OsNRAMP5 Is a Major Transporter for Lead Uptake in Rice

Impact of different sulfur sources on cadmium accumulation and metabolic responses of glutathione and cysteine in peppers

BACKGROUND

Sulfur (S) is essential for life while cadmium (Cd) is known to be both extremely toxic and ubiquitous in natural environments. Interactions between S and Cd from soil to plant could provide insights into the dynamics of environmental contaminants and the mechanisms of regulation. Geological sulfur from pyrite (FeS2) and atmospheric sulfur from coal combustion deposits (H2SO4) significantly affect soil acidification, potentially increasing the bioavailability of Cd. To investigate the influence of different sulfur sources on cadmium (Cd) uptake, transfer, accumulation, and the regulation of physiological responses in pepper, we conducted a controlled pot experiment. The study utilized yellow soil and 'Z2' line pepper (Capsicum spp.) as the test plant. We assessed the fresh weight, Cd concentration in various plant parts, and the levels of reduced glutathione (GSH) and cysteine (Cys) during the fruiting phase under varying concentrations of Cd.

RESULTS

Under Cd stress, supplementation with FeS2 and H2SO4 markedly enhanced pepper growth, increasing biomass by over 30%. Among these, geological sulfur (FeS2) demonstrated a more pronounced effect compared to H2SO4. Overall, the cadmium content of each part of FeS2 treatment was lower than H2SO4 under cadmium-induced stress. And at medium cadmium level, a concentration of 100 mg/kg of FeS2 resulted in a 16.51% relative reduction in fruit Cd content from 2.12 mg/kg in the 1.5 mg/kg Cd treatment to 1.78 mg/kg. Both sulfur sources increased GSH and Cys content, particularly under high Cd stress, with FeS2 raising GSH and Cys levels by 3.33-61.87% and 43.29-71.94%, H2SO4 increased 15.65-66.43% and 48.02-74.58%, respectively (P ≤ 0.05). The highest GSH content occurred in leaves, whereas fruits had the highest Cys content.

CONCLUSION

FeS2 and H2SO4 can promote pepper growth and enhancing the synthesis of GSH and Cys, which mitigates Cd toxicity. On the whole, the cadmium content of each part of FeS2 treatment was lower than H2SO4 under cadmium stress conditions.

Complex regulatory network of ZmbZIP54-mediated Pb tolerance in maize

Lead (Pb) is highly toxic and widely distributed in the soil, causing adverse effects on plant growth and yield formation. Herein, the combination of transmission electron microscope (TEM), energy dispersive X-ray Spectroscopy (EDS), and comparative transcriptome analyses was conducted to reveal the cytological mechanism and regulatory network of in the ZmbZIP54-mediated Pb tolerance in maize. As results, ZmbZIP54 helped in Pb2+ retention in the cell wall and intercellular space, inhibiting Pb2+ entering the cells and reducing its toxic effects on cell ultrastructure. Meanwhile, ZmbZIP54 was involved in the transition between the HCl-extracted and CH3COOH-extracted Pb speciations. At the molecular level, ZmbZIP54 affected the macromolecule metabolism, thus decreasing Pb accumulation in the roots. Moreover, ZmZIFL1 and NRT1/PTR were the direct targets of ZmbZIP54, which participated in heavy metal binding, nitrogen uptaking, and IAA transport and thus mediated Pb transport, Pb speciation transition, and antioxidant enzyme activation. Collectively, we proposed a model to explain the complex regulatory network mediated by ZmbZIP54 and its target genes in maize tolerance to Pb stress.

An efficient fungi-biochar-based system for advancing sustainable management of combined pollution

Heavy metal (HM) contamination poses significant global environmental threats, impacting ecosystems, public health, and sustainable development. Fungi, as eco-friendly alternatives to chemical treatments, have the potential to reduce HM bioavailability in contaminated soils while promoting plant growth. However, current fungal remediation methods face limitations in efficiency, long-term effectiveness, and the ability to address combined contamination, particularly with naturally occurring strains. Herein, we developed a Trichoderma reesei-Laccase (LAC)-Biochar coupling system (TLBS), based on the structural and electrostatic analyses of LAC's metal-chelated active site (T1 Cu), for the sustainable remediation of combined pollutants, including HMs. In the TLBS, genetically engineered T. reesei produces a mutated LAC with enhanced binding capability for HMs (Ni and Cd). The TLBS enables high-efficiency remediation through three steps. First, lignin-derived biochar serves as both a supportive carrier and an inducer, initiating LAC expression. Second, natural mediators are released due to the interaction between biochar and T. reesei, and LAC is activated by environmental HMs and natural mediators. Finally, TLBS achieved significant reductions in the available concentrations of Ni (93.63%) and Cd (89.68%) and efficiently remediated multiple organic pollutants (71.41-96.79%), including antibiotics and pesticides. Furthermore, the synergistic interaction among TLBS components ensures long-term remediation effects in environments rich in agricultural biomass, making it ideal for eco-friendly farming practices. This in situ amendment strategy, utilizing only green, biodegradable lignocellulosic wastes and environmentally friendly fungi, offers new pathways for the sustainable management of combined contamination and the improvement of human health.

The Effect of Rhizophagus intraradices on Cadmium Uptake and OsNRAMP5 Gene Expression in Rice

The molecular mechanism of arbuscular mycorrhizal fungi (AMF) in reducing cadmium (Cd) accumulation in plants remains unclear. In this respect, the effects of Rhizophagus intraradices (Ri) inoculation under Cd stress on rice growth, the uptake of Cd along with other elements, and the expression of Cd transport genes, including OsNRAMP1/5, were studied using wild-type (WT) and osnramp5 mutant rice. The results showed that Ri inoculation did not affect rice growth. The uptake of Cd of the osnramp5 mutant was much lower than the WT, as 27.6%, 17.5%, and 39.9% of Cd were noted in the grains, shoots, and roots, respectively. For the WT, Cd alone significantly promoted the OsNRAMP5 expression in shoots, but Ri inoculation significantly suppressed OsNRAMP5 expression and significantly reduced its grain and shoot Cd by 44.4% and 62.3%, respectively, compared to the Cd alone treatment. In contrast, for the osnramp5 mutant, Ri inoculation did not influence OsNRAMP5 expression or the grain and shoot Cd. Furthermore, the expression of other Cd transporters (OsIRT1, OsZIP3/7, OsCAX1a) in both varieties were not changed under the treatments. In conclusion, Ri inoculating significantly reduced Cd uptake by rice, with the molecular mechanism by negative regulation of expression of the OsNRAMP5 gene.

Analysis of NRAMP genes in the Triticeae reveals that TaNRAMP5 positively regulates cadmium (Cd) tolerance in wheat (Triticum aestivum)

Natural Resistance-Associated Macrophage Protein (NRAMP), a class of metal transporter proteins widely distributed in plants, is mainly involved in the uptake and transport by plants of metal ions, such as iron, manganese and cadmium. The current study is the first to fully investigate the Triticum aestivum (T. aestivum) NRAMP gene family. 33 NRAMP members were identified from the entire T. aestivum genome and classified into three main groups based on related genes found in five other species. Among the TaNRAMP genes, the exon-intron structure and motif composition exhibited significant similarity among members of the same evolutionary branch of the phylogenetic tree. Based on RNA-seq and qRT-PCR analyses, we identified the expression patterns of the TaNRAMP genes in different tissues and under various stress conditions. TaNRAMP genes expression were responsive to induction by cadmium (Cd). Overexpression of the TaNRAMP5 gene enhanced wheat and tobacco tolerance to Cd toxicity. Additionally, the TaNRAMP5 protein physically interacted with protein phosphatase 2A (PP2A) in yeast cells. This study provides a valuable reference point for further investigations into the functional and molecular mechanisms of the NRAMP gene family.

Silicon reduces lead accumulation in Moso bamboo via immobilization and suppression of metal cation transporter genes in roots

Lead (Pb) is a hazardous element that affects the growth and development of plants, while silicon (Si) is a beneficial element for alleviating the stress caused by heavy metals, including Pb. However, the mechanisms by which Si reduces Pb accumulation in Moso bamboo (Phyllostachys edulis (Carr ·) H · de Lehaie) remain unclear. In this study, physiological assessments and transcriptome analyses were conducted to investigate the interaction between Si and Pb. Our findings showed that Si application has no significant effect on alleviating Pb-induced inhibition of root elongation and dry weight in short-term and long-term experiments, respectively. However, it did rescue leaf yellowing and reduce Pb accumulation, particularly in the shoot. Pre-treatment with Si led to a reduction in Pb uptake, translocation and accumulation, coupled with an increase in Pb fixation within the hemicellulose of the root cell wall, resulting in a lower Pb concentration in the cell sap. At the cellular level, Pb was found to be distributed in all cells of roots, and Si pretreatment did not alter Pb distribution. Additionally, Si application downregulated the expression of genes related to ABC and metal cation transporters. These findings indicate that Si reduces Pb accumulation in Moso bamboo by immobilizing Pb in the hemicellulose of root cell walls and downregulating the expression of transporter genes involved in Pb uptake and transport.

The Tolerance Differences of Two Industrial Hemp Varieties Under Lead (Pb) Stress

Industrial hemp is a crop with a high tolerance and accumulation of lead (Pb). Improving the Pb tolerance and accumulation capacity of industrial hemp is of great scientific and practical importance. This study utilized a pot with soil contaminated with Pb to investigate the differences in Pb tolerance between two industrial hemp varieties, Yunma1 (YM) and Shaanxi Industrial Hemp (SM), under Pb stress. The results indicated that Pb mainly accumulates in the roots of YM and SM (70-80%), with YM having a higher Pb accumulation than SM. It is worth nothing that under high Pb concentration conditions (5000 mg/kg), the Pb accumulation capacity of YM is twice that of SM. Accumulation characteristics of Pb in different plant tissues followed the pattern: roots > stems > leaves > fibers > seeds. In YM, approximately 70% of the absorbed Pb was fixed in the roots and 30% was transported to the above-ground parts. In contrast, SM transported more than 50% of absorbed Pb by roots to the above-ground areas, causing some degree of damage to stems and leaves. Even when Pb concentrations exceed 4000 mg/kg, YM exhibits strong tolerance (tolerance index greater than 90%), with normal growth and no signs of toxicity. However, SM showed a tolerance level of < 50% at high Pb concentrations, with significant heavy metal toxicity symptoms in the above-ground areas. These results provide important information for the remediation of Pb contaminated soils in mining areas.

RsNRAMP5, a major metal transporter, promotes cadmium influx and ROS accumulation in radish (Raphanus sativus L.)

Arable soil contamination with heavy metals (HMs) poses a great potential threat to vegetable crops and human health. Radish (Raphanus sativus L.), an economical and popular root vegetable crop, is easily absorbed HMs by its taproot. Although the Natural Resistance-Associated Macrophage Proteins (NRAMPs) were participated in transporting a number of HMs in plants, whether and how the NRAMP genes involved in cadmium (Cd) uptake and transport remains elusive in radish. In this study, a total of nine RsNRAMP gene members were identified, which were classified into three subgroups and dispersed on six radish chromosomes. Three RsNRAMPs (RsNRAMP3, RsNRAMP4 and RsNRAMP5) displayed high expression in the vascular cambium, and they exhibited obviously Cd-induced expression, among which the expression of RsNRAMP4 and RsNRAMP5 reached to the highest level at 24h. Moreover, the RsNRAMP5 was localized to the plasma membrane and its promoter activity was dramatically induced by Cd exposure. Heterologous expression analysis indicated that over-expression of RsNRAMP5 significantly promoted the uptake of Cd, lead (Pb), iron (Fe) and manganese (Mn) in yeast cells. In addition, the transient over-expression of RsNRAMP5 promoted Cd uptake and enhanced ROS (reactive oxygen species) accumulation in radish cotyledons. These findings would expedite unraveling the molecular mechanism underlying RsNRAMP5-mediated Cd uptake and transport in radish.

Uptake and Accumulation of Cobalt Is Mediated by OsNramp5 in Rice

Cobalt (Co) contamination in soils potentially affects human health through the food chain. Although rice (Oryza sativa) as a staple food is a major dietary source of human Co intake, it is poorly understood how Co is taken up by the roots and accumulated in rice grain. In this study, we physiologically characterized Co accumulation and identified the transporter for Co2+ uptake in rice. A dose-dependent experiment showed that Co mainly accumulated in rice roots. Further analysis with LA-ICP-MS showed Co deposited in most tissue of the roots, including exodermis, endodermis and stele region. Co accumulation analysis using mutants defective in divalent cation uptake showed that Co2+ uptake in rice is mediated by the Mn2+/Cd2+/Pb2+ transporter OsNramp5, rather than OsIRT1 for Fe2+ and OsZIP9 for Zn2+. Knockout of OsNramp5 enhanced tolerance to Co toxicity. Heterologous expression of OsNramp5 showed transport activity for Co2+ in Saccharomyces cerevisiae. Co2+ uptake was inhibited by either Mn2+ or Cd2+ supply. At the reproductive stage, the Co concentration in the straw and grains of the OsNramp5 knockout lines was decreased by 41%-48% and 28%-36%, respectively, compared with that of the wild-type rice. The expression level of OsNramp5 in the roots was not affected by Co2+. Taken together, our results indicate that OsNramp5 is a major transporter for Co2+ uptake in rice, which ultimately mediates Co accumulation in the grains.

Multi-omics profiling reveals elevated CO2-enhanced tolerance of Trifolium repens L. to lead stress through environment-plant-microbiome interactions

The increasing atmospheric CO2 resulting from human activities over the past two centuries, which is projected to persist, has significant implications for plant physiology. However, our predictive understanding of how elevated CO2 (eCO2) modifies plant tolerance to metal stress remains limited. In this study, we collected roots and rhizosphere soils from Trifolium repens L. subjected to lead (Pb) stress under ambient and elevated CO2 conditions, generating transcriptomic data for roots, microbiota data for rhizospheres, and conducting comprehensive multi-omics analyses. Our findings show that eCO2 reduced the accumulation of Pb-induced reactive oxygen species (ROS) and promoted plant growth by 72% to 402%, as well as increases shoot Pb uptake by 79% compared to ambient CO2. Additionally, eCO2 triggers specific defense response in T. repens, elevating the threshold for stress response. We observed a adaptive reconfiguration of transcriptional network that enhances energy efficiency and optimizes photosynthetic product utilization. Notably, eCO2 induces salicylic acid biosynthesis and activates defense pathways related to redox balance and ROS scavenging processes, thereby enhancing abiotic stress resistance. Through weighted gene co-expression network analysis, our comprehensive investigation reveals a holistic regulatory network encompassing plant traits, gene expression patterns, and bacterial structure potentially linked to metal accumulation as well as tradeoffs between growth and defense in plants under elevated CO2. These insights shed light on the plant stress responses under elevated CO2 and while contributing to a broader comprehension of plant-environment interactions.

Sufficient manganese supply is necessary for OsNramp5 knockout rice plants to ensure normal growth and less Cd uptake

The development of crop cultivars with less Cd uptake in roots and accumulation in shoots is a most efficient and environment-friendly approach to deal with soil Cd contamination. Recently repression of Nramp5 expression or its knockout is commonly recognized to be efficient for reducing Cd accumulation in plants, but such mutant plants suffer from manganese deficiency. In this study, we assessed the efficacy of exogenous Mn addition in mitigating Cd stress in a japonica rice cultivar Xidao 1 (Wild Type, WT) and its OsNramp5 knockout mutant. Exposure to Cd stress resulted in notable low photosynthetic rate, growth inhibition, and high Cd accumulation in rice seedlings. Although the mutant plants contained much lower Cd concentration in both roots and shoots than the WT plants, their growth was significantly inhibited relative to the WT plants under the normal condition. Exogenous application of Mn (40 μM) dramatically reduces root and shoot Cd concentrations and alleviates the toxic effect of Cd stress in both rice types, with the mutant plants demonstrating lower Cd concentration and less Cd toxicity in comparison with WT plants. The alleviation of Cd toxicity by Mn addition was more effective in higher Cd level (1.0 μM) than in lower Cd level (0.1 μM). Mn increases the expression of OsNramp5 and other genes, including OsHMA2, OsHMA3, OsIRT1, and OsIRT2, which encode ion transporters related to Mn uptake and transportation, and meanwhile reduces Cd uptake and accumulation in rice seedlings. In short, the knockout of OsNramp5 results in the significant reduction of Cd uptake, but accompanies with Mn deficiency in rice plants, which can be efficiently overcome through exogenous Mn addition.

Significant lead isotope 'fractionation' in maize records plant lead uptake, transfer, and detoxification mechanisms

Lead isotope analysis is the main method to trace the sources and cycling of Pb in the biosphere system. The linchpin of such application hinges on the assumption that there is negligible or no biologically mediated isotopic fractionation of Pb occurs in the environment. However, recent measurements by high-precision multi-collector mass spectrometry revealed that biological isotope fractionation of heavy mass elements is a prevalent phenomenon. This study shows that compared with the Pb sources, the maize plant (Zea mays L.) organs exhibit a wider range of Pb isotope compositions and a depletion of radioactive Pb isotopes (206Pb, 207Pb, and 208Pb). Moreover, three independent studies consistently indicate that the 206Pb/207Pb ratio of maize organs varies as root/leaf > stem/grain, reflecting a continuous loss of light Pb isotopes during transportation. The conventional wisdom fails to account for these phenomena, suggesting that maize may undergo Pb isotope fractionation during the absorption and transportation of Pb. However, compared with other non-traditional metal isotopes, Pb isotope exhibits a more significant fractionation magnitude. We tentatively attribute this fractionation to the Pb tolerance mechanism of maize and its selective absorption of various forms of Pb, which requires further research to validate. Findings from this study mandate caution in future Pb source tracing in plants using Pb isotope methods and open up applications in using Pb isotopic fractionation to track Pb uptake and transfer pathways and decipher the associated detoxification mechanisms in plants.

Toxic Metals and Metalloids in Food: Current Status, Health Risks, and Mitigation Strategies

PURPOSE OF REVIEW

Exposure to toxic metals/metalloids, such as arsenic (As), cadmium (Cd), and lead (Pb), through food consumption is a global public health concern. This review examines the contamination status of these metals/metalloids in food, assesses dietary intake across different populations, and proposes strategies to reduce metal/metalloid exposures throughout the food chain.

RECENT FINDINGS

For the general population, dietary intake of metals/metalloids is generally lower than health-based guidance values. However, for vulnerable populations, such as infants, children, and pregnant women, their dietary intake levels are close to or even higher than the guidance values. Among different food categories, seafood shows higher total As, but largely present as organic species. Rice accumulates higher As concentration than other cereals, with inorganic As (iAs) and dimethylarsinic acid (DMA) being the main As species. Methylated thioarsenate species, such as dimethylmonothioarsenate, have also been detected in rice. The distribution of iAs and DMA in rice shows geographical variation. Additionally, seafood and cocoa products generally contain more Cd than other food, but seafood consumption does not significantly increase in adverse health effects due to its high zinc and iron content. Compared to As and Cd, Pb concentrations in food are generally lower. To minimize the health risks of metal/metalloid exposure, several strategies are proposed. Food contamination with toxic metals/metalloids poses significant concerns for human health, particularly for vulnerable populations. This review provides scientific evidence and suggestions for policy makers to reduce human exposure of metals/metalloids via dietary intake.

Mitigating toxic metals contamination in foods: Bridging knowledge gaps for addressing food safety

Background

Reducing exposure to harmful substances in food is highly desired, especially for infants, young children, and pregnant women. A workshop focused on understanding and reducing toxic metal contamination in food was conducted involving leading scientists, educators, practitioners, and key stakeholders in conjunction with the USDA National Institute of Food and Agriculture.

Scope and approach

The goal of this review and the workshop was to advance the current knowledge of major toxic metals concerning food safety, viz. arsenic (As), lead (Pb), cadmium (Cd), mercury (Hg), and chromium (Cr), preventive measures, identify critical knowledge gaps, and the need for research, extension, and education. Being a part of the "Closer to Zero (C2Z)" initiative of the USDA, FDA, and other federal agencies, the workshop adopted a "One Health" approach to mitigate dietary exposure and environmental pollution of hazardous elements.

Key findings and conclusions

The experts discussed the accumulation of toxic metals in food crops and drinking water in relation to soil biogeochemistry, plant uptake, and multidisciplinary factors such as food processing, detection, regulatory standards, etc. To forward food safety, this workshop critically examined toxic metals contamination, exposure and toxicity along the farm-to-fork-to-human continuum, research gaps, prevailing regulations, and sustainable remediation approaches, and offered significant recommendations. This review paper provides perspective on key findings of the workshop relative to addressing this important aspect of food safety, emphasizing interdisciplinary research that can effectively investigate and understand the complex and dynamic relationships between soil biogeochemistry, the microbiome, plant tolerance and accumulation strategies, uniform standards for acceptable and safe toxic element levels in food and water, and raising public awareness. This article also provides a foundation for decision-making regarding toxic metal fate and effects, including risk management strategies, in the face of modern industrialization and a changing climate.

Hydroxamate Siderophores Intensify the Co-Deposition of Cadmium and Silicon as Phytolith-Like Particulates in Rice Stem Nodes: A Natural Strategy to Mitigate Grain Cadmium Accumulation

Sequestration of cadmium (Cd) in rice phytolith can effectively restrict its migration to the grains, but how hydroxamate siderophore (HDS) affects phytolith formation within rice plants especially the fate of Cd and silicon (Si) remains poorly understood. Here, we found that the addition of HDS increased the content of dissolved Si and Cd in soil pore water as well as its absorption by the rice roots during the reproductive growth stage. HDS effectively trapped orthosilicic acid and Cd ions at the third stem nodes of rice plants via hydrogen bonds and chelation interactions, which then rapidly deposited on the xylem cell wall through hydrophobic interactions. Ultimately, Cd was immobilized as phytolith-like particulates in the form of CdSiO3. Field experiments verified that Cd accumulation was significantly reduced by 46.4% in rice grains but increased by 41.2% in rice stems after HDS addition. Overall, this study advances our understanding of microbial metabolites enhancing the instinctive physiological barriers within rice plants.

The transcription factor OsNAC5 regulates cadmium accumulation in rice

Cadmium (Cd) is a hazardous heavy metal that threatens human health through the consumption of contaminated rice. To mitigate Cd accumulation in rice grains, it is crucial to reduce Cd uptake. Nevertheless, the transcriptional mechanisms governing Cd uptake in rice remain largely unknown. This research identifies the transcription factor OsNAC5 in Oryza sativa as a positive regulator of the Cd transporter gene OsNRAMP1, thereby influencing Cd uptake. OsNAC5 is predominantly expressed in the roots, resides in the nucleus, and is upregulated by Cd-induced hydrogen peroxide (H2O2). Knocking out OsNAC5 results in lower Cd concentrations in both shoots and roots and heightens sensitivity to Cd. The expression of OsNRAMP1, enhanced by Cd stress, is dependent on OsNAC5. OsNAC5 binds to "CATGTG" motifs in the OsNRAMP1 promoter, activating its expression. The loss of OsNAC5 function leads to reduced Cd accumulation in rice grains. Our findings provide insights into the transcriptional regulation of Cd stress response in rice and propose biotechnological strategies to lower Cd uptake in crops.

Transcriptomics Combined with Physiology and Metabolomics Reveals the Mechanism of Tolerance to Lead Toxicity in Maize Seedling

Lead (Pb) exposure can induce molecular changes in plants, disrupt metabolites, and impact plant growth. Therefore, it is essential to comprehend the molecular mechanisms involved in Pb tolerance in plants to evaluate the long-term environmental consequences of Pb exposure. This research focused on maize as the test subject to study variations in biomass, root traits, genes, and metabolites under hydroponic conditions under Pb conditions. The findings indicate that high Pb stress significantly disrupts plant growth and development, leading to a reduction in catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) activities by 17.12, 5.78, and 19.38%, respectively. Conversely, Pb stress led to increase malondialdehyde (MDA) contents, ultimately impacting the growth of maize. The non-targeted metabolomics analysis identified 393 metabolites categorized into 12 groups, primarily consisting of organic acids and derivatives, organ heterocyclic compounds, lipids and lipid-like molecules and benzenoids. Further analysis indicated that Pb stress induced an accumulation of 174 metabolites mainly enriched in seven metabolic pathways, for example phenylpropanoid biosynthesis and flavonoid biosynthesis. Transcriptome analysis revealed 1933 shared differentially expressed genes (DEGs), with 1356 upregulated and 577 downregulated genes across all Pb treatments. Additionally, an integrated analysis identified several DEGs and differentially accumulated metabolites (DAMs), including peroxidase, alpha-trehalose, and D-glucose 6-phosphate, which were linked to cell wall biosynthesis. These findings imply the significance of this pathway in Pb detoxification. This comprehensive investigation, employing multiple methodologies, provides a detailed molecular-level insight into maize's response to Pb stress.

A review of phytoremediation of environmental lead (pb) contamination

An estimated one billion people globally are exposed to hazardous levels of lead (Pb), resulting in intellectual disabilities for over 600,000 children each year. This critical issue aligns with the expanding worldwide population and the demand for food security, emphasizing the urgency of effectively addressing heavy metal pollution especially from Pb for sustainable development. Phytoremediation, a highly favoured approach in conjunction with conventional physical, chemical, and microbial methods, is a promising approach to mitigating soil and environmental contamination. In this review, we delve into a range of soil pollution mitigation strategies, with focus on the mechanisms that underpin the phytoremediation of environmental Pb. This detailed exploration sheds light on the efficacy and complexities of utilizing plants for the detoxification and removal of lead from contaminated environments. It also examines strategies to enhance phytoremediation by incorporating microbiology, composting, nanotechnology, and foliar spraying. The potential remediation strategies largely depend on the investigation and incorporation of environmentally friendly catalysts, as well as the utilization of innovative methods such as genetic engineering to improve phytoremediation processes. Studies have also shown that biochar has the capability to lower heavy metal concentrations in plant branches by over 50%, without affecting the pH of the soil. Specifically, magnetic biochar (MBC) has been shown to decrease lead levels in plants by up to 42%. Employing these methods showcases an effective strategy to enhance the efficacy of remediation techniques and fosters sustainable solutions to the pervasive issue of Pb pollution, thereby contributing to sustainable development efforts globally.

A review of amendments for simultaneously reducing Cd and As availability in paddy soils and rice grain based on meta-analysis

Arsenic (As) and cadmium (Cd) accumulation in rice grains is a global food safety issue, and various methods and materials have been used to remove or reduce As and Cd in agricultural soils and rice grains. Despite the availability of synthesized materials capable of simultaneous As and Cd reduction from soil and rice grains, the contributions, efficiency, and main ingredients of the materials for As and Cd immobilization remain unclear. The present study first summarized the biogeochemistry of As and Cd in paddy soils and their transfer in the soil-food-human continuum. We also reviewed a series of reported inorganic and organic materials for simultaneous immobilization of As and Cd in paddy soils, and their reduction efficiency of As and Cd bioavailability were listed and compared. Based on the abovementioned materials, the study conducted a meta-analysis of 38 articles with 2565 observations to quantify the impacts of materials on simultaneous As and Cd reduction from soil and rice grains. Meta-analysis results showed that combining organic and inorganic amendments corresponded to effect sizes of -62.3% and -67.8% on As and Cd accumulation in rice grains, while the effect sizes on As and Cd reduction in paddy soils were -44.2% and -46.2%, respectively. Application of Fe based materials significantly (P < 0.05) reduced As (-54.2%) and Cd (-74.9%), accounting for the highest immobilization efficiency of As and Cd in rice grain among all the reviewed materials, outweighing S, Mn, P, Si, and Ca based materials. Moreover, precipitation, surface complexation, ion exchange, and electrostatic attraction mechanisms were involved in the co-immobilization tactics. The present study underlines the application of combined organic and inorganic amendments in simultaneous As and Cd immobilization. It also highlighted that employing Fe-incorporated biochar material may be a potential strategy for co-mitigating As and Cd pollution in paddy soils and accumulation in rice grains.

Sub-lethal effects of metal(loid) contamination on the halophyte Sarcocornia quinqueflora with links to plant photosynthetic performance and biomass - A field study

Two saltmarsh locations within Lake Macquarie, NSW, Australia were selected to investigate the uptake and partitioning of metal(loid)s Cu, Zn, As, Se, Cd and Pb in the Australian saltmarsh halophyte, Sarcocornia quinqueflora and the associated sub-lethal effects of metal(loid)s on plant health, including photosynthetic performance, biomass, and productivity. Metal(loid)s primarily accumulated to roots (BCF > 1). Barriers to transport were observed at the root to non-photosynthetic stem transition (TF < 1) for all metal(loid)s, suggesting this species is suitable for phytostabilisation. Sediment and plant tissue metal(loid) concentrations were significantly correlated with photosynthetic performance and plant biomass. As such, the action of sediment and tissue metal(loid)s on photosynthetic performance and the subsequent effect on biomass of S.quinqueflora appear to be suitable targets for molecular analyses to further elucidate mechanisms responsible for the observed adverse effects and the development of adverse outcome pathways.

Genome-wide association study reveals genetic loci for ten trace elements in foxtail millet (Setaria italica)

KEY MESSAGE

One hundred and fifty-five QTL for trace element concentrations in foxtail millet were identified using a genome-wide association study, and a candidate gene associated with Ni-Co-Cr concentrations was detected. Foxtail millet (Setaria italica) is an important regional crop known for its rich mineral nutrient content, which has beneficial effects on human health. We assessed the concentrations of ten trace elements (Ba, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sr, and Zn) in the grain of 408 foxtail millet accessions. Significant differences in the concentrations of five elements (Ba, Co, Ni, Sr, and Zn) were observed between two subpopulations of spring- and summer-sown foxtail millet varieties. Moreover, 84.4% of the element pairs exhibited significant correlations. To identify the genetic factors influencing trace element accumulation, a comprehensive genome-wide association study was conducted, identifying 155 quantitative trait locus (QTL) for the ten trace elements across three different environments. Among them, ten QTL were consistently detected in multiple environments, including qZn2.1, qZn4.4, qCr4.1, qFe6.3, qFe6.5, qCo6.1, qPb7.3, qPb7.5, qBa9.1, and qNi9.1. Thirteen QTL clusters were detected for multiple elements, which partially explained the correlations between elements. Additionally, the different concentrations of five elements between foxtail millet subpopulations were caused by the different frequencies of high-concentration alleles associated with important marker-trait associations. Haplotype analysis identified a candidate gene SETIT_036676mg associated with Ni accumulation, with the GG haplotype significantly increasing Ni-Co-Cr concentrations in foxtail millet. A cleaved amplified polymorphic sequence marker (cNi6676) based on the two haplotypes of SETIT_036676mg was developed and validated. Results of this study provide valuable reference information for the genetic research and improvement of trace element content in foxtail millet.

Populus euphratica R2R3-MYB transcription factor RAX2 binds ANN1 promoter to increase cadmium enrichment in Arabidopsis

The expression of R2R3-MYB transcription factor PeRAX2 increased transiently upon CdCl2 exposure (100 μM, 48 h) in leaves and roots of Populus euphratica. We observed that overexpression of PeRAX2 increased Cd2+ concentration in Arabidopsis root cells and Cd2+ amount in whole plant, which was due to the increased Cd2+ influx into root tips. However, the Cd2+ influx facilitated by PeRAX2 overexpression was substantially reduced by LaCl3 (an inhibitor of Ca2+-channels), suggesting that PeRAX2 could promote the Cd2+ entering through PM Ca2+-permeable channels (CaPCs) in the roots. It is noting that the expression of annexin1 (AtANN1), which mediates the influx of divalent cations through the PM calcium channels, was upregulated by Cd2+ in PeRAX2-transgenic Arabidopsis. Bioinformatic analysis revealed that the AtANN1 promoter (AtANN1-pro) contains four cis-elements for MYB binding. The PeRAX2 interaction with AtANN1-pro was validated by LUC reporter assay, EMSA, and Y1H assay. Our data showed that PeRAX2 binds to the AtANN1 promoter region to regulate gene transcription and that AtANN1 mediates the Cd2+ entry through CaPCs in the PM, leading to a Cd2+ enrichment in transgenic plants. The PeRAX2-stimulated Cd2+ enrichment consequently resulted in high H2O2 production in root cells of transgenic plants. The expression of AtSOD and AtPOD and activities of CAT, SOD, POD increased in the transgenic lines under Cd2+ stress. However, the Cd2+-upregulated expression and activity of antioxidative enzymes were less pronounced in the PeRAX2-overexpressed lines, compared to the wildtype and vector controls. As a result, root length and plant growth were more suppressed by Cd2+ in the transgenic lines. Our data suggest that transcriptional regulation of AtANN1 by PeRAX2 can be utilized to improve Cd2+ enrichment and phytoremediation, although the enriched Cd2+ affected antioxidant defense system and plant growth in the model species.

Uptake and distribution of metal(loid)s in two rare species of saltmarsh, blackseed samphire, Tecticornia pergranulata, and narrow-leafed wilsonia, Wilsonia backhousei, in New South Wales, Australia

The uptake and distribution of copper, zinc, arsenic, and lead was examined in two rare Australian saltmarsh species, Tecticornia pergranulata and Wilsonia backhousei. The bioconcentration factors and translocation factors were generally much lower than one, except for the Zn translocation factors for T. pergranulata. When compared to other Australian saltmarsh taxa, these species generally accumulated the lowest levels observed among taxa, especially in terms of their BCFs. Essential metals tended to be regulated, while non-essential metals increased in concentration with dose during transport among compartments, a pattern not previously observed in Australian saltmarsh taxa. The uptake of metals into roots was mainly explained by total sediment metal loads as well as more acidic pH, increased soil organic matter, and decreased salinity. The low uptake and limited translocation observed in these rare taxa may offer a competitive advantage for their establishment and survival in the last urbanised populations, where legacy metal contamination acts as a selective pressure.

Alleviated lead toxicity in rice plant by co-augmented action of genome doubling and TiO2 nanoparticles on gene expression, cytological and physiological changes

Lead is a very toxic and futile heavy metal for rice plants because of its injurious effects on plant growth and metabolic processes. Polyploidy or whole genome doubling increases the ability of plants to withstand biotic and abiotic stress. Considering the beneficial effects of nanoparticles and tetraploid rice, this research was conducted to examine the effectiveness of tetraploid and titanium dioxide nanoparticles (TiO2 NPs) in mitigating the toxic effects of lead. A diploid (E22-2x) and it's tetraploid (T-42) rice line were treated with Pb (200 μM) and TiO2 NPs (15 mg L-1). Lead toxicity dramatically reduced shoot length (16 % and 4 %) and root length (17 % and 9 %), biological yield (55 % and 36 %), and photosynthetic activity, as evidenced by lower levels of chlorophyll a and b (30 % and 9 %) in E-22 and T-42 rice cultivars compared to the control rice plants, respectively. Furthermore, lead toxicity amplified the levels of reactive oxygen species (ROS), such as malondialdehyde and H2O2, while decreasing activities of all antioxidant enzymes, such as superoxidase, peroxidase, and glutathione predominately in the diploid cultivar. Transmission electron microscopy and semi-thin section observations revealed that Pb-treated cells in E22-2x had more cell abnormalities than T-42, such as irregularly shaped mitochondria, cell wall, and reduced root cell size. Polyploidy and TiO2 reduced Pb uptake in rice cultivars and expression levels of metal transporter genes such as OsHMA9 and OsNRAMP5. According to the findings, genome doubling alleviates Pb toxicity by reducing Pb accumulation, ROS, and cell damage. Tetraploid rice can withstand the toxic effect of Pb better than diploid rice, and TiO2 NPs can alleviate the toxic impact of Pb. Our study findings act as a roadmap for future research endeavours, directing the focus toward risk management and assessing long-term impacts to balance environmental sustainability and agricultural growth.

Local distribution of manganese to leaf sheath is mediated by OsNramp5 in rice

To play essential roles of manganese (Mn) in plant growth and development, it needs to be transported to different organs and tissues after uptake. However, the molecular mechanisms underlying Mn distribution between different tissues are poorly understood. We functionally characterized a member of rice natural resistance-associated macrophage protein (NRAMP) family, OsNramp5 in terms of its tissue specificity of gene expression, cell-specificity of protein localization, phenotypic analysis of leaf growth and response to Mn fluctuations. OsNramp5 is highly expressed in the leaf sheath. Immunostaining revealed that OsNramp5 is polarly localized at the proximal side of xylem parenchyma cells of the leaf sheath. Both the gene expression and protein abundance of OsNramp5 are unaffected by different Mn concentrations. Knockout of OsNramp5 decreased the distribution of Mn to the leaf sheath, but increased the distribution to the leaf blade at both low and high Mn supplies, resulting in reduced growth of leaf sheath. Furthermore, expression of OsNramp5 under the control of root-specific promoter in osnramp5 mutant complemented Mn uptake, but could not complement Mn distribution to the leaf sheath. These results indicate that OsNramp5 expressed in the leaf sheath plays an important role in unloading Mn from the xylem for the local distribution in rice.

Metagenomic and proteomic insights into the self-adaptive cell surface hydrophobicity of Sphingomonas sp. strain PAH02 reducing the migration of cadmium-phenanthrene co-pollutant in rice

Cell surface hydrophobicity (CSH) dominates the interactions between rhizobacteria and pollutants at the soil-water interface, which is critical for understanding the dissipation of pollutants in the rhizosphere microzone of rice. Herein, we explored the effects of self-adaptive CSH of Sphingomonas sp. strain PAH02 on the translocation and biotransformation behaviour of cadmium-phenanthrene (Cd-Phe) co-pollutant in rice and rhizosphere microbiome. We evidenced that strain PAH02 reduced the adsorption of Cd-Phe co-pollutant on the rice root surface while enhancing the degradation of Phe and adsorption of Cd via its self-adaptive CSH in the hydroponic experiment. The significant upregulation of key protein expression levels such as MerR, ARHDs and enoyl-CoA hydratase/isomerase, ensures self-adaptive CSH to cope with the stress of Cd-Phe co-pollutant. Consistently, the bioaugmentation of strain PAH02 promoted the formation of core microbiota in the rhizosphere soil of rice (Oryza sativa L.), such as Bradyrhizobium and Streptomyces and induced gene enrichment of CusA and PobA that are strongly associated with pollutant transformation. Consequently, the contents of Cd and Phe in rice grains at maturity decreased by 17.2% ± 0.2% and 65.7% ± 0.3%, respectively, after the bioaugmentation of strain PAH02. These findings present new opportunities for the implementation of rhizosphere bioremediation strategies of co-contaminants in paddy fields.

Robust Potentiometric Microelectrodes for In Situ Sensing of Ion Fluxes with High Sensitivity

Simple, reproducible, and reliable preparation of robust potentiometric microelectrodes is both challenging and of great importance for noninvasive real-time ion sensing. Herein, we report a simple strategy for the large-scale synthesis of nickel cobalt sulfide (NiCo2S4) nanowire arrays grown on carbon fibers for potentiometric microelectrodes. The highly uniform NiCo2S4 nanowire array serving as a transduction layer can provide a high capillary pressure and viscous resistance for loading the ion sensing membrane and exhibit a large redox capacitance for improving the stability. An all-solid-state lead-selective microelectrode, which presents a detection limit of 2.5 × 10-8 M in the simulated soil solution, was designed as a model for noninvasive, in situ, and real-time detection of ion fluxes near the rice root surface. Importantly, the microsensor enables sensitive detection of trace-level ion-fluxes. This work provides a simple yet general strategy for designing potentiometric microelectrodes.

Benzotriazole Ultraviolet Stabilizers (BUVSs) as Potential Protein Kinase Antagonists in Rice

The ubiquitous occurrence of benzotriazole ultraviolet stabilizers (BUVSs) in the environment and organisms has warned of their potential ecological and health risks. Studies showed that some BUVSs exerted immune and chronic toxicities to animals by disturbing signaling transduction, yet limited research has investigated the toxic effects on crop plants and the underlying mechanisms of signaling regulation. Herein, a laboratory-controlled hydroponic experiment was conducted on rice to explore the phytotoxicity of BUVSs by integrating conventional biochemical experiments, transcriptomic analysis, competitive sorption assays, and computational studies. The results showed that BUVSs inhibited the growth of rice by 6.30-20.4% by excessively opening the leaf stomas, resulting in increased transpiration. BUVSs interrupted the transduction of abscisic acid (ABA) signal through competitively binding to Ca2+-dependent protein kinase (CDPK), weakening the CDPK phosphorylation and further inhibiting the downstream signaling. As structural analogues of ATP, BUVSs acted as potential ABA signaling antagonists, leading to physiological dysfunction in mediating stomatal closure under stresses. This is the first comprehensive study elucidating the effects of BUVSs on the function of key proteins and the associated signaling transduction in plants and providing insightful information for the risk evaluation and control of BUVSs.

Identifying cadmium and lead co-accumulation from living rice blade spectrum

Neither cadmium (Cd) nor lead (Pb) is necessary for crop growth, but they both can accumulate in soil and crop tissues, resulting in land degradation and crop reduction. Few researchers have explored how to detect Cd-Pb co-accumulation in leaves using proximal sensing techniques, especially by low-cost, easy-to-use leaf clips that capture hyperspectral reflections at suitable foliar positions. In this study, a hyperspectral imager was employed to collect images of the rice canopy from a designed greenhouse experiment that included 16 pretreatments of Cd-Pb co-accumulation, followed by spectral extractions from 3 foliar positions: the blade root, the middle of the leaf, and the leaf apex. A support vector machine with leave-one-out cross-validation was performed to diagnose the contaminative levels based on the feature wavelengths selected by an improved successive projection algorithm. Partial least squares regression was used to predict Cd-Pb concentrations in rice blades. The results indicated that diagnostic accuracies were varied using spectra of different foliar positions. The blade root and leaf apex of rice blades were the optimal foliar position for detecting Cd and Pb contamination, respectively. At the optimal foliar positions, diagnostic accuracies exceeded 0.80 for distinguishing whether the rice is subject to Cd-Pb contamination. The Cd prediction performed 'very good' with a residual prediction deviation (RPD) of 2.21, a R2 of 0.79, and a root mean square error (RMSE)of 6.14, while that of Pb was 1.62, 0.61, and 186.54. Important wavelengths were identified at 659-694 nm and 667-694 nm to detect Cd and Pb contamination. In summary, our results verified the feasibility and clarified the optimal foliar positions of rice blades to detect Cd-Pb contamination. The wavelengths selecting have the great potential in the design of future leaf clips, and the optimal foliar position can provide suggestions to improve diagnostic performances in field applications.

CRISPR-Cas System, a Possible "Savior" of Rice Threatened by Climate Change: An Updated Review

Climate change has significantly affected agriculture production, particularly the rice crop that is consumed by almost half of the world's population and contributes significantly to global food security. Rice is vulnerable to several abiotic and biotic stresses such as drought, heat, salinity, heavy metals, rice blast, and bacterial blight that cause huge yield losses in rice, thus threatening food security worldwide. In this regard, several plant breeding and biotechnological techniques have been used to raise such rice varieties that could tackle climate changes. Nowadays, gene editing (GE) technology has revolutionized crop improvement. Among GE technology, CRISPR/Cas (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein) system has emerged as one of the most convenient, robust, cost-effective, and less labor-intensive system due to which it has got more popularity among plant researchers, especially rice breeders and geneticists. Since 2013 (the year of first application of CRISPR/Cas-based GE system in rice), several trait-specific climate-resilient rice lines have been developed using CRISPR/Cas-based GE tools. Earlier, several reports have been published confirming the successful application of GE tools for rice improvement. However, this review particularly aims to provide an updated and well-synthesized brief discussion based on the recent studies (from 2020 to present) on the applications of GE tools, particularly CRISPR-based systems for developing CRISPR rice to tackle the current alarming situation of climate change, worldwide. Moreover, potential limitations and technical bottlenecks in the development of CRISPR rice, and prospects are also discussed.

Two novel transporters NtNRAMP6a and NtNRAMP6b are involved in cadmium transport in tobacco (Nicotiana tabacum L.)

Plant natural resistance-associated macrophage protein (NRAMP) plays important roles in metal transport and tolerance. Tobacco is a typical cadmium (Cd) accumulator, while research on NRAMP in tobacco has been limited. In the current study, two novel NRAMP genes (NtNRAMP6a and NtNRAMP6b) were identified from the allotetraploid plant Nicotiana tabacum L. Real time‒PCR and GUS (β-glucuronidase) staining results showed that the two genes were expressed in roots, stems, leaves and flowers and induced by Cd stress. Subcellular localization revealed that they were located in the plasma membrane. Heterologously expressed NtNRAMP6a and NtNRAMP6b significantly increased the Cd sensitivity of the Δycf1 mutant, indicating that NtNRAMP6a and NtNRAMP6b had Cd transport functions in yeast. The difference in the manganese (Mn) transport activity of the two genes was demonstrated by point mutation, which was caused by the difference in the 18th amino acid. NRAMP6-N18K is a new key active site for manganese transport. After 50 μM Cd treatment for 7 days, the contents of Cd and Mn of the ntnramp6a/6b mutants was significantly lower than those of wild type in shoots, while the contents in roots were higher. Additionally, the mutant lines showed higher chorphyll contentration and lighter leaf damage. Knockout of NtNRAMP6a and NtNRAMP6b reduced Cd and Mn accumulation in tobacco shoots by influence root-to-shoot translocation. This provides new idea for cultivating tobacco varieties with low cadmium accumulation and high cadmium tolerance.

Functional Group Properties and Position Drive Differences in Xenobiotic Plant Uptake Rates, but Metabolism Shares a Similar Pathway

Plant uptake of xenobiotic compounds is crucial for phytoremediation (including green stormwater infrastructure) and exposure potential during crop irrigation with recycled water. Experimentally determining the plant uptake for every relevant chemical is impractical; therefore, illuminating the role of specific functional groups on the uptake of trace organic contaminants is needed to enhance predictive power. We used benzimidazole derivatives to probe the impact of functional group electrostatic properties and position on plant uptake and metabolism using the hydroponic model plant Arabidopsis thaliana. The greatest plant uptake rates occurred with an electron-withdrawing functional group at the 2 position; however, uptake was still observed with an electron-donating group. An electron-donating group at the 1 position significantly slowed uptake for both benzimidazole- and benzotriazole-based molecules used in this study, indicating possible steric effects. For unsubstituted benzimidazole and benzotriazole structures, the additional heterocyclic nitrogen in benzotriazole increased plant uptake rates compared to benzimidazole. Analysis of quantitative structure-activity relationship parameters for the studied compounds implicates energy-related molecular descriptors as uptake drivers. Despite significantly varied uptake rates, compounds with different functional groups yielded shared metabolites, including an impact on endogenous glutathione production. Although the topic is complex and influenced by multiple factors in the field, this study provides insights into the impact of functional groups on plant uptake, with implications for environmental fate and consumer exposure.

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.

Genome-Wide Identification and Expression Analysis Reveals Roles of the NRAMP Gene Family in Iron/Cadmium Interactions in Peanut

The natural resistance-associated macrophage protein (NRAMP) family plays crucial roles in metal uptake and transport in plants. However, little is known about their functions in peanut. To understand the roles of AhNRAMP genes in iron/cadmium interactions in peanut, genome-wide identification and bioinformatics analysis was performed. A total of 15 AhNRAMP genes were identified from the peanut genome, including seven gene pairs derived from whole-genome duplication and a segmental duplicated gene. AhNRAMP proteins were divided into two distinct subfamilies. Subfamily I contains eight acid proteins with a specific conserved motif 7, which were predicted to localize in the vacuole membrane, while subfamily II includes seven basic proteins sharing specific conserved motif 10, which were localized to the plasma membrane. Subfamily I genes contained four exons, while subfamily II had 13 exons. AhNRAMP proteins are perfectly modeled on the 5m94.1.A template, suggesting a role in metal transport. Most AhNRAMP genes are preferentially expressed in roots, stamens, or developing seeds. In roots, the expression of most AhNRAMPs is induced by iron deficiency and positively correlated with cadmium accumulation, indicating crucial roles in iron/cadmium interactions. The findings provide essential information to understand the functions of AhNRAMPs in the iron/cadmium interactions in peanuts.