Comprehensive analysis of variation of cadmium accumulation in rice and detection of a new weak allele of OsHMA3

Ectopic expression of SaCTP3 from the hyperaccumulator Sedum alfredii in sorghum increases Cd accumulation for phytoextraction

The Cd tolerance protein SaCTP3, which responds to Cd stress, was identified in Sedum alfredii; however, how to improve the efficiency of phytoremediation of Cd-contaminated soil using the CTP gene remains unknown. In this study, the phytoremediation potential of SaCTP3 of Sedum alfredii was identified. In the yeast Cd-sensitive strain Δycf1 overexpressing SaCTP3, the accumulation of Cd was higher than that in the Δycf1 strain overexpressing an empty vector. Transgenic sorghum plants overexpression SaCTP3 were further constructed to verify the function of SaCTP3. Compared to wild-type plants, the SaCTP3-overexpressing lines exhibited higher Cd accumulation under 500 μM Cd conditions. The average Cd content inSaCTP3-overexpressing plants is more than four times higher than that of WT plants. This was accompanied by an enhanced ability to scavenge ROS, as evidenced by the significantly increased activities of peroxidase, catalase, and superoxide dismutase in response to Cd stress. Pot experiments further demonstrated that SaCTP3 overexpression resulted in improved soil Cd scavenging and photosynthetic abilities. After 20 days of growth, the average Cd content in the soil planted with SaCTP3-overexpressing sorghum decreased by 19.4%, while the residual Cd content in the soil planted with wild-type plants was only reduced by 5.4%. This study elucidated the role of SaCTP3 from S.alfredii, highlighting its potential utility in genetically modifying sorghum for the effective phytoremediation of Cd.

Chronic cadmium exposure triggered ferroptosis by perturbing the STEAP3-mediated glutathione redox balance linked to altered metabolomic signatures in humans

Cadmium (Cd), a predominant environmental pollutant, is a canonical toxicant that acts on the kidneys. However, the nephrotoxic effect and underlying mechanism activated by chronic exposure to Cd remain unclear. In the present study, male mice (C57BL/6J, 8 weeks) were treated with 0.6 mg/L cadmium chloride (CdCl2) administered orally for 6 months, and tubular epithelial cells (TCMK-1 cells) were treated with low-dose (1, 2, and 3 μM) CdCl2 for 72 h (h). Our study results revealed that environmental Cd exposure triggered ferroptosis and renal dysfunction. Spatially resolved metabolomics enabled delineation of metabolic profiles and visualization of the disruption to glutathione homeostasis related to ferroptosis in mouse kidneys. Multiomics analysis revealed that chronic Cd exposure induced glutathione redox imbalance that depended on STEAP3-driven lysosomal iron overload. In particular, glutathione metabolic reprogramming linked to ferroptosis emerged as a metabolic hallmark in the blood of Cd-exposed workers. In conclusion, this study provides the first evidence indicating that chronic Cd exposure triggers ferroptosis and renal dysfunction that depend on STEAP3-mediated glutathione redox imbalance, greatly increasing our understanding of the metabolic reprogramming induced by Cd exposure in the kidneys and providing novel clues linking chronic Cd exposure to nephrotoxicity.

Strontium alleviated the growth inhibition and toxicity caused by cadmium in rice seedlings

Cadmium (Cd) contamination of rice is an urgent ecological and agricultural problem. Strontium (Sr) has been shown to promote plant growth. However, the effect of Sr on rice seedlings under Cd stress is currently unclear. In this work hydroponic experiments were used to assess the impact of Sr on rice seedling growth under Cd stress. The findings demonstrated that foliar application of 0.5 mg L-1 Sr had no discernible impact on the development of rice seedlings. However, Sr significantly alleviated growth inhibition and toxicity in rice seedlings when threatened by Cd. Compared with the Cd treatment (Cd, 2.5 mg L-1), the root length, shoot height, and whole plant length of rice seedlings in the Cd + Sr treatment (Cd, 2.5 mg L-1; Sr, 0.5 mg L-1) increased by 4.96 %, 12.47 % and 9.60 %, respectively. The content of Cd in rice decreased by 23.34 % (roots) and 5.79 % (shoots). Sr lessened the degree of membrane lipid peroxidation damage (lower MDA concentration) among the seedlings of rice under Cd stress by controlling the activities of antioxidant enzymes and GSH content. By changing the expression of antioxidant enzyme-encoding genes and downregulating the heavy metal transporter gene (OsNramp5), Sr reduced accumulation and the detrimental effects of Cd on rice seedlings. Our study provides a new solution to the problem of Cd contamination in rice, which may promote the safe production of rice and benefit human health.

Roles of Chloride and Sulfate Ions in Controlling Cadmium Transport in a Soil-Rice System as Evidenced by the Cd Isotope Fingerprint

Anions accompanying inorganic fertilizers, such as chloride and sulfate ions, potentially affect the solubility, uptake, and transport of Cd to rice grains. However, the role of anions in controlling Cd transport in the soil-soil solution-Fe plaque-rice plant continuum remains poorly understood. Cd isotope ratios were applied to Cd-contaminated soil pots, hydroponic rice, and adsorption experiments with or without KCl and K2SO4 treatments to decipher transport processes in the complex soil-rice system. The chloride and sulfate ions increased the Cd concentrations in the soil solution, Fe plaque, and rice plants. Accordingly, the magnitude of positive fractionation from soil to the soil solution was less pronounced, but that between soil and Fe plaque or rice plant is barely varied. The similar isotope composition of Fe plaque and soil, and the similar fractionation magnitude between Fe plaque and the solution and between goethite and the solution, suggested that desorption-sorption between iron oxides and the solution could be important at the soil-soil solution-Fe plaque continuum. This study reveals the roles of chloride and sulfate ions: (i) induce the mobility of light Cd isotopes from soil to the soil solution, (ii) chloro-Cd and sulfato-Cd complexes contribute to Cd immobilization in the Fe plaque and uptake into roots, and (iii) facilitate second leaves/node II-to-grain Cd transport within shoots. These results provide insights into the anion-induced Cd isotope effect in the soil-rice system and the roles of anions in facilitating Cd migration and transformation.

Cd isotope fractionation in a soil-rice system: Roles of pH and mineral transformation during Cd immobilization and migration processes

Cd speciation in soil and its transport to rice roots are influenced by the soil pH, oxidation-reduction potential, and mineral transformation; however, the immobilization and migration of Cd in soil-rice systems with different pH values under distinct water regimes remain unclear. This study used Cd isotope fractionation, soil physical analysis, and root gene quantification to elucidate the immobilization and transport of Cd in different soil-rice systems. In drainage soils, the high soil pH enhanced the transformation and magnitude of negative fractionation of Cd from MgCl2 extract to FeMn oxide-bound pool; however, it favored Cd uptake and root-to-grain transport. Compared with drainage regimes, the flooding regimes shifted fractionation toward heavy isotopes from MgCl2-extracted Cd to FeMn oxide-bound Cd in acidic soils (∆114/110CdMgCl2 extract - FeMn oxide-bound Cd = -0.09 ± 0.03 ‰) and to light isotopes from MgCl2-extracted Cd to carbonate-bound Cd in neutral and alkaline soils (∆114/110CdMgCl2 extract - carbonate-bound Cd = 0.29-0.40 ‰). The submerged soils facilitated the forming of carbonate and poorly crystalline minerals (such as ferrihydrite), which were transformed into highly crystalline forms (such as goethite). These results demonstrated that the dissolution-precipitation process of iron oxides was essential for controlling soil Cd availability under flooding regimes, and the relative contribution of carbonate minerals to Cd immobilization was promoted by a high soil pH. Flooding regimes induced lower expressions of OsNRAMP1 and OsNRAMP5 to limit the uptake of light Cd isotopes from MgCl2-extract pool, whereas a teeter-totter effect on gene expression patterns in roots (including those of OsHMA3 and OsHMA2) limited the transport of heavy Cd isotopes from root to grain. These findings demonstrate that flooding regimes could exert multiple effects on soil Cd immobilization and Cd transport to grain. Moreover, alkaline soil was conducive to forming carbonate minerals to sequester Cd.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

Cadmium induced a non-coding RNA microRNA535 mediates Cd accumulation in rice

Identifying key regulators related to cadmium (Cd) tolerance and accumulation is the main factor for genetic engineering to improve plants for bioremediation and ensure crop food safety. MicroRNAs (miRNAs), as fine-tuning regulators of genes, participate in various abiotic stress processes. MiR535 is an ancient conserved non-coding small RNA in land plants, positively responding to Cd stress. We investigated the effects of knocking out (mir535) and overexpressing miR535 (mir535 and OE535) under Cd stress in rice plants in this study. The mir535 plants showed better Cd tolerance than wild type (WT), whereas the OE535 showed the opposite effect. Cd accumulated approximately 71.9% and 127% in the roots of mir535 and OE535 plants, respectively, compared to WT, after exposure to 2 µmol/L Cd. In brown rice, the total Cd accumulation of OE535 and mir535 was about 78% greater and 35% lower than WT. When growing in 2 mg/kg Cd of soil, the Cd concentration was significantly lower in mir535 and higher in OE535 than in the WT; afterward, we further revealed the most possible target gene SQUAMOSA promoter binding-like transcription factor 7(SPL7) and it negatively regulates Nramp5 expression, which in turn regulates Cd metabolism. Therefore, the CRISPR/Cas9 technology may be a valuable strategy for creating new rice varieties to ensure food safety.

A phytoexclusion strategy for reducing contamination risk of rice based on low-Cd natural variations pyramid of root transporters

Cadmium pollution in rice is a threat to human health. Phytoexclusion is an effective strategy to reduce the Cd accumulation. Soil-to-root is the first step of Cd entering rice and plays a crucial role in Cd accumulation, so targeting root transporters could be an effective approach for phytoexclusion. This study utilized single-gene & multi-gene joint haplotype analysis to reveal the law of natural variations. The result showed that natural variations of rice root transporters assembled regularly following a certain pattern, rather than randomly. A total of 3 dominant nature variation combinations with 2 high-Cd combinations and 1 low-Cd combination were identified. In addition, indica-japonica differentiation was observed, with indica germplasms harboring high-Cd combinations while japonica germplasms harboring. In Chinese rice landraces, most of the collected indica landraces contained high-Cd combinations, indicating a high Cd contamination risk in indica landraces in terms of both phenotype and genotype. To address this issue, multiple superior low-Cd natural variations were pyramided to create two new low-Cd germplasms. In both pond and farmland trials, the ameliorated rice grain Cd did not exceed safety standards. This research provided a framework for future phytoexclusion, thus to reduce Cd-contamination risk in soil-rice system.

Cadmium isotope fractionation and gene expression evidence for tracking sources of Cd in grains during grain filling in a soil-rice system

Grain filling is the key period that causes excess cadmium (Cd) accumulation in rice grains. Nevertheless, uncertainties remain in distinguishing the multiple sources of Cd enrichment in grains. To better understand the transport and redistribution of Cd to grains upon drainage and flooding during grain filling, Cd isotope ratios and Cd-related gene expression were investigated in pot experiments. The results showed that the Cd isotopes in rice plants were much lighter than those in soil solutions (∆^114/110Cd_{rice-soil solution} = -0.36 to -0.63 ‰) but moderately heavier than those in Fe plaques (∆^114/110Cd_{rice-Fe plaque} = 0.13 to 0.24 ‰). Calculations revealed that Fe plaque might serve as the source of Cd in rice (69.2 % to 82.6 %), particularly upon flooding at the grain filling stage (82.6 %). Drainage at the grain filling stage yielded a larger extent of negative fractionation from node I to the flag leaves (∆^114/110Cd_{flag leaves-node I} = -0.82 ± 0.03 ‰), rachises (∆^114/110Cd_{rachises-node I} = -0.41 ± 0.04 ‰) and husks (∆^114/110Cd_{rachises-node I} = -0.30 ± 0.02 ‰), and significantly upregulated the OsLCT1 (phloem loading) and CAL1 (Cd-binding and xylem loading) genes in node I relative to that upon flooding. These results suggest that phloem loading of Cd into grains and transport of Cd-CAL1 complexes to flag leaves, rachises and husks were simultaneously facilitated. Upon flooding of grain filling, the positive fractionation from the leaves, rachises and husks to the grains (∆^114/110Cd_{flag leaves/rachises/husks-node I} = 0.21 to 0.29 ‰) is less pronounced than those upon drainage (∆^114/110Cd_{flag leaves/rachises/husks-node I} = 0.27 to 0.80 ‰). The CAL1 gene in flag leaves is down-regulated relative to that upon drainage. Thus, the supply of Cd from the leaves, rachises and husks to the grains is facilitated during flooding. These findings demonstrate that the excess Cd was purposefully transported to grain via xylem-to-phloem within nodes I upon the drainage during grain filling, and the expression of genes responsible for encoding ligands and transporters together with isotope fractionation could be used to tracking the source of Cd transported to rice grain.

Abscisic-Acid-Regulated Responses to Alleviate Cadmium Toxicity in Plants

High levels of cadmium (Cd) in soil can cause crop yield reduction or death. Cadmium accumulation in crops affects human and animal health as it passes through the food chain. Therefore, a strategy is needed to enhance the tolerance of crops to this heavy metal or reduce its accumulation in crops. Abscisic acid (ABA) plays an active role in plants' response to abiotic stress. The application of exogenous ABA can reduce Cd accumulation in shoots of some plants and enhance the tolerance of plants to Cd; therefore, ABA may have good application prospects. In this paper, we reviewed the synthesis and decomposition of ABA, ABA-mediated signal transduction, and ABA-mediated regulation of Cd-responsive genes in plants. We also introduced physiological mechanism underlying Cd tolerance because of ABA. Specifically, ABA affects metal ion uptake and transport by influencing transpiration and antioxidant systems, as well as by affecting the expression of metal transporter and metal chelator protein genes. This study may provide a reference for further research on the physiological mechanism of heavy metal tolerance in plants.

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

Transcriptional analysis of heavy metal P1B-ATPases (HMAs) elucidates competitive interaction in metal transport between cadmium and mineral elements in rice plants

Cadmium (Cd) pollution has become a major threat to crop production and quality globally. The heavy metal P_1B-ATPases (HMAs) play a crucial role in metal transport in plants. In the present study, we investigated the interaction in metal transport by HMAs between Cd and mineral elements in rice plants. Rice seedlings were treated with cadmium nitrate either in the nutrient solution ("Cd+M") or in the ultrapure water ("Cd-M"). Result showed that phytotoxicity of Cd to rice seedlings was evident from both Cd treatments, judged by relative growth rate (RGR), where more severe repression (p < 0.05) of RGR was observed in the "Cd-M" treatments than the "Cd+M" treatments. More Cd (p < 0.05) was accumulated in rice tissues from the "Cd-M" treatments than the "Cd+M" treatments, while there is a significant difference (p < 0.05) in distribution and translocation of mineral elements in rice tissues between the "Cd+M" and the "Cd-M" treatments. RT-qPCR analysis displayed that the expression patterns of HMAs related genes were quite different between "Cd+M" and "Cd-M" treatments, suggesting their different regulatory effects during the transport of Cd and mineral elements within rice plants. The competition in metal transport by HMAs mainly occurs between Cd and micro-elements of Zn and Cu in rice tissues during Cd exposure. Overall, this study provides new evidence to clarify the different translocation mechanisms of HMAs in metal transport between Cd and mineral elements in rice seedlings during Cd exposure.

Toxic metals and metalloids: Uptake, transport, detoxification, phytoremediation, and crop improvement for safer food

Agricultural soils are under threat of toxic metal/metalloid contamination from anthropogenic activities, leading to excessive accumulation of arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in food crops that poses significant risks to human health. Understanding how these toxic metals and their methylated species are taken up, translocated, and detoxified is prerequisite to developing strategies to limit their accumulation for safer food. Toxic metals are taken up and transported across different cellular compartments and plant tissues via various transporters for essential or beneficial nutrients, e.g. As by phosphate and silicon transporters, and Cd by manganese (Mn), zinc (Zn), and iron (Fe) transporters. These transport processes are subjected to interactions with nutrients and the regulation at the transcriptional and post-translational levels. Complexation with thiol-rich compounds, such as phytochelatins, and sequestration in the vacuoles are the common mechanisms for detoxification and for limiting their translocation. A number of genes involved in toxic metal uptake, transport, and detoxification have been identified, offering targets for genetic manipulation via gene editing or transgenic technologies. Natural variations in toxic metal accumulation exist within crop germplasm, and some of the quantitative trait loci underlying these variations have been cloned, paving the way for marker-assisted breeding of low metal accumulation crops. Using plants to extract and remove toxic metals from soil is also possible, but this phytoremediation approach requires metal hyperaccumulation for efficiency. Knowledge gaps and future research needs are also discussed.

The role of polysaccharides functional groups in cadmium binding in root cell wall of a cadmium-safe rice line

Exploring the mechanism of cadmium (Cd) accumulation in Cd-safe rice lines is beneficial for ensuring rice safety. D62B, a Cd-safe rice line, accumulates less than 0.2 mg Cd kg^-1 in the brown rice due to strong capacity of Cd retention in the roots, and the root cell wall (RCW) polysaccharides play important roles. However, specific underlying mechanism of Cd binding on the polysaccharides is little known. In this study, the role of polysaccharides, especially pectin and hemicellulose 1 (HC1), in RCW of D62B was investigated by adsorption experiments and Fourier Transform Infrared Spectroscopy (FTIR) analysis compared with a common rice line (Luhui17). Cadmium was adsorbed on RCW of two rice lines by a multilayer and inhomogeneous chemisorption way with the force of ion transfer or exchange. Cadmium was adsorbed on RCW rapidly at first stage with the limit of internal and external diffusion, and gradually reached saturation. With the removal of pectin, the Cd adsorption rate, maximum Cd adsorption amount and the shift degree of carboxyl groups in the RCW of D62B sharply decreased, which showed advantages compared with Luhui17. Sequential removal of HC1 further decreased the maximum Cd adsorption amount and the shift degree of hydroxyl groups. The results showed that more available functional groups, especially carboxyl groups in pectin and hydroxyl groups in HC1, contributed to Cd immobilization within the RCW of Cd-safe rice line, thus limiting Cd translocation to the shoot and reducing Cd accumulation in the brown rice.

Overexpression of OsLCT2, a Low-Affinity Cation Transporter Gene, Reduces Cadmium Accumulation in Shoots and Grains of Rice

Cadmium (Cd)-contaminated rice is a serious issue affecting food safety. Understanding the molecular regulatory mechanisms of Cd accumulation in rice grains is crucial to minimizing Cd concentrations in grains. We identified a member of the low-affinity cation transporter family, OsLCT2 in rice. It was a membrane protein. OsLCT2 was expressed in all tissues of the elongation and maturation zones in roots, with the strongest expression in pericycle and stele cells adjacent to the xylem. When grown in Cd-contaminated paddy soils, rice plants overexpressing OsLCT2 significantly reduced Cd concentrations in the straw and grains. Hydroponic experiment demonstrated its overexpression decreased the rate of Cd translocation from roots to shoots, and reduced Cd concentrations in xylem sap and in shoots of rice. Moreover, its overexpression increased Zn concentrations in roots by up-regulating the expression of OsZIP9, a gene responsible for Zn uptake. Overexpression of OsLCT2 reduces Cd accumulation in rice shoots and grains by limiting the amounts of Cd loaded into the xylem and restricting Cd translocation from roots to shoots of rice. Thus, OsLCT2 is a promising genetic resource to be engineered to reduce Cd accumulation in rice grains.

The Road to Practical Application of Cadmium Phytoremediation Using Rice

Cadmium (Cd) is a toxic heavy metal that causes severe health issues in humans. Cd accumulates in the human body when foods produced in Cd-contaminated fields are eaten. Therefore, soil remediation of contaminated fields is necessary to provide safe foods. Rice is one of the primary candidates for phytoremediation. There is a genotypic variation of Cd concentration in the shoots and grains of rice. Using the world rice core collection, ‘Jarjan’, ‘Anjana Dhan’, and ‘Cho-ko-koku’ were observed with a significantly higher level of Cd accumulation in the shoots and grains. Moreover, OsHMA3, a heavy metal transporter, was identified as a responsive gene of quantitative trait locus (QTL) for high Cd concentration in the shoots of these three varieties likewise. However, it is difficult to apply practical phytoremediation to these varieties because of their unfavorable agricultural traits, such as shatter and easily lodged. New rice varieties and lines were bred for Cd phytoremediation using OsHMA3 as a DNA marker selection. All of them accumulated Cd in the shoots equal to or higher than ‘Cho-ko-koku’ with improved cultivation traits. Therefore, they can be used for practical Cd phytoremediation.

Cd tolerance and accumulation in barley: screening of 36 North African cultivars on Cd-contaminated soil

In North Africa, barley (Hordeum vulgare L) is the second most cultivated cereal. In Tunisia, barley is cultivated in mining areas with possible Cd soil contamination. The accumulation of Cd was studied in the 36 most cultivated North African barley cultivars cultured during 6 months on control soil and on soil containing 10 ppm of Cd. Cadmium did not affect germination and morphology in any cultivar. However, Cd induced variable effects on the biomass according to the cultivar. The cultivar Lemsi was the most sensitive one and Gisa 127 the most tolerant to Cd. The spike morphology did not show any differences between control and Cd-treated plants. The number of grains per spike and the weight of kernels were differently affected by Cd. On this basis, we identified Manel, Temassine, Giza 130, and Firdaws as the most tolerant cultivars and Raihane, Giza 123, Adrar, and Amira as the most sensitive ones. Cd accumulated at a higher concentration in straw than in the grains, but for both organs, we observed a significant intraspecific variability. In the straw, Lemsi and Massine showed the highest Cd concentration, while the lowest concentration was recorded in Temassine. In the kernels, Amalou showed the highest Cd concentration, 14 μgg^-1 of dry weight (DW), but the lowest Cd concentration was 1.7 μg g^-1 DW in Kebelli. Based on the official allowable limit of Cd in the grain, all cultivars represent a potential risk when cultivated on soil contaminated with 10 ppm Cd. The molecular and physiological basis responsible for the differences in Cd tolerance and accumulation among barley cultivars will require more investigations.

Analysis of Cadmium Root Retention for Two Contrasting Rice Accessions Suggests an Important Role for OsHMA2

Two rice accessions, Capataz and Beirao, contrasting for cadmium (Cd) tolerance and root retention, were exposed to a broad range of Cd concentrations (0.01, 0.1, and 1 μM) and analyzed for their potential capacity to chelate, compartmentalize, and translocate Cd to gain information about the relative contribution of these processes in determining the different pathways of Cd distribution along the plants. In Capataz, Cd root retention increased with the external Cd concentration, while in Beirao it resulted independent of Cd availability and significantly higher than in Capataz at the lowest Cd concentrations analyzed. Analysis of thiol accumulation in the roots revealed that the different amounts of these compounds in Capataz and Beirao, as well as the expression levels of genes involved in phytochelatin biosynthesis and direct Cd sequestration into the vacuoles of the root cells, were not related to the capacity of the accessions to trap the metal into the roots. Interestingly, the relative transcript abundance of OsHMA2, a gene controlling root-to-shoot Cd/Zn translocation, was not influenced by Cd exposure in Capataz and progressively increased in Beirao with the external Cd concentration, suggesting that activity of the OsHMA2 transporter may differentially limit root-to-shoot Cd/Zn translocation in Capataz and Beirao.

The critical role of the shoot base in inhibiting cadmium transport from root to shoot in a cadmium-safe rice line (Oryza sativa L.)

Cadmium (Cd) is harmful to rice and human, thus screening and understanding the mechanism of Cd-safe rice lines, which accumulate little Cd in brown rice, is necessary. D62B was screened as a Cd-safe rice line with low Cd translocation from roots to shoots, and there must be a switch restricting Cd transport from roots to shoots. Here we found that shoot base played the role as switch. Cd concentration in the shoot base of D62B was 1.57 times higher compared with a high Cd-accumulating rice line (Wujin4B) and lower Cd translocation under Cd stress. Glutathione (GSH) and phytochelatins (PCs) were important in this process. GSH and PCs concentrations in the shoot bases of D62B were 1.01- 1.83 times higher than Wujin4B as well as the glutathione S-transferase (GST) and phytochelatin synthase (PCS) concentrations, keeping in consistent with up-regulation of the genes OsGST and OsPCS1. PCs synthesis was further promoted by exogenous GSH. Our results prove the role of shoot bases as switch for restricting Cd transport in D62B due to its great potential for GSH and PCs biosynthesis, and thereby Cd chelation. This could be considered a key mechanism for low Cd accumulation in brown rice of the Cd-safe rice line.

Genetic loci regulating cadmium content in rice grains

It has been estimated that up to 90% of human exposure to cadmium is through food, and that cadmium within rice grains can be a major contributor to that dietary source. In this study genome wide association mapping was conducted on the Bengal and Assam Aus Panel (BAAP) of rice to identify quantitative trait loci and candidate genes for lowering grain cadmium. Field experiments were conducted over two years under two different irrigation systems: continually flooded and alternate wetting and drying (AWD). There was significant effects of water treatment, genotype, and genotype by water treatment interaction. Importantly, AWD increased grain cadmium, on average, by 49.6% and 108.8% in year 1 and 2 respectively. There was between 4.6 and 28 fold variation in cadmium concentration. A total of 58 QTLs were detected but no loci are clearly specific to one water regime despite approximately 20% of variation attributable to genotype by water regime interaction. A number of QTLs were consistent across most water treatments and years. These included QTLs on chromosome 7 (7.23-7.61, 8.93-9.04, and 29.12-29.14 Mbp), chromosome 5 (8.66-8.72 Mbp), and chromosome 9 (11.46-11.64 Mbp). Further analysis of the loci on chromosome 7 (8.93-9.04 Mbp), identified the candidate gene OsNRAMP1, where cultivars with a deletion upstream of the gene had higher concentrations of cadmium compared to the cultivars that did not have the deletion. The distribution of alleles within the BAAP suggest this QTL is easily detected in this population because it is composed of aus cultivars. Local genome cluster analysis suggest high Cd alleles are uncommon, but should be avoided in breeding.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at (10.1007/s10681-020-02752-1).

Cadmium stress in paddy fields: Effects of soil conditions and remediation strategies

Cadmium (Cd) toxicity in paddy soil and accumulation in rice plants and grains have got global concern due to its health effects. This review highlights the effects of soil factors including soil organic matter, soil pH, redox potential, and soil microbes which influencing Cd uptake by rice plant. Therefore, a comprehensive review of innovative and environmentally friendly management practices for managing Cd stress in rice is lacking. Thus, this review discusses the effect of Cd toxicity in rice and describes management strategies to offset its effects. Moreover, future research thrusts to reduce its uptake by rice has also been highlighted. Through phytoremediation, Cd may be extracted and stabilized in the soil while through microbes Cd can be sequestrated inside the microbial bodies. Increased Cd uptake in hyperaccumulator plants to remediate and convert the toxic form of Cd into non-toxic forms. While in chemical remediation, Cd can be washed out, immobilized and stabilized in the soil through chemical amendments. The organic amendments may help through an increase in soil pH, adsorption in its functional groups, the formation of complexations, and the conversion of exchangeable to residual forms. Developing rice genotypes with restricted Cd uptake and reduced accumulation in grain through conventional and marker-assisted breeding are fundamental keys for safe rice production. In this regard, the use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics may be quite helpful.

Differential cadmium translocation and accumulation between Nicotiana tabacum L. and Nicotiana rustica L. by transcriptome combined with chemical form analyses

Cadmium (Cd) is a severely toxic and carcinogenic heavy metal. Cigarette smoking is one of the major source of Cd exposure in humans. Nicotiana tabacum is primarily a leaf Cd accumulator, while Nicotiana rustica is a root Cd accumulator among Nicotiana species. However, little is known about the mechanisms of differential Cd translocation and accumulation in Nicotiana. To find the key factors, Cd concentration, Cd chemical forms, and transcriptome analysis were comparatively studied between N. tabacum and N. rustica under control or 10 μM Cd stress. The leaf/root Cd concentration ratio of N. tabacum was 2.26 and that of N. rustica was 0.14. The Cd concentration in xylem sap of N. tabacum was significantly higher than that of N. rustica. The root of N. tabacum had obviously higher proportion of ethanol extractable Cd (40%) and water extractable Cd (16%) than those of N. rustica (16% and 6%). Meanwhile the proportion of sodium chloride extracted Cd in N. rustica (71%) was significantly higher than that in N. tabacum (30%). A total of 30710 genes expressed differentially between the two species at control, while this value was 30,294 under Cd stress, among which 27,018 were collective genes, manifesting the two species existed enormous genetic differences. KEGG pathway analysis showed the phenylpropanoid biosynthesis pathway was overrepresented between the two species under Cd stress. Several genes associated with pectin methylesterase, suberin and lignin synthesis, and heavy metal transport were discovered to be differential expressed genes between two species. The results suggested that the higher accumulation of Cd in the leaf of N. tabacum depends on a comprehensive coordination of Cd transport, including less cell wall binding, weaker impediment by the Casparian strip, and efficient xylem loading.

Overexpression of the manganese/cadmium transporter OsNRAMP5 reduces cadmium accumulation in rice grain

Rice is a major dietary source of the toxic metal cadmium (Cd), and reducing its accumulation in the grain is therefore important for food safety. We selected two cultivars with contrasting Cd accumulation and generated transgenic lines overexpressing OsNRAMP5, which encodes a major influx transporter for manganese (Mn) and Cd. We used two different promoters to control the expression, namely OsActin1 and maize Ubiquitin. Overexpression of OsNRAMP5 increased Cd and Mn uptake into the roots, but markedly decreased Cd accumulation in the shoots, whilst having a relatively small effect on Mn accumulation in the shoots. The overexpressed OsNRAMP5 protein was localized to the plasma membrane of all cell types in the root tips and lateral root primordia without polarity. Synchrotron X-ray fluorescence mapping showed that the overexpression lines accumulated more Cd in the root tips and lateral root primordia compared with the wild-type. When grown in three Cd-contaminated paddy soils, overexpression of OsNRAMP5 decreased concentration of Cd in the grain by 49-94% compared with the wild type. OsNRAMP5-overexpression plants had decreased Cd translocation from roots to shoots as a result of disruption of its radial transport into the stele for xylem loading, demonstrating the effect of transporter localization and polarity on ion homeostasis.

Overexpression of Rice OsHMA3 in Wheat Greatly Decreases Cadmium Accumulation in Wheat Grains

Cereals are a major dietary source of the toxic metal cadmium (Cd). Reducing Cd accumulation in cereal crops such as wheat (Triticum aestivum) is important for food safety and human health. In this study, we show that three diverse cultivars of wheat had a high Cd translocation from roots to shoots, similar to a rice (Oryza sativa) cultivar possessing a nonfunctional tonoplast Cd transporter OsHMA3. We investigated the function of TaHMA3 genes in wheat. Three TaHMA3 genes were identified in wheat, all of which encode tonoplast-localized proteins. However, heterologous expression of TaHMA3 genes in yeast showed no transport activities for Cd, which likely explains the low Cd sequestration in wheat roots and subsequently the high Cd translocation to wheat shoots. To increase Cd sequestration in wheat roots, we overexpressed a rice functional OsHMA3 gene in wheat driven by a strong constitutive Ubiquitin promoter. Overexpression of the OsHMA3 gene decreased root-to-shoot Cd translocation in wheat by nearly 10-fold and Cd accumulation in wheat grain by 96%. The results suggest that high Cd translocation is a common trait in wheat caused by a loss of the Cd transport function of TaHMA3 proteins. Transgenic wheat overexpressing a functional OsHMA3 gene offers a highly effective solution to decrease Cd accumulation in wheat grain.

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.

Reducing Cadmium Accumulation in Plants: Structure-Function Relations and Tissue-Specific Operation of Transporters in the Spotlight

Cadmium (Cd) is present in many soils and, when entering the food chain, represents a major health threat to humans. Reducing Cd accumulation in plants is complicated by the fact that most known Cd transporters also operate in the transport of essential nutrients such as Zn, Fe, Mn, or Cu. This work summarizes the current knowledge of mechanisms mediating Cd uptake, radial transport, and translocation within the plant. It is concluded that real progress in the field may be only achieved if the transport of Cd and the above beneficial micronutrients is uncoupled, and we discuss the possible ways of achieving this goal. Accordingly, we suggest that the major focus of research in the field should be on the structure-function relations of various transporter isoforms and the functional assessment of their tissue-specific operation. Of specific importance are two tissues. The first one is a xylem parenchyma in plant roots; a major "controller" of Cd loading into the xylem and its transport to the shoot. The second one is a phloem tissue that operates in the last step of a metal transport. Another promising and currently underexplored avenue is to understand the role of non-selective cation channels in Cd uptake and reveal mechanisms of their regulation.