Enhanced expression of SaHMA3 plays critical roles in Cd hyperaccumulation and hypertolerance in Cd hyperaccumulator Sedum alfredii Hance

Cadmium uptake and detoxification in tomato plants: Revealing promising targets for genetic improvement

Cadmium (Cd) is a hazardous heavy metal known for its detrimental effects on plants, human health, and the environment. This review article delves into the dynamics of Cd uptake, long-distance transport, and its impact on plant performance, with a specific focus on tomato plants. The process of Cd uptake by roots and its subsequent long-distance transport in the xylem and phloem are explored to understand how Cd influences plants operation. The toxic effects of Cd on tomato plants are discussed, highlighting on the challenges it poses to plant growth and development. Furthermore, the review investigates various Cd tolerance mechanisms in plants, including avoidance or exclusion by the root cell wall, root-to-shoot translocation, detoxification pathways, and antioxidative defence systems against Cd-induced stress. In addition, the transcriptomic analyses of tomato plants under Cd stress provide insights into the molecular responses and adaptations of plants to Cd toxicity. Overall, this comprehensive review enhances our understanding of Cd-plant interactions and reveal promising genes for tomato genetic improvement to increase its tolerance to cadmium.

Variety-dependent responses of common tobacco with differential cadmium resistance: Cadmium uptake and distribution, antioxidative activity, and gene expression

Cadmium (Cd), which accumulates in tobacco leaves, enters the human body through inhalation of smoke, causing harmful effects on health. Therefore, identifying the pivotal factors that govern the absorption and resistance of Cd in tobacco is crucial for mitigating the harmful impact of Cd. In the present study, four different Cd-sensitive varieties, namely, ZhongChuan208 (ZC) with resistance, ZhongYan100 (ZY), K326 with moderate resistance, and YunYan87 (YY) with sensitivity, were cultivated in hydroponic with different Cd concentrations (20 µM, 40 µM, 60 µM and 80 µM). The results indicated that plant growth was significantly decreased by Cd. Irrespective of the Cd concentration, ZC exhibited the highest biomass, while YY had the lowest biomass; ZY and K326 showed intermediate levels. Enzymatic (APX, CAT, POD) and nonenzymatic antioxidant (Pro, GSH) systems showed notable variations among varieties. The multifactor analysis suggested that the ZC and ZY varieties, with higher levels of Pro and GSH content, contribute to a decrease in the levels of MDA and ROS. Among all the Cd concentrations, ZC exhibited the lowest Cd accumulation, while YY showed the highest. Additionally, there were significant differences observed in Cd distribution and translocation factors among the four different varieties. In terms of Cd distribution, cell wall Cd accounted for the highest proportion of total Cd, and organelles had the lowest proportion. Among the varieties, ZC showed lower Cd levels in the cell wall, soluble fraction, and organelles. Conversely, YY exhibited the highest Cd accumulation in all tissues; K326 and ZY had intermediate levels. Translocation factors (TF) varied among the varieties under Cd stress, with ZC and ZY showing lower TF compared to YY and K326. This phenomenon mainly attributed to regulation of the NtNramp3 and NtNramp5 genes, which are responsible for the absorption and transport of Cd. This study provides a theoretical foundation for the selection and breeding of tobacco varieties that are resistant to or accumulate less Cd.

CIP1, a CIPK23-interacting transporter, is implicated in Cd tolerance and phytoremediation

Environmental pollution from cadmium (Cd) presents a serious threat to plant growth and development. Therefore, it's crucial to find out how plants resist this toxic metal to develop strategies for remediating Cd-contaminated soils. In this study, we identified CIP1, a transporter protein, by screening interactors of the protein kinase CIPK23. CIP1 is located in vesicles membranes and can transport Cd2+ when expressed in yeast cells. Cd stress specifically induced the accumulation of CIP1 transcripts and functional proteins, particularly in the epidermal cells of the root tip. CIKP23 could interact directly with the central loop region of CIP1, phosphorylating it, which is essential for the efficient transport of Cd2+. A loss-of-function mutation of CIP1 in wild-type plants led to increased sensitivity to Cd stress. Conversely, tobacco plants overexpressing CIP1 exhibited improved Cd tolerance and increased Cd accumulation capacity. Interestingly, this Cd accumulation was restricted to roots but not shoots, suggesting that manipulating CIP1 does not risk Cd contamination of plants' edible parts. Overall, this study characterizes a novel Cd transporter, CIP1, with potential to enhance plant tolerance to Cd toxicity while effectively eliminating environmental contamination without economic losses.

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

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

Copper-dependent control of uptake, translocation and accumulation of cadmium in hyperaccumlator Sedum alfredii

Cadmium (Cd) is detrimental to plant growth and threatens human health. Here, we investigated the potential for remediation of Cd-contaminated soil with high copper (Cu) background using Cd hyperaccumulator ecotype (HE) Sedum alfredii. We assessed effects of Cu on Cd accumulation, compartmentation and translocation in HE S. alfredii, and compared with those in a related non-accumulator ecotype (NHE). We found that Cu supply significantly induced Cd accumulation in roots and shoots of long-term soil-cultivated HE S. alfredii. A large fraction of root Cd was accumulated in the organelles, but a small fraction was stored in the cell wall. Importantly, Cu addition reduced Cd accumulation in the cell wall and the organelles in root cells. Furthermore, leaf cell capacity to sequestrate Cd in the organelles was greatly improved upon Cu exposure. We also found that genes involving metal transport and cell wall remodeling were distinctly regulated to mediate Cd accumulation in HE S. alfredii. These findings indicate that Cu-dependent decrease of root cell-wall-bound Cd, and stimulation of efflux/influx of organelle Cd transport in root and leaf cells plays a role in the dramatic Cd hyperaccumulation expressed in naturally survived HE S. alfredii.

Unveiling Innovative Approaches to Mitigate Metals/Metalloids Toxicity for Sustainable Agriculture

Due to anthropogenic activities, environmental pollution of heavy metals/metalloids (HMs) has increased and received growing attention in recent decades. Plants growing in HM-contaminated soils have slower growth and development, resulting in lower agricultural yield. Exposure to HMs leads to the generation of free radicals (oxidative stress), which alters plant morpho-physiological and biochemical pathways at the cellular and tissue levels. Plants have evolved complex defense mechanisms to avoid or tolerate the toxic effects of HMs, including HMs absorption and accumulation in cell organelles, immobilization by forming complexes with organic chelates, extraction via numerous transporters, ion channels, signaling cascades, and transcription elements, among others. Nonetheless, these internal defensive mechanisms are insufficient to overcome HMs toxicity. Therefore, unveiling HMs adaptation and tolerance mechanisms is necessary for sustainable agriculture. Recent breakthroughs in cutting-edge approaches such as phytohormone and gasotransmitters application, nanotechnology, omics, and genetic engineering tools have identified molecular regulators linked to HMs tolerance, which may be applied to generate HMs-tolerant future plants. This review summarizes numerous systems that plants have adapted to resist HMs toxicity, such as physiological, biochemical, and molecular responses. Diverse adaptation strategies have also been comprehensively presented to advance plant resilience to HMs toxicity that could enable sustainable agricultural production.

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

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

Plant response to heavy metal stress toxicity: the role of metabolomics and other omics tools

Metabolomic investigations offers a significant foundation for improved comprehension of the adaptability of plants to reconfigure the key metabolic pathways and their response to changing climatic conditions. Their application to ecophysiology and ecotoxicology help to assess potential risks caused by the contaminants, their modes of action and the elucidation of metabolic pathways associated with stress responses. Heavy metal stress is one of the most significant environmental hazards affecting the physiological and biochemical processes in plants. Metabolomic tools have been widely utilised in the massive characterisation of the molecular structure of plants at various stages for understanding the diverse aspects of the cellular functioning underlying heavy metal stress-responsive mechanisms. This review emphasises on the recent progressions in metabolomics in plants subjected to heavy metal stresses. Also, it discusses the possibility of facilitating effective management strategies concerning metabolites for mitigating the negative impacts of heavy metal contaminants on the growth and productivity of plants.

Inventory of cadmium-transporter genes in the root of mangrove plant Avicennia marina under cadmium stress

Mangrove Avicennia marina has the importantly potential for cadmium (Cd) pollution remediation in coastal wetlands. Unfortunately, the molecular mechanisms and transporter members for Cd uptake by the roots of A. marina are not well documented. In this study, photosynthetic and phenotypic analysis indicated that A. marina is particularly tolerant to Cd. The content and flux analysis indicated that Cd is mainly retained in the roots, with greater Cd influx in fine roots than that in coarse roots, and higher Cd influx in the root meristem zone as well. Using transcriptomic analysis, a total of 5238 differentially expressed genes were identified between the Cd treatment and control group. Moreover, we found that 54 genes were responsible for inorganic ion transport. Among these genes, AmHMA2, AmIRT1, and AmPCR2 were localized in the plasma membrane and AmZIP1 was localized in both plasma membrane and cytoplasm. All above gene encoding transporters showed significant Cd transport activities using function assay in yeast cells. In addition, the overexpression of AmZIP1 or AmPCR2 in Arabidopsis improved the Cd tolerance of transgenic plants. This is particularly significant as it provides insight into the molecular mechanism for Cd uptake by the roots of mangrove plants and a theoretical basis for coastal wetland phytoremediation.

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 characterization of a DNA-damage repair/tolerance 100 (DRT100) gene in Sedum alfredii Hance for genome stability maintenance and Cd hypertolerance

Cd contamination is a world-wild concern for its toxicity and accumulation in food chain. Sedum alfredii Hance (Crassulaceae) is a zinc (Zn) and cadmium (Cd) hyperaccumulator native to China and widely applied for the phytoremediation at Zn or Cd contaminated sites. Although many studies report the uptake, translocation and storage of Cd in S. alfredii Hance, limited information is known about the genes and underlying mechanisms of genome stability maintenance under Cd stress. In this study, a gene resembling DNA-damage repair/toleration 100 (DRT100) was Cd inducible and designated as SaDRT100. Heterologous expression of SaDRT100 gene in yeasts and Arabidopsis thaliana enhanced Cd tolerance capability. Under Cd stress, transgenic Arabidopsis with SaDRT100 gene exhibited lower levels of reactive oxygen species (ROS), fewer Cd uptake in roots and less Cd-induced DNA damage. Evidenced by the subcellular location in cellular nucleus and expression in aerial parts, we suggested the involvement of SaDRT100 in combating Cd-induced DNA damage. Our findings firstly uncovered the roles of SaDRT100 gene in Cd hypertolerance and genome stability maintenance in S. alfredii Hance. The potential functions of DNA protection make SaDRT100 gene a candidate in genetic engineering for phytoremediation at multi-component contaminated sites.

Significance and genetic control of membrane transporters to improve phytoremediation and biofortification processes

Humans frequently consume plant-based foods in their daily life. Contamination of agricultural soils by heavy metals (HMs) is a major food and nutritional security issue. The crop plants grown in HM-contaminated agricultural soil may accumulate more HMs in their edible part, further transferring into the food chain. Consumption of HM-rich crops can cause severe health issues in humans. On the other hand, the low content of the essential HM in the edible part of the crop also causes health problems. Therefore, researchers must try to reduce the non-essential HM in the edible part of the crop plants and improve the essential HMs. Phytoremediation and biofortification are the two strategies for resolving this problem. The genetic component helps to improve the efficiency of phytoremediation and biofortification processes in plants. They help eliminate HMs from soil and improve essential HM content in crop plants. The membrane transporter genes (genetic components) are critical in these two strategies. Therefore, engineering membrane transporter genes may help reduce the non-essential HM content in the edible part of crop plants. Targeted gene editing by genome editing tools like CRISPR could help plants achieve efficient phytoremediation and biofortification. This article covers gene editing's scope, application, and implication to improve the phytoremediation and biofortification processes in non-crop and crop plants.

Enhancing the potential for cadmium phytoremediation by introducing Perilla frutescens genes in tobacco

To improve the potential of cadmium phytoremediation, distant hybridization between tobacco (Nicotiana tabacum L. var. 78-04), a high-biomass crop, and Perilla frutescens var. frutescens, a wild Cd-hyperaccumulator, was carried out, developing a new variety N. tabacum L. var. ZSY. Seedlings at the six-leaf stage were grown in hydroponics and treated with 0 (control), 10 µM, 180 µM, and 360 µM CdCl₂ for 7 days; then, the differences in Cd tolerance and accumulation and physiological and metabolic responses were evaluated among "ZSY" and its parents. At high Cd dose, the growth of "ZSY," such as fresh weight, plant height, and root length, was evidently better than "78-04." In contrast to P. frutescens and "78-04," "ZSY" could accumulate more Cd in shoots than roots. Under the same treatment, "ZSY" accumulated greater amounts of Cd in both shoots (195-1523 mg kg^-1) and roots (140-1281 mg kg^-1) than "78-04" (shoots: 35-89 mg kg^-1, roots: 39-252 mg kg^-1), followed by P. frutescens (shoots: 156-454 mg kg^-1, roots: 103-761 mg kg^-1). BCF and TF values of "ZSY" reached 38-195 and 1.2-1.4, which were far higher than those of "78-04" (BCF: 2.2-35.3, TF: 0.35-0.9). Perilla frutescens was found with BCF and TF of 11-156 and 0.5-1.5. Cd stress obviously promoted the production of ROS and MDA in seedlings but reduced chlorophyll contents, especially in "78-04." As a response to Cd stress, "ZSY" had higher SOD and CAT activities when compared to P. frutescens and "78-04," while "78-04" produced more POD and proline than those of P. frutescens and "ZSY." Cd stress could affect the production and accumulation of alkaloids and phenolic compounds in root (endodermis and cortex) and mesophyll. At high Cd doses, P. frutescens and "ZSY" had more alkaloids in tissues than "78-04." Phenolic compounds in "78-04" were more obviously inhibited compared with P. frutescens and "ZSY." These secondary metabolites may play an important role in eliminating oxidative damage and enhancing Cd tolerance and accumulation in "ZSY" and P. frutescens. Results indicated that distant hybridization could be one of effective methods for introducing excellent genes from metal-hyperaccumulators into high biomass species, creating plants with superior phytoremediation potential.

Overexpression of Sedum SpHMA2, SpHMA3 and SpNramp6 in Brassica napus increases multiple heavy metals accumulation for phytoextraction

Phytoextraction is an environmentally friendly phytoremediation technology that can reduce the total amount of heavy metals (HMs) in the soil. Hyperaccumulators or hyperaccumulating transgenic plants with biomass are important biomaterials for phytoextraction. In this study, we show that three different HM transporters from the hyperaccumulator Sedum pumbizincicola, SpHMA2, SpHMA3, and SpNramp6, possess Cd transport. These three transporters are located at the plasma membrane, tonoplast, and plasma membrane, respectively. Their transcripts could be strongly stimulated by multiple HMs treatments. To create potential biomaterials for phytoextraction, we overexpressed the three single genes and two combining genes, SpHMA2&SpHMA3 and SpHMA2&SpNramp6, in rapes having high biomass and environmental adaptability, and found that the aerial parts of the SpHMA2-OE3 and SpHMA2&SpNramp6-OE4 lines accumulated more Cd from single Cd-contaminated soil because SpNramp6 transports Cd from root cells to the xylem and SpHMA2 from the stems to the leaves. However, the accumulation of each HM in the aerial parts of all selected transgenic rapes was strengthened in multiple HMs-contaminated soils, probably due to the synergistic transport. The HMs residuals in the soil after the transgenic plant phytoremediation were also greatly reduced. These results provide effective solutions for phytoextraction in both Cd and multiple HMs-contaminated soils.

Zinc Supply Affects Cadmium Uptake and Translocation in the Hyperaccumulator Sedum Plumbizincicola as Evidenced by Isotope Fractionation

This study employs stable isotope analysis to investigate the mechanisms of cadmium (Cd) and zinc (Zn) interaction in the metal hyperaccumulating plant species Sedum plumbizincicola. To this end, the Cd and Zn isotope compositions of root, stem, leaf, and xylem sap samples were determined during metal uptake and translocation at different Cd and Zn concentrations. The enrichment of light isotopes of both elements in plants during uptake was less pronounced at low metal supply levels, likely reflecting the switch from a low-affinity to a high-affinity transport system at lower levels of external metal supply. The lower δ114/110Cd values of xylem sap when treated with a metabolic inhibitor decreasing the active Cd uptake further supports the preference of heavier Cd isotopes during high-affinity transport. The Δ66Znplant-initial solution or Δ66Znplant-final solution values were similar at different Cd concentrations, indicating negligible interaction of Cd in the Zn uptake process. However, decreasing Zn supply levels significantly increased the enrichment of light Cd isotopes in plants (Δ114/110Cd = -0.08‰) in low-Cd treatments but reduced the enrichment of light Cd isotopes in plants (Δ114/110Cd = 0.08‰) under high Cd conditions. A systematic enrichment of heavy Cd and light Zn isotopes was found in root-to-shoot translocation of the metals. The Cd concentrations of the growth solutions thereby had no significant impact on Zn isotope fractionation during root-to-shoot translocation. However, the Δ114/110Cdtranslocation values hint at possible competition between Cd and Zn for transporters during root-to-shoot transfer and this may impact the transport pathway of Cd. The stable isotope data demonstrate that the interactions between the two metals influenced the uptake and transport mechanisms of Cd in S. plumbizincicola but had little effect on those of Zn.

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 survey of HMA gene family and its characterization in wheat (Triticum aestivum)

Background

Abiotic stresses, particularly drought and heavy metal toxicity, have presented a significant risk to long-term agricultural output around the world. Although the heavy-metal-associated domain (HMA) gene family has been widely explored in Arabidopsis and other plants, it has not been thoroughly studied in wheat (Triticum aestivum). This study was proposed to investigate the HMA gene family in wheat.

Methods

To analyze the phylogenetic relationships, gene structure, gene ontology, and conserved motifs, a comparative study of wheat HMA genes with the Arabidopsis genome was performed.

Results

A total of 27 T. aestivum proteins belonging to the HMA gene family were identified in this study, with amino acid counts ranging from 262 to 1,071. HMA proteins were found to be grouped into three subgroups in a phylogenetic tree, and closely related proteins in the tree showed the same expression patterns as motifs found in distinct subgroups. Gene structural study elucidated that intron and exon arrangement differed by family.

Conclusion

As a result, the current work offered important information regarding HMA family genes in the T. aestivum genome, which will be valuable in understanding their putative functions in other wheat species.

Physiology and proteomics reveal Fulvic acid mitigates Cadmium adverse effects on growth and photosynthetic properties of lettuce

Understanding the molecular mechanisms of plants in response to Cd stress is crucial for improving plants adaptation to Cd stress. Fulvic acid (FA) is an active humic substance that is often used as a soil conditioner. However, there are few reports on the role of FA against Cd stress. The aim of this study was to determine the effects of Fulvic acid on alleviation of Cd toxicity in lettuce (Lactuca sativa L) under hydroponic conditions. Our results showed that 20 μmol/L Cd stress significantly reduced photosynthetic pigment metabolism and the expression of photosynthetic apparatus-related proteins, thereby inhibiting photosynthetic electron transport, net photosynthetic rate and negatively affecting photosynthetic carbon assimilation and growth of lettuce. However, proteomic findings suggest that the application of FA can reduce the adverse effects of Cd contamination. Compared to Cd stress alone, FA significantly increased the expression of Light-harvesting proteins, reaction center and electron transport-related proteins. Further results showed that FA at 0.5 g/L reduced the uptake of Cd by the roots, resulting in a 23.5% reduction in total Cd content in lettuce. Moreover, FA enhanced S metabolism and rebuilt redox homeostasis in cells. Overall, these findings provide new insights into the mechanism of cadmium toxicity mitigation in lettuce by FA. Which is recommended as an eco-friendly tool for improving the photosynthesis performance and biomass of lettuce under Cd stress.

Transcriptome analysis reveals candidate genes involved in multiple heavy metal tolerance in hyperaccumulator Sedum alfredii

Sedum alfredii Hance is a perennial herb native to China that can particularly be found in regions with abandoned Pb/Zn mines. It is a Cd/Zn hyperaccumulator that is highly tolerant to Pb, Cu, Ni, and Mn, showing potential for phytoremediation of soils contaminated with multiple heavy metals. A better understanding of how this species responds to different heavy metals would advance the phytoremediation efficiency. In this study, transcriptomic regulation of S. alfredii roots after Cd, Zn, Pb, and Cu exposure was analyzed to explore the candidate genes involved in multi-heavy metal tolerance. Although Zn and Cd, Pb and Cu had similar distribution patterns in S. alfredii, distinct expression patterns were exhibited among these four metal treatments, especially about half of the differentially expressed genes were upregulated under Cu treatment, suggesting that it utilizes distinctive and flexible strategies to cope with specific metal stress. Most unigenes regulated by Cu were enriched in catalytic activity, whereas the majority of unigenes regulated by Pb had unknown functions, implying that S. alfredii may have a unique strategy coping with Pb stress different from previous cognition. The unigenes that were co-regulated by multiple heavy metals exhibited functions of antioxidant substances, antioxidant enzymes, transporters, transcription factors, and cell wall components. These metal-induced responses at the transcriptional level in S. alfredii were highly consistent with those at the physiological level. Some of these genes have been confirmed to be related to heavy metal absorption and detoxification, and some were found to be related to heavy metal tolerance for the first time in this study, like Metacaspase-1 and EDR6. These results provide a theoretical basis for the use of genetic engineering technology to modify plants by enhancing multi-metal tolerance to promote phytoremediation efficiency.

Advances in Genes-Encoding Transporters for Cadmium Uptake, Translocation, and Accumulation in Plants

Cadmium (Cd) is a heavy metal that is highly toxic for plants, animals, and human beings. A better understanding of the mechanisms involved in Cd accumulation in plants is beneficial for developing strategies for either the remediation of Cd-polluted soils using hyperaccumulator plants or preventing excess Cd accumulation in the edible parts of crops and vegetables. As a ubiquitous heavy metal, the transport of Cd in plant cells is suggested to be mediated by transporters for essential elements such as Ca, Zn, K, and Mn. Identification of the genes encoding Cd transporters is important for understanding the mechanisms underlying Cd uptake, translocation, and accumulation in either crop or hyperaccumulator plants. Recent studies have shown that the transporters that mediate the uptake, transport, and accumulation of Cd in plants mainly include members of the natural resistance-associated macrophage protein (Nramp), heavy metal-transporting ATPase (HMA), zinc and iron regulated transporter protein (ZIP), ATP-binding cassette (ABC), and yellow stripe-like (YSL) families. Here, we review the latest advances in the research of these Cd transporters and lay the foundation for a systematic understanding underlying the molecular mechanisms of Cd uptake, transport, and accumulation in plants.

Knowledge Mapping of the Phytoremediation of Cadmium-Contaminated Soil: A Bibliometric Analysis from 1994 to 2021

Cadmium pollution of soil threatens the environmental quality and human health. Phytoremediation of cadmium-contaminated soil has attracted global attention in recent decades. This study aimed to conduct a comprehensive and systematic review of the literature on phytoremediation of cadmium-contaminated soil based on bibliometric analysis. A total of 5494 articles published between 1994 and 2021 were retrieved from the Web of Science Core Collection. Our knowledge mapping presented the authors, journals, countries, institutions, and other basic information to understand the development status of phytoremediation of cadmium-contaminated soil. Based on a keyword cluster analysis, the identified major research domains were "biochar", "Thlaspi caerulescens", "endophytic bacteria", "oxidative stress", "EDTA", and "bioconcentration factor". Overall, this study provided a detailed summary of research trends and hotspots. Based on the keyword co-occurrence and burst analysis, the core concepts and basic theories of this field were completed in 2011. However, the pace of theoretical development has been relatively slow. Finally, future research trends/frontiers were proposed, such as biochar addition, rhizosphere bacterial community manipulation, cadmium subcellular distribution, and health risk assessment.

Rhodococcus qingshengii facilitates the phytoextraction of Zn, Cd, Ni, and Pb from soils by Sedum alfredii Hance

The enhanced heavy metal (HM) phytoextraction efficiency of hyperaccumulating plants via plant-growth-promoting microbes has been proposed as an effective strategy to remove HMs from contaminated soil. Nevertheless, it remains unclear whether catabolizing the abscisic acid (ABA) in hyperaccumulating plants via rhizobacteria can facilitate HM phytoextraction. In the present study, a hyperaccumulator, Sedum alfredii Hance, inoculated with an ABA-catabolizing bacterium Rhodococcus qingshengii, showed higher concentrations of Zn, Cd, Ni, and Pb in the contaminated paddy-grown plant shoots by 35%, 63%, 49%, and 49%, and in plants grown in mine soils by 112%, 105%, 46%, and 49%, respectively, than in the control_{bacteria-free} plants. However, no significant changes were observed in Cu content between these plants. Furthermore, parameters indicating phytoremediation potential, including the translocation factor (TF) and bioconcentration factor (BCF), revealed that bacterial inoculation could markedly increase the efficacy of Zn, Cd, Ni, and Pb phytoextraction from the soil. Notably, the bioavailabilities of HMs in soils were not influenced by R. qingshengii; however, the expression of transporters related to the uptake of these HMs, including SaIRT1, SaZIP1, SaZIP2, SaZIP3, SaNramp1, SaNramp3, SaNramp6, SaHMA2, and SaHMA3, was upregulated. These findings indicate that R. qingshengii inoculation could increase the HM-uptake ability of plants by catabolizing ABA and may provide a promising strategy for enhancing the phytoremediation efficacy in HM-contaminated soils.

Lowered Cd toxicity, uptake and expression of metal transporter genes in maize plant by ACC deaminase-producing bacteria Achromobacter sp

In this study, an ACC deaminase-producing bacterial strain Achromobacter sp. A1 was isolated from maize rhizosphere soil, characterized and evaluated for the effects on cadmium (Cd) immobilization in solution/rhizosphere, physiological characteristics and the tissue Cd contents in maize and the molecular mechanisms involved by hydroponic and pot experiments. ACC deaminase activity of strain A1 was significantly enhanced by Cd addition and Cd concentration decreased (55.54-63.62%) in solution supplemented with various Cd concentrations. Strain A1 significantly increased the maize dry weights (30.77-105%) and chlorophyll content (7.46-14.46%), decreased MDA content (25.16-36.87%) and ethylene production (20.93-35.86%) in hydroponic experiment. Strain A1 significantly reduced the above-ground tissue Cd uptake by 12.64-33.68% and 42-48% in hydroponic and pot experiments, reduced the DTPA-extractable Cd content and elevated invertase, urease and catalase activity in rhizosphere soils. In addition, the expression levels of Cd transporter genes HMA3 and Nramp5 were significantly reduced in root and shoot after strain A1 inoculation. These results indicate that strain A1 has great potential for application as a novel and environmentally friendly inoculant to immobilize Cd and reduce maize Cd uptake in Cd-contaminated environments, and will improve the understanding of the relative molecular mechanisms underlying the response to strain A1 in maize plant.

Roles of exogenous plant growth regulators on phytoextraction of Cd/Pb/Zn by Sedum alfredii Hance in contaminated soils

Plant growth regulators (PGRs) assisted phytoextraction was investigated as a viable phytoremediation technology to increase the phytoextraction efficiency in contaminated soils. This study aimed to evaluate the cadimum (Cd)/lead (Pb)/zinc (Zn) phytoextraction efficiency by a hyperaccumulator Sedum alfredii Hance (S. alfredii) treated with 9 PGRs, including indole-3-acetic acid (IAA), gibberellin (GA₃), cytokinin (CKs), abscisic acid (ABA), ethylene (ETH), brassinosteroid (BR), salicylic acid (SA), strigolactones (SL) and jasmonic acid (JA), in slightly or heavily contaminated (SC and HC, respectively) soil. Results demonstrated that PGRs were able to improve S. alfredii biomass, the most significant increases were observed in GA₃ and SL for HC soil, while for SC soil, IAA and BR exhibited positive effects. The levels of Cd, Pb and Zn in the shoots of S. alfredii treated with ABA and SL were noticeably greater than in the CK treatment in HC soil, while the uptake of metals were increased by IAA and CKs in SC soil. Combined with the results of biomass and metal contents in shoots, we found that ABA showed the highest Cd removal efficiency and the maximum Pb and Zn removal efficiency was observed with GA₃, which was 62.99%, 269.23%, and 41.18%, respectively higher than the control in HC soil. Meanwhile, compared to control, the maximum removal efficiency of Cd by IAA treatment (52.80%), Pb by JA treatment (165.1%), and Zn by BR treatment (44.97%) in the SC soil. Overall, our results suggested that these PGRs, especially, ABA, SL, IAA, BR and GA₃ had great potential in improving phytoremediation efficiency of S. alfredii grown in contaminated soils.

Exposure of cerium oxide nanoparticles to the hyperaccumulator Sedum alfredii decreases the uptake of cadmium via the apoplastic pathway

Cadmium (Cd) is harmful to the environment and threatens human health. With the increasing use of cerium oxide nanoparticles (CeO₂NPs) in extensive industries, investigating the combination of CeO₂NPs and plants has attracted research interests for phytoremediation. Here, we explored the effects of CeO₂NPs on Cd uptake, transport and the consequent Cd accumulation in Sedum alfredii. Exposure of 50 or 500 mg L^-1 CeO₂NPs alone had no apparent damaging effects on plant growth. However, upon Cd condition, the consistent CeO₂NPs decreased Cd concentrations in the roots and shoots by up to 37%. Furthermore, the application of a metabolic inhibitor revealed that CeO₂NPs mainly decreased the Cd uptake in roots by the apoplastic pathway. Simultaneously, CeO₂NPs accelerated the development of Casparian strips (CSs) and suberin, which was further proven by the elevated expression levels of genes associated with their formation, SaCASP, SaGPAT5, SaKCS20 and SaCYP86A1. Compared to CeO₂NPs added alone, the concurrent Cd decreased the Ce contents in the roots and altered its translocation from root to shoot. Taken together, both CeO₂NPs and Cd influence the interactional uptake of both chemicals in roots of S. alfredii mainly via the apoplastic pathway which is primarily regulated by the development of CSs and suberin.

The Different Faces of Arabidopsis arenosa-A Plant Species for a Special Purpose

The following review article collects information on the plant species Arabidopsis arenosa. Thus far, A. arenosa has been known as a model species for autotetraploidy studies because, apart from diploid individuals, there are also tetraploid populations, which is a unique feature of this Arabidopsis species. In addition, A arenosa has often been reported in heavy metal-contaminated sites, where it occurs together with a closely related species A. halleri, a model plant hyperaccumulator of Cd and Zn. Recent studies have shown that several populations of A. arenosa also exhibit Cd and Zn hyperaccumulation. However, it is assumed that the mechanism of hyperaccumulation differs between these two Arabidopsis species. Nevertheless, this phenomenon is still not fully understood, and thorough research is needed. In this paper, we summarize the current state of knowledge regarding research on A. arenosa.

Exogenous application of Mn significantly increased Cd accumulation in the Cd/Zn hyperaccumulator Sedum alfredii

Sedum alfredii is a Cd/Zn hyperaccumulator native to China, which was collected from a mined area where Mn content in soil was extremely high, together with Zn and Cd content. We investigated the tolerance and accumulation ability of Mn and its possible association with Cd hyperaccumulation in this plant species by using MP-AES, SR-μ-XRF, and RT-PCR. The results showed that the hyperaccumulating ecotype (HE) S. alfredii exhibited high tolerance to Mn and accumulating around 10,000 and 12,000 mg kg^-1 Mn in roots and shoots, respectively, without exhibiting toxicity under 5000 mg kg^-1 Mn treatment for 4 weeks. Exposure to Cd significantly reduced plant uptake of Mn. In contrast, exogenous Mn application significantly improved root uptake and root-to-shoot translocation of Cd, resulting in the increased Cd accumulation in the shoots of HE S. alfredii. SR-μ-XRF analysis demonstrated that high Mn (20 μM) exposure resulted in higher intensities of Cd localized in both stem vascular bundles and cortex, as well as leaf mesophyll cells, than in those treated with low Mn levels (0.2 μM or 2.0 μM). RT-PCR analysis of several genes possibly involved in Mn/Cd transportation showed that expression of SaNramp3 in roots was significantly reduced under high Mn exposure. These results suggested a significant interaction between Cd and Mn in the HE S. alfredii plants, possibly through their competition for transporters and theoretically provided a strategy to improve the efficiency of Cd extraction from polluted soils by this plant species, after using appropriate nutrient management of Mn.

Bio-remediation approaches for alleviation of cadmium contamination in natural resources

Cadmium (Cd) is a harmful heavy metal that can cause potent environmental and health hazards at different trophic levels through food chain. Cd is relatively non-biodegradable and persists for a long time in the environment. Considering the potential toxicity and non-biodegradability of Cd in the environment as well as its health hazards, this is an urgent issue of international concern that needs to be addressed by implicating suitable remedial approaches. The current article specifically attempts to review the different biological approaches for remediation of Cd contamination in natural resources. Further, bioremediation mechanisms of Cd by microbes such as bacteria, fungi, algae are comprehensively discussed. Studies indicate that heavy metal resistant microbes can be used as suitable biosorbents for the removal of Cd (up to 90%) in the natural resources. Soil-to-plant transfer coefficient (TC) of Cd ranges from 3.9 to 3340 depending on the availability of metal to plants and also on the type of plant species. The potential phytoremediation strategies for Cd removal and the key factors influencing bioremediation process are also emphasized. Studies on molecular mechanisms of transgenic plants for Cd bioremediation show immense potential for enhancing Cd phytoremediation efficiency. Thus, it is suggested that nano-technological based integrated bioremediation approaches could be a potential futuristic path for Cd decontamination in natural resources. This review would be highly useful for the biologists, chemists, biotechnologists and environmentalists to understand the long-term impacts of Cd on ecology and human health so that potential remedial measures could be taken in advance.

The Role of Aquaporin Overexpression in the Modulation of Transcription of Heavy Metal Transporters under Cadmium Treatment in Poplar

Populus alba ‘Villafranca’ clone is well-known for its tolerance to cadmium (Cd). To determine the mechanisms of Cd tolerance of this species, wild-type (wt) plants were compared with transgenic plants over-expressing an aquaporin (aqua1, GenBank GQ918138). Plants were maintained in hydroponic conditions with Hoagland’s solution and treated with 10 µM of Cd, renewed every 5 d. The transcription levels of heavy metal transporter genes (PaHMA2, PaNRAMP1.3, PaNRAMP2, PaNRAMP3.1, PaNRAMP3.2, PaABCC9, and PaABCC13) were analyzed at 1, 7, and 60 d of treatment. Cd application did not induce visible toxicity symptoms in wt and aqua1 plants even after 2 months of treatment confirming the high tolerance of this poplar species to Cd. Most of the analyzed genes showed in wt plants a quick response in transcription at 1 d of treatment and an adaptation at 60 d. On the contrary, a lower transcriptional response was observed in aqua1 plants in concomitance with a higher Cd concentration in medial leaves. Moreover, PaHMA2 showed at 1 d an opposite trend within organs since it was up-regulated in root and stem of wt plants and in leaves of aqua1 plants. In summary, aqua1 overexpression in poplar improved Cd translocation suggesting a lower Cd sensitivity of aqua1 plants. This different response might be due to a different transcription of PaNRAMP3 genes that were more transcribed in wt line because of the importance of this gene in Cd compartmentalization.

Identification and functional characterization of ABCC transporters for Cd tolerance and accumulation in Sedum alfredii Hance

Cd is one of the potential toxic elements (PTEs) exerting great threats on the environment and living organisms and arising extensive attentions worldwide. Sedum alfredii Hance, a Cd hyperaccumulator, is of great importance in studying the mechanisms of Cd hyperaccumulation and has potentials for phytoremediation. ATP-binding cassette sub-family C (ABCC) belongs to the ABC transporter family, which is deemed to closely associate with multiple physiological processes including cellular homeostasis, metal detoxification, and transport of metabolites. In the present work, ten ABCC proteins were identified in S. alfredii Hance, exhibiting uniform domain structure and divergently clustering with those from Arabidopsis. Tissue-specific expression analysis indicated that some SaABCC genes had significantly higher expression in roots (Sa23221 and Sa88F144), stems (Sa13F200 and Sa14F98) and leaves (Sa13F200). Co-expression network analysis using these five SaABCC genes as hub genes produced two clades harboring different edge genes. Transcriptional expression profiles responsive to Cd illustrated a dramatic elevation of Sa14F190 and Sa18F186 genes. Heterologous expression in a Cd-sensitive yeast cell line, we confirmed the functions of Sa14F190 gene encoding ABCC in Cd accumulation. Our study performed a comprehensive analysis of ABCCs in S. alfredii Hance, firstly mapped their tissue-specific expression patterns responsive to Cd stress, and characterized the roles of Sa14F190 genes in Cd accumulation.

Evolution of the metal hyperaccumulation and hypertolerance traits

To succeed in life, living organisms have to adapt to the environmental issues to which they are subjected. Some plants, defined as hyperaccumulators, have adapted to metalliferous environments, acquiring the ability to tolerate and accommodate high amounts of toxic metal into their shoot, without showing symptoms of toxicity. The determinants for these traits and their mode of action have long been the subject of research, whose attention lately moved to the evolution of the hypertolerance and hyperaccumulation traits. Genetic evidence indicates that the evolution of both traits includes significant evolutionary events that result in species-wide tolerant and accumulating backgrounds. Different edaphic environments are responsible for subsequent refinement, by local adaptive processes, leading to specific strategies and various degrees of hypertolerance and hyperaccumulation, which characterize metallicolous from non-metallicolous ecotypes belonging to the same genetic unit. In this review, we overview the most updated concepts regarding the evolution of hyperaccumulation and hypertolerance, highlighting also the ecological context concerning the plant populations displaying this fascinating phenomenon.

A comparative study of root cadmium radial transport in seedlings of two wheat (Triticum aestivum L.) genotypes differing in grain cadmium accumulation

The radial transport of cadmium (Cd) is essential for Cd influx in roots. The role of radial transport pathway on the Cd translocation from root to shoot among wheat genotypes are still poorly understood. This study explored the role of apoplastic and symplastic pathway on root Cd uptake and root-to-shoot translocation in Zhenmai 10 (ZM10, high Cd in grains) and Aikang 58 (AK58, low Cd in grains). Under Cd treatment, the deposition of Casparian strips (CSs) and suberin lamellae (SL) initiated closer to the root apex in ZM10 than that in AK58, which resulted in the lower Cd concentration in apoplastic fluid of ZM10. Simultaneously, Cd-induced expression levels of genes related to Cd uptake in roots were significantly higher in AK58 by contrast with ZM10, contributing to the symplastic Cd accumulation in AK58 root. Moreover, the addition of metabolic inhibitor CCCP noticeably decreased the Cd accumulation in root of both genotypes. Intriguingly, compared to ZM10, greater amounts of Cd were sequestrated in the cell walls and vacuoles in roots of AK58, limiting the translocation of Cd from root to shoot. Furthermore, the elevated TaHMA2 expression in ZM10 indicates that ZM10 had a higher capacity of xylem loading Cd than AK58. All of these results herein suggest that the radial transport is significant for Cd accumulation in roots, but it cannot explain the difference in root-to-shoot translocation of Cd in wheat genotypes with contrast Cd accumulation in grains.

Plant Cadmium Resistance 2 (SaPCR2) Facilitates Cadmium Efflux in the Roots of Hyperaccumulator Sedum alfredii Hance

Hyperaccumulators are the preferred materials for phytoremediation. Sedum alfredii Hance is a cadmium (Cd) hyperaccumulator plant in China, although its detoxification mechanism remains unresolved. In our study, we cloned a gene belonging to the plant cadmium resistance (PCR) family, named SaPCR2, from the hyperaccumulating ecotype (HE) of S. alfredii. Sequence analysis indicated that SaPCR2 contained a cysteine-rich domain highly conserved in the PCR family and played an important role in Cd detoxification. Based on the relative quantitative results, SaPCR2 was highly expressed in the roots of HE S. alfredii, but not the shoots and Cd exposure did not significantly affect SaPCR2 expression. In contrast, the expression level of SaPCR2 was very low in plants of its non-hyperaccumulating ecotype (NHE). The subcellular localization of SaPCR2 in tobacco leaves and yeasts showed that SaPCR2 was localized on the plasma membrane and the expression of the SaPCR2 protein in a Zn/Cd-sensitive yeast Δzrc1 significantly increased its tolerance to Cd stress by decreasing the Cd content in cells. Heterologous expression of SaPCR2 in plants of both Arabidopsis thaliana and NHE S. alfredii significantly reduced the Cd levels in the roots, but not in the shoots. These results suggest that the overexpression of SaPCR2 in plants provides a route for Cd leak out of the root cells and protects the root cells against phytotoxicity of Cd stress. To the best of our knowledge, this is the first study of transporter-mediated root efflux of Cd in hyperaccumulator S. alfredii.

Pseudomonas fluorescens accelerates a reverse and long-distance transport of cadmium and sucrose in the hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote root uptake and shoot accumulation of cadmium (Cd) in hyperaccumulator plants, but the mechanisms by which PGPB accelerate root-to-shoot transport of Cd is still unknown. A better understanding of these mechanisms is necessary to develop the strategies that can promote the practical phytoextraction of Cd-polluted soils. In this study, we found that Pseudomonas fluorescens accelerates a reversed and a long-distance transport of Cd and sucrose in Sedum alfredii, by examining the xylem and phloem sap and by quantifying the concentrations of Cd and sucrose in shoot and root. The transcriptome sequencing has revealed the up-regulated expressions of starch metabolism and sucrose biosynthesis related genes in the shoots of Cd hyperaccumulator plant S. alfredii that was inoculated with PGPB P. fluorescens. In addition, the genes of sugar, cation and anion transporters were also up-regulated by bacterial treatment, showing a complicated co-expression network with sucrose biosynthesis related genes. The expression levels of Cd transporter genes, such as ZIP1, ZIP2, HMA2, HMA3 and CAX2, were elevated after PGPB inoculation. As a result, the PGPB successfully colonized the root, and promoted the sucrose shoot-to-root transport and Cd root-to-shoot transport in S. alfredii. Since non-photosynthetic root-associated bacteria usually obtain sugars from photosynthetic plants, our results highlight the importance of PGPB-induced changes in hyperaccumlator plants for both the host and the PGPB.

Fava bean intercropping with Sedum alfredii inoculated with endophytes enhances phytoremediation of cadmium and lead co-contaminated field

Phytoremediation coupled with agro-production is considered a sustainable strategy for remediation of trace element contaminated fields without interrupting crop production. In this study hyperaccumulator Sedum alfredii was intercropped with a leguminous plant fava bean (Vicia fava) in cadmium (Cd) and lead (Pb) co-contaminated field to evaluate the effects of intercropping on growth performance and accumulations of trace elements in plants with plant growth promoting endophyte (PGPE) consortium application. The results showed, compared with monoculture, intercropping coupled with inoculation application promoted biomass as well as Cd and Pb concentrations in individual parts of both plants, thus increasing the removal efficiencies of trace elements (4.49-folds for Cd and 5.41-folds for Pb). Meanwhile, this superposition biofortification measure maintained normal yield and nutrient content, and limited the concentration of Cd and Pb within the permissible limit (<0.2 mg kg^-1 FW) in fava bean during the grain production. These results demonstrated a feasible technical system for phytoremediation coupled with agro-production in slightly or moderately Cd and Pb co-contaminated field, and also provided useful information for further investigation of interaction mechanisms between intercropping and PGPEs inoculation.

The plant-growth promoting bacteria promote cadmium uptake by inducing a hormonal crosstalk and lateral root formation in a hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) that inhabit hyperaccumulating plants assist cadmium (Cd) absorption, but the underlying mechanism has not been comprehensively studied. For this reason, we combined the fluorescence imaging, and transcriptomic and metabolomic methods in a Cd hyperaccumulator, Sedum alfredii, inoculated or not with PGPB Pseudomonas fluorescens. The results showed that the newly emerged lateral roots, that were heavily colonized by P. fluorescens, are the main entry for Cd influx in S. alfredii. Inoculation with P. fluorescens promoted a lateral root formation of its host plant, leading to a higher Cd phytoremediation efficiency. Furthermore, the plant transcriptome revealed that 146 plant hormone related genes were significantly up-regulated by the bacterial inoculation, with 119 of them showing a complex interaction, which suggests that a hormonal crosstalk participated root development. The targeted metabolomics analysis showed that P. fluorescens inoculation significantly increased indole acetic acid concentration and significantly decreased concentrations of abscisic acid, brassinolide, trans-zeatin, ethylene and jasmonic acid in S. alfredii roots, thereby inducing lateral root emergence. Altogether, our results highlight the importance of PGPB-induced lateral root formation for the increased Cd uptake in a hyperaccumulating plant.

Phytoremediation of Cadmium: Physiological, Biochemical, and Molecular Mechanisms

Cadmium (Cd) is one of the most toxic metals in the environment, and has noxious effects on plant growth and production. Cd-accumulating plants showed reduced growth and productivity. Therefore, remediation of this non-essential and toxic pollutant is a prerequisite. Plant-based phytoremediation methodology is considered as one a secure, environmentally friendly, and cost-effective approach for toxic metal remediation. Phytoremediating plants transport and accumulate Cd inside their roots, shoots, leaves, and vacuoles. Phytoremediation of Cd-contaminated sites through hyperaccumulator plants proves a ground-breaking and profitable choice to combat the contaminants. Moreover, the efficiency of Cd phytoremediation and Cd bioavailability can be improved by using plant growth-promoting bacteria (PGPB). Emerging modern molecular technologies have augmented our insight into the metabolic processes involved in Cd tolerance in regular cultivated crops and hyperaccumulator plants. Plants’ development via genetic engineering tools, like enhanced metal uptake, metal transport, Cd accumulation, and the overall Cd tolerance, unlocks new directions for phytoremediation. In this review, we outline the physiological, biochemical, and molecular mechanisms involved in Cd phytoremediation. Further, a focus on the potential of omics and genetic engineering strategies has been documented for the efficient remediation of a Cd-contaminated environment.

Pseudomonas fluorescens promote photosynthesis, carbon fixation and cadmium phytoremediation of hyperaccumulator Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote photosynthesis and biomass production of hyperaccumulators, achieving enhanced phytoremediation efficiency of cadmium (Cd). A better understanding of the mechanisms controlling photosynthesis of hyperaccumulating plants by PGPB is necessary for developing strategies that promote the practical phytoextraction of Cd-polluted soils. In this study, chlorophyll fluorescence, gas exchange, and transcriptome sequencing were conducted to evaluate the physiological and transcriptional changes on photosynthesis and carbon fixation in hyperaccumulator Sedum alfredii after inoculation with PGPB Pseudomonas fluorescens. The results showed that bacterial inoculation significantly enhanced maximum quantum yield of PS II (Fv/Fm), effective quantum yield of PS II (ΦPSII), photochemical quenching (qP) and chlorophyll concentration, while reduced non-photochemical quenching (NPQ) of S. alfredii. Further, inoculation resulted in an increased net photosynthetic rates (Pn), intercellular CO₂ concentration (Ci), transpiration rate (Tr) and stomatal conductance (Gs) of the studied plant. At the transcriptional level, 70 photosynthetic genes and 42 C4-pathway carbon fixation related genes were significantly up-regulated in response to inoculation, which could be the reason for enhanced photosynthesis and dry biomass. To sum up, this P. fluorescens strain can simultaneously promote growth and Cd uptake of S. alfredii, which can be a promising bacterial agent applied to Cd phytoremediation practices.

Low calcium-induced delay in development of root apoplastic barriers enhances Cd uptake and accumulation in Sedum alfredii

Mineral nutrients play an important role in heavy metal uptake and accumulation in plant. However, the effects of calcium (Ca) supply level on apoplastic transport in roots and consequences for uptake of cadmium (Cd) in hyperaccumulators are poorly understood. Here, we investigated how Ca regulated the development of apoplastic barriers in the roots of two ecotypes of Sedum alfredii and assessed its effects on Cd uptake. Results of correlation analysis indicated that Ca content was positively correlated with the development of Casparian strips (CSs) and suberin lamellae (SL) in the absence or presence of Cd. Simultaneously, low Ca supply was proven to delay the formation of endodermis CSs and suberin accumulation by decreasing the relative expressions of genes associated with CSs localization and lignin/suberin synthesis. Moreover, Cd in apoplastic fluid and cell walls (regarding the apoplastic transport) and symplastic fractions were elevated by low Ca supply. Contrary to high Ca supply, the expression levels of genes related to Cd influx and xylem loading were increased upon low Ca addition in roots of both ecotypes. All the results above suggested that low Ca supply promotes root Cd uptake via apoplastic pathway by delaying apoplastic barriers development and also regulating Cd transport to the xylem in S. alfredii.

A Single Amino Acid Change in Nramp6 from Sedum Alfredii Hance Affects Cadmium Accumulation

SaNramp6 in Sedum alfredii encodes a membrane-localized metal transporter. We isolated the SaNramp6h allele from the hyperaccumulating ecotype (HE) of S. alfredii. When this allele was expressed in transgenic yeast and Arabidopsis thaliana, it enhanced their cadmium (Cd) sensitivity by increased Cd transport and accumulation. We isolated another allele, SaNramp6n, from a nonhyperaccumulating ecotype (NHE) of S. alfredii. Amino acid sequence comparisons revealed three amino acid differences between SaNramp6h and SaNramp6n. We investigated the Cd transport activity of the Nramp6 allele, and determined which residues are essential for the transport activity. We conducted structure-function analyses of SaNramp6 based on site-directed mutagenesis and functional assays of the mutants in yeast and Arabidopsis. The three residues that differed between SaNramp6h and SaNramp6n were mutated. Only the L157P mutation of SaNramp6h impaired Cd transport. The other mutations, S218N and T504A, did not affect the transport activity of SaNramp6h, indicating that these residues are not essential for metal selectivity. Transgenic plants overexpressing SaNramp6hL157P showed altered metal accumulation in shoots and roots. Our results suggest that the conserved site L157 is essential for the high metal transport activity of SaNramp6h. This information may be useful for limiting or increasing Cd transport by other plant natural resistance associated macrophage protein (NRAMP) proteins.

Exogenous abscisic acid (ABA) promotes cadmium (Cd) accumulation in Sedum alfredii Hance by regulating the expression of Cd stress response genes

Sedum alfredii Hance is a zinc (Zn) and cadmium (Cd) hyperaccumulator plant. However, the regulatory role of plant hormones in the Zn or Cd uptake and accumulation of S. alfredii remains unclear. In this work, the growth, Cd accumulation, abscisic acid (ABA) synthesis and catabolism, malonaldehyde (MDA) content, and transcriptional level of some Cd stress response genes under ABA and Cd co-treatment were investigated to reveal the impact of ABA on Cd resistance and Cd accumulation of S. alfredii. The results show that 0.2 mg/L ABA and 100 μmol/L Cd co-treatment enhanced Cd accumulation and growth in S. alfredii, whereas lower or higher ABA concentrations weaken or even reverse this effect, which was positively correlated with endogenous ABA content. The increase in endogenous ABA content might be the results of the increasing ABA synthetase activities and decreasing ABA lytic enzyme, which was induced by the application of 0.2 mg/L ABA under 100 μmol/L Cd treatment. Principal component analysis (PCA) indicated that ABA impacted the expression pattern of Cd stress response genes, which coincided with the Cd accumulation pattern in the shoots of S. alfredii. Cross-over analysis of partial least squares-discriminant analysis (PLS-DA) and correlation analysis indicated that HsfA4c, HMA4 expression in roots, and HMA2, HMA3, CAD, NAS expression in shoots were correlated with endogenous ABA, which suggests that endogenous ABA improves Cd resistance of seedlings, switches the root-to-shoot transporter from HMA2 to HMA4, and transports more Cd into apoplasts to promote Cd accumulation in the shoots of S. alfredii. Taken together, ABA plays an essential role not only in Cd resistance but also in Cd transport from root to shoot in S. alfredii under Cd stress.

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

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

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

cDNA Library for Mining Functional Genes in Sedum alfredii Hance Related to Cadmium Tolerance and Characterization of the Roles of a Novel SaCTP2 Gene in Enhancing Cadmium Hyperaccumulation

Heavy metal contamination presents serious threats to living organisms. Functional genes related to cadmium (Cd) hypertolerance or hyperaccumulation must be explored to enhance phytoremediation. Sedum alfredii Hance is a Zn/Cd cohyperaccumulator exhibiting abundant genes associated with Cd hypertolerance. Here, we developed a method for screening genes related to Cd tolerance by expressing a cDNA-library for S. alfredii Hance. Yeast functional complementation validated 42 of 48 full-length genes involved in Cd tolerance, and the majority of them were strongly induced in roots and exhibited diverse expression profiles across tissues. Coexpression network analysis suggested that 15 hub genes were connected with genes involved in metabolic processes, response to stimuli, and metal transporter and antioxidant activity. The functions of a novel SaCTP2 gene were validated by heterologous expression in Arabidopsis, responsible for retarding chlorophyll content decrease, maintaining membrane integrity, promoting reactive oxygen species (ROS) scavenger activities, and reducing ROS levels. Our findings suggest a highly complex network of genes related to Cd hypertolerance in S. alfredii Hance, accomplished via the antioxidant system, defense genes induction, and the calcium signaling pathway. The proposed cDNA-library method is an effective approach for mining candidate genes associated with Cd hypertolerance to develop genetically engineered plants for use in phytoremediation.

Comparative expression analysis of heavy metal ATPase subfamily genes between Cd-tolerant and Cd-sensitive turnip landraces

The heavy metal ATPase (HMA) subfamily is mainly involved in heavy metal (HM) tolerance and transport in plants, but an understanding of the definite roles and mechanisms of most HMA members are still limited. In the present study, we identified 14 candidate HMA genes named BrrHMA1-BrrHMA8 from the turnip genome and analyzed the phylogeny, gene structure, chromosome distribution, and conserved domains and motifs of HMAs in turnip (Brassica rapa var. rapa). According to our phylogenetic tree, the BrrHMAs are divided into a Zn/Cd/Co/Pb subclass and Cu/Ag subclass. The BrrHMA members show similar structural characteristics within subclasses. To explore the roles of BrrHMAs in turnip, we compared the gene sequences and expression patterns of the BrrHMA genes between a Cd-tolerant landrace and a Cd-sensitive landrace. Most BrrHMA genes showed similar spatial expression patterns in both Cd-tolerant and Cd-sensitive turnip landraces; some BrrHMA genes, however, were differentially expressed in specific tissue in Cd-tolerant and Cd-sensitive turnip. Specifically, BrrHMA genes in the Zn/Cd/Co/Pb subclass shared the same coding sequence but were differentially expressed in Cd-tolerant and Cd-sensitive turnip landraces under Cd stress. Our findings suggest that the stable expression and up-regulated expression of BrrHMA Zn/Cd/Co/Pb subclass genes under Cd stress may contribute to the higher Cd tolerance of turnip landraces.

Ideal Cereals With Lower Arsenic and Cadmium by Accurately Enhancing Vacuolar Sequestration Capacity

Cereals are a staple food for many people around the world; however, they are also a major dietary source of toxic metal(loid)s. Many agricultural regions throughout the world are contaminated with toxic metal(loid)s, which can accumulate to high levels in the grains of cereals cultivated in these regions, posing serious health risks to consumers. Arsenic (As) and cadmium (Cd) are efficiently accumulated in cereals through metal transport pathways. Therefore, there is an urgent need to develop crops that contain greatly reduced levels of toxic metal(loid)s. Vacuolar sequestration of toxic metal(loid)s is a primary strategy for reducing toxic metal(loid)s in grains. However, until recently, detailed strategies and mechanisms for reducing toxic metal(loid)s in grain were limited by the lack of experimental data. New strategies to reduce As and Cd in grain by enhancing vacuolar sequestration in specific tissues are critical to develop crops that lower the daily intake of As and Cd, potentially improving human health. This review provides insights and strategies for developing crops with strongly reduced amounts of toxic metal(loid)s without jeopardizing agronomic traits.

Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge

Hyperaccumulator plants are the material basis for phytoextraction research and for practical applications in decontaminating polluted soils and industrial wastes. China's high biodiversity and substantial mineral resources make it a global hotspot for hyperaccumulator plant species. Intensive screening efforts over the past 20 years by researchers working in China have led to the discovery of many different hyperaccumulators for a range of elements. In this review, we present the state of knowledge on all currently reported hyperaccumulator species from China, including Cardamine hupingshanensis (selenium, Se), Dicranopteris dichotoma (rare earth elements, REEs), Elsholtzia splendens (copper, Cu), Phytolacca americana (manganese, Mn), Pteris vittata (arsenic, As), Sedum alfredii, and Sedum plumbizincicola (cadmium/zinc, Cd/Zn). This review covers aspects of the ecophysiology and molecular biology of tolerance and hyperaccumulation for each element. The major scientific advances resulting from the study of hyperaccumulator plants in China are summarized and synthesized.

Ectopic expression of SaNRAMP3 from Sedum alfredii enhanced cadmium root-to-shoot transport in Brassica juncea

SaNRAMP3 gene cloned from a Zn/Cd hyperaccumulator Sedum alfredii was ectopicly expressed in Brassica juncea, a fast-growing and high-biomass crop plant. In a tissue culture experiment, transgenic plants were grown on MS medium with 0, 25, 50, 100, 200 μM Cd. It was shown that, at the same Cd treatment, the Cd tolerance of transgenic plants had no significant difference with those of wild-type plants (WT). However, the shoot Cd content and accumulation were improved significantly while the root Cd content and accumulation were descended significantly by SaNRAMP3 gene expression, which obviously enhanced the Cd root-to-shoot translocation factor (TF). In the hydroponic experiment, plants were cultured in nutrition solution with 0, 2.5, 25 μM Cd. Data showed that the Cd tolerance of transgenic plants had no significant difference with that of WT under the same Cd exposure. Whereas, the shoot Cd content and accumulation was increased 1.43-1.81 times and the TF was enhanced 3.09-3.51 times by SaNRAMP3 gene expression. Those results indicated that ectopic expression of SaNRAMP3 in B. juncea didn't lead to Cd sensitivity, but enhanced Cd root-to-shoot transport, so that increased shoot Cd accumulation. This study provided a possibility to improve phytoextraction efficiency of heavy metal through gene engineering.