The Uptake, Transfer, and Detoxification of Cadmium in Plants and Its Exogenous Effects

Cadmium (Cd) exerts a toxic influence on numerous crucial growth and development processes in plants, notably affecting seed germination rate, transpiration rate, chlorophyll content, and biomass. While considerable advances in Cd uptake and detoxification of plants have been made, the mechanisms by which plants adapt to and tolerate Cd toxicity remain elusive. This review focuses on the relationship between Cd and plants and the prospects for phytoremediation of Cd pollution. We highlight the following issues: (1) the present state of Cd pollution and its associated hazards, encompassing the sources and distribution of Cd and the risks posed to human health; (2) the mechanisms underlying the uptake and transport of Cd, including the physiological processes associated with the uptake, translocation, and detoxification of Cd, as well as the pertinent gene families implicated in these processes; (3) the detrimental effects of Cd on plants and the mechanisms of detoxification, such as the activation of resistance genes, root chelation, vacuolar compartmentalization, the activation of antioxidant systems and the generation of non-enzymatic antioxidants; (4) the practical application of phytoremediation and the impact of incorporating exogenous substances on the Cd tolerance of plants.


Introduction
It is well known that the rapid evolution of modernized industrial and agricultural practices has led to various environmental challenges.One of the prominent problems is cadmium (Cd) pollution.Between 1950 and 1990, global cadmium production doubled, reaching approximately 20,000 tons annually.Anthropogenic sources deposited an estimated 900 to 3600 tons of Cd into aquatic environments [1], resulting in severe Cd pollution.This pollution poses a significant threat to both human health and environmental safety.
Cd pollution is a global concern, evident in various regions.In the suburbs of Dera Ismail Khan, Pakistan, vegetables cultivated using wastewater irrigation exhibited significantly higher levels of Cd accumulation compared to those grown with freshwater irrigation [2].In southern China, tobacco-rice rotation causes soil pH to decrease, thereby enhancing the flow of cadmium to crops [3].Notably, the transfer of cadmium from the soil to the human body through crops such as vegetables and rice can lead to a variety of health problems, such as central nervous system depression and kidney and liver damage [4].In the last century, many people in Japan suffered immensely from Itai-itai disease due to cadmium contamination of farmland and water sources [4].
To cope with these formidable challenges, numerous scientists have dedicated their efforts to environmental science research, trying to explore strategies for mitigating Cd pollution through a variety of pathways, including physical, chemical, and phytoremediation.Based on previous studies, this review will focus on the mechanisms of Cd transport by plants, the forms of Cd toxicity suffered/coped by plants, and the effects of exogenous modes on phytoremediation.

The Pollution Status and Hazards of Cadmium
Cd is a rare dispersed element commonly found in soils and zinc (Zn) minerals in the form of divalent cations (Cd 2+ ) and soluble complexes with a variety of toxic effects.Cd has an extremely long biological half-life, predominantly accumulating in the liver and kidneys of the human body where it is difficult to eliminate [5].

Sources and Distribution of Cadmium
Cd levels vary in different countries around the world due to geographic location, latitude and longitude, and environmental climate.Cd levels in soils are currently higher than the original environmental background values in all countries as a result of atmospheric deposition and overuse of phosphate fertilizers.For example, the average concentration of Cd in European soils is 0.33 mg kg −1 [6], while in agricultural land in China, it is 0.19 mg kg −1 , with an environmental background value of 0.097 mg kg −1 [7].In the United States agricultural soils, the average Cd concentration is 0.265 mg kg −1 [8].
Soil Cd comes from a wide range of sources and can be categorized into two main sources, including natural and anthropogenic sources.The presence of Cd in natural soils mostly originates from rock weathering and suspended soil particles transported through the air.Soil particle sources encompass various natural occurrences such as forest fires, volcanic emissions, and atmospheric dust [9].In contrast, anthropogenic Cd emissions predominantly originate from activities like phosphate fertilizer application, tailings disposal, metal industry practices, mining operations, and fossil fuel combustion [10,11].Since the Industrial Revolution, diverse industrial, mining, and agricultural activities worldwide have led to substantial heavy metal diffusion into soils and water bodies (Figure 1).By the beginning of the 21st century, global anthropogenic Cd production during the industrial era had accumulated to 1.1 million tons, with the global per capita burden estimated at 0.18 kg [12].

Hazards of Cadmium on the Human Body
With the escalation of mining activities and metal smelting, there has been a corresponding increase in Cd levels found in soil surfaces, air, and water sources.This raises a significant hazard to the health of animals, plants, and human beings.Cd in animals and humans mainly comes from drinking water, eating, and respiration, and a very small part is absorbed through the skin and hair.When Cd enters the body through the gastrointestinal or respiratory tract, it is transported into the bloodstream through erythrocytes and albumin, and then accumulates in the kidneys and liver [5].Of note, the biological half-life of Cd in the kidney is 45 years [13].
To minimize the potential harm caused by exposure to or inhalation of Cd, individuals should consume foods that are rich in polyphenols [14], such as mint and strawberries.These foods possess antioxidant properties and can aid in chelating Cd 2+ .Additionally, incorporating more seafood, legume products, melon seeds, and other foods with high Zn content into one's diet can help counteract the excessive accumulation of Cd in the body.Collectively, these dietary measures operate through distinct mechanisms to support overall physical well-being.
Figure 1.Sources of Cd and its transmission pathways in the environment.This figure shows the process of Cd deposition, accumulation, cycling in the environment, and enrichment in plants, animals, and humans through the food chain.Cd exists in different chemical forms in each pathway, such as in water bodies, where it is mainly in the form of chloride [15], in coal combustion and dust formed in non-ferrous metal production, where it mainly contains CdS and CdSO4 [16], and in phosphorus fertilizers in the form of Cd(H2PO4)2 and CdHPO4 [17].

Forms and Bioavailability of Cadmium in Soils
The chemical forms of Cd in different soils vary depending on conditions such as soil redox potential, moisture, texture, and inter-root environment.According to the five-step sequential extraction method proposed by Tessier et al. (1979), Cd in soils can be grouped into five forms: soluble and exchangeable state (Cd 2+ ), carbonate-bound state (CdCO3), ferromanganese-oxidized state, organic-bound state, and residue [18], with soluble and exchangeable states dominating.
Not all Cd in soils can be absorbed by plants.Plants mainly take up the exchangeable and carbonate-bound states of Cd.However, the bioavailability of Cd can also be influenced by the soil's properties.Among them, soil acidity and alkalinity will significantly affect the Cd solubility and morphological distribution in soils, and increasing soil pH is negatively correlated with the effectiveness of heavy metals in plants [19,20].The main reason for the increased Cd pollution in recent years is soil acidification.In addition, soil properties are also a key factor influencing Cd morphology and utilization, and plants in sandy soils are more capable of Cd uptake compared with clay soils [21].It is worth mentioning that the chelating agents such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) can increase the effectiveness of Cd in soils, facilitate its movement from roots to aboveground tissues, and increase Cd accumulation in leaves [22].In addition to Cd bioavailability, Cd uptake and accumulation in plants are also influenced by other factors, such as plant species, genotype, inter-root environment, and mineral nutrients such as silicon (Si), selenium (Se), and iron (Fe) [23][24][25][26].

Mechanism of Cadmium Uptake by Plants
Cd uptake and accumulation in plants mainly consist of the following physiological processes: uptake of Cd from the outside (soil or air), lateral and radial transport of Cd, and phloem-mediated aboveground redistribution (Figure 2).[15], in coal combustion and dust formed in non-ferrous metal production, where it mainly contains CdS and CdSO 4 [16], and in phosphorus fertilizers in the form of Cd(H 2 PO 4 ) 2 and CdHPO 4 [17].

Forms and Bioavailability of Cadmium in Soils
The chemical forms of Cd in different soils vary depending on conditions such as soil redox potential, moisture, texture, and inter-root environment.According to the five-step sequential extraction method proposed by Tessier et al. (1979), Cd in soils can be grouped into five forms: soluble and exchangeable state (Cd 2+ ), carbonate-bound state (CdCO 3 ), ferromanganese-oxidized state, organic-bound state, and residue [18], with soluble and exchangeable states dominating.
Not all Cd in soils can be absorbed by plants.Plants mainly take up the exchangeable and carbonate-bound states of Cd.However, the bioavailability of Cd can also be influenced by the soil's properties.Among them, soil acidity and alkalinity will significantly affect the Cd solubility and morphological distribution in soils, and increasing soil pH is negatively correlated with the effectiveness of heavy metals in plants [19,20].The main reason for the increased Cd pollution in recent years is soil acidification.In addition, soil properties are also a key factor influencing Cd morphology and utilization, and plants in sandy soils are more capable of Cd uptake compared with clay soils [21].It is worth mentioning that the chelating agents such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) can increase the effectiveness of Cd in soils, facilitate its movement from roots to aboveground tissues, and increase Cd accumulation in leaves [22].In addition to Cd bioavailability, Cd uptake and accumulation in plants are also influenced by other factors, such as plant species, genotype, inter-root environment, and mineral nutrients such as silicon (Si), selenium (Se), and iron (Fe) [23][24][25][26].

Mechanism of Cadmium Uptake by Plants
Cd uptake and accumulation in plants mainly consist of the following physiological processes: uptake of Cd from the outside (soil or air), lateral and radial transport of Cd, and phloem-mediated aboveground redistribution (Figure 2).

Pathways of Cadmium Transport from Soil to Root Epidermis
The root is the first organ exposed to Cd in soils and the first barrier for plants to resist Cd toxicity.Plants absorb Cd from the soil through root hairs and epidermal cells in the mature zone of the root tip.
The entry of Cd into roots mainly includes the following pathways: 1. Cd 2+ is exchanged with H + produced during plant respiration and is thus adsorbed on the surface of root epidermal cells and then enters the cortex via the apoplast pathway [27].2. As Cd is a non-essential metallic element lacking a specific transporter in plants, its entry into plant tissues typically occurs through the symplast pathway.This process involves competing for binding sites on metal transporter proteins, including IRT1 (a bivalent iron transporter protein) and NRAMP5 (a manganese transporter protein) [28].Additionally, Cd can also enter plants through ion channels of divalent metals such as calcium (Ca).3. To enhance ion utilization within the inter-root soil, certain low molecular compounds, such as erucic acid and oxalic acid, are secreted by plant roots.These compounds form metal-ligand complexes with Cd 2+ , thus facilitating Cd to enter the root epidermis as a chelate transported by yellow-stripe-Like (YSL) proteins [29].
Although Cd 2+ binds to both organic ligands (such as dissolved organic substances like low-molecular-weight organic acids secreted by plant roots) and inorganic ligands

Pathways of Cadmium Transport from Soil to Root Epidermis
The root is the first organ exposed to Cd in soils and the first barrier for plants to resist Cd toxicity.Plants absorb Cd from the soil through root hairs and epidermal cells in the mature zone of the root tip.
The entry of Cd into roots mainly includes the following pathways: 1. Cd 2+ is exchanged with H + produced during plant respiration and is thus adsorbed on the surface of root epidermal cells and then enters the cortex via the apoplast pathway [27].

2.
As Cd is a non-essential metallic element lacking a specific transporter in plants, its entry into plant tissues typically occurs through the symplast pathway.This process involves competing for binding sites on metal transporter proteins, including IRT1 (a bivalent iron transporter protein) and NRAMP5 (a manganese transporter protein) [28].Additionally, Cd can also enter plants through ion channels of divalent metals such as calcium (Ca).3.
To enhance ion utilization within the inter-root soil, certain low molecular compounds, such as erucic acid and oxalic acid, are secreted by plant roots.These compounds form metal-ligand complexes with Cd 2+ , thus facilitating Cd to enter the root epidermis as a chelate transported by yellow-stripe-Like (YSL) proteins [29].
Although Cd 2+ binds to both organic ligands (such as dissolved organic substances like low-molecular-weight organic acids secreted by plant roots) and inorganic ligands (e.g., NO 3− , Cl − ) to form soluble complexes, subsequently enhancing the mobility of Cd to the root surface, it is widely accepted that Cd 2+ is the primary form of translocation through the plasma membrane into the root tip cells [30].

Lateral Transport of Cadmium from Root Epidermis to Xylem
Cd is synergistically transported through both the symplast and the apoplast pathways after entering the epidermal cells of the root hair zone (Figure 2).
In the symplast pathway, Cd is mainly transported by intracellular protoplasmic flow and plasmodesmata channels between cells, first across the endodermis into the stele, then through the pericycle sheaths into the parenchyma cell, and ultimately into the xylem conductance ducts [31].In the apoplast pathway, Cd enters the xylem along the apoplast space such as the cell wall, cell interstitial space, intercellular layer, and conduit cavity.This pathway is impeded by casparian strips through the endodermis.Additionally, a fraction of the Cd 2+ is sequestered into vacuoles through transporter proteins like OsHMA3 in rice [32], redirected to the symplast pathway for Cd transportation to the xylem of the stele [33].
Cd transport from the epidermis to the xylem completes within the root, and the entire mechanism is mediated by transporter proteins in the plasma membrane, which directs the ions to specific locations [34].

Radial Transport of Cadmium through the Xylem to Aboveground Parts
Upon Cd entering into roots, the xylem pathway subsequently plays a dominant role in the long-distance transport of Cd from roots to aboveground (Figure 2), with the main driving forces being transpiration pull and root pressure [35].
It is known that Cd accumulates mainly in the roots of plants and is partially translocated from roots to aboveground parts.The efficiency of translocation depends on root vacuole sequestration and xylem loading capacity [30].The more vacuole sequestration in the root, the less Cd 2+ is transferred to the aboveground part.As documented, the transporter proteins CAX2, CAX4, and HMA3 play key roles in chelating Cd to the vacuole [32,36,37].In addition, the lignified casparian strips in the endodermis of plant roots serve as a barrier, impeding the entry of Cd into the shoots.This may account for the higher accumulation of Cd in the roots in comparison to the shoots [38].

Phloem Mediates Cadmium Redistribution
The phloem is mainly responsible for the redistribution of Cd in the aboveground part based on studies in Arabidopsis thaliana [39] and rice [40].
Cd absorbed by plant roots is mainly translocated to the aboveground part through the xylem and then to the rice grain through the phloem [41], suggesting that most of the Cd needs to be translocated from the xylem to the phloem before it can be re-translocated to the grain.In recent years, new insights into the process of Cd translocation from roots to stems have been gained, suggesting that past studies have overlooked the value of long-distance transport in the phloem [42].It was found that the phloem is an important pathway for Cd long-distance transport from root to leaf in eggplant and oilseed rape [35,43], and that Cd-GS 2 complexes contribute to Cd long-distance movement in the phloem [42].

Cadmium-Related Transporter Proteins in Plants
Cd is one of the non-essential metal ions; the uptake of Cd by plants from soils must be mediated by transporter proteins for essential cations [44,45].As shown in Figure 3 and Table 1, the following protein families are commonly involved in Cd transport or detoxification.

The Natural Resistance-Associated Macrophage Protein Family
The natural resistance-associated macrophage protein (NRAMP) family has been identified in model plants such as Arabidopsis thaliana and rice.As revealed, NRAMP plays a key role in metal homeostasis.

The Natural Resistance-Associated Macrophage Protein Family
The natural resistance-associated macrophage protein (NRAMP) family has been identified in model plants such as Arabidopsis thaliana and rice.As revealed, NRAMP plays a key role in metal homeostasis.
A total of seven NRAMP family members in rice have been functionally characterized.NRAMP1, NRAMP2, and NRAMP5 are involved in Cd transport [46].OsNRAMP2 is localized to the tonoplast and is primarily expressed in seeds, roots, and leaves [47].Knockout of OsNRAMP2 resulted in a reduction in Cd translocation from vegetative tissues to rice grains; conversely, overexpression lines of OsNRAMP2 exhibited a significant increase in grain Cd concentrations [48,49].OsNRAMP5 is localized in the plasma membrane and expressed in rice roots [50].OsNRAMP5 serves as the primary transporter protein for manganese (Mn) and Cd, facilitating their translocation from soil solution to root cells.The knockout lines of OsNRAMP5 have a substantial reduction in Cd in rice, and overexpressing OsNRAMP5 increases Cd uptake in roots, but Cd levels in shoots remain at low levels [51,52].OsNRAMP1 is localized in root and leaf cells and is a close homolog to OsNRAMP5 (72.78% amino acid sequence similarity) with similar but non-redundant functions [53][54][55].Knockout of OsNRAMP1 or OsNRAMP5 results in reduced levels of Cd in rice.Moreover, the loss of function of OsNRAMP5 has a greater impact than OsNRAMP1.Further studies have revealed that double knockout mutants of these two genes have a significant reduction in Cd [53].This indicates that gene editing of OsNRAMP1 or/and OsNRAMP5 can help in the breeding of low-Cd varieties of rice, especially by altering their expression levels via editing the promoter sequence in the future.It must be pointed out that the uptake of Mn is also impaired by the knockout of these two genes.As Mn is one of the essential elements for plant growth and development, how to balance the relationship between the two elements is an important goal for future investigations.

The Zinc/Iron-Regulated Transporter-like Protein Family
Lots of studies have demonstrated that the zinc-iron-regulated transport proteins (ZIPs) family plays an important role in metal uptake in roots and distribution in plants.
To date, 15 ZIP family members have been reported in Arabidopsis thaliana [56].AtIRT1, the first identified ZIP family member in Arabidopsis thaliana, is mainly expressed in roots and plays a key role in the uptake of divalent iron from the soil [57,58].Moreover, AtIRT1 is involved in the transport of divalent cations such as Zn, Mn, cobalt (Co), and Cd [58][59][60][61].Knockout of AtIRT1 results in lower accumulation of the heavy metal Cd [60].Although AtIRT2 shares phylogenetic similarity with AtIRT1, it is not directly responsible for Fe uptake from soils.Instead, AtIRT2 collaborates with AtIRT1 to maintain Fe homeostasis in plants [62,63].Overexpression of AtIRT2 in Arabidopsis thaliana enhances the uptake of metals such as Fe, Cd, and Zn.However, the sensitivity of yeast cells to Cd remains unaffected when overexpressing AtIRT2 [62], possibly due to the indirect synergistic interaction between AtIRT2 and AtIRT1 in response to Cd stress.AtZIP1 is a vacuole transporter that transfers Mn [64].AtZIP2 participates in the uptake of Mn and Zn [64].Interestingly, AtZIP1, AtZIP3, and AtZIP4 respond to Zn deficiency [65].In addition, AtZIP2 and AtZIP4 are also involved in copper (Cu) transport [66].In short, ZIP family members have versatile roles in Mn, Zn, and Cd transportation.
There are 16 ZIP family members in rice [67].OsIRT1 and OsIRT2 are mainly responsible for the uptake of Zn and Fe in the rice root system [68] and also play a role in Cd uptake because of the similar physicochemical properties of Zn 2+ , Fe 2+ , and Cd 2+ [69,70].OsZIP1, localized in the endoplasmic reticulum and plasma membrane, is a metal detoxification transporter.Overexpressing OsZIP1 can reduce the overaccumulation of Zn, Cu, and Cd in rice and promote rice growth [71,72].OsZIP3 is preferred for Zn uptake over other divalent cations such as Cd.OsZIP3 co-regulates Cd transport and uptake together with OsHMA2 and OsLCT1 [73,74].Both OsZIP5 and OsZIP9 are redundantly involved in Zn and Cd uptake.OsZIP9 is responsible for the broad regulation of Zn in roots and shoots, and OsZIP5 fine-tunes Zn uptake to maintain Zn homeostasis.Accordingly, rice with a single or both genes knocked out exhibits reduced uptake of Zn and Cd, whereas overexpression of OsZIP5 or OsZIP9 has the opposite effect [75].OsZIP6 demonstrates transport activity for Fe 2+ , Cd 2+ , and Co 2+ , exhibiting the highest efficiency under acidic environmental conditions [76].OsZIP7 is expressed in parenchyma cells of vascular bundles in rice roots and nodes and is involved in the transport of Zn and Cd.In line with this, the Cd levels in the roots and internode of knockout lines of OsZIP7 are higher [77].
Different species have different expression patterns in Cd stress, Arabidopsis thaliana up-regulates ZIP family genes in the roots, and rice mainly up-regulates aboveground ZIP family genes.It has been verified that AtIRT1, OsZIP1, and OsZIP3 play more important roles in Cd uptake [78].

The Heavy Metal ATPases
Known as P 1B -ATPase, heavy metal ATPase (HMAs) plays an important role in the translocation or detoxification of heavy metals in plants [79], especially in hyperaccumulators.These transporters are reported to exhibit variations in various aspects, including subcellular localization and metal specificity.When plants are exposed to low concentrations of Cd stress, only a limited number of HMA genes are up-regulated.The majority of transporters are mobilized only in response to elevated concentrations [80].
The rice genome encodes nine heavy metal ATPases.OsHMA1 and OsHMA4 have higher expression levels when stressed with Cd.Generally, OsHMA1 and OsHMA4 play roles in maintaining homeostasis in plants under heavy metal stress [80], but their specific biological functions remain to be explored.OsHMA2 is localized in the plasma membrane of the root stele.OsHMA2 is mainly involved in mediating the xylem loading of Cd and Zn, and the translocation to the shoots [69,81].OsHMA3 is expressed in the vesicular membrane of root cells and is responsible for chelating extra-membranous Cd and Zn to the vacuole to prevent their translocation to the aboveground organs [32,45].OsHMA9 is expressed in root and mesophyll tissue and appears to be responsible for Cd, Cu, Zn, and Pb efflux [82].
Arabidopsis thaliana genome encodes eight HMAs.AtHMA1 is localized in the chloroplast periplasm and has been found to transport not only Cu and Zn [83,84], but also Cd and Ca after heterologous expression in yeast [85].AtHMA2 and AtHMA4 are localized in the plasma membrane and mediate the translocation of Cd from roots to shoots [86][87][88][89].AtHMA3 showed similar properties to OsHMA3 and mediated the vacuole sequestration of Cd in roots.Consistently, overexpression of AtHMA3 results in enhanced tolerance of Arabidopsis thaliana to Zn, Co, Cd, and Pb [90].
In summary, the heavy metal ATPase family can be divided into two groups based on the properties of the metal substrates; the first group is the Zn/Co/Cd/Pb subgroup as exemplified by rice OsHMA1-OsHMA3 and Arabidopsis thaliana AtHMA1-AtHMA4, and the second is the Cu/Ag subgroup, containing rice OsHMA4-OsHMA9, Arabidopsis thaliana AtHMA5-AtHMA8 [91].As the second group of HMA is not related to Cd, we do not introduce them here.

Others
In addition to the above family of transporter proteins, there are many other transporters also involved in the uptake, transport, and efflux of Cd (Table S1).
In rice, the common ones are Cd Accumulation in Leaves 1 (OsCAL1), Plant Cd Resistance 1 (OsPCR1), Low Cd (OsLCD), Low-affinity Cation Transporter genes 1 (OSLCT1), Cd transporter genes 1 (OsCd1) and Hypersensitive Induced Reaction Protein 1 (OsHIR1).OsCAL1 is responsible for chelating Cd and exports it from the cytoplasm to the outside of the cell, thus reducing Cd concentration in the cells [92,93].OsPCR1 is involved in the transport of rice from roots to aerial parts [94].OsLCD is mainly expressed in roots and leaves, and its absence reduces the accumulation of Cd in plants [95].OsLCT1 is localized at the plasma membrane and exhibits Cd efflux activity in yeast [96,97].As reported, knockout lines of OsLCT1 have lower Cd levels in phloem and grains [96,97].OsCd1 belongs to the Major Facilitator Superfamily (MFS) family of transporter proteins.OsCd1 is localized in the plasma membrane of roots.OsCd1 is associated with Cd uptake in roots and contributes to Cd accumulation in rice grains [93,98].Heterogeneous overexpressing OsHIR1 significantly reduces Cd and arsenic (As) accumulation, thus increasing plant tolerance to Cd and As [99].

Cadmium-Related Transcription Factors in Plants
Apart from transporter proteins, transcription factors (TFs) also play crucial roles in mediating plants' responses to Cd stress.It has been reported that TF families such as WRKY, MYB, and NAC play direct or indirect roles in regulating Cd tolerance in plants [191][192][193].This regulation is achieved through the control of Cd-related genes, activation of specific signaling pathways, or interaction with other proteins.
The WRKY family is one of the largest families of TF in plants.A total of seven WRKYs (AtWRKY12, AtWRKY13, AtWRKY18, AtWRKY33, AtWRKY40, AtWRKY45, and AtWRKY60) related to Cd tolerance are currently reported in Arabidopsis thaliana [133,191,[194][195][196][197].In particular, AtWRKY45 facilitated the synthesis of phytochelatins by activating AtPCS1 and AtPCS2, thereby enhancing Cd tolerance in A. thaliana [191].Moreover, there are many other families of transcription factors (Table S2) that play important roles in the alleviation of Cd stress.AtMYB49 regulates Cd accumulation through activation of the iron transport protein IRT1 and the abscisic acid (ABA) signaling pathway [192].AtbHLH38/AtbHLH39 increased the expression of NAS1 and NAS2 and reduced Cd accumulation.Collectively, these TFs alter plant sensitivity and tolerance to Cd in a manner that regulates downstream genes or modulates signaling pathways.
TFs are widely involved in plant growth and development.However, the large number of TF and the complex regulatory networks have led to the fact that fewer Cd-related TFs have been identified.Deciphering the functions and regulatory networks of various types of Cd-related TFs is an important goal, which can provide the basis for solving Cd pollution at the molecular level.

Toxicity of Cadmium to Plants
Excessive Cd usually negatively affects plant growth, development, and proliferative metabolism [198], such as plant biomass accumulation, germination rate, stomatal conductance and transpiration rate (Figure 4).Potential hazards include inhibition of photosynthetic pigment formation, reduction in photosynthetic efficiency, disruption of cellular homeostasis, chromosomal aberrations, damage to mitochondria, disruption of antioxidant mechanisms and metabolic pathways, and disruption of ATP synthesis [14,199].Of note, the proton gradient generated by the electron transport chain and photochemical reactions is the main source of ATP synthase [200].Cd stress inhibits electron transfer during the photoresponse by acting on different sites of the PSI and PSII electron transport chain (oxygen-evolving complex on the electron donor side of PSII and sites such as Q A and Q B on the electron acceptor side of PSII) [201,202], thereby affecting the synthesis of plant ATPase and other physiological processes.In addition, Cd induces the production of reactive oxygen species (ROS), leading to protein oxidation, DNA damage, malondialdehyde (MDA) accumulation, and even damage to cell membranes [203].Apart from polluting the environment and poisoning plants and animals, Cd has been reported to act as a "hormone", i.e., at low concentrations, it activates plant defense mechanisms without causing severe oxidative stress [29,204,205].Currently, there are few studies in this area, which need to be verified in different environments and plants.The toxic effects of Cd are manifested in physiological and ecological aspects, but the degree of its toxic effects is related to Cd concentration and treatment time, plant species, and cultivars [206].For instance, a low Cd concentration of 100 nM affected the growth of sunflower [207], while Sesuvium portulacastrum showed significant cellular damage only after 300 µM CdCl2 treatment [208].Furthermore, certain plants are classified as Cd hyperaccumulators for their ability to accumulate Cd in their aerial tissues in excess of 100 mg Kg −1 , showing high tolerance and uptake capacity [209,210].
Hence, when exploring the mechanism of plants to cope with Cd toxicity, researchers The toxic effects of Cd are manifested in physiological and ecological aspects, but the degree of its toxic effects is related to Cd concentration and treatment time, plant species, and cultivars [206].For instance, a low Cd concentration of 100 nM affected the growth of sunflower [207], while Sesuvium portulacastrum showed significant cellular damage only after 300 µM CdCl 2 treatment [208].Furthermore, certain plants are classified as Cd hyperaccumulators for their ability to accumulate Cd in their aerial tissues in excess of 100 mg Kg −1 , showing high tolerance and uptake capacity [209,210].
Hence, when exploring the mechanism of plants to cope with Cd toxicity, researchers use different concentrations of Cd in hydroponic or soil culture treatments to screen and explore Cd high-tolerant plant species based on physiological data such as phenotype, plant height, root parameters, and photosynthetic efficiency, and changes in antioxidant enzyme activities.

Mechanisms of Plant Response to Cadmium Stress
To cope with the stress of the heavy metal Cd, plants have evolved elegant defense mechanisms (Figure 4).Firstly, when confronting Cd in soils, the plant root system secretes substances such as malic acid and citric acid, which bind to Cd 2+ to prevent their uptake by the root system [211].Secondly, after entering into the roots, Cd binds to polygalacturonic acid and pectin in the cell wall [212,213], thus reducing the amount of Cd in the cytosol.On the other hand, the Casparian strips on the endodermis of the root also prevent Cd from entering the cell [214].Thirdly, chelation and vacuole isolation of Cd is also one of the important detoxification pathways.On the one hand, free Cd 2+ binds to glutathione (GSH), phytochelatins (PCs), and metallothioneins (MTs) to form non-toxic complexes such as Cd-GS 2 , PC-Cd, MT-Cd, and so on [29].On the other hand, plants isolate Cd 2+ and complexes by transporting them from the cytoplasm to the vacuole through transporter proteins, thereby reducing the toxic effects of Cd on plants and enhancing their tolerance to Cd.Finally, plants keep more Cd in their roots by decreasing the long-distance transport of Cd in the xylem from root to shoot, thus mitigating the negative effects of Cd on leaves or productive organs.
When Cd crosses the barrier of plant cells, it will trigger a burst of ROS.Then the activation of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase, (POD) and increased production of non-enzymatic antioxidants tocopherol and flavonoids follow (Figure 3).They are widely present in various organelles and act to eliminate the excessive accumulation of O 2-, H 2 O 2 and malonaldehyde to maintain intracellular environmental homeostasis.In addition, sugars, amino acids, and polyols, as an osmotic pressure regulator in plants, can maintain intracellular balance and improve plant tolerance when plants are subjected to abiotic stresses [215,216], and they can also inhibit the production of oxyradicals, scavenge excess ROS, and mitigate oxidative damage in plants [217].It is noteworthy that salicylic acid (SA), gibberellin (GA), and ABA have been demonstrated to play pivotal roles in mitigating Cd-induced oxidative stress [29,192,218,219].While other hormones appear to exert a regulatory influence, their specific mechanisms remain to be thoroughly explored.
According to previous studies, the above-mentioned approaches can be classified into two different strategies: avoidance and tolerance of Cd.The former is to prevent Cd from entering the cells of plants to protect plants from Cd stress, and the latter is dependent on the plant's own tolerance and mitigation mechanisms to alleviate the negative effects of Cd.The two strategies are complementary to each other and together constitute the plant's defense mechanism against Cd toxicity.

Effect of Exogenous Substances on Phytoremediation of Soil Cadmium Pollution
It is obvious that high levels of heavy metals negatively affect plant growth and development.Under the strong selective pressures exerted by heavy metal in soils, many plants have evolved sophisticated biological mechanisms for resisting, tolerating, or thriving in metal-bearing soils, and are collectively referred to as heavy-metal plants [220].Of these, those that can only survive in contaminated areas are known as obligate metallophytes [221].

Status of Phytoremediation and Its Application
Phytoremediation is a remediation technique via plants or soil microorganisms to reduce pollutants in the surrounding environment [222], which include heavy metals, radionuclides, or organic pollutants such as polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and pesticides in soils or air [223].Compared with traditional physical and chemical methods, phytoremediation is green, low-cost, and highly effective [224].
Currently, the most used plants for phytoremediation of Cd-excessive soils are hyperenriched plants, which accumulate more than 0.01 percent of Cd in their aboveground dry weight [225].Although more than 450 plant species have been identified as metal hyper-enriched plants, only a few plant species are recognized as Cd hyper-enriched, such as Solanum nigrum, Phytolacca acinosa, and Sedum plumbizincicola [226].More interestingly, these Cd-hyper-enriched plants were also Zn-hyper-enriched plants and vice versa [227].The Cd extraction capacity of plants was different in soils, with the Cd bioconcentration factor (BCF) of plant leaves being larger in acidic soils and smaller in alkaline soils [226].
In addition, woody plants are receiving increased attention as an alternative phytoremediation technique.Recently, it has been found that fast-growing woody plants can accumulate high levels of heavy metals.For instance, a one-year-old willow can extract 17% of the Cd in soils [228], and a four-year-old Averrhoa carambola can remove 0.3% to 51.8% of the total Cd from soils at the surface of 20 cm [229].This suggests that woody plants have greater potential for absorbing and accumulating Cd.
However, plants are prone to toxicity during heavy metal uptake and growth is affected.This greatly limits the efficiency of phytoremediation.With the deterioration of the environment, staple food crops such as rice and maize are also at risk of exceeding the heavy metal content standards.Therefore, enhancing plant tolerance and improving the efficiency of phytoremediation through the addition of different exogenous substances, or converting soil Cd into a non-absorbable form is a valuable approach in agriculture.

Effects of Adding Exogenous Substances to Plants
It has been verified that exogenous supplementation of beneficial elements, plant growth regulators, or nanomaterials can alleviate the Cd stress, thus approaching normal levels of plant height, leaf photosynthesis, and respiration rate [93,[230][231][232].
Ca is essential for plant growth and development [233].When used as an exogenous substance, Ca can reduce Cd-induced physiological and biochemical disorders [234], and also down-regulate Cd accumulation by decreasing the negative charge of the cell membrane surface [235] in Salix matsudana [236] and Fagopyrum esculentum [237].As one of the beneficial elements, Se plays an important role in improving plant stress tolerance.Low concentrations of Se can enhance antioxidant capacity and membrane stability and reduce the uptake of heavy metals and the accumulation of ROS [238].Se has been reported to alleviate Cd stress in rice, oilseed rape, and sunflower by counteracting Cd-induced nutritional changes and reducing oxidative stress [239][240][241].It is worth noting that Se is like a "rapier" in plants, with low concentrations producing beneficial effects and high concentrations causing stress instead [230,238].
In short, when plants are subjected to Cd stress, if the exogenous elements are metallic elements such as iron (Fe), Ca, and potassium (K), the mitigation mechanism may be because they regulate the biochemical and physiological aspects of the plants, mitigate toxicity, or compete with Cd to the transporter, thereby reducing the amount absorbed by the plant.If non-metallic elements such as boron (B), Se, and Si, are added as exogenous elements, the mitigation mechanism may be because these elements act as nutrients for plants and enhance the tolerance of plants, or they may form complexes with Cd and reduce the uptake of the plants.
Polyamines, along with hormones like 1-naphthaleneacetic acid (NAA) and ABA, serve as plant growth regulators for the modulation of plant development and the augmentation of plant tolerance to Cd [232,242,243].Wherein, supplementation of exogenous NAA increases the content of Arabidopsis hemicellulose 1, which immobilizes more Cd in the roots [243].Exogenous application of polyamines, on the other hand, mitigates the adverse effects of Cd contamination on wheat by activating antioxidant enzyme activities [242].In addition, with the development of technology, nanomaterials and biochar materials have also become ideal candidates for solving Cd pollution.Biogenic hydroxyapatite nanoparticles effectively mitigate the toxicity of Cd to mung beans by adsorbing Cd 2+ from the environment and forming a protective layer on plant roots [231].Biochar materials reduce Cd availability and lignocellulosic biochar and herbaceous biochar have a broader range of remediation applications than manure biochar [244].
Of note, in addition to the exogenous substances mentioned above, the addition of melatonin, citric acid, and amino acids can also lead to higher Cd tolerance in plants [245][246][247].Therefore, exploring more beneficial exogenous substances, forms, and proportions of additions are of practical significance to improve the efficiency of phytoremediation.

Conclusions and Perspectives
Cd initiates signal transduction cascades in plants.The exposure to Cd-induced stress in Arabidopsis reduced endogenous auxin content.Concurrently, exogenous supplementation with NAA enhanced the fixation of Cd to the cell wall through an elevation in hemicellulose 1 levels in A. thaliana [243].ABA mitigates oxidative stress following exposure to Cd through the ABI5-MYB49-bHLH cascade, activation of the glutathione pathway, and the formation of an apoplastic barrier [232,248].In contrast to ABA, ethylene amplifies the deleterious effects of Cd on plants in two distinct manners: by augmenting the generation of reactive oxygen species via RBOHC or by impeding the establishment of the apoplastic barrier through unidentified pathways [232,248].Currently, the influences of auxin, ABA, ethylene, and other hormones on Cd tolerance in plants have been initially revealed [232], yet their molecular mechanisms and regulatory networks remain elusive.Given that distinct hormones may elicit contrasting responses under Cd-induced stress, how do they intricately interplay?How do hormone signals crosstalk when encountering ROS or other signals?In addition to the aforementioned circumstances, plant roots may experience heterogeneous Cd stress, characterized by intense pressure on one side and minimal or absent pressure on the other.At this point, plants remodel the root structure and avoid the side with the high level of Cd stress through the RBOH-ROS-growth hormone signaling cascade [249].The exploration of whether other signaling cascades exist in this process and how they function necessitates further investigation.
A shows the screening of Cd-hyperaccumulator plants and fast-growing woody plants that can accumulate large amounts of Cd.B shows the application of different exogenous substances to plants or genetic modification to make plants have higher Cd tolerance.
Throughout plant evolution, certain species have developed robust resistance mechanisms as a result of prolonged adaptation to high Cd pollution.Therefore, these highly tolerant plants can be regarded as potential candidates for mitigating excessive Cd pollution in the environment.At present, Cd-hyperaccumulator plants, predominantly wild herbs, are frequently employed in phytoremediation.However, due to their limited aboveground biomass, the overall Cd absorption capacity remains relatively modest.Conversely, fastgrowing woody plants, such as poplars and willows, boast larger size, greater biomass, and a swifter growth rate, thereby enabling them to absorb more Cd (Figure 5A).Phytoremediation technology has a broad application prospect in Cd-contaminated soil remediation, but many aspects have not yet been clarified, and for Cd-hyperaccumulator plants, it is worth exploring whether they can improve their growth rate, biomass and accumulated heavy metal content, and stress resistance.For fast-growing woody plants, it is necessary to continue to explore their absorption mechanisms at the molecular level and apply transgenic technology or gene editing to improve the uptake of Cd (Figure 5B).

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cells13110907/s1,Table S1: Gene family related to Cd Chelation, accumulation, and detoxification.Table S2: Transcription factors (TFs) families related to Cd uptake, transport, and tolerance.Refs.To optimize the efficiency of Cd pollution phytoremediation, we propose the supplementation of plant regulators (exogenous hormones or polyamines) and beneficial elements to plant nutrition (Figure 5B).Studies have reported that employing this approach confers beneficial effects on plant metabolic pathways and enhances stress resilience [232,236,240,242].At present, there are fewer studies on the compound addition of multiple substances, and it is worthwhile to explore in depth under what ratio different elements or hormones are added to produce better results.In addition, the application of nanomaterials, biochar materials, and microorganisms can effectively reduce the effectiveness of Cd in the soil, so the combination of nanomaterials/biochar, clumping rhizobial fungi, plant growth-promoting bacteria with Ca, K, B, Si may be an effective way to reduce the uptake of Cd by plants [250].In

Figure 1 .
Figure1.Sources of Cd and its transmission pathways in the environment.This figure shows the process of Cd deposition, accumulation, cycling in the environment, and enrichment in plants, animals, and humans through the food chain.Cd exists in different chemical forms in each pathway, such as in water bodies, where it is mainly in the form of chloride[15], in coal combustion and dust formed in non-ferrous metal production, where it mainly contains CdS and CdSO 4[16], and in phosphorus fertilizers in the form of Cd(H 2 PO 4 ) 2 and CdHPO 4[17].

Figure 2 .
Figure 2. Pathways of Cd uptake and transfer by plants through roots, stems, and leaves.(A) The pathway of Cd uptake in plant roots from root hair cells in the maturation zone of the root tip, through the exodermis and endodermis to the stele (Xylem).(B) The translocation process of Cd to the aboveground parts through the xylem and phloem after reaching the stele.(C) Schematic diagram of Cd uptake and transport by plant leaves under Cd stress.

Figure 2 .
Figure 2. Pathways of Cd uptake and transfer by plants through roots, stems, and leaves.(A) The pathway of Cd uptake in plant roots from root hair cells in the maturation zone of the root tip, through the exodermis and endodermis to the stele (Xylem).(B) The translocation process of Cd to the aboveground parts through the xylem and phloem after reaching the stele.(C) Schematic diagram of Cd uptake and transport by plant leaves under Cd stress.

Figure 4 .
Figure 4. Toxic effects of Cd on plants and plant coping mechanisms.The blue part on the left shows the negative effects of Cd stress on plants, and the pink color on the right shows the mitigation mechanisms that plants have evolved over millions of years to cope with Cd stress.

Figure 4 .
Figure 4. Toxic effects of Cd on plants and plant coping mechanisms.The blue part on the left shows the negative effects of Cd stress on plants, and the pink color on the right shows the mitigation mechanisms that plants have evolved over millions of years to cope with Cd stress.

Figure 5 .
Figure 5. Hot spots for future phytoremediation of Cd pollution.(A) Screening of Cd hyperaccumulation-tolerant plants and woody plants that can take up more Cd; (B) Enhancement of Cd tolerance in plants by exogenous means (chemical regulators or genetic engineering).

Figure 5 .
Figure 5. Hot spots for future phytoremediation of Cd pollution.(A) Screening of Cd hyperaccumulationtolerant plants and woody plants that can take up more Cd; (B) Enhancement of Cd tolerance in plants by exogenous means (chemical regulators or genetic engineering).

Table 1 .
Gene family related to Cd uptake, transport and efflux.