Gibberellins (GA) can also protect plants from negative impacts of trace metals by reducing oxidative stress and increasing antioxidant mechanisms (Nguyen et al., 2020). GAs enhance sulfate assimilation, which promotes GSH/phytochelatin production, as S-containing metabolites are important for plant defense mechanisms. Application of GAs increased biomass accumulation, Cd uptake efficacy of Lolium perenne L. (He et al., 2014) and Helianthus annuus L. (Long et al., 2017), hampered MDA contents and oxidative stress of B. juncea under Cd toxicity (Meng et al., 2019), and in lupin plants and broad beans, it mitigated Cd toxicity by increasing soluble proteins under Cd stress (Sharaf et al., 2009). GAST1, a GA-stimulated transcript implicated in the control of ROS buildup, was also upregulated after an exogenous gibberellin was applied and subsequent transcriptomic techniques were employed (Sun et al., 2013). GA signaling boosted the expression of adenosine 50-phosphosulfate reductase, an enzyme crucial in sulfate assimilation, in A. thaliana under stress (Koprivova et al., 2008).

Abscisic acid (ABA) is a plant hormone that regulates many aspects of plant development, growth, and stress responses (Nian et al., 2021). Reduced seed dormancy and wilty phenotypes are seen in ABA-deficient mutants from a variety of plant species, indicating that these important ABA activities are maintained across the plant kingdom (Haider et al., 2021b). In Bidens pilosa, the application of a stress hormone, e.g., ABA, enhanced plant tolerance to Cd stress and Cd extraction from Cd polluted-soil (Liu et al., 2017; Pompeu et al., 2017). Li et al. (2014) concluded that pretreatment of ABA decreased the activities of ascorbic acid, CAT, SOD, APX, POD, and GSH in roots of Vigna radiate L. under Cd stress. ABA, when used as a pretreatment before Cd treatment, did not result in increased contents of cysteine (CYS) and phytochelation (PC). This suggests the role of ABA in the regulation of PCS (Stroiński et al., 2013). The protective role of ABA against Cd stress has also been demonstrated by experiments comparing wild-type Arabidopsis plants and ABA-deficient plants, in which the mutants proved to be more sensitive to Cd metal stress (Sharma and Kumar, 2002). These findings strongly suggest that ABA may be involved in signal pathways during Cd stress.

Nitric oxide is a free radical that reacts with oxygen molecules and controls the deposition of oxygen in plant tissues (Tran et al., 2011). NO is a signal molecule that activates cell defense mechanisms in response to a variety of stressors (Tran and Popova, 2013). Tran and Popova (2013) and Xu et al. (2015) documented that NO application diminished the structural modification of leaves, increased nutritional value, and improved antioxidant enzyme activities under Cd Stress. In other studies, application of NO on Cucumis sativus L. under Cd stress augmented chlorophyll contents and biomass accumulation, and decreased chlorosis symptoms and oxidative stress in plants (Yu et al., 2013).

An endogenous phenolic PGR such as salicylic acid (SA) governs an imperative role in plant physiological processes (photosynthesis, growth, and development), specifically under abiotic stresses, including Cd toxicity (Tran and Popova, 2013; Gruznova et al., 2018). Ahmad et al. (2011) highlighted that SA in initial growth stages assisted plants in mitigating increased damage caused by Cd toxicity by expressing specific proteins and defense-related enzymes (Çanakci and Dursun, 2012; Roychoudhury et al., 2016). Pretreatment of SA abridged Cd accumulation, electrolyte leakage, and level of MDA in wheat shoots under Cd stress (Shakirova et al., 2016), enriched the level of lipids, upregulated the antioxidant system, and caused variations in fatty acid composition of vegetable seedlings (Tran and Popova, 2013; Semida et al., 2015; Rizwan et al., 2017). Increased levels of SA, in response to Cd stress, are reported in pea (Rodríguez-Serrano et al., 2006), maize (Krantev et al., 2008), Arabidopsis (Zawoznik et al., 2007), and halophyte Kosteletzkya virginica (Han et al., 2013). The role of SA in modulating the oxidative stress caused by Cd toxicity is evident by comparing the SA accumulating and deficient lines of Arabidopsis (Zawoznik et al., 2007; Tao et al., 2013). Mutants showed variable levels of H₂O₂ contents, lipid peroxidation, and antioxidant enzymes in comparison with wild plants. Increased levels of endogenous SA led to growth retardation while mutants having decreased endogenous SA showed least retardation in growth because of Cd stress. However, majority of studies disclose the protective role of SA in reduction of oxidative stress caused by Cd.

Jasmonates are oxylipins, which are oxygenated fatty acid derivatives (Ahmad et al., 2017). Methyl JA (MeJA) is a volatile molecule that may have a role in plant-to-plant communication. Exogenous application jasmonic acid (JA) resulted in reduced Cd, H₂O₂, and malondialdehyde accumulation in Vicia faba L. (Ahmad et al., 2017). Similarly, exogenous application of GB reduced the oxidative stress caused by Cd stress and increased the biomass of wheat (Rasheed et al., 2014). Application of ethylene on Arabidopsis thaliana under Cd stress increased root proliferation by modulating superoxide anion (Abozeid et al., 2017). At low levels (10−4 mol/L), it may incur a protective role in mitigation of Cd stress, but at high concentrations, JA might induce retardation in growth by degrading chlorophyll and photosynthetic enzymes (Maksymiec and Krupa, 2002).

The auxin IAA is a well characterized hormone and is involved in growth regulation and physiological development of plant; however, it is still a less studied and less understood mystery in terms of response to Cd stress (Farooq et al., 2015; Teiri et al., 2018). Exogenous application of IAA on B. Juncea promoted uptake and accumulation of Cd that might be attributed to increased cell division, formation of vascular tissue and development of a broader root system that as a result, reduces the toxic effect of Cd (Teiri et al., 2018; Rostami and Azhdarpoor, 2019). Farooq et al. (2015) opined augmented growth and yield of rice when a precursor of an auxin was applied in Cd-contaminated soil. Elobeid et al. (2012), in their experiment, reported disturbed homeostasis of auxin in response to exogenously applied Cd. Yu et al. (2017) stated that an auxin transporter (OsAUX1) induced the extension of root hair and primary roots of rice under Cd stress. Ostrowski et al. (2016) reported that the application of an auxin conjugate (IAA-Asp) induced reduction of H₂O₂, and upregulated POD and CAT activity under Cd stress.

Cytokinins (CKs) are a kind of plant hormone that promotes cytokinesis (cell division) in plant roots and shoots (Singh and Prasad, 2016). They have a role in cell proliferation and differentiation, as well as apical dominance, axillary bud development, and leaf senescence. Exogenous application of CKs inhibited Cd biosorption and augmented the activities of antioxidant enzymes in Chlorella vulgaris (Piotrowska-Niczyporuk et al., 2012) and tomato under Cd-contaminated soil conditions (Singh and Prasad, 2016).

Brassinosteroids (BRs) are endogenous plant hormones that regulate a variety of physiological processes that are necessary for appropriate plant growth and development (Rehman et al., 2022a,b). Application of 28-homobrassinolide (homoBL) on the foliage of Brassica juncea improved Cd tolerance because of enhanced activity of antioxidant enzymes (CAT, POD, and SOD) (Hayat et al., 2007). The effect of 24-epibrassinolide (24-epiBL) was examined on Phaseolus vulgaris in response to Cd stress (Shahzad et al., 2018). 24-epiBL improved membrane stability index, proline content, and antioxidant system (Rady, 2011). Plants exposed to Cd stress have impaired electron transport (ETC) because of demolished photochemical reaction centers, whereas epi-brassinolide (EBL) reduced Cd toxicity and impaired the reaction centers of photosystems and ETC (Janeczko et al., 2005). Moreover, Cd stress helped older tissues more effectively and EBL improved the photosynthetic activity in radish leaves (Anuradha and Rao, 2009). Similarly, in tomatoes, BR application reduced the phytotoxic effects of Cd and improved fruit quality and yield (Hayat et al., 2012). Application of EBL in bean plants subjected to Cd stress increased the levels of antioxidant enzymes. Rise in an antioxidant system (superoxide dismutase, catalase, peroxidase and glutathione reductase, and proline) leads to increased tolerance, enhanced photosynthetic machinery, and growth. Similar kind of results were reported on other crops, i.e., mustard (Hayat et al., 2007) and chickpea (Rady, 2011), by exogenous application of EBL and HBL, respectively. The application of BRs as a shotgun approach (EBL and HBL) improved the chlorophyll content and photosynthesis efficiency of Cd- stressed tomato plants (Hayat et al., 2010). Besides this, BR treatment significantly increased the number of fruits, fruit yield, lycopene, and β-carotene contents in fruits of plants grown under Cd stress.

Polyamines are necessary for cell growth. Polyamine content is greater in quickly expanding tissues, and growth-promoting and regenerative hormone cues boost polyamine production and content (Hasanuzzaman et al., 2019). Rady and Hemida (2015) discussed that presoaking wheat seeds with polyamines, spermine or spermidine resulted in enhanced seedling growth, relative water contents, starch, ascorbic acid, membrane stability index, total glutathione, and concentration of protein, and that H₂O₂, total soluble sugars, concentration of proline, electrolyte leakage and MDA were decreased under Cd stress.

Some other compounds (paclobutrazol, daminozide, humic acid, and melatonin) are extensively used worldwide to decrease the devastating effects of Cd stress in cultivable plants (Khan et al., 2017; Hasan et al., 2019). In conclusion, the use of the above-mentioned PGRs might be an effective, eco-friendly strategy to enhance the growth and development of plants cultivated in a Cd-stressed environment.

Plant growth-promoting rhizobacteria (PGPRs) are rhizosphere inhabitants that enhance plant growth by improving plant nutrient availability, water relationship, and antioxidant activity to improve abiotic stress tolerance (Saeed et al., 2021). They are categorized based on (a) inherent characters (Kloepper, 1994): they (i) enhance root colonization, (ii) improve plant growth, (iii) acclimatize, survive, reproduce, and compete until expression of their potential in plant growth promotion/protection; (b) functional properties (Ahemad, 2014): (i) phytostimulators (phytohormones improve growth), (ii) biofertilizers (regulate nutrient uptake), (iii) rhizoremediators (solubilization of metals), and (iv) biopesticides (regulate plant diseases and pathogens by producing metabolic compounds and lytic enzymes). PGPRs help in phytoremediation; production of soluble minerals, siderophore, phytohormones, rhamnolipid, extracellular polymeric substances, osmo-protectants, 1-aminocyclopropane-1-carboxylate deaminase (ACCD), immobilization of metals (Mahajan and Kaushal, 2018), bioremediation; accumulation or transformation of contaminants, rhizoremediation; and remediation of contaminated soils by symbiotic relationship between plant roots and suitable microbial species. Inoculation of PGPRs has been reported to decrease Cd uptake and alleviate Cd-induced oxidative stress by producing phytohormones (Glick, 2014; Wang et al., 2015), ammonia, and siderophores that mediate nutrient availability, plant biomass accumulation, and plant water status (Ahmad et al., 2015; Hassan et al., 2015; Figure 4). However, Cd uptake may be improved by inoculation of Cd-tolerant bacteria in plants (Bojorquez et al., 2016; Sharma and Archana, 2016), which depicts the specificity of PGPRs and aim of experiment, i.e., phytostabilization vs. phytoextraction. Under heavy metal stress conditions, PGPR-induced IAA acts as a phytohormone to improve cell division and elongation to stimulate root growth, and enhances root nodulation, vascular bundle development, and plant growth (Goswami et al., 2016; Chen et al., 2017). Pseudomonas aeruginosa strains ZGKD5 and ZGKD2 augment to synthesize IAA in Solanum nigrum to improve tolerance against Cd stress (Shi et al., 2016). Huang et al. (2016) observed that Pse-w-MT induces Cd tolerance in Pisum sativum L. by improving the production of IAA. A Bacillus megaterium strain regulated cytokinin production by mediating the transcriptional level of the roots and shoots s receptor (AHK3/AHK4) that induced root morphogenesis in Arabidopsis thaliana (Jianfeng et al., 2017). Cd toxicity and uptake have been decreased by inoculation of Cd-resistant Micrococcus sp. TISTR2221 in maize (Suksabye et al., 2016) and Pseudomonas aeruginosa and Bacillus subtilis in rice (Sangthong et al., 2016).

Extracellular polymeric substances (EPSs), mucopolysaccharides, and proteins are produced by different PGPRs that help to bind toxic HMs for thriving plant growth (Upadhyay et al., 2011; Rajkumar et al., 2012). PGPR strains have been identified that produce EPS to counter the toxic effect of Cd contamination by decreasing the uptake of Cd in wheat (Joshi and Juwarkar, 2009). Pseudomonas aeruginosa inoculation with a PGPR strain helps to detoxify Cd stress based on extracellular biosorption, bioaccumulation, controlled siderophore production, formation of biofilm, enhanced respiration, and modified protein profile (Zivkovic et al., 2018). Recent studies have also shown the detoxification of Cd stress by P. aeruginosa (Zivkovic et al., 2018) and its adapted cells (Tang et al., 2018). Cd stress involved in the uptake of different nutrients influences metal solubility, and P solubilization, and improves nutrient mobility (chelation, precipitation, and immobilization) (Rafique et al., 2017; Saeid et al., 2018). PGPRs could be a promising option to enhance phyto-stabilization efficiency and bioleaching of Cu and Cd in heavily polluted soils owing to biosorption or bioaccumulation (Yang et al., 2018; Ke et al., 2021). Plants produce ROS-scavenging enzymes to detoxify ROS, which are produced under HM stress (Zainab et al., 2020). Exogenous applications of PGPR a strain, Streptomyces strain IT25, decreased catalase (CAT),and it is reported that PGPRs upregulated SOD (superoxide dismutase), POD (peroxidase), and PPO (plant polyphenol oxidases) genes to sequester Cd stress (Khanna et al., 2019; Abbasi et al., 2020) (Table 6). Azospirillum brasilense may ameliorate the negative effects of Cd stress by upregulation of lower Na/K ratio, TaSOS1 transcript level, proline, higher pigments, and antioxidant activities to improve dry weight (Ghassemi and Mostajeran, 2018). PGPR-mediated remediation of HMs and promotion of plant growth depend on PGPR strains coded by several genes and must be understood to build a multidimensional PGPR strain to perform multidimensional functions. Nevertheless, use of genetically engineered PGPR strains with hyper-accumulator plants to remove HMs is less common (Ullah et al., 2015). Several studies have also documented that genetic engineering mediated the improvement in remediation of HM stress in plants (Verma et al., 2017). Qiu et al. (2014) incorporated a bifunctional glutathione synthase gene (gcsgs) into Enterobacter sp. CBSB1 improved the efficiency of HM removal in B. juncea.

To overcome the deleterious effects of HMs, biochar addition to soil is a practical approach (Yuan et al., 2019). Soil productivity and growth of plants have been increased with biochar (Qayyum et al., 2017). In the past years, biochar (BC) has been used in HM-contaminated soils because of its higher adsorption capacity and ability to lower HM contents in soils (Boostani et al., 2019). Use of biochar is an efficient approach to minimize HMs in soils, but it is expensive compared to other amendments in terms of production (Sohail et al., 2020). Biochar, an organic soil additive, has been very helpful in immobilizing the in soils (Hamid et al., 2020b; Haider et al., 2022). Its role in Cd immobilization is auspicious because of its basic nature, porous texture, energetic functional groups, and higher CEC (Medynska-Juraszek and Cwielag-Piasecka, 2020). Biochar-amended soils showed lower Cd transport and accumulation (Abbas et al., 2018). Biochar can easily adsorb Cd, Pb, and Cu by forming complexes and cation exchange mechanisms in soils (Farooq et al., 2020). Breakdown of natural organic material under controlled temperature and limited or no oxygen resulted in biochar production (Hussain et al., 2021). It is well-reported in the published literature that the use of biochar in pots and field tests has significantly improved growth, biomass, and economic productivity in Cd-contaminated soils (Table 7).

Moreover, application of biochar enhances soil pH, water holding capacity, and porosity (Yuan et al., 2019). The role of biochar in growth enhancement is due to the presence of essential nutrients available for plants (Sohail et al., 2019). Rizwan et al. (2018) reported that application of biochar has a positive influence on plant growth. With application of biochar, the exchange portion of Cd in soils is reduced by up to 28% because of increase in soil pH (Bashir et al., 2018). Application of rice straw considerably reduced the concentration of extractable Cd and Cd in soils (Elyamine et al., 2019). Zia-ur-Rehman et al. (2020) reported that various types of biochar showed significant increase in wheat grain in addition to reduced Cd bioavailability. Different low-cost amendments enhanced the photosynthetic pigments and activity of enzymatic antioxidants (SOD, POD, CAT, and ASP) in maize (Shahkolaie et al., 2020).

In contrast to various fertilizers, utilization of compost is a valuable practice to increase soil fertility and crop yield (Abbas et al., 2020). Being a saturated organic carbon, compost enhances the HM-holding capacity of soils along with increase in soil absorption capacity (Medyńska-Juraszek et al., 2020). The bioavailability of HMs in soils has been decreased with the application of compost because of the mechanism of chelation, degradation of microbes, co-precipitation, and de-methylation (Hussain et al., 2021). Naturally, the process of composting helps to stabilize HMs and solid wastes from agricultural and municipal sources by degradation of various microbes (Awasthi et al., 2015). Addition of compost modifies soil aggregation, soil moisture content, and percentage of organic matter, and different nutrients resulted in higher crop growth and yield (Abd El-Mageed et al., 2018). This improved aggregation of soil resulted in altered soil physical and chemical properties, which helps to improve seed germination and roots of seedlings (Abd-El-Mageed et al., 2019). Addition of compost to soils showed 20, 19, and 10% decrease in shoot Cd, Cu, and Zn, respectively (Eissa, 2019). Application of vegetable waste compost reduced Cd concentration by up to 50 and 46% in maize shoots and roots, respectively, along with significant increase in plant growth and NPK contents (Bashir et al., 2021). An increase of 39–85 and 29–63% was observed in shoot and root fresh weight of pak choi cabbage, respectively, along with decrease in the concentration of Cd both in roots and shoots by up to 21–44 and 26–53%, respectively, when green waste compost was applied to HM-contaminated soils (Li et al., 2021).

Manure is used as a replacement for fertilizers, as it is a proper organic fertilizer for plant production (Zhen et al., 2020). Being an organic soil amendment, manure has been extensively used to mobilize HM-contaminated soils (Huang et al., 2018). Its application increased soil pH because of mineralization of carbon and excessive addition of basic cations (Hussain et al., 2021). Application of manure increased the organic matter in soils that resulted in reduced Cd mobilization and phytotoxicity (Liu et al., 2015). Higher accumulation of organic matter in soils stops Cd mobilization by forming organic compounds or, more importantly, adsorption (Halim et al., 2015). Long-term use of manure in uncontaminated soils introduced the availability of HMs as there is little or no information regarding the available or total HMs in the soil with the addition of manure in uncontaminated soils (Zhen et al., 2020). Li F. et al. (2019) stated that SOM had been increased in addition to reduced bioavailability of Cd with the addition of chicken manure in a Cd-contaminated paddy field. Farmyard manure application significantly decreased Cd bioavailability and increased wheat yield (Bashir et al., 2020). Treatment of contaminated soils with composted manure increased the growth of maize plants in addition to decreased HM uptake (Gul et al., 2016). Huang et al. (2020) observed a promising decrease in Cd and Zn bioavailability and enhanced biomass of B. juncea with the application of manure. Bashir et al. (2021) used animal manure to observe the growth of maize in polluted soil and found a prominent decrease of 58 and 52.4% in shoot and root Cd along with enhanced NPK contents.

Screening of genotypes for low Cd accumulation is the primary step to develop Cd-tolerant genotypes. Utilization of these genotypes will help in the development of transgenic crops with enhanced tolerance. Rice has a main role in the supply of Cd to humans, which causes Cd toxicity in them. The risk of Cd toxicity in human beings can be reduced by identifying rice genotypes with low Cd-accumulating capacity. Considerable genetic variability has been found in polished rice and brown rice ranging from 0.14-1.43 and 0.06–0.99 mg kg^–1, respectively (He et al., 2006; Liu et al., 2007). Indica rice displayed higher Cd accumulation in shoots and grains than japonica rice (He et al., 2006; Uraguchi et al., 2009).

In cereals, a substantial amount of genetic variability has been observed in grain legume crops, i.e., soybean (Arao et al., 2003; Sugiyama et al., 2011; Salazar et al., 2012; Vollmann et al., 2015). High or low Cd accumulating genotypes were identified in different crops by analyzing genetic variations present among them. In wheat, genotypes with low Cd accumulation capacity included durum lines 8982-TL-L, CDC-Verona, and Strongfield (Clarke et al., 2002; Pozniak et al., 2009). On the other hand, high Cd accumulating wheat genotypes were Joppa and Carpio (Elias et al., 2015; Elias and Manthey, 2016). Introgression of genes from one plant to another is also a promising genetic engineering technique, as many plants express genes that help in Cd tolerance (Lan et al., 2012; Yuan et al., 2012; Menguer et al., 2013). Aegilops tauschii accessions, i.e., AS623194, AS623402, and AS623402, were recommended to be rich sources of Cd-tolerant genes, as wild species have high genetic variability for Cd accumulation. Identification and inclusion of these genotypes in crop improvement programs will be helpful in developing new Cd-tolerant cultivars. Genes or QTLs controlling the accumulation of Cd can be identified using these lines in mapping populations (Qin et al., 2015).

Genetic engineering along with the development of transgenic plants may help in the cultivation of crops in soils contaminated with Cd. Various approaches in genetic engineering can help in the identification of desirable genes that can be used to develop plants with increased HM tolerance. These genes may assist in translocation and deposition of metals in tissues of transgenic plants along with other characteristics that help in tolerance of metals (Mosa et al., 2016; Sheoran et al., 2016). Recent research has displayed that several genes are involved in regulation of HM tolerance. This may be performed either by overexpression of one gene or by overexpression of multiple genes simultaneously (Bhargava et al., 2012). Therefore, genetic modifications involving transfer or regulation of genes should be studied thoroughly to develop crops tolerant to HM stress (Gerszberg and Hnatuszko-Konka, 2017). Metallothioneins and phytochelatins are key components that can be utilized to protect plants against toxic metals. Several studies have shown the importance of these components in phytoremediation, as HM tolerance was increased in transgenic plants developed using metallothionein genes and synthases (Sunitha et al., 2012).

Gene manipulation is one the most important strategies for producing Cd-tolerant plants. It involves complete understanding of molecular mechanisms involved in Cd stress and can be utilized to develop methods to reduce damages that are caused by toxic metals. This may also help reducing Cd concentration in edible plant parts by breeding crop varieties with low Cd accumulation and more agronomical importance. Most effective molecular techniques used worldwide for Cd tolerance in crop plants have been discussed.

The identification of chromosomal regions governing genes that control Cd tolerance has become an important genetic tool with the discovery of quantitative trait loci (QTLs) using family based method. This method has helped in the illustration of underlying genetic basis of Cd accumulation in various crops (Ishikawa et al., 2005, 2010; Knox et al., 2009; Ueno et al., 2009a,b; Jegadeesan et al., 2010; Jha and Bohra, 2016). In rice, QTLs are supposed to be located on LGs 3, 6, and 8 in chromosome segment substitution lines (CSSLs) developed from the cross between Kasalath (indica) and Koshihikari (japonica) (Ishikawa et al., 2005).

A QTL responsible for Cd accumulation in roots and shoots was found on LGs 6 and 7, while LG 3 consisted of a QTL for root or shoot Cd content. LGs 1, 3, 5, and 8 governed a total of six QTLs responsible for Cd tolerance (Xue et al., 2009). In F2 resulting from a cross between Badari Dhan and Shwe War, it was demonstrated that QTLs controlling 16.1% of total phenotypic variation (PV) related to Cd tolerance was mapped on LG 11 (Ueno et al., 2009a). QTL qLCdG11 for Cd tolerance was also mapped on LG 11 in an RIL population developed from a cross between Fukuhibiki 9 and LAC23. Major QTLs elucidating 85% of PV related to Cd accumulation were reported on LG 7 in rice (Ueno et al., 2009b). Afterward, QTL qGCd7 found on LG 7 explained 35.5% of phenotypic variation by analyzing backcross inbred lines developed by crossing Sasanishiki (japonica) with Habataki (indica). Major QTL qlGCd3 controlling Cd levels in rice grains was discovered by mapping CSSLs developed from a cross between LAC23 and Koshihikari (Abe et al., 2013). A total of 18 Cd-accumulating QTLs in milled rice and 14 in brown rice were reported to be located on LGs 2, 3, 4, 5, and 7 (Da-wei et al., 2018).

Luo et al. (2018) demonstrated CAL1 as a QTL in rice causing 13% change in Cd accumulation in leaves of a double haploid population. CAL1 is responsible for Cd translocation from root to shoot, and rice mutants with CAL1 knockout displayed decreased Cd concentration in leaves (Luo et al., 2018). Cd-accumulating QTLs were mapped in rice by growing an inbred population (743/Katy) derived from Xiang in soil contaminated with Cd revealing two QTLs, i.e., qCd-7 and qCd-2 involved in the process of Cd uptake and accumulation in rice (Li W. et al., 2016; McCouch et al., 2016). Sato et al. (2011) discovered two QTLs in brown rice affecting Cd accumulation, i.e., qLCdG11 and qLCdG3, explaining 9.4–12.9 and 8.3–13.9% of phenotypic variation, respectively. Five most important QTLs were discovered; three of them (gcc3, gcc9, and gcc1) were responsible for grain Cd concentration; scc10 was linked with Cd accumulation in shoots, and the QTL sgr5 was involved in supply of Cd to roots and shoots. Grain Cd accumulation was highly influenced by Srg5 in rice (Abe et al., 2013).

In durum wheat, the gene Cdu1 plays a role in Cd uptake, as reported by Knox et al. (2009), whereas Wiebe et al. (2010) illustrated the role of Cdu1 in accumulation of Cd in wheat grains. Recently, a major QTL on 5BL has been located in durum wheat controlling 54.3% PV for Cd uptake with the help of high-density genotyping (AbuHammad et al., 2016). In an RIL population developed by crossing Divide with D041735, another QTL, “QCdu. ndsu-5B,” has been discovered on 5B for Cd intake and tolerance. In soybean, a QTL for lower accumulation of Cd has been mapped on LGK controlling up to 57.3% of phenotypic variation (Jegadeesan et al., 2010).

Advancement in methods used for genome mapping has led to revelation of various significant market trait associations (SMTAs) present in genomes for several traits including Cd intoxication (Huang and Han, 2014). Additionally, elucidation of crop genomes and other techniques, such as genome resequencing and genotyping by sequencing (GNB) to identify SNPs has helped in performing GWAS to study complex plant traits, i.e., HM toxicity and tolerance. Six SMTAs (such as pdil5-1, TaAP2-B, TaAP2-D, DME-5A, and Acc-1) related to genes controlling Cd tolerance were revealed in 235 accessions of Aegilops tauschii by GWAS using 7,185 SNPs.

A total of 17 QTLs for grain Cd accumulation were discovered in rice by GWAS conducted on 276 rice accessions using 416 K SNPs (Liu et al., 2019). A total of 312 out of 1,568 accessions of rice displaying more diversity were selected and evaluated to develop low Cd accumulating germplasm (Li W. et al., 2016; McCouch et al., 2016). Twenty-four rice accessions including 3 Indica accessions linked with reduced accumulation of Cd by up to 0.2 mg kg–1 were selected based on GWAS. A total of 312 accessions including subpopulations of Indica and Japonica rice were analyzed, leading to discovery of 28 QTLs related to Cd uptake and accumulation in plant tissues. Genes, i.e., OsNRAMP5, OsNRAMP1, OsLCD, and OsHMA3, that have been already discovered were also reported in recent GWASs (Li W. et al., 2016; McCouch et al., 2016).

A GWAS was carried out on 100 barley accessions and resulted in identification of several QTLs, i.e., 9 for root Cd accumulation, 21 for shoot Cd, 14 for transport of Cd from roots to shoots, and 15 for grain Cd amassment (Wu et al., 2015). Recently, 63 SMTAs on 5 LGs for leaf Cd were discovered in maize by GWAS using 43,737 SNPs (Zhao et al., 2018). This study also revealed several genes such as GRMZM2G45549 and GRMZM2G124103 coding for vacuolar ATPase, and GRMZM2G175576 for ATPase controlling Zn/Cd transport. Identification of specific DNA markers and accurate phenotyping techniques for study of Cd stress in crop plants will help in better understanding of Cd accumulation mechanism and improvement of genotypes for Cd tolerance.

Introduction and overexpression of genes associated with metal uptake and translocation are the most effective and commonly used methods for phytoremediation using transgenics (Shukla et al., 2013; Mani and Kumar, 2014; Das et al., 2016). Various pathways including metal intake by roots, formation of metal-ligands and metal-chelator complexes, deposition of metals in vacuole, and long-distance translocation to shoots through symplast and apoplast can be exploited to enhance metal accumulation in plant tissues, as metal intake is a complex process (Nakamura et al., 2014; Das and Jayalekshmy, 2015). Genes coding for metal ion transporters and chelators are being used to manipulate plant metal uptake and translocation.

Metal transporters are also linked to gene families coding for ZIP proteins that are involved in Fe and Zn transport in cytoplasm. Conolly et al. (2002) reported 150% enhanced accumulation of Cd and Zn when gene coding for AtIRT1 was overexpressed in A. thaliana. However, some ZIP genes are highly specific, and their overexpression does not lead to improved accumulation of HMs like Cd (Tiong et al., 2014). Therefore, another strategy has been exploited in which genes coding for metal transporters and involved in microelements uptake and transport are transformed into plants. When absorption microelements are increased, HM uptake is reduced. These genes manipulate plant’s ability to collect HMs by enhancing or decreasing the effect of ZIP genes. This method may be utilized for phytoremediation of soil contaminated with HMs and for biofortification of crops (Siemianowski et al., 2014; Gong et al., 2015).

Nakamura et al. (2014) developed transgenic plants of Nicotine tabacum by overexpression of cysteine synthase (CS) and serine acetyltransferase (SAT) involved in cysteine biosynthesis. The plants produced by overexpression of these genes displayed enhanced Cd tolerance. He et al. (2016) inserted a homologous corn gene in Arabidopsis by activating methyltransferase gene (CIMT1 protein). Results displayed enhanced Cd tolerance in Arabidopsis by the overexpression of this gene. This gene could be utilized to enhance Cd tolerance in agricultural crops. Lee and Back (2017) overexpressed melatonin-related genes using transgenic OsSNAT1 in rice. The study revealed that transgenic OsSNAT1 provides tolerance to Cd. An expansin gene, TaEXPA2, was isolated from wheat and overexpressed in tobacco plants. This enhanced tolerance to Cd toxicity in tobacco plants and improved seed germination and growth of seedling and roots (Ren et al., 2018).

Expression of OXS3 (OXIDATIVE STRESS 3)-like gene fragments led to reduced accumulation of Cd in rice grains and other plant tissues without affecting yield and amount of minerals, i.e., Cu, Mn, Zn, and Fe (Blanvillain et al., 2009). Hence, this approach seems to be promising for production of Cd-tolerant genotypes without any loss in yield or minerals. OsHMA3 was involved in the transport of Zn to the vacuoles in root cells (Sasaki et al., 2012). The expression of OsHMA3 along with the OsHMA2 promoter had no effect on the growth of plants in vegetative stage. Moreover, when OsHMA3 was expressed under OsHMA2 control, the expression and localization of OsHMA3 in tissues were increased, and significant reduction in Cd concentration was observed because of increased Cd sequestration in roots, nodes, and vacuoles (Shao et al., 2018).

The functional gene OsHMA3 from rice was overexpressed in wheat for Cd sequestration in roots. Cd accumulation in roots was enhanced by up to 110–125%. Reduction in Cd accumulation in wheat grains was observed to be up to 40-folds, while translocation of Cd from roots to shoots was decreased by approximately 10-folds (Zhang L. et al., 2020). This technique offers an effective solution to reduce Cd accumulation in plants, leading to reduced health risks.