Impact of osmoregulation on the differences in Cd accumulation between two contrasting edible amaranth cultivars grown on Cd-polluted saline soils

NtGCN2 confers cadmium tolerance in Nicotiana tabacum L. by regulating cadmium uptake, efflux, and subcellular distribution

General control non-derepressible-2 (GCN2) is widely expressed in eukaryotes and responds to biotic and abiotic stressors. However, the precise function and mechanism of action of GCN2 in response to cadmium (Cd) stress in Nicotiana tabacum L. (tobacco) remains unclear. We investigated the role of NtGCN2 in Cd tolerance and explored the mechanism by which NtGCN2 responds to Cd stress in tobacco by exposing NtGCN2 transgenic tobacco lines to different concentrations of CdCl2. NtGCN2 was activated under 50 μmol·L-1 CdCl2 stress and enhanced the Cd tolerance and photosynthetic capacities of tobacco by increasing chlorophyll content and antioxidant capacity by upregulating NtSOD, NtPOD, and NtCAT expression and corresponding enzyme activities and decreasing malondialdehyde and O2·- contents. NtGCN2 enhanced the osmoregulatory capacity of tobacco by elevating proline (Pro) and soluble sugar contents and maintaining low levels of relative conductivity. Finally, NtGCN2 enhanced Cd tolerance in tobacco by reducing Cd uptake and translocation, promoting Cd efflux, and regulating Cd subcellular distribution. In conclusion, NtGCN2 improves the tolerance of tobacco to Cd through a series of mechanisms, namely, increasing antioxidant, photosynthetic, and osmoregulation capacities and regulating Cd uptake, translocation, efflux, and subcellular distribution. This study provides a scientific basis for further exploration of the role of NtGCN2 in plant responses to Cd stress and enhancement of the Cd stress signaling network in tobacco.

Mechanisms of low cadmium accumulation in crops: A comprehensive overview from rhizosphere soil to edible parts

Cadmium (Cd) is a toxic heavy metal often found in soil and agricultural products. Due to its high mobility, Cd poses a significant health risk when absorbed by crops, a crucial component of the human diet. This absorption primarily occurs through roots and leaves, leading to Cd accumulation in edible parts of the plant. Our research aimed to understand the mechanisms behind the reduced Cd accumulation in certain crop cultivars through an extensive review of the literature. Crops employ various strategies to limit Cd influx from the soil, including rhizosphere microbial fixation and altering root cell metabolism. Additional mechanisms include membrane efflux, specific transport, chelation, and detoxification, facilitated by metalloproteins such as the natural resistance-associated macrophage protein (Nramp) family, heavy metal P-type ATPases (HMA), zinc-iron permease (ZIP), and ATP-binding cassette (ABC) transporters. This paper synthesizes differences in Cd accumulation among plant varieties, presents methods for identifying cultivars with low Cd accumulation, and explores the unique molecular biology of Cd accumulation. Overall, this review provides a comprehensive resource for managing agricultural lands with lower contamination levels and supports the development of crops engineered to accumulate minimal amounts of Cd.

NaCl-mediated strategies for the trade-off between Cd bioconcentration and translocation in Solanum nigrum L

Salt interference significantly affects the behavior of heavy metals in the environment. This study compared and analyzed the response process, migration, and transformation of cadmium (Cd) in the hyperaccumulator Solanum nigrum (S. nigrum) under different NaCl levels to reveal the interference mechanisms of salt in plant remediation of Cd-contaminated soil. The results showed that Cd and salt stress significantly inhibited the growth of plants. The stress effect had more potent growth inhibition at the root than aboveground, thus inducing changes in the spatial configuration of the plants (decreased root-to-aboveground biomass ratio). Salt could activate Cd in plants, enhancing the inhibitory effect on plant growth. Salt increased Cd bioavailability due to the rhizosphere acidification effect, increasing plants' Cd accumulation. The Cd bioconcentration factor in plant roots peaked during the high Cd-high salt treatment (117.10), but the Cd accumulation of plants peaked during the high Cd-low salt treatment (233.04 μg plant-1). Salt additions and increased Cd concentrations enhanced root compartmentalization, inhibiting Cd transport to the aboveground. Changes in Fourier-transform infrared spectroscopy (FTIR) measurements confirmed that the functional groups in plants provided binding sites for Cd. These findings can help guide the phytoremediation of Cd contamination under saline soil conditions.

Hydrogel-potassium humate composite alleviates cadmium toxicity of tobacco by regulating Cd bioavailability

Cadmium (Cd) removal from soil to reduce Cd accumulation in plants is essential for agroecology, food safety, and human health. Cd enters plants from soil and affects plant growth and development. Hydrogels can easily combine with Cd, thereby altering its bioavailability in soil. However, few studies have evaluated the effects of hydrogel on the complex phytotoxicity caused by Cd uptake in plants and the microbial community structure. Herein, a new poly (acrylic acid)-grafted starch and potassium humate composite (S/K/AA) hydrogel was added to soil to evaluate its impact on tobacco growth and the soil microenvironment. The results indicate that the addition of S/K/AA hydrogel can significantly improve the biomass, chlorophyll (Chl) content, and photosynthetic capacity of tobacco plants during Cd stress conditions, and decrease Cd concentration, probably by affecting Cd absorption through the expression of Cd absorption transporters (e.g., NRAMP5, NRAMP3, and IRT1). Moreover, the application of S/K/AA hydrogel not only reduced the accumulation of reactive oxygen species (ROS), but also reduced the antioxidant activities of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT), suggesting that S/K/AA hydrogel alleviates Cd toxicity via a non-antioxidant pathway. Notably, we further analyzed the effectiveness of the hydrogel on microbial communities in Cd-contaminated soil and found that it increased the Cd-tolerant microbial community (Arthrobacter, Massilia, Streptomyces), enhancing the remediation ability of Cd-contaminated soil and helping tobacco plants to alleviate Cd toxicity. Overall, our study provides primary insights into how S/K/AA hydrogel affects Cd bioavailability and alleviates Cd toxicity in plants.

Mapping and Identifying Candidate Genes Enabling Cadmium Accumulation in Brassica napus Revealed by Combined BSA-Seq and RNA-Seq Analysis

Rapeseed has the ability to absorb cadmium in the roots and transfer it to aboveground organs, making it a potential species for remediating soil cadmium (Cd) pollution. However, the genetic and molecular mechanisms underlying this phenomenon in rapeseed are still unclear. In this study, a 'cadmium-enriched' parent, 'P1', with high cadmium transport and accumulation in the shoot (cadmium root: shoot transfer ratio of 153.75%), and a low-cadmium-accumulation parent, 'P2', (with a cadmium transfer ratio of 48.72%) were assessed for Cd concentration using inductively coupled plasma mass spectrometry (ICP-MS). An F₂ genetic population was constructed by crossing 'P1' with 'P2' to map QTL intervals and underlying genes associated with cadmium enrichment. Fifty extremely cadmium-enriched F₂ individuals and fifty extremely low-accumulation F₂ individuals were selected based on cadmium content and cadmium transfer ratio and used for bulk segregant analysis (BSA) in combination with whole genome resequencing. This generated a total of 3,660,999 SNPs and 787,034 InDels between these two segregated phenotypic groups. Based on the delta SNP index (the difference in SNP frequency between the two bulked pools), nine candidate Quantitative trait loci (QTLs) from five chromosomes were identified, and four intervals were validated. RNA sequencing of 'P1' and 'P2' in response to cadmium was also performed and identified 3502 differentially expressed genes (DEGs) between 'P1' and 'P2' under Cd treatment. Finally, 32 candidate DEGs were identified within 9 significant mapping intervals, including genes encoding a glutathione S-transferase (GST), a molecular chaperone (DnaJ), and a phosphoglycerate kinase (PGK), among others. These genes are strong candidates for playing an active role in helping rapeseed cope with cadmium stress. Therefore, this study not only sheds new light on the molecular mechanisms of Cd accumulation in rapeseed but could also be useful for rapeseed breeding programs targeting this trait.

The beneficial effects of bare and CMC-supported α-FeOOH, Fe3O4, and α-Fe2O3 nanoparticles on growth, nutrient content, and essential oil of summer savory (Satureja hortensis L.) under Cd, Pb and Zn stresses

This research studies the impacts of iron oxide nanoparticles (FeONPs) on alleviating the toxic effects of cadmium (Cd), lead (Pb), and zinc (Zn) on summer savory (Satureja hortensis L.). Different types of soil additives, including bare and carboxymethylcellulose (CMC)-supported hematite (α-Fe₂O₃), goethite (α-FeOOH), and magnetite (Fe₃O₄), were applied at three rates (0, 0.25, and 0.5% w/w) to a Cd, Pb, and Zn-contaminated soil sample. The experimental results showed that the application of FeONPs increased plant height, dry weights of shoot and root, and yield and content of essential oil. Bare and CMC-supported FeONPs increased the content of K, P, and Fe in the aerial parts of summer savory. However, these soil additives reduced the contents of Cd, Pb, and Zn in plant tissues. CMC-supported FeONPs proved to be more efficient additives in diminishing the toxic effects of Cd, Pb, and Zn in summer savory compared to their bare forms. Bare and CMC-supported goethite NPs were able to restrict the uptake of Cd, Pb, and Zn by summer savory roots in the metal-contaminated soil. The application of CMC-supported goethite at an application dose of 0.5% (w/w) increased shoot dry weight, shoot concentrations of K, P, and Fe, and yield of essential oil by about 62.6, 76.6, 77.1, 210, and 230%, respectively. Conversely, they reduced shoot concentrations of Cd, Pb, and Zn by about 64.6, 68.7, and 40.6%, respectively, compared to the control. These are significant results and indicate that CMC-supported goethite is likely to be the most effective soil additive in diminishing the toxicity of Cd, Pb, and Zn to metal-stressed summer savory.

Salt stress-induced changes in soil metabolites promote cadmium transport into wheat tissues

Soil salinity is known to improve cadmium (Cd) mobility, especially in arid soils. However, the mechanisms involved in how salt stress-associated metabolic profiles participate in mediating Cd transport in the soil-plant system remain poorly understood. This study was designed to investigate the effects of salinity-induced changes in soil metabolites on Cd bioavailability. Sodium salts in different combinations according to molar ratio (NaCl:Na₂SO₄=1:1; NaCl:Na₂SO₄:NaHCO₃=1:2:1; NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃=1:9:9:1; NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃=1:1:1:1) were applied to the Cd-contaminated soils, which increased soil Cd availability by 22.36% and the Cd content in wheat grains by 36.61%, compared to the control. Salt stress resulted in soil metabolic reprogramming, which might explain the decreased growth of wheat plants and increased Cd transport from the soil into wheat tissues. For example, down-regulation of starch and sucrose metabolism reduced the production of sugars, which adversely affected growth; up-regulation of fatty acid metabolism allowed wheat plants to maintain a normal intracellular environment under saline conditions; up-regulation of the tricarboxylic acid (TCA) cycle was triggered, causing an increase in organic acid synthesis and the accumulation of organic acids, which facilitated the migration of soil Cd into wheat tissues. In summary, salt stress can facilitate Cd transport into wheat tissues by the direct effect of salt-based ions and the combined effect of altered soil physicochemical properties and soil metabolic profiles in Cd-contaminated soils.

Salinity elevates Cd bioaccumulation of sea rice cultured under co-exposure of cadmium and salt

Salt-tolerant rice (sea rice) is a key cultivar for increasing rice yields in salinity soil. The co-existence of salinity and cadmium (Cd) toxicities in the plant-soil system has become a great challenge for sustainable agriculture, especially in some estuaries and coastal areas. However, little information is available on the Cd accumulating features of sea rice under the co-stress of Cd and salinity. In this work, a hydroponic experiment with combined Cd (0, 0.2, 0.8 mg/L Cd²⁺) and saline (0, 0.6%, and 1.2% NaCl, W/V) levels and a pot experiment were set to evaluate the Cd toxic risks of sea rice. The hydroponic results showed that more Cd accumulated in sea rice than that in the reported high-Cd-accumulating rice, Chang Xianggu. It indicated an interesting synergistic effect between Cd and Na levels in sea rice, and the Cd level rose significantly with a concomitant increase in Na level in both shoot (r = 0.54, p < 0.01) and root (r = 0.66, p < 0.01) of sea rice. Lower MDA content was found in sea rice, implying that the salt addition probably triggered the defensive ability against oxidative stress. The pot experiment indicated that the coexistent Cd and salinity stress further inhibited the rice growth and rice yield, and the Cd concentration in rice grain was below 0.2 mg/kg. Collectively, this work provides a general understanding of the co-stress of Cd and salinity on the growth and Cd accumulation of sea rice. Additional work is required to precisely identify the phytoremediation potential of sea rice in Cd-polluted saline soil.

Effects of Arbuscular Mycorrhizal Fungi on Alleviating Cadmium Stress in Medicago truncatula Gaertn

Heavy metal contamination is a global problem for ecosystems and human health. Remediation of contaminated soils has received much attention in the last decade. Aided mitigation of heavy metal phytotoxicity by arbuscular mycorrhizal fungi (AMF) is a cost-effective and environmentally friendly strategy. This study was carried out to investigate the mitigation effect of AMF inoculation on heavy metal toxicity in Medicago truncatula under soil cadmium stress. Therefore, a pot experiment was designed to evaluate the growth, chlorophyll fluorescence, Cd uptake and distribution, malondialdehyde (MDA) content, root soil physicochemical properties, and metabolite profile analysis of M. truncatula with/without AMF inoculation in Cd (20 mg/Kg)-contaminated soil. The results showed that inoculating AMF under Cd stress might enhance photosynthetic efficiency, increase plant biomass, decrease Cd and MDA content, and improve soil physicochemical properties in M. truncatula. Non-targeted metabolite analysis revealed that inoculation with AMF under Cd stress significantly upregulated the production of various amino acids in inter-root metabolism and increase organic acid and phytohormone synthesis. This study provides information on the physiological responses of mycorrhizal plants to heavy metal stress, which could help provide deeper insight into the mechanisms of heavy metal remediation by AMF.

Zinc oxide nanoparticles mediated biostimulant impact on cadmium detoxification and in silico analysis of zinc oxide-cadmium networks in Zea mays L. regulome

Cadmium (Cd) toxicity can significantly limit plant growth and development. To eliminate the toxic effects of Cd stress, we intended to evaluate the biochemical mediated physiological responses in maize treated with biostimulant and zinc oxide nanoparticles (ZnPs). In silico analysis exhibited that the maize treated with Cd stress (200 μM) had an adverse impact on CAT1, CAT2, CAT3 and gor1 proteins, which are influential in managing the machinery of redox homeostasis. While maize inoculated with bacteria-based biostimulant and ZnPs (10 ppm) showed prominently improved biomass, chlorophyll a, b and carotenoid content. We found a significant increase in the total sugar, protein, proline content and antioxidants under the effect of Cd stress. However, these parameters are further enhanced by applying biostimulants and ZnPs. Declined lipid peroxidation and membrane solubilization index under the effect of biostimulant and ZnPs was observed. Furthermore, these treatments improved maize's zinc, copper, sodium, magnesium, iron, potassium and calcium content. Based on these results, an antagonistic relationship between Zn and Cd uptake that triggered efficient Cd detoxification in maize shoot was found. Scanning electron micrography showed distorted leaf structure of the Cd stressed plants while the biostimulant and ZnPs reduced the structural cell damage of maize leaves. In silico study showed that ZnO positively regulates all protein interactors, including GRMZM2G317386_P01 (Metallo endo proteinase 1-MMP), GRMZM2G110220_P01 (Metallo endo proteinase 5-MMP), GRMZM2G103055_P01 (Alpha-amylase) and GRMZM2G006069_P01 (Zn-dependent exo peptidase superfamily) proteins which are involved in energy generating processes, channels formation, matrix re-localization and stress response. This suggests that ZnO offers an ideal role with protein interactors in maize. Our findings depict that these treatments, i.e., biostimulant and ZnPs alone, are efficient enough to exhibit Cd remediation potential in maize; however, their combination showed synergistic effects.

Zinc Biofortification through Basal Zinc Supply Reduces Grain Cadmium in Mung Beans: Metal Partitioning and Health Risks Assessment

Grain zinc (Zn) biofortification with less cadmium (Cd) accumulation is of paramount importance from human health and environmental point of view. A pot experiment was carried out to determine the influence of Zn and Cd on their accumulations in Mung bean tissues (Vigna radiata) in two contrast soil types (Dermosol and Tenosol). The soil types with added Zn and Cd exerted a significant effect on translocation and accumulation of metals in different tissues. The accumulation of Zn and Cd was higher for Tenosol than that for Dermosol. At control, the concentration of Cd followed a pattern, e.g., root > stem > petiole > pod > leaflet > grain for both soils. A basal Zn supply (5 mg kg−1) increased the grain Zn concentration to a significant amount (up to 67%). It also reduced Cd accumulation in tissues, including grains (up to 34%). No non-carcinogenic effect was observed for either the children or the adults as the EDI and PTDI values were below the safety limit; however, the ILCR values exceeded the safety limit, indicating the possibility of some carcinogenic effects. Added Zn helped to reduce the carcinogenic and non-carcinogenic health risks on humans.

Elevated CO2 differentially mitigates chromium (VI) toxicity in two rice cultivars by modulating mineral homeostasis and improving redox status

Chromium (Cr) contamination reduces crop productivity worldwide. On the other hand, the expected increase in the future CO₂ levels (eCO₂) would improve plant growth under diverse growth conditions. However, the synergetic effect of eCO₂ has not been investigated at both physiological and biochemical levels in Cr-contaminated soil. This study aims to analyze the mitigating effect of eCO₂ on Cr VI phytotoxicity in two rice cultivars (Giza 181 and Sakha 106). Plants are exposed to different Cr concentrations (0, 200 and 400 mg Cr/kg Soil) at ambient (aCO₂) and eCO₂ (410 and 620 ppm, respectively). Unlike the stress parameters (MDA, H2O2 and protein oxidation), growth and photosynthetic reactions significantly dropped with increasing Cr concentration. However, in eCO₂ conditions, plants were able to mitigate the Cr stress by inducing antioxidants as well as higher concentrations of phytochelatins to detoxify Cr. Notably, the expression levels of the genes involved in mineral nutrition i.e., OsNRAMP1, OsRT1, OsHMA3, OsLCT1 and iron chelate reductase were upregulated in Cr-stressed Giza 181 plants grown under eCO₂. Mainly in Sakha 106, eCO₂ induced ascorbate-glutathione (ASC/GSH)-mediated antioxidative defense system. The present study brings the first ever comprehensive assessment of how future eCO₂ differentially mitigated Cr toxicity in rice.

Indole pyruvate decarboxylase gene regulates the auxin synthesis pathway in rice by interacting with the indole-3-acetic acid-amido synthetase gene, promoting root hair development under cadmium stress

This research focused on cadmium (Cd), which negatively affects plant growth and auxin hemostasis. In plants, many processes are indirectly controlled through the expression of certain genes due to the secretion of bacterial auxin, as indole-3-acetic acid (IAA) acts as a reciprocal signaling molecule in plant-microbe interaction. The aim of current studies was to investigate responsible genes in rice for plant-microbe interaction and lateral root development due to the involvement of several metabolic pathways. Studies revealed that GH3-2 interacts with endogenous IAA in a homeostasis manner without directly providing IAA. In rice, indole-3-pyruvate decarboxylase (IPDC) transgenic lines showed a 40% increase in lateral roots. Auxin levels and YUCCA (auxin biosynthesis gene) expression were monitored in osaux1 mutant lines inoculated with Bacillus cereus exposed to Cd. The results showed an increase in root hairs (RHs) and lateral root density, changes in auxin levels, and expression of the YUCCA gene. B. cereus normalizes the oxidative stress caused by Cd due to the accumulation of O 2 - and H₂O₂ in osaux1 mutant lines. Furthermore, the inoculation of B. cereus increases DR5:GUS expression, indicating that bacterial species have a positive role in auxin regulation. Thus, the current study suggests that B. cereus and IPDC transgenic lines increase the RH development in rice by interacting with IAA synthetase genes in the host plant, alleviating Cd toxicity and enhancing plant defense mechanisms.

Toxic effects of microplastics in plants depend more by their surface functional groups than just accumulation contents

Differentially charged microplastics (MPs) engendered by plastic aging (e.g., plastic film) widely existed in the agricultural ecosystem, yet minimal was known about the toxic effects of MPs on plants and their absorption and accumulation characteristics. Root absorption largely determined the migration and accumulation risks of MPs in the soil-crop food chain. Here, five types of MPs exposure experiments of leaf lettuce were implemented to simulate root absorption by hydroponics. MPs exposure caused different degrees of growth inhibition, root lignification, root cell apoptosis, and oxidative stress responses; accelerated chlorophyll decomposition and hampered normal electron transfer within the PSII photosystem. Moreover, the uptake of essential elements by roots was inhibited to varying degrees due to the pore blockage in the cell wall and the hetero-aggregation of opposite charges after MPs exposure. MPs exposure observably up-regulated the organic metabolic pathways in roots, thus affecting MPs mobility and absorption through the electrostatic and hydrophobic interactions between the root exudations and MPs. Importantly, MPs penetrated the root extracellular cortex into the stele and were transported to the shoots by transpiration through xylem vessels based on confocal laser scanning microscopy and scanning electron microscopy images. Quantitative analysis of MPs indicated that their toxic effects on plants were determined to a greater extent by the types of surface functional groups than just their accumulation contents, that is, MPs were confirmed edible risks through crop food chain transfer, but bioaccumulation varied by surface functional groups.

Rhizobacteria communities reshaped by red mud based passivators is vital for reducing soil Cd accumulation in edible amaranth

Red mud (RM) was constantly reported to immobilize soil cadmium (Cd) and reduce Cd uptake by crops, but few studies investigated whether and how RM influenced rhizobacteria communities, which was a vital factor determining Cd bioavailability and plant growth. To address this concern, high-throughput sequencing and bioinformatics were used to analyze microbiological mechanisms underlying RM application reducing Cd accumulation in edible amaranth. Based on multiple statistical models (Detrended correspondence analysis, Bray-Curtis, weighted UniFrac, and Phylogenetic tree), this study found that RM reduced Cd content in plants not only through increasing rhizosphere soil pH, but by reshaping rhizobacteria communities. Special taxa (Alphaproteobacteria, Gammaproteobacteria, Actinobacteriota, and Gemmatimonadota) associated with growth promotion, anti-disease ability, and Cd resistance of plants preferentially colonized in the rhizosphere. Moreover, RM distinctly facilitated soil microbes' proliferation and microbial biofilm formation by up-regulating intracellular organic metabolism pathways and down-regulating cell motility metabolic pathways, and these microbial metabolites/microbial biofilm (e.g., organic acid, carbohydrates, proteins, S^2-, and PO₄^3-) and microbial cells immobilized rhizosphere soil Cd via the biosorption and chemical chelation. This study revealed an important role of reshaped rhizobacteria communities acting in reducing Cd content in plants after RM application.

Nitrogen fertilizer affects rhizosphere Cd re-mobilization by mediating gene AmALM2 and AmALMT7 expression in edible amaranth roots

In-situ stabilization of Cd-contaminated farmland is a commonly used remediation technology. Yet, rhizosphere metabolites (e.g., organic acids) during crop cultivation may cause Cd re-mobilization and over-accumulation. Here, we identified four pivotal cytomembrane-localized genes underlying Cd accumulation difference between two contrasting edible amaranth cultivars based on root gene expression profile, studied their subcellular localization and functional characteristics, and then investigated effects of nitrogen fertilizer on their expression and rhizosphere Cd re-mobilization. Results showed that more Cd accumulated by edible amaranth was due to rhizosphere Cd mobilization by mediating high expression of AmALMT2 and AmALMT7 genes, not Cd transporters in roots. This was confirmed by heterologous expression of AmALMT2 and AmALMT7 genes in Arabidopsis thaliana, since they mediated malic, fumaric, succinic, and aspartic acids efflux. Furthermore, nitrogen influencing rhizosphere acidification might be closely associated with organic acids efflux genes. Compared with N-NO₃^- application, N-NH₄⁺ was massively assimilated into glutamates and oxaloacetates through up-regulating glutamine synthetase and alanine-aspartate-glutamate metabolic pathways, thereby enhancing TCA cycle and organic acids efflux dominated by binary carboxylic acids via up-regulating AmALMT2 and AmALMT7 genes, which finally caused Cd re-mobilization. Therefore, N-NO₃^--dominated nitrogen retarded rhizosphere Cd re-mobilization via inhibiting organic acids efflux function of AmALMT2 and AmALMT7 proteins.

ROS and NO Phytomelatonin-Induced Signaling Mechanisms under Metal Toxicity in Plants: A Review

Metal toxicity in soils, along with water runoff, are increasing environmental problems that affect agriculture directly and, in turn, human health. In light of finding a suitable and urgent solution, research on plant treatments with specific compounds that can help mitigate these effects has increased, and thus the exogenous application of melatonin (MET) and its role in alleviating the negative effects of metal toxicity in plants, have become more important in the last few years. MET is an important plant-related response molecule involved in growth, development, and reproduction, and in the induction of different stress-related key factors in plants. It has been shown that MET plays a protective role against the toxic effects induced by different metals (Pb, Cd, Cu, Zn, B, Al, V, Ni, La, As, and Cr) by regulating both the enzymatic and non-enzymatic antioxidant plant defense systems. In addition, MET interacts with many other signaling molecules, such as reactive oxygen species (ROS) and nitric oxide (NO) and participates in a wide variety of physiological reactions. Furthermore, MET treatment enhances osmoregulation and photosynthetic efficiency, and increases the concentration of other important antioxidants such as phenolic compounds, flavonoids, polyamines (PAs), and carotenoid compounds. Some recent studies have shown that MET appeared to be involved in the regulation of metal transport in plants, and lastly, various studies have confirmed that MET significantly upregulated stress tolerance-related genes. Despite all the knowledge acquired over the years, there is still more to know about how MET is involved in the metal toxicity tolerance of plants.

ROS and NO Phytomelatonin-Induced Signaling Mechanisms under Metal Toxicity in Plants: A Review

Metal toxicity in soils, along with water runoff, are increasing environmental problems that affect agriculture directly and, in turn, human health. In light of finding a suitable and urgent solution, research on plant treatments with specific compounds that can help mitigate these effects has increased, and thus the exogenous application of melatonin (MET) and its role in alleviating the negative effects of metal toxicity in plants, have become more important in the last few years. MET is an important plant-related response molecule involved in growth, development, and reproduction, and in the induction of different stress-related key factors in plants. It has been shown that MET plays a protective role against the toxic effects induced by different metals (Pb, Cd, Cu, Zn, B, Al, V, Ni, La, As, and Cr) by regulating both the enzymatic and non-enzymatic antioxidant plant defense systems. In addition, MET interacts with many other signaling molecules, such as reactive oxygen species (ROS) and nitric oxide (NO) and participates in a wide variety of physiological reactions. Furthermore, MET treatment enhances osmoregulation and photosynthetic efficiency, and increases the concentration of other important antioxidants such as phenolic compounds, flavonoids, polyamines (PAs), and carotenoid compounds. Some recent studies have shown that MET appeared to be involved in the regulation of metal transport in plants, and lastly, various studies have confirmed that MET significantly upregulated stress tolerance-related genes. Despite all the knowledge acquired over the years, there is still more to know about how MET is involved in the metal toxicity tolerance of plants.

Red mud based passivator reduced Cd accumulation in edible amaranth by influencing root organic matter metabolism and soil aggregate distribution

Red mud was a highly alkaline hazardous waste, and their resource utilization was a research hotspot. In this study, influencing mechanisms of red mud based passivator on the transformation of Cd fraction in acidic Cd-polluted soil, photosynthetic property, and Cd accumulation in edible amaranth were investigated based on the evaluation of Cd adsorption capacity, root metabolic response, and soil aggregate distribution. Results showed that red mud exhibited good Cd adsorption capacities at about 35 °C and pH 9 in an aqueous solution, and the adsorption behavior of red mud on Cd in rhizosphere soil solution was considered to have some similarity. In the soil-pot trial, red mud application significantly facilitated edible amaranth growth by enhancing the maximum photochemical efficiency and light energy absorption by per unit leaf area by activating more reaction centers. The main mechanisms of rhizosphere soil Cd immobilisation by red mud application included: i) the reduction of mobilized Cd caused by the increasing negative surface charge of soil and precipitation of Cd hydroxides and carbonates at high pH; ii) the increase of organics-Cd complexes caused by the increasing -OH and -COOH amounts adsorbed on the surface of rhizosphere soil after red mud application; and iii) the decrease of available Cd content in soil aggregates caused by the increasing organic matters after red mud application. This study would provide the basis for the safe utilization of red mud remediating acidic Cd-polluted soil.

Cadmium toxicity in plants: Impacts and remediation strategies

Cadmium (Cd) is an unessential trace element in plants that is ubiquitous in the environment. Anthropogenic activities such as disposal of urban refuse, smelting, mining, metal manufacturing, and application of synthetic phosphate fertilizers enhance the concentration of Cd in the environment and are carcinogenic to human health. In this manuscript, we reviewed the sources of Cd contamination to the environment, soil factors affecting the Cd uptake, the dynamics of Cd in the soil rhizosphere, uptake mechanisms, translocation, and toxicity of Cd in plants. In crop plants, the toxicity of Cd reduces uptake and translocation of nutrients and water, increases oxidative damage, disrupts plant metabolism, and inhibits plant morphology and physiology. In addition, the defense mechanism in plants against Cd toxicity and potential remediation strategies, including the use of biochar, minerals nutrients, compost, organic manure, growth regulators, and hormones, and application of phytoremediation, bioremediation, and chemical methods are also highlighted in this review. This manuscript may help to determine the ecological importance of Cd stress in interdisciplinary studies and essential remediation strategies to overcome the contamination of Cd in agricultural soils.

Cadmium accumulation, subcellular distribution and chemical fractionation in hydroponically grown Sesuvium portulacastrum [Aizoaceae]

Sesuvium portulacastrum is a well-known halophyte with considerable Cd accumulation and tolerance under high Cd stress. This species is also considered as a good candidate of Cd phytoremediation in the polluted soils. However, the mechanism of Cd accumulation, distribution and fractionation in different body parts still remain unknown. Seedlings of Sesuvium portulacastrum were studied hydroponically under exposure to a range of Cd concentrations (50 μM or μmol/L to 600 μM or μmol/L) for 28 days to investigate the potential accumulation capability and tolerance mechanisms of this species. Cd accumulation in roots showed that the bio-concentration factor was > 10, suggesting a strong ability to absorb and accumulate Cd. Cd fractionation in the aboveground parts showed the following order of distribution: soluble fraction > cell wall > organelle > cell membrane. In roots, soluble fraction was mostly predominant than other fractions. Cd speciation in leaves and stems was mainly contained of sodium chloride and deionised water extracted forms, suggesting a strong binding ability with pectin and protein as well as with organic acids. In the roots, inorganic form of Cd was dominant than other forms of Cd. It could be suggested that sodium chloride, deionised water and inorganic contained form of Cd are mainly responsible for the adaption of this plant in the Cd stress environment and alleviating Cd toxicity.

Comparative transcriptome combined with metabolome analyses revealed key factors involved in nitric oxide (NO)-regulated cadmium stress adaptation in tall fescue

BACKGROUND

It has been reported that nitric oxide (NO) could ameliorate cadmium (Cd) toxicity in tall fescue; however, the underlying mechanisms of NO mediated Cd detoxification are largely unknown. In this study, we investigated the possible molecular mechanisms of Cd detoxification process by comparative transcriptomic and metabolomic approaches.

RESULTS

The application of Sodium nitroprusside (SNP) as NO donor decreased the Cd content of tall fescue by 11% under Cd stress (T1 treatment), but the Cd content was increased by 24% when treated with Carboxy-PTIO (c-PTIO) together with Nitro-L-arginine methyl ester (L-NAME) (T2 treatment). RNA-seq analysis revealed that 904 (414 up- and 490 down-regulated) and 118 (74 up- and 44 down-regulated) DEGs were identified in the T1 vs Cd (only Cd treatment) and T2 vs Cd comparisons, respectively. Moreover, metabolite profile analysis showed that 99 (65 up- and 34-down- regulated) and 131 (45 up- and 86 down-regulated) metabolites were altered in the T1 vs Cd and T2 vs Cd comparisons, respectively. The integrated analyses of transcriptomic and metabolic data showed that 81 DEGs and 15 differentially expressed metabolites were involved in 20 NO-induced pathways. The dominant pathways were antioxidant activities such as glutathione metabolism, arginine and proline metabolism, secondary metabolites such as flavone and flavonol biosynthesis and phenylpropanoid biosynthesis, ABC transporters, and nitrogen metabolism.

CONCLUSIONS

In general, the results revealed that there are three major mechanisms involved in NO-mediated Cd detoxification in tall fescue, including (a) antioxidant capacity enhancement; (b) accumulation of secondary metabolites related to cadmium chelation and sequestration; and (c) regulation of cadmium ion transportation, such as ABC transporter activation. In conclusion, this study provides new insights into the NO-mediated cadmium stress response.

Phytoremediation of cadmium-polluted soil assisted by D-gluconate-enhanced Enterobacter cloacae colonization in the Solanum nigrum L. rhizosphere

Microbe-assisted phytoremediation for Cd-polluted soil is being regarded increasingly. However, the availability of microbes that can collaborate with Cd-hyperaccumulators effectively has become one of bottlenecks restricting the remediation efficiency. A siderophore-producing bacterium (Y16; Enterobacter cloacae) isolated from the rhizospheric soil of Cd-hyperaccumulator Solanum nigrum L. was identified by 16S rRNA gene sequencing and biochemical analysis, and then used for analyzing microbial chemotaxis, carbon source utilization, and insoluble P/Cd mobilization capacities. Besides, a soil-pot trial was performed to underlie the phytoremediation mechanism of Cd-polluted soil assisted by D-gluconate-enhanced Enterobacter cloacae colonization (DEYC) in the Solanum nigrum L. rhizosphere. Results displayed that D-gluconate was an effective chemoattractant and carbon source strengthening Y16 colonization, and Y16 exhibited strong abilities to mobilize insoluble P/Cd in shake flask by extracellular acidification (p < 0.05). In the soil-pot trial, DEYC observably enhanced soil Cd phytoextraction by Solanum nigrum L., and increased microbial diversity according to alpha- and beta-diversity analysis (p < 0.05). Taxonomic distribution and co-occurrence network analysis suggested that DEYC increased relative abundances of dominant microbial taxa associated with soil acidification (Acidobacteria-6), indoleacetic acid secretion (Ensifer adhaerens), soil fertility improvement (Flavisolibacter, Bdellovibrio bacteriovorus, and Candidatus nitrososphaera), and insoluble Cd mobilization (Massilia timonae) at different classification levels. Importantly, COGs analysis further shown that DEYC aroused the up-regulation of key genes related to chemotactic motility, carbon fixation, TCA cycle, and propanoate metabolism. These results indicated that DEYC drove the rhizospheric enrichment of pivotal microbial taxa directly or indirectly involved in soil Cd mobilization, meanwhile distinctly promoted plant growth for accumulating more mobilizable Cd. Therefore, Y16 could be used as bio-inoculants for assisting phytoremediation of Cd-polluted soil.

Biological mechanisms of cadmium accumulation in edible Amaranth (Amaranthus mangostanus L.) cultivars promoted by salinity: A transcriptome analysis

Strategies to prevent cadmium (Cd) mobilization by crops under salinity conditions differs among distinct genotypes, but the biological mechanisms of Cd accumulation in different genotype crops promoted by salinity have remained scarce. In this study, we investigated the biological mechanisms of Cd accumulation in two quite different amaranth cultivars of low-Cd accumulator Quanhong (QH) and high-Cd accumulator Liuye (LY) in response to salt stress. Transcriptomes analysis was carried out on leaves and roots tissues of LY and QH grown with exchangeable Cd 0.27 mg kg^-1 and salinity 3.0 g kg^-1 treatment or control conditions, respectively. A total of 3224 differentially expressed genes (DEGs) in LY (1119 in roots, 2105 in leaves) and 848 in QH (207 in roots, 641 in leaves) were identified. Almost in each fold change category (2-2⁵, 2⁵-2¹⁰, >2¹⁰), the numbers of DEGs induced by salinity in LY treatments were much more than those in QH treatments, indicating that LY is more salt sensitive. Gene ontology (GO) analysis revealed that salinity stress promoted soil acidification and Cd mobilization in LY treatments through the enhancive expression of genes related to adenine metabolism (84-fold enrichment) and proton pumping ATPase (50-fold enrichment) in roots, and carbohydrate hydrolysis (2.5-fold enrichment) in leaves compared with that of whole genome, respectively. The genes expression of organic acid transporter (ALMT) was promoted by 2.71- to 3.94-fold in roots, facilitating the secretion of organic acids. Salt stress also inhibited the expression of key enzymes related to cell wall biosynthesis in roots, reducing the physical barriers for Cd uptake. All these processes altered in LY were more substantially compared with that of QH, suggesting that salt sensitive cultivars might accumulate more Cd and pose a higher health risk.

Transcriptome analysis revealed cadmium accumulation mechanisms in hyperaccumulator Siegesbeckia orientalis L

Siegesbeckia orientalis L. was identified as a novel Cd-hyperaccumulator and valuable phytoremediation material. However, the molecular mechanisms underlying Cd accumulation in S. orientalis are largely unknown. In this study, RNA-Seq analysis was performed to study the Cd-accumulating mechanisms in its roots with or without Cd treatment. The RNA-seq analysis generated 312 million pairs of clean reads and 78G sequencing data. De novo transcriptome assembly produced 355,070 transcripts with an average length of 823.59 bp and 194,207 unigenes with an average length of 605.68 bp. Comparative transcriptome analyses identified a large number of differentially expressed genes in roots under Cd stress, and functional annotation suggested that S. orientalis utilizes various biological pathways involving many gene networks working simultaneously to cope with the stress. This study revealed that four biological pathways were mainly involved in S. orientalis tolerance to Cd stress, including reactive oxygen species scavenging, phenylpropanoid biosynthesis pathway, Cd absorption and transport, and ABA signaling pathway. The genes related to photosynthesis and heavy metal transport are likely the potential candidates and could be further investigated to determine their roles in Cd tolerance in S. orientalis roots. These findings will be useful to understand the Cd accumulation mechanisms in S. orientalis and facilitate the study of phytoremediation at the molecular level in plants.

A mechanism for efficient cadmium phytoremediation and high bioethanol production by combined mild chemical pretreatments with desirable rapeseed stalks

Cadmium (Cd) is one of the most hazardous trace metals, and rapeseed is a major oil crop over the world with considerable lignocellulose residues applicable for trace metal phytoremediation and cellulosic ethanol co-production. In this study, we examined that two distinct rapeseed cultivars could accumulate Cd at 72.48 and 43.70 ug/g dry stalk, being the highest Cd accumulation among all major agricultural food crops as previously reported. The Cd accumulation significantly increased pectin deposition as a major factor for trace metal association with lignocellulose. Meanwhile, the Cd-accumulated rapeseed stalks contained much reduced wall polymers (hemicellulose, lignin) and cellulose degree of polymerization, leading to improved lignocellulose enzymatic hydrolysis. Notably, three optimal chemical pretreatments were performed for enhanced biomass enzymatic saccharification and bioethanol production by significantly increasing cellulose accessibility and lignocellulose porosity, along with a complete Cd release for collection and recycling. Hence, this study proposed a mechanism model interpreting why rapeseed stalks are able to accumulate much Cd and how the Cd-accumulated stalks are of enhanced biomass saccharification. It has also provided a powerful technology for both cost-effective Cd phytoremediation and value-added bioethanol co-production with minimum waste release.

Salinity influences Cd accumulation and distribution characteristics in two contrasting halophytes, Suaeda glauca and Limonium aureum

The potential for the phytoremediation of halophytes has been widely recognized. However, the effects of salt on Cd accumulation characteristics in different halophytic species, which may also be related to their salt tolerance, are still unclear. This study investigated the effects of salinity on Cd accumulation and distribution in two distinct halophytes, Suaeda glauca (euhalophyte) and Limonium aureum (recretohalophyte). Seedlings of the two species were treated with 0, 3, and 6 mg kg^-1 soil Cd in combination with or without 0.3% NaCl in a pot experiment. The amount of Cd within the rhizosphere and plant tissues, plant biomass, and the subcellular distribution and chemical forms of Cd were examined. Results showed that the addition of NaCl significantly increased Cd bioavailability at high Cd levels due to the rhizosphere acidification effect. Meanwhile, salinity differently impacted plant biomass allocation, and enhanced Cd uptake and translocation in both studied halophytes. Excess Cd was excreted from the leaf surface, possibly by salt glands of L. aureum, with the salinity facilitating this process. Majority of the Cd was found within the cell walls and vacuolar compartments of two species. However, S. glauca plants had higher proportions of inactive Cd (extracted by 2% HAc and 0.6 M HCl) and lower proportions of active Cd (extracted by 80% ethanol and water), as opposed to L. aureum, which would better inform S. glauca's higher Cd accumulation. Based on these results, S. glauca seems more applicable for phytomanagement of Cd-contaminated saline soils due to its higher capacity for Cd enrichment and tolerance amplified by NaCl.

Influence of irrigation with microalgae-treated biogas slurry on agronomic trait, nutritional quality, oxidation resistance, and nitrate and heavy metal residues in Chinese cabbage

Biogas slurry (BS) is a main byproduct of biogas production that is commonly used for agricultural irrigation because of its abundant nutrients and microelements. However, direct application of BS may cause quality decline and nitrate and heavy metal accumulation in crops. To address this issue, a microalgae culture experiment and an irrigation experiment were performed to evaluate the removal efficiencies of nutrients and heavy metals from diluted BS by microalgae Scenedesmus sp. and to investigate the effects of irrigation with microalgae-treated BS (MBS-25, MBS-50, MBS-75, and MBS-100) on nutritional quality, oxidation resistance, and nitrate and heavy metal residues in Chinese cabbage. After 8 days of continuous culture, a ratio of 1/1 for BS/tap water mixture (BS-50) was the optimal proportion for microalgal growth (3.73 g dry cell L^-1) and efficient removal of total nitrogen (86.1%), total phosphorus (94.3%), COD (87.5%), Cr (50%), Pb (60.7%), and Cd (59.7%). The pH in MBS-50 medium recovered to the highest level in a shorter period of time and accelerated the gas stripping of ammonia nitrogen and the formation of insoluble phosphate and metals, which partly contributed to the high removal efficiencies. MBS irrigation significantly promoted crop growth; improved nutritional quality, edible taste, and oxidation resistance; and reduced nitrate and heavy metal residues in Chinese cabbage at a large scale. Therefore, microalgae culture was beneficial to reduce negative impacts of BS irrigation in crop growth and agricultural product safety. This study may provide a theoretical basis for the safe utilization of BS waste in agricultural irrigation.

Dynamics and potential roles of abundant and rare subcommunities in the bioremediation of cadmium-contaminated paddy soil by Pseudomonas chenduensis

Microbial bioremediation of heavy metal-contaminated soil is a potential technique to reduce heavy metals in crop plants. However, the dynamics and roles of the local microbiota in bioremediation of heavy metal-contaminated soil following microbial application are rarely reported. In this study, we used Pseudomonas chenduensis strain MBR for bioremediation of Cd-contaminated paddy soil and investigated its effects on the dynamics of the local soil bacterial community and Cd accumulation in rice. Cd accumulation in rice grains and roots were significantly reduced by the addition of the strain MBR. The addition of the strain MBR caused greater changes in bacterial communities in rhizosphere soil than in bulk soil. MBR enhanced the roles of microbial communities in transformation of Cd fractions, especially in rhizosphere soil. The strain MBR likely regulated abundant subcommunities more than rare subcommunities to improve Cd bioremediation, especially in rhizosphere soil. Consequently, the dynamics and functional roles of the local microbial communities differed significantly during bioremediation between abundant and rare subcommunities and between rhizosphere soil and bulk soil. This study provides new insight into the microbiota-related mechanisms underlying bioremediation.

A comprehensive mitigation strategy for heavy metal contamination of farmland around mining areas - Screening of low accumulated cultivars, soil remediation and risk assessment

A three-year field test was conducted in an area surrounding past mining activity (mining area) to investigate the value of a novel comprehensive remediation strategy for Cd and Pb contamination, which included screening of low accumulated vegetable cultivars that take up Cd and Pb less than normal cultivars, in situ soil remediation using different soil amendments, and health risk assessment that evaluates the possibility of safe consumption for the vegetables. Results showed that cultivar Huoqing 91-5C of which vegetable was selected as a low accumulator of Cd and Pb in a soil contaminated with 0.5 mg kg^-1 and 8180 mg kg^-1 total Cd and Pb concentrations, respectively. Addition of 20 t ha^-1 of biochar with 2 t ha^-1 of calcium superphosphate in 10 cm depth could decrease available Cd and Pb by 70% and 85% after 1 year, respectively. Following treatments, hazard quotients for adults and children were below 1, indicating that the vegetables grown were safe for human consumption. The total cost of remediation was $3885 ha^-1, so the cost of the remediation of the combination of Cd and Pb was economic in this mining area.

Low root/shoot (R/S) biomass ratio can be an indicator of low cadmium accumulation in the shoot of Chinese flowering cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee) cultivars

Chinese flowering cabbage is a commonly consumed vegetable that accumulates Cd easily from Cd-contaminated soils. Cultivations of low-Cd cultivars are promising strategies for food safety, but low-Cd-accumulating mechanisms are not fully elucidated. To address this issue, 37 cultivars were screened to identify high- and low-Cd cultivars upon exposure to sewage-irrigated garden soil pretreated with different Cd concentrations (1.81, 2.90, and 3.70 mg kg^-1dry soil). The results showed that shoot Cd concentrations differed among the cultivars by maximum degrees of 2.67-, 3.71-, and 3.00-fold under control and treatments, respectively. Soil-pot trial and hydroponic trial found no significant difference in Cd and Ca mobilization, uptake, and transport ability by root per weight between high- and low-Cd cultivars. Interestingly, a stable R/S ratio difference among cultivars (p < 0.01) was observed, and the cultivar variation of Cd accumulation in shoots was mainly dependent on their R/S ratios. R/S ratio was also statistically positively associated with Cd and Ca accumulation in high- and low-Cd cultivars (p < 0.05), both in soil and hydroponics culture. This was mainly due to the lower root biomass of low-Cd cultivars resulted in lower total release of root exudates, lower total Cd and Ca mobilization in rhizosphere soil, and lower total Cd and Ca uptake and transport. The higher shoot biomass of low-Cd cultivars also has dilution effects on Cd concentration in shoot. Overall, low R/S ratio may be regarded as a direct and efficient indicator of low Cd accumulation in the shoot of Chinese flowering cabbage. These findings provided the possibilities to screening low-Cd cultivars using their R/S ratio.

Evaluation of cadmium transfer from soil to leafy vegetables: Influencing factors, transfer models, and indication of soil threshold contents

Food chain contamination by soil cadmium (Cd) through leafy vegetable consumption poses a threat to human health. It is imperative to understand the relationship between Cd phytoavailability in soils and its uptake in common leafy vegetables. A large-scale field survey in Zhejiang Province, southeast China, was conducted to develop models to evaluate the Cd phytoavailability to leafy vegetables based on soil properties and to establish soil Cd thresholds based on food safety. The empirical models developed in this study explained the combined effects of soil properties and diethylenetriaminepentaacetic acid (DTPA)-extractable Cd content on Cd phytoavailability to leafy vegetables. The Cd accumulation in celery, pak choi, and amaranth was quantitatively predicted by measurement of DTPA-extractable soil Cd and soil pH, organic matter, cation exchange capacity and clay content. For predicting Cd accumulation, the DTPA-extractable Cd, pH and clay content had a major influence in lettuce; and for water spinach, the DTPA-extractable Cd, pH, and cation exchange capacity had a major influence. Soil DTPA-extractable Cd was suitable to be used as Cd thresholds in soils cultivating celery, amaranth, pak choi, lettuce, and water spinach, with values of 0.24, 0.13, 0.23, 0.32, and 0.37 mg kg^-1, respectively. However, the threshold values of soil total Cd were 0.26, 0.34, and 0.83 mg kg^-1 for amaranth, celery, and pak choi fields, indicating that the current soil quality standard (GB 15618-1995) for soils cultivating different types of vegetables could be overestimated or underestimated for Cd contamination and the associated risk. This study will provide a useful reference for controlling Cd contamination in common leafy vegetables and developing sustainable production of leafy vegetables.

Response of edible amaranth cultivar to salt stress led to Cd mobilization in rhizosphere soil: A metabolomic analysis

The present study aimed to investigate the metabolic response of edible amaranth cultivars to salt stress and the induced rhizosphere effects on Cd mobilization in soil. Two edible amaranth cultivars (Amaranthus mangostanus L.), Quanhong (low-Cd accumulator; LC) and Liuye (high-Cd accumulator; HC), were subject to salinity treatment in both soil and hydroponic cultures. The total amount of mobilized Cd in rhizosphere soil under salinity treatment increased by 2.78-fold in LC cultivar and 4.36-fold in HC cultivar compared with controls, with 51.2% in LC cultivar and 80.5% in HC cultivar being attributed to biological mobilization of salinity. Multivariate statistical analysis generated from metabolite profiles in both rhizosphere soil and root revealed clear discrimination between control and salt treated samples. Tricarboxylic acid cycle in root was up-regulated to cope with salinity treatment, which promoted release of organic acids from root. The increased accumulation of organic acids in rhizosphere under salt stress obviously promoted soil Cd mobility. These results suggested that salinity promoted release of organic acids from root and enhanced soil Cd mobilization and accumulation in edible amaranth cultivar in soil culture.

Effect of biochar on alleviation of cadmium toxicity in wheat (Triticum aestivum L.) grown on Cd-contaminated saline soil

Soil degradation by salinity and accumulation of trace elements such as cadmium (Cd) in the soils are expected to become one of the most critical issues hindering sustainable production and feeding the increasing population. Biochar (BC) has been known to protect the plants against soil salinity and heavy metal stress. A soil culture study was performed to evaluate the effect of BC on wheat (Triticum aestivum L.) growth, biomass, and reducing Cd and sodium (Na) uptake grown in Cd-contaminated saline soil under ambient conditions. Soil salinity decreased the plant growth, biomass, grain yield, chlorophyll contents, and gas exchange parameters and caused oxidative stress in plants compared with Cd stress alone. Salt stress increased Cd and Na uptake and reduced the potassium (K) and zinc (Zn) uptake by plants. AB-DTPA-extractable Cd and soil electrical conductivity (ECe) increased under salt stress compared to the soil without NaCl stress. Biochar application improved the plant growth and reduced the Cd and Na uptake except in plants treated with higher BC and salt stress (5.0% BC + 50 mM NaCl). Biochar application reduced the oxidative stress in plants and modified the antioxidant enzyme activities, and reduced the bioavailable Cd under salt stress. The positive effects of BC under lower salt stress while the negative effects of BC under higher BC and salt levels indicated that BC doses should be used with great care in higher soil salinity levels simultaneously contaminated with Cd to avoid the negative effects of BC on growth and metal uptake.

Low-Cd tomato cultivars (Solanum lycopersicum L.) screened in non-saline soils also accumulated low Cd, Zn, and Cu in heavy metal-polluted saline soils

Many reclaimed tidal flat soils feature high salinity and heavy metal (HM) accumulation. Consumption of vegetables cultivated in this type of cropland may cause health risks. Low-Cd tomato cultivars (Solanum lycopersicum L.) were identified in non-saline soil in our previous studies (Tan et al. 2014). However, further research should determine whether these low-Cd cultivars will maintain in the repeatability and stability in saline soil and whether they have low accumulation abilities for accompanying metals (such as Zn and Cu). A soil-pot trial was implemented to measure Cd, Zn, and Cu concentrations in low- and high-Cd cultivars of both common and cherry-type tomatoes grown on HM-polluted reclaimed tidal flat saline soil. Then, cultivar differences in dissolution of Cd, Zn, and Cu in soil and their uptake and redistribution in plants were analyzed. Results showed that the cherry type accumulated more Cd, Zn, and Cu than the common type. Low-Cd cultivars of both types in saline soil accumulated low concentrations of Cd, Zn, and Cu in fruits. Low HM accumulation in fruits is partly attributed to a low root/shoot (R/S) biomass ratio. Low amounts of soil HMs were dissolved because of the low level of rhizosphere organic compounds, which possibly decreased HM uptake by the roots. Low-Cd cultivars of both tomato types had a higher ability to retain HMs in the roots than their high-Cd cultivars. These findings may provide a scientific guidance for the safe cultivation of HM-polluted saline soils.

Exogenous Glycinebetaine Promotes Soil Cadmium Uptake by Edible Amaranth Grown during Subtropical Hot Season

Exogenous glycinebetaine treatment is an effective measure for preventing crops from being exposed to drought and high temperature; however, the effects of this approach on the soil Cd uptake and accumulation by crops remain unclear. Pot experiments were conducted in this study to analyze the effect of glycinebetaine on the soil Cd uptake and accumulation by edible amaranth cultivated in Cd-contaminated soil. Results revealed that after exogenous glycinebetaine treatment on amaranth leaves during the vigorous growth period, the plant biomass, the Cd concentrations in the roots and shoots, and the Cd translocation factor (TF) were significantly higher than those of the control group. The highest Cd concentrations in the roots and shoots and the TF were higher by 91%, 96% and 23.8%, respectively, than the corresponding values in the control group. In addition, exogenous glycinebetaine treatment significantly increased leaf chlorophyll content and promoted the photosynthesis of edible amaranth. Consequently, the contents of soluble sugar, dissolved organic carbon, and low-molecular-weight organic acids significantly increased in the rhizosphere, resulting in Cd mobilization. Significant positive correlations were observed among the contents of leaf chlorophyll, Mg, Fe, pectin and Ca. Given that Cd shares absorption and translocation channels with these elements, we speculated that the increased leaf chlorophyll and pectin contents promoted the absorption and accumulation of Mg, Fe and Ca, which further promoted the absorption and translocation of Cd. These results indicated that exogenous glycinebetaine treatment during hot season would aggravate the health risks of crops grown in Cd-contaminated soils.