelate complex is stable across an extensive pH range, its leaching is unavoidable, and it could enter groundwater and contaminate it [218].

Phytochelatins and Metallothionein for Cadmium Phytoremediation
The binding of Cd to high-affinity chelators, including phytochelatins (PCs) and metallothioneins (MTs), and organic acids and amino acids within plant cells is one of the vital adaptative mechanisms [46]. Moreover, the character of these ligands varies, depending on their position inside the plants and plant age [219]. Notably, among metals, Cd is known as a potent stimulator of PCs in different types of plants [220]. Figure 4 represents a schematic summary of Cd tolerance and its accumulation by PCs in leaf cells of Cd-hyperaccumulating plants. Generally, the disadvantages of the application of chelating agents in soils can reduce the growth and biomass of the plant, as well as the adverse effects on the soil quality [213]. Moreover, some studies have reported that EDTA elevates Cd mobility in soils and its absorption by the roots, though EDTA is not able to overcome the restrictions of the translocation of Cd from the root to shoot [214,215]. Since the absorbed EDTA-Cd complex moves through the apoplastic pathway, the existence of the Casparian strip and suberin deposits disrupt the transfer of this complex from roots to aerial parts, so greater amounts of Cd are accumulated in roots than aerial parts [216,217]. Despite the enhanced levels of soluble Cd in the soil by chelators, regarding the existence of high amounts of Ca and Fe in the soil and their competition for binding to these chelators, higher amounts of these chelating agents must enter the soil to bind to Cd. On the other hand, plants able to absorb only a small portion of the soluble Cd, and a high amount of the soluble Cd-chelate complexes persist in the soil [217]. Notably, the Cd-chelate complex is stable across an extensive pH range, its leaching is unavoidable, and it could enter groundwater and contaminate it [218].

Phytochelatins and Metallothionein for Cadmium Phytoremediation
The binding of Cd to high-affinity chelators, including phytochelatins (PCs) and metallothioneins (MTs), and organic acids and amino acids within plant cells is one of the vital adaptative mechanisms [46]. Moreover, the character of these ligands varies, depending on their position inside the plants and plant age [219]. Notably, among metals, Cd is known as a potent stimulator of PCs in different types of plants [220]. Figure 4 represents a schematic summary of Cd tolerance and its accumulation by PCs in leaf cells of Cd-hyperaccumulating plants.  PCs are Cys-rich polypeptides that include duplicate units of γ-glutamyl-cysteine, followed by one glycine (Gly) in the C-terminal [(γ-Glu-Cys)n-Gly] with 2 to 11 repeating units. Albeit, depending on the plant species; Gly in the C-terminal can be substituted by Ala, Ser, Gln, or Glu. These metal ligands are enzymatically synthesized by γ-Glu-Cys dipeptide transpeptidase (PC synthase) from GSH in the cytosol to transfer Cd-PCs complexes to vacuoles or the apoplastic space by ATP-dependent pumps [221,222]. PC synthase is activated by direct Cd binding to the enzyme [223]. While the Cd concentration in the cytosol is elevated, PC synthase enhances tolerance and detoxification of Cd by biosynthesis of PCs [222]. Lee and Hwang [224] found that the overexpression of the NtPCS1 gene involved in PC synthase biosynthesis in transgenic tobacco plants results in high tolerance to Cd and As, and improved growth and development in these plants. By binding Cd to PCs, it forms a constant complex that is less toxic than the free Cd ions existing in the cells. Once the Cd-PCs complexes are formed inside the cytosol, they can finally be sequestered within vacuoles by transferring the ATP-binding cassette (ABC) transporters through the tonoplast (Figure 4) [225]. So far, three types of vacuolar ABC transporter, AtABCC1, AtABCC2 [143], and AtABCC3 [145], have been recognized in the Arabidopsis thaliana that are involved in the transfer of Cd-PCs complexes across the tonoplast into the vacuole and Cd tolerance [226]. Current research has determined that metals compartmentalize as a critical resistance mechanism to reduce oxidative stress and damage to the photosynthetic apparatus in the shoot cells of hyperaccumulator plants [227]. Similarly, it has been identified that mature leaves contain higher amounts of S-rich chelators (PCs and MTs) than in young leaves and can store high concentrations of Cd [228]. Sun et al. [229] found that the levels of PCs in leaf mesophilic tissues of the Cd hyperaccumulator Solanum nigrum were dramatically higher than the non-accumulator Solanum melongena, which caused the accumulation of significant amounts of Cd and their detoxification in shoots of the Cd-hyperaccumulator population. Similarly, analysis of the EDX spectrum in potato exposed to different Cd concentrations indicated that the highest amount of Cd was stored in the mesophilic tissue of leaves by binding to S-rich compounds [230]. A study on alfalfa under Cd stress conditions indicated that Cd tolerance in seedling roots was enhanced by increasing levels of the expression of genes associated with sulfur metabolism, especially genes implicated in GSH and PCs biosynthesis, also ABC transporters [231]. Due to the low pH of vacuoles, Cd-PCs complexes disassociate, and Cd can be stabilized in vacuoles by binding to ligands, including organic acids and probably amino acids [232]. PCs may be by destroyed hydrolase enzymes inside the vacuole or returned to the cytosol, where they able to keep their role as Cd shuttles [233]. PCs are also implicated in the long-distance transfer of excess Cd from the root to aerial parts to reduce Cd accumulation in roots [234]. For instance, PCs and GSH could play a role as long-distance Cd carriers by the xylem and phloem in rapeseed. Likewise, the rate of Cd transfer by the xylem depends on the Cd concentration in the root [235].
Like PCs, MTs belong to a small Cys-rich protein family with binding sites to metal that exists in an extensive range of organisms [236,237]. In contrast to PCs, which are biosynthesized enzymatically by PC synthase, MTs are directly produced by translating mRNA [238]. MT gene regulation is different in various plants and abiotic stress conditions, including heavy metal-induced expression ( Figure 5) [239]. So far, four types of MTs have been recognized in plants, containing MT1, MT2, MT3, and MT4 types, which are categorized according to the arrangement of Cys residues in their C-and N-terminal domains [240]. Regarding the diversity of plants as well as the organization or distribution of Cys residues in the MT structure, there may be different isoforms for each type of MT, e.g., in A. thaliana, two isoforms (MT4a and MT4b) have been identified for MT4 [241]. The expression genes of MTs are varied in diverse plant tissues and different growth and development steps of plants [237]. Thus, the genes involved in MT-type1 produce are mostly expressed in roots. In contrast, the genes of MT-type2 are often expressed in the plant shoots and help to store high concentrations of heavy metals in shoots by biding to them, and besides, play an essential role in ROS tolerance. Besides, genes related to MT-type3 are usually involved in fruit ripening and are also expressed in leaves.  In contrast, the expression of genes associated with MT-type4 has been observed in the seed development stage [237]. However, the role of different types of MTs as metal chelators has been identified, which can lead to the tolerance and accumulation of heavy metals in plants [238]. MTs have a great tendency to bind to heavy metals containing Cd, Zn, Cu, and As, and can eliminate them even at a low amount; nevertheless, in terms of the importance as a Cd chelator, MT is considered after PCs [242]. Generally, MTs play multiple roles in plants, such as tolerance and maintenance of the cellular ion balance by detoxification of heavy metals, ROS scavenging, damaged DNA repair, and as well as various physiological processes, including seed germination and fruit ripening in plants [240,243]. However, the mechanism of action of MT plants in stressful conditions relative to mammalian MT remains unknown. It has been established that MT2, in Coptis japonica, has 14 Cys-residues in its C-and N-terminal that can bind to four Cd(II) ions [239]. Li et al. [242], proved that in Ziziphus jujuba 24 h after Cd exposure, ZjMTs levels elevated and caused increased Cd tolerance and detoxification. This MTs also has six Cys residues in the C-and N-terminal structure and belongs to the MT-Type1 group [242]. It was found that the expression of a gene of CsMTL2 screened from cucumber fruit was regulated by induction of metal stress, especially Cd in the transformed cells of Escherichia. coli. Besides, the highest levels of Cd and Zn were observed consequent of heterologous overexpression of CsMTL2 in E. coli cells compared to the control [242]. In a similar experiment, the same results were obtained on the expression of CsMT4 screened from cucumber fruit in E. coli cells under Cd and Zn stress [244]. Overexpression of PjMT1 and PjMT2 genes transferred to tobacco plants from Prosopis juliflora caused significantly increased accumulation of Cd in transgenic tobacco plants compared to wild-type plants [245]. Overall, these outcomes suggest that different types of MTs are implicated in the tolerance and accumulation of high amounts of Cd in plant cells. Gonzalez-Mendoza et al. [246] examined Cd and Cu stress in Avicennia germinate plants, and they observed a direct relationship between the overexpression of AvPCS and AvMT2 genes and homeostasis and detoxification of Cd and Cu in these plant cells. Additionally, it was shown that the expression of MT genes in yeast strains screened form A. thaliana, which were exposed to several metal stress conditions, MT3 was the best candidate for metal phytoremediation [247]. Today, genetic engineering techniques can improve the effectiveness of phytoremediation in plants [248]. For instance, given the importance of vacuoles as a subcellular organelle for storing Cd, the engineering of vacuolar carriers in particular cells, as well as overexpression of proteins in the cell wall with a high affinity to bind to Cd as another site of Cd storage, can help accumulate more Cd in shoots [249]. Further research for understanding the mechanisms of tolerance and accumulation of Cd by genetic engineering techniques might bring a new milestone for the evolution of phytoremediation of Cd by plants.

Omics Approaches for Cadmium Phytoremediation
Omics technologies, such as genomics, transcriptomics, proteomics, and metabolomics, have been widely applied to study the genetic insights, metabolic pathways, and molecular changes in response to external heavy metal stress response, its transport, and accumulation (Tables 4-6).

Genomics
Genomics approaches' advancement enhances the identification of multiple genes involved in phytoremediation, plant stress tolerance, and transport of heavy metals, such as DNA mismatch repair (MMR) in a soybean cultivar [250], targeted induced local lesions in genomes (TILLING) in rapeseed [8], clustered regularly interspaced short palindromic repeats (CRISPER/Cas9) in rice [251], and genome-wide association studies (GWAS) in rapeseed [252,253].
Navarro-Leon et al. [8] studied the role of the HMA4 gene of Brassica rapa through TILLING under 100 µM CdCl 2 stress. The mutated plant showed increased Cd accumulation in the leaf with an increased level of GSH/GSSG and photosynthetic pigments and reduced biomass and oxidative stress. GWAS using the 60K Brassica Infinium ® SNP array was performed with phenotypic and genotypic data to understand the mechanism of Cd tolerance in rapeseed. In total, 12 Cd-tolerant genotypes, 9 single nucleotide polymorphisms (SNPs) loci, and 7 genes linked to Cd tolerance were identified [252]. Chen et al. [253] performed GWAS of 419 rapeseed and identified 25 QTLs integrated with 98 SNPs that reside on 15 chromosomes linked to Cd-accumulated traits.
Ma et al. [254] studied Oryza nivara to detect quantitative trait loci (QTL) related to Cd tolerance. Seven QTLs residing on chromosome 2, 4, 6, and 8 were identified along with five genes related to oxidoreductase, terpene synthase, carboxypeptidase, and cysteine-rich receptor protein through GeneChip data. Ra44 was obtained as a Cd-tolerant line and further used to study the Cd tolerance in rice. GWAS analysis of 349 wild A. thaliana was performed to check the variability in Cd accumulation and the HMA3 locus was found as being potentially responsible for Cd accumulation in the leaf [255].
CRISPER/Cas9 technology was used to obtain three mutants (LCH1, LCH2, and LCH3) of the OSNramp5 gene from rice that were involved in the uptake of Cd and other metal ions from root cells [251]. Tang et al. [256] used the CRISPER/Cas9 system in the rice indica gene OsNramp5 to reduce Cd accumulation in rice for food safety, and demonstrated a 0.05 mg kg −1 decrease in the Cd concentration whereas the plant yield was not affected.
Wang et al. [257] studied the adaptability of the ornamental plant Verbena. bonariensis under Cd stress. Plants transcriptome analysis under Cd stress revealed 237,866 transcripts and 191,370 unigenes whose enrichment analysis revealed differentially expressed genes (DEGs) from all major process, especially lignin synthesis, anthocyanins synthase (ANS), and chalcone synthase (CHS), under Cd stress, confirming the plant has a great Cd phytoremediation property through Cd tolerance and distillation of it. B. juncea is a well-known plant used for phytoremediation studies utilized for microarray analysis to understand the functional genes associated with Pb and Cd stress. The microarray chip of A. thaliana probes was used to study the DEGs in roots of B. juncea (variety P78) [258].
The genome-wide transcriptomic profile of Brassica rapa var. parachinesis (Chinese flowering cabbage) was performed using Solexa sequencing. They identified 1404 upregulated genes and 1669 downregulated genes, and precisely three Cd tolerance genes identified as HMA3, HMA4, and Nramp1 [259]. In a recent study, Cd accumulation and distribution was investigated in the peanut plant through RNA-seq analysis. Here, 8cDNA libraries identified 4484 novel genes from which 6798 were grouped to Cd-related DEGs among two cultivars, first Fenghua 1 (low-Cd) and second Silihong (high-Cd). A total of 183 DEGs were to be found linked to ion transport-related proteins (ZIPs, MTPs, Nramps, and YSLs), among them 9 genes related to the cell wall and 9 genes specifically related to metal transport from which 4 were linked to endomembrane-tonoplast transported genes (MTP4, ABCC4, ABCC15, and ZIP11), four Cd efflux genes (YSL7, FRD3, PDR12, and ZIP1), and one Cd influx gene (IRT1); higher expression in Fenghua 1 might be related to the difference in Cd transport and accumulation among the two cultivars [260]. Creeping bentgrass transcriptional analysis showed four transcription factors (bZIP, WRKY, MYB, and ERF) linked with cd stress [261].
RNA-seq integrated with PacBio ISO-seq was developed to study the full transcriptomic data of Italian rye grass, a potential phytoremediation plant. Out of 2367 DEGs, the overexpression of the LmAUX1 gene significantly enhanced the Cd concentration in A. thaliana. The transcriptome analysis helped in the construction of full-length UniTransModels; out of this, 26.76% had isoforms [262]. RNA-seq analysis of Pokeweed revealed 10.63 Gb transcriptomic data consist of up to 97,000 unigenes covering 72 metabolic pathways. It had excellent phytoremediation ability, and different heavy metal-tolerance genes were identified, including nicotianamine synthase (8), ABC transporter (3), expansins (11), metallothioneine (3), ZRT/IRT protein (4), and aquaporins (4) [263].
Proteomic analysis of xylem tissue of Cd-treated rapeseed plant revealed the proteins related to energy production, carbohydrate metabolism, and redox reduction [267]. Cd stress caused a reduction in growth and photosynthesis when Populus. tremula x Populus. alba was exposed to 360 mg kg −1 Cd for 61 days [268]. Proteomic analysis (MALDI-TOF/TOF, 2DE) of a Cd-treated Sorghum bicolor plant revealed increased expression of 15 proteins while it downregulated 8 proteins. Mostly, Cd reduces the activity of ATP production, carbon fixation, and protein synthesis regulation [269]. Suspension cells of rice were subjected to iTRAQ and ICP-MS analysis to study improved Cd stress with silicon [270].
Under 500 µM Cd stress, Microsorum pteropus fern had a higher ability to hyperaccumulate Cd (up to 4000 mg kg −1 ) in its root and leaf dry mass. Proteomic analysis of fern leaves and roots through MALDI-TOF revealed eight proteins majorly involved in energy metabolism. It enhanced the antioxidant activity to reduce damage from Cd, whereas 20 proteins differentially expressed in leaves were mainly involved in the regulation of photosynthesis and cellular metabolism [271].

Metabolomics
Different analytical approaches have been developed to understand the plant metabolic response, including nuclear magnetic resonance spectroscopy (NMR), gas and liquid chromatography (GC and LC), and inductively joined mass spectrometry (Table 6). Amino acids, phenols, carotenoids, α-tocopherol, and glutathione are major metabolites whose synthesis varies under metal stress. Cd stress caused a vital change on the metabolism of amino acids, sugar, and organic acids [278,279]. Siriporansdulsil et al. [280] reported an increased level of proline under Cd stress in microalgae whereas an increased level of α-tocopherol was observed in A. thaliana under Cd stress [281]. Metabolic pathways and metabolite enhance Cd tolerance and absorption by plants by promoting PCs [282].
LC-MS/MS and HPLC analysis for metabolites and thiol compounds in Amaranthus hypochondriacus revealed that the plant accumulates 40 times more Cd inside leaves as compared to the control under Cd stress. Among 41 SDMs, 12 were significantly related to PCs in 3 metabolic pathways as Lue, Val, and Ile biosynthesis; Asp, Ala, and Glu metabolism; and Pro and Arg metabolism [282]. Navarro-Reig et al. [283] identified 112 metabolites in rice through LC-MS (HPLC joined with Q-Exactive mass spectrometer). Mengdi et al. [284] identified nine metabolic pathways in A. hypochondriacus (K472) through LC-MS analysis. The intermediate vegetative stage had the highest Cd tolerance ability, which expressed 29 significantly different metabolites involved in 4 metabolic processes, including purine metabolism; Gly, Thr, and Ser metabolism; Asp, Ala, and Glu metabolism; and Arg and Pro metabolism. Further, 100 µM Cd caused an increase in asparagine, tyrosine, and α-tocopherol levels in the leaves of a tomato plant but overall causes severe metabolic and physiological deformities in the plant whereas tomato could bear the exposure to 20 µM Cd adequately [285]. In radish, GC-MS analysis revealed that Cd stress caused an alteration in amino acid metabolism, energy production, and oxidative phosphorylation pathways [278].

Genetic Engineering for Cadmium Phytoremediation
A series of studies addressed the issue of remediating Cd through genetically engineered plants (Table 7) as it is known that metals with similar chemical and physical properties biologically antagonize each other [288]. Interestingly, there is a high resemblance between the ionic hydrated radius of Fe 2+ (4.28 a.m.) and Cd 2+ (4.26 a.m.) [248]. In line with this, a bts-1 lack-of-function mutant in Arabidopsis displayed a higher Cd accumulation capacity in its roots and shoots in comparison to wild-type plants. The enhanced Cd accumulation in the roots and shoots of the bts-1 mutant was because of the positive regulation of Fe nutrition via the upregulation of Fe-related genes [248]. In another study, the detoxification and accumulation of Cd in rice shoots was investigated. The CAL2 gene, which is located in the cell wall, has shown a good Cd chelation ability. To confirm this, ectopic overexpression of CAL2 in Arabidopsis and rice strongly induced their Cd accumulation capacity without affecting the uptake of other essential nutrients [289]. However, the CAL2 gene-overexpressing transgenic lines showed sensitivity to Cd stress. The reason could be the high abundance of accumulated Cd in the shoots of transgenic plants [289]. The Arabidopsis AtPDF2.5 was triggered when the plant was subjected to Cd stress. The results showed the responsive nature of AtPDF2.5 to Cd [35]. The AtPDF2.5 possessed eight cysteine residues, which are involved in the tolerance and chelation of Cd. To validate its role, the AtPDF2.5 gene-overexpressed plants were generated, which displayed tolerance to Cd and also enhanced the Cd accumulation in shoots. Further, AtPDF2.5 facilitates Cd efflux in the cytoplasm via its chelation activity [35]. Similarly, the Arabidopsis AtPDF2.6 possessed Cd chelation properties and were also induced significantly under Cd treatment [290]. Taken together, these lines of evidence affirmed the importance of genetic engineering in the production of Cd-tolerant crops, which could also be helpful in the phytoremediation of soil. Table 7. Role of genetically modified plants in the detoxification and phytoremediation of Cd in soil.

Plant Species Genes Phytoremediation Activity References
Indian mustard γ-glutamylcysteine synthetase (γ-ECS) Higher phytochelatins production directly enhanced the Cd phytoremediation capacity of transgenic plants [291] Oryza sativa OsHMA3 Keep the Cd at roots via sequestrating into root vacuoles [148] Solanum lycopersicum FER FER lack of function mutant indirectly impaired the Cd translocation from root to aerial parts [292] Arabidopsis NRT1.1 Controlled the uptake of Cd in roots [293] Arabidopsis AtHMA4 Better root to shoot translocation of Cd [294] Arabidopsis AtBCC3 Facilitated the Cd phytoremediation in Arabidopsis by enhancing the chelation properties [145] Oryza sativa OsHMA3 Loss-of-function enhanced root-shoot Cd translocation [295] Arabidopsis AtFC1 Increased in the accumulation of Cd was observed [296] Arabidopsis AtPDF2.5, AtPDF2.6 Improved the Cd tolerance accumulation in shoot [35,290] Arabidopsis CAL2 Hastened the Cd accumulation in shoots [289] Arabidopsis BTS BTS lack of function mutant enhanced the Cd accumulative characteristics of Arabidopsis plants [248]

Employing Microbes for Cadmium Phytoremediation
Transformation of heavy metals from an unavailable to available form is an important factor that decides the fate of phytoremediation. Numerous microbes have been reported to initiate the phytoremediation of Cd by fractionating it in the soil and allow the plant to uptake it. Different classes of microbes (bacteria and fungi) play a crucial role in Cd phytoremediation by activating various mechanisms and producing different compounds, such as siderophores, organic acid, polymeric substances, bioaccumulation, and biosorption ( Figure 6). The siderophores are the iron chelators with la ow molecular weight and generally help in the phytoremediation of Cd [297]. Organic acids, which enhance the bioavailability of Cd, are produced by soil microbes. Additionally, organic acid influences the soil pH level by keeping it low, which facilitate the phytoextraction process and is thus crucial for the phytoremediation of Cd [298]. The secretion of extracellular polymeric compounds (exopolysaccharides, mucopolysaccharides, and glomalin) reduces the mobility and bioavailability of Cd, which makes it an essential element in the Cd phytoremediation process [299]. Biosorption and bioaccumulation refer to the processes of metal absorption and accumulation by plants with the help of soil microbes [300]. The biosorption and bioaccumulation contribute largely to Cd phytoremediation by phytostabilizing Cd at the root zone. Below, we discuss the role of bacteria and fungi in promoting phytoremediation of Cd in soil.

Employing Microbes for Cadmium Phytoremediation
Transformation of heavy metals from an unavailable to available form is an important factor that decides the fate of phytoremediation. Numerous microbes have been reported to initiate the phytoremediation of Cd by fractionating it in the soil and allow the plant to uptake it. Different classes of microbes (bacteria and fungi) play a crucial role in Cd phytoremediation by activating various mechanisms and producing different compounds, such as siderophores, organic acid, polymeric substances, bioaccumulation, and biosorption ( Figure 6). The siderophores are the iron chelators with la ow molecular weight and generally help in the phytoremediation of Cd [297]. Organic acids, which enhance the bioavailability of Cd, are produced by soil microbes. Additionally, organic acid influences the soil pH level by keeping it low, which facilitate the phytoextraction process and is thus crucial for the phytoremediation of Cd [298]. The secretion of extracellular polymeric compounds (exopolysaccharides, mucopolysaccharides, and glomalin) reduces the mobility and bioavailability of Cd, which makes it an essential element in the Cd phytoremediation process [299]. Biosorption and bioaccumulation refer to the processes of metal absorption and accumulation by plants with the help of soil microbes [300]. The biosorption and bioaccumulation contribute largely to Cd phytoremediation by phytostabilizing Cd at the root zone. Below, we discuss the role of bacteria and fungi in promoting phytoremediation of Cd in soil. Figure 6. Schematic illustration of Cd phytoremediation by microbes. In the phytoextraction process, the production of siderophores and organic acids by the soil microbes influences the phytoextraction capacity of the accumulator in a positive manner. The generation of polymeric substances by soil microbes keeps the Cd metal in a static form, which facilitates the biosorption and bioaccumulation process.

Role of Arbuscular Mycorrhizal Fungi
The arbuscular mycorrhizal fungi (AMF) is a portent regulator of plant growth under various stress conditions, including heavy metals. In general, the AMF makes a symbiotic association with the host plant by increasing the availability of solubilized P [301]. According to previous research, Figure 6. Schematic illustration of Cd phytoremediation by microbes. In the phytoextraction process, the production of siderophores and organic acids by the soil microbes influences the phytoextraction capacity of the accumulator in a positive manner. The generation of polymeric substances by soil microbes keeps the Cd metal in a static form, which facilitates the biosorption and bioaccumulation process.

Role of Arbuscular Mycorrhizal Fungi
The arbuscular mycorrhizal fungi (AMF) is a portent regulator of plant growth under various stress conditions, including heavy metals. In general, the AMF makes a symbiotic association with the host plant by increasing the availability of solubilized P [301]. According to previous research, the uptake of Cd largely depends on the amount of accumulation of P [301]. This suggests the critical role of phytoremediation of Cd by AMF. For instance, the application of AMF in Cd-polluted soil significantly enhanced Cd accumulation and phytoavailability in the root and shoot tissues by lowering the soil pH and chemical fractions [302]. Additionally, it increased the tolerance of Solanum nigrum to Cd stress [302]. Eichhornia crassipes is considered as a metal hyperaccumulator and plays an important role in soil phytoremediation. The combination of E. crassipes with AMF reduced the Cd concentration in soil to a large extent. Additionally, the inoculation of AMF with E. crassipes substantially induced Cd accumulation and translocation in the roots and shoots [303]. In Table 8, we list studies featuring the role of AMF and other strains of fungi in Cd phytoremediation.
Therefore, this phytoremediation strategy can be applied to minimize the Cd toxicity in soil and water [303]. The growth of Cassia italica Mill has been restricted by the increased amount of Cd in soil [304]. The inoculation of the AMF to Cassia italica Mill medicinal plant significantly hastened the tolerance to Cd by limiting its translocation to shoots. Meanwhile, a higher amount of Cd uptake was observed in the roots of the C. italica Mill plant inoculated with AMF [304]. Tomato fruits are mainly consumed throughout the world; however, the Cd toxicity severely affected the growth and yield of it. On the other hand, the accumulation of high amounts of Cd in tomato fruit could be extremely harmful to human health [305]. The inoculation of AMF in tomato plants reduced the Cd toxicity and prevented the translocation of Cd from the roots to the aerial parts of the plants [305]. The application of Cd could be a useful strategy to protect hazardous environmental changes, such as the removal of Cd from the soil, and reduced the usage of inorganic fertilizer. Table 8. Enlisted studies of arbuscular mycorrhizal fungi (AMF)/fungi and its role in Cd phytoremediation.

Salix viminalis Glomus intraradices
Increased Cd phytoextraction and retained Cd in roots. root and restricted its translocation to shoots [306] Solanum nigrum Paecilomyces lilacinus Improved phytoextraction capacity along with enhanced antioxidant systems [307] Tagetes erecta

Glomus intraradices, Glomus constrictum and
Glomus mosseae Better phytoextraction of Cd from the soil and also restrict the translocation of Cd from roots to shoots [308] Lpomoea aquatica Not specified Increased the accumulation of Cd along with induced antioxidant enzymes activity and nutrient uptake [309] Cajanus cajan Glomus mosseae Amended the generation of phytochelatins and uptake of Cd by roots without accumulating a higher amount of toxic ions [310] Helianthus

Host Plants AMF/Fungi Species Specific Function References
Eichhornia crassipes Not specified Enhanced the Cd accumulation capacity of water hyacinth in Cd contaminated soil and water [303] Solanum nigrum Not specified Boosted the phytoavailability of Cd by lowering the soil pH and altered Cd chemical fractions. [302]

Phragmites communis Simplicillium chinense
Enhanced the Cd phytoremediation by triggering the biosorption process in the host plant [315]

Role of Plant Growth-Promoting Bacteria (PGRB) in the Phytoremediation of Cd
A plethora of studies highlighted the role of PGRB in the phytoremediation of toxic metals, including Cd (Table 9). Among the group of PGRB, some of them detoxify and break down heavy metals by releasing various essential binding compounds, such as organic acid, siderophores, exopolymers, and biosurfactants, which makes the metals available to the plants ( Figure 6) [316]. For example, Arthrobacter inoculated to Ocimum gratissimum helps in the removal of Cd from the soil by inducing the uptake of Cd through roots [317]. Similarly, the Arthrobacter sp. was inoculated to the Glycine max under Cd-contaminated soil. The application of Arthrobacter sp. remarkably enhanced the bioavailability of Cd to the plant. Additionally, increased accumulation of Cd in the roots was also observed in the Arthrobacter sp.-inoculated plants [318]. The Bradyrhizobium sp. is a Cd-tolerant PGRB that reduces Cd toxicity and improves agronomic traits of plants grown in Cd-contaminated soil [319]. The inoculation of Bradyrhizobium sp. over Lolium multiflorum triggered the uptake of Cd from the soil and increased the biomass. The induction in Cd accumulation was also observed in the shoots of Lolium multiflorum [319]. The Mesorhizobium huakuii subsp. rengei B3 was inoculated to an Astragalus sinicus plant. The results showed a 19-fold increase in the cell-mediated Cd +2 binding capacity [320]. In rapeseed, the Arthrobacter sp. SrN1 and Bacillus altitudinis SrN9 was applied to alleviate the deleterious effects of Cd. The Arthrobacter sp. SrN1 and Bacillus altitudinis SrN9 inoculation not only boosted the resistance of rapeseed to Cd stress but also increased the uptake and translocation of Cd [321]. This indicates the potential role of Arthrobacter sp. SrN1 and Bacillus altitudinis SrN9 in Cd phytoremediation without altering the plant productivity. Likewise, enhanced Cd uptake was observed in the roots of the Sedum plumbizincicolaa plant after being inoculated with the bacterial strain Rhodococcus erythropolis NSX2 [322]. The application of PGRB to soil could be a useful, cost-effective, and environmentally friendly strategy of Cd phytoremediation. However, more work is required to understand and explore the different strains of microbes involved in the phytoremediation of Cd without hindering plant growth. Table 9. List of studies featuring plant growth-promoting bacteria (PGRB) role in Cd phytoremediation.

Host Plants PGRB Species Specific Functions References
Solanum nigrum Serratia nematodiphila LRE07 Serratia nematodiphila LRE07 inoculated plants induced the phytoaccumulation ability of the host plant resulted in to 70% more uptake of Cd [323] Sorghum bicolor Bacillus sp. SLS18 Augmented phytoextraction and phytostabilization of Cd at root was observed in the inoculated plants resulted in 65% more uptake of Cd from the contaminated soil [323] Brassica napus Burkholderia sp. J62 and Pseudomonas thivervalensis Y-1-3-9 The inoculated plants showed high level of Cd accumulated in their shots than the control plants [324]  Ocimum ratissimum Arthrobacter sp.
Cd resistant bacteria along with host plant enhanced the phytoextraction process without altering the quality of grain [317] Eruca sativa Pseudomonas putida ATCC 39,213 Better phytoextraction resulted in increased Cd uptake (29%) in the inoculated plants than the control [326] Salix dasyclados Streptomyces sp.
Generation of Siderophores molecules triggered the phytoextraction capacity of the host plant [327] Sedum plumbizincicolaa Endophytic bacterium E6S Production of IAA, ACC and organic acid maintained the pH at low level resulted in better phytoextraction of Cd [328] Vetiveria zizanioides Bacillus cereus Induced production of phosphate and Siderophores which helps in the phytoremediation of Cd [329]

Conclusions and Future Perspectives
Phytoremediation of Cd provides a way forward for the restoration of the polluted environment and has provided many positive and desirable results. Through an extensive literature review, it is evident that Cd interferes with plant functions and as an external stimulus; it activates the defense mechanism through various physiological and metabolic pathways. The multiple phytoremediation strategies offer cost-efficient optimal prospects for the in-situ remediation of Cd in a most environment-friendly way. For a successful rehabilitation, it is essential to utilize the prominent physiological features of Cd hyperaccumulators for the extraction, transformation, and/or stabilization of Cd. Meanwhile, it is important to evaluate the effectiveness of phytoremediation technologies and integrate the available resources. This will not only facilitate the phytoextraction process but also boost the plant productivity in areas with suboptimal soil metal levels by utilizing multi-omics approaches, microbes' potential, amendments (like AMF and PGPBs), and techniques, such as genetic engineering. Furthermore, the phytoremediation potential and improved tolerance to Cd could be counted as a first step towards leveraging the accumulation potential of plant species. On the other hand, it is equally important to consider the antagonistic and synergistic behavior of contaminants for remediation potential and ensure an ecologically responsible alternative for further use and/or processing of plants, which must be done under a strictly controlled environment.
Notably, the phytoremediation tool is still under the examination and progress phase, and numerous technical barriers need to be resolved. The multifaceted connections that occur under site-specific environments demand a multi-disciplinary approach for metal phytoextraction. This accomplishment will eventually depend upon utilization of a complete analytical tool to integrate the works of plant scientists, soil microbiologists, agronomists, and environmental engineers. Nevertheless, phytoremediation promises to be a vital waste managing choice for the present century.
Moreover, research is required to understand the molecular mechanism of hyperaccumulators in field trials at different locations, as a majority of the current research is based on lab studies. The molecular mechanism of soil amendment-mediated phytoremediation is also unclear. Therefore, omics approaches could be employed to elaborate on how AMF and PGPBs induce the phytoremediation of Cd by regulating numerous genetic, metabolic, and hormonal pathways. An exploration of different genetic pathways will not only enhance the phytoremediation capacity of food crops but also improve their productivity in Cd-contaminated soil.