Tolerance to cadmium in plants: the special case of hyperaccumulators

Influences of earthworm activity and mucus on Cd phytoremediation based on harvesting different leaf types of tall fescue (Festuca arundinacea)

To explore cost-effective and efficient phytoremediation strategies, this study investigated the distinct roles of earthworm activity and mucus in enhancing Cd phytoextraction from soils contaminated by Festuca arundinacea, focusing on the comparative advantages of selective leaf harvesting versus traditional whole-plant harvesting methods. Our study employed a horticultural trial to explore how earthworm activity and mucus affect Festuca arundinacea' s Cd phytoremediation in soils using control, earthworm, and mucus treatments to examine their respective effects on plant growth and Cd distribution. Earthworm activity increased the dry weight of leaves by 13.5% and significantly increased the dry weights of declining and senescent leaves, surpassing that of the control by more than 40%. Earthworm mucus had a similar, albeit less pronounced, effect on plant growth than earthworm activity. This study not only validated the significant role of earthworm activity in enhancing Cd phytoextraction by Festuca arundinacea, with earthworm activity leading to over 85% of Cd being allocated to senescent tissues that comprise only approximately 20% of the plant biomass, but also highlighted a sustainable and cost-effective approach to phytoremediation by emphasizing selective leaf harvesting supported by earthworm activity. By demonstrating that earthworm mucus alone can redistribute Cd with less efficiency compared to live earthworms, our findings offer practical insights into optimizing phytoremediation strategies and underscore the need for further research into the synergistic effects of biological agents in soil remediation processes.

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.

BcaSOD1 enhances cadmium tolerance in transgenic Arabidopsis by regulating the expression of genes related to heavy metal detoxification and arginine synthesis

Cadmium (Cd), which is a nonessential heavy metal element for organisms, can have a severe impact on the growth and development of organisms that absorb excessive Cd. Studies have shown that Brassica carinata, a semiwild oil crop, has strong tolerance to various abiotic stresses, and RNA-seq has revealed that the B. carinata superoxide dismutase gene (BcaSOD1) likely responds to Cd stress. To elucidate the BcaSOD1 function involved in tolerance of Cd stress, we cloned the coding sequences of BcaSOD1 from a purple B. carinata accession and successfully transferred it into Arabidopsis thaliana. The subcellular localization results demonstrated that BcaSOD1 was primarily located in the plasma membrane, mitochondria and nucleus. Overexpression of BcaSOD1 in transgenic Arabidopsis (OE) effectively decreased the toxicity caused by Cd stress. Compared to the WT (wild type lines), the OE lines exhibited significantly increased activities of antioxidant enzymes (APX, CAT, POD, and SOD) after exposure to 2.5 mM CdCl2. The Cd content of underground (root) in the OE line was dominantly higher than that in the WT; however, the Cd content of aboveground (shoot) was comparable between the OE and WT types. Moreover, the qRT‒PCR results showed that several heavy metal detoxification-related genes (AtIREG2, AtMTP3, AtHMA3, and AtNAS4) were significantly upregulated in the roots of OE lines under Cd treatment, suggesting that these genes are likely involved in Cd absorption in the roots of OE lines. In addition, both comparable transcriptome and qRT-PCR analyses revealed that exogenous BcaSOD1 noticeably facilitates detoxification by stimulating the expression of two arginine (Arg) biosynthesis genes (AtGDH1 and AtGDH2) while inhibiting the expression of AtARGAH1, a negative regulator in biosynthesis of Arg. The Arg content was subsequently confirmed to be significantly enhanced in OE lines under Cd treatment, indicating that BcaSOD1 likely strengthened Cd tolerance by regulating the expression of Arg-related genes. This study demonstrates that BcaSOD1 can enhance Cd tolerance and reveals the molecular mechanism of this gene, providing valuable insights into the molecular mechanism of Cd tolerance in plants.

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.

Contrasting Cd accumulation of Arabidopsis halleri populations: a role for (1→4)-β-galactan in pectin

Cadmium (Cd) accumulation is highly variable among Arabidopsis halleri populations. To identify cell wall (CW) components that contribute to the contrasting Cd accumulation between PL22-H (Cd-hyperaccumulator) and I16-E (Cd-excluder), Cd absorption capacity of CW polysaccharides, CW mono- and poly- saccharides contents and CW glycan profiles were compared between these two populations. PL22-H pectin contained 3-fold higher Cd concentration than I16-E pectin in roots, and (1→4)-β-galactan pectic epitope showed the biggest difference between PL22-H and I16-E. CW-related differentially expressed genes (DEGs) between PL22-H and I16-E were identified and corresponding A. thaliana mutants were phenotyped for Cd tolerance and accumulation. A higher Cd translocation was observed in GALACTAN SYNTHASE1 A. thaliana knockout and overexpressor mutants, which both showed a lengthening of the RG-I sidechains after Cd treatment, contrary to the wild-type. Overall, our results support an indirect role for (1→4)-β-galactan in Cd translocation, possibly by a joint effect of regulating the length of RG-I sidechains, the pectin structure and interactions between polysaccharides in the CW. The characterization of other CW-related DEGs between I16-E and PL22-H selected allowed to identify a possible role in Zn translocation for BIIDXI and LEUNIG-HOMOLOG genes, which are both involved in pectin modification.

Cadmium contributes to heat tolerance of a hyperaccumulator plant species Sedum alfredii

Hyperaccumulators are plant species that tolerate and accumulate very high concentrations of toxic metals, including Cd. Hyperaccumulation of heavy metals is reported to benefit plant biotic resistance; however, no prior study has examined the possible role of toxic metals on abiotic stress resistance in hyperaccumulators. A preliminary experiment found that Cd significantly improved plant growth of a hyperaccumulator, Sedum alfredii Hance, under heat stress. This study investigated the possible role of Cd in S. alfredii's heat resistance, using infrared thermography, transmission electron microscopy (TEM), real-time quantitative polymerase chain reaction (RTqPCR), and high-throughput sequencing. The results showed that high temperatures irreversibly damaged stomatal function, chloroplast structure, photosynthesis in S. alfredii, and lowered survival rates to 25%. However, Cd application significantly decreased the leaf temperature of S. alfredii and increased the survival rate to 75%. Cd penetrated the guard cells, restored stomatal function, and mitigated excessive water loss from S. alfredii under heat stress. Moreover, it activated antioxidant enzymes, promoted phytohormone biosynthesis, and upregulated a series of unigenes, thereby augmenting heat resistance in S. alfredii. These results indicate that Cd effectively improved thermotolerance in S. alfredii by regulating stomatal movement and antioxidant systems via upregulation of phytohormones and heat shock proteins.

Bioaccumulation and physiological changes in the fruiting body of Agaricus bisporus (Large) sing in response to cadmium

The bioremediation of heavy metals contaminated soils with macrofungi is a new and promising approach; hence Agaricus bisporus (Large) sing has potentially shown accumulating ability to Cd contamination. This study focused on the tolerance response by A. bisporus to different contents of Cd in the closed cup and the flat stage of fruiting body development. The contents of Cd, soluble protein, sugar, low molecular weight organic acids (LMWOAs), and antioxidant activity were investigated. The bioaccumulation factor and transfer factor results revealed that Cd accumulated in the cap of A. bisporus more than that in the stipe with the highest content being 18.38 mg kg^-1 dry weight at the closed cup stage under 414.28 mg kg^-1 Cd stress. High Cd content stress increased soluble protein, proline, and malonaldehyde contents at both stages; while higher peroxidase, catalase, ascorbic acid peroxidase activities, and LMWOAs contents were only recorded at the closed cup stage. On the other hand, Superoxide dismutase activities and soluble sugar content showed a complex trend. Overall, these results have successfully established that A. bisporus could resort to modulating its metabolism to avoid the destructive effects of Cd stress and could successfully accumulate Cd in the soil, which is a promising prospect for the remediation of Cd-contaminated soils.

Phytoextraction of Zn and Cd with Arabidopsis halleri: a focus on fertilization and biological amendment as a means of increasing biomass and Cd and Zn concentrations

The current work aims to investigate the influence of fertilization (fertilizer) and fungal inoculation (Funneliformis mosseae and Serendipita indica (formerly Piriformospora indica), respectively arbuscular mycorrhizal (AMF) and endophytic fungi) on the phytoextraction potential of Arabidopsis halleri (L.) O'Kane & Al-Shehbaz (biomass yield and/or aboveground part Zn and Cd concentrations) over one life plant cycle. The mycorrhizal rates of A. halleri were measured in situ while the fungal inoculation experiments were carried out under controlled conditions. For the first time, it is demonstrated that the fertilizer used on A. halleri increased its biomass not only at the rosette stage but also at the flowering and fruiting stages. Fertilizer reduced the Zn concentration variability between developmental stages and increased the Cd concentration at fruiting stage. A. halleri roots did not show AMF colonization at any stage in our field conditions, neither in the absence nor in the presence of fertilizer, thus suggesting that A. halleri is not naturally mycorrhizal. Induced mycorrhization agreed with this result. However, S. indica has been shown to successfully colonize A. halleri roots under controlled conditions. This study confirms the benefit of using fertilizer to increase the phytoextraction potential of A. halleri. Overall, these results contribute to the future applicability of A. halleri in a phytomanagement strategy by giving information on its cultural itinerary.

Integrated glycolysis and pyrolysis process for multiple utilization and cadmium collection of hyperaccumulator Sedum alfredii

Phytoremediation is a cost-effective and environmentally-friendly method to treat cadmium (Cd) contaminated soils, however, there is still a lack of safe disposal methods of harvested hyperaccumulators. In this study, by integrating glycolysis and pyrolysis, we investigated the possibility of bioproduct production and Cd collection from the hyperaccumulator Sedum alfredii. By means of acid-alkali pretreatment, the degree of cellulose polymerization was reduced by 36.24% while the surface accessibility was increased by 115.80%, resulting in a bioethanol yield of 9.29%. Meanwhile, 99.22% of total Cd of biomass could be reclaimed by collecting H₂SO₄-pretreatment waste. The saccharification residue was subsequently modified by NaOH-pretreatment-filtrate and converted into biochar at 500 °C which possessed a maximum Cd²⁺ sorption capacity of 60.52 mg g^-1 based on the Langmuir model. Furthermore, sustainability analysis indicated that the economic input of this process is acceptable when considering its good environmental benefits. Taken together, our study provides a strategy for simultaneous bioethanol and biochar production during Cd collection from the hyperaccumulator S. alfredii, which could be a promising alternative for the suitable treatment of metal-enriched plants.

Genome-Wide Characterization of Sedum plumbizincicola HMA Gene Family Provides Functional Implications in Cadmium Response

Heavy-metal ATPase (HMA), an ancient family of transition metal pumps, plays important roles in the transmembrane transport of transition metals such as Cu, Zn, Cd, and Co. Although characterization of HMAs has been conducted in several plants, scarcely knowledge was revealed in Sedum plumbizincicola, a type of cadmium (Cd) hyperaccumulator found in Zhejiang, China. In this study, we first carried out research on genome-wide analysis of the HMA gene family in S. plumbizincicola and finally identified 8 SpHMA genes and divided them into two subfamilies according to sequence alignment and phylogenetic analysis. In addition, a structural analysis showed that SpHMAs were relatively conserved during evolution. All of the SpHMAs contained the HMA domain and the highly conserved motifs, such as DKTGT, GDGxNDxP, PxxK S/TGE, HP, and CPx/SPC. A promoter analysis showed that the majority of the SpHMA genes had cis-acting elements related to the abiotic stress response. The expression profiles showed that most SpHMAs exhibited tissue expression specificity and their expression can be regulated by different heavy metal stress. The members of Zn/Co/Cd/Pb subgroup (SpHMA1-3) were verified to be upregulated in various tissues when exposed to CdCl₂. Here we also found that the expression of SpHMA7, which belonged to the Cu/Ag subgroup, had an upregulated trend in Cd stress. Overexpression of SpHMA7 in transgenic yeast indicated an improved sensitivity to Cd. These results provide insights into the evolutionary processes and potential functions of the HMA gene family in S. plumbizincicola, laying a theoretical basis for further studies on figuring out their roles in regulating plant responses to biotic/abiotic stresses.

Melatonin Protects Tobacco Suspension Cells against Pb-Induced Mitochondrial Dysfunction

Recent studies have shown that melatonin is an important molecule in plant physiology. It seems that the most important is that melatonin effectively eliminates oxidative stress (direct and indirect antioxidant) and switches on different defence strategies (preventive and interventive actions) during environmental stresses. In the presented report, exogenous melatonin potential to protect Nicotiana tabacum L. line Bright Yellow 2 (BY-2) exposed to lead against death was examined. Analyses of cell proliferation and viability, the level of intracellular calcium, changes in mitochondrial membrane potential (ΔΨm) as well as possible translocation of cytochrome c from mitochondria to cytosol and subsequent caspase-like proteolytic activity were conducted. Our results indicate that pretreatment BY-2 with melatonin protected tobacco cells against mitochondrial dysfunction and caspase-like activation caused by lead. The findings suggest the possible role of this indoleamine in the molecular mechanism of mitochondria, safeguarding against potential collapse and cytochrome c release. Thus, it seems that applied melatonin acted as an effective factor, promoting survival and increasing plant tolerance to lead.

Adaptation of Arabidopsis halleri to extreme metal pollution through limited metal accumulation involves changes in cell wall composition and metal homeostasis

Metallophytes constitute powerful models for the study of metal homeostasis, adaptation to extreme environments and the evolution of naturally selected traits. Arabidopsis halleri is a pseudometallophyte which shows constitutive zinc/cadmium (Zn/Cd) tolerance and Zn hyperaccumulation but high intraspecific variability in Cd accumulation. To examine the molecular basis of the variation in metal tolerance and accumulation, ionome, transcriptome and cell wall glycan array profiles were compared in two genetically close A. halleri populations from metalliferous and nonmetalliferous sites in Northern Italy. The metallicolous population displayed increased tolerance to and reduced hyperaccumulation of Zn, and limited accumulation of Cd, as well as altered metal homeostasis, compared to the nonmetallicolous population. This correlated well with the differential expression of transporter genes involved in trace metal entry and in Cd/Zn vacuolar sequestration in roots. Many cell wall-related genes were also more highly expressed in roots of the metallicolous population. Glycan array and histological staining analyses demonstrated that there were major differences between the two populations in terms of the accumulation of specific root pectin and hemicellulose epitopes. Our results support the idea that both specific cell wall components and regulation of transporter genes play a role in limiting accumulation of metals in A. halleri at contaminated sites.

Accumulator plants and hormesis

Accumulation of metals by plants is an important area of investigation in plant ecology and evolution as well as in soil contamination/phytoremediation practices. This paper reports that hormetic-biphasic dose-response relationships were commonly observed for multiple agents (i.e. arsenic, cadmium, chromium, fluoride, lead, and zinc) and 20 species in plant (hyper)accumulator studies. The hormetic stimulation was related to metal accumulation in affected tissues, with the metal stimulation concentration zone unique for each metal, species, tissue, and endpoint studied. However, quantitative features of the hormetic dose response were similar across all (hyper)accumulation studies, with results independent of plant species, endpoints measured, and metal. The dose-dependent stimulatory and inhibitory/toxic plant responses were often associated with the up- and down-regulation of adaptive mechanisms, especially those involving anti-oxidative enzymatic processes. These findings provide a mechanistic framework to account for both the qualitative and quantitative features of the hormetic dose response in plant (hyper)accumulator studies.

Heavy metals modulate DNA compaction and methylation at CpG sites in the metal hyperaccumulator Arabidopsis halleri

Excess heavy metals affect plant physiology by inducing stress symptoms, however several species have evolved the ability to hyperaccumulate metals in above-ground tissues without phytotoxic effects. In this study we assume that at subcellular level, different strategies were adopted by hyperaccumulator versus the non-accumulator plant species to face the excess of heavy metals. At this purpose the comet assay was used to investigate the nucleoid structure modifications occurring in response to Zn and Cd treatments in the I16 and PL22 populations of the hyperaccumulator Arabidopsis halleri versus the nonaccumulator species Arabidopsis thaliana. Methy-sens comet assay and RT-qPCR were also performed to associate metal induced variations in nucleoids with possible epigenetic modifications. The comet assay showed that Zn induced a mild but non significant reduction in the tail moment in A. thaliana and in both I16 and PL22. Cd treatment induced an increase in DNA migration in nuclei of A. thaliana, whereas no differences in DNA migration was observed for I16, and a significant increase in nucleoid condensation was found in PL22 Cd treated samples. This last population showed higher CpG DNA methylation upon Cd treatment than in control conditions, and an up-regulation of genes involved in symmetric methylation and histone deacetylation. Our data support the hypothesis of a possible role of epigenetic modifications in the hyperaccumulation trait to cope with the high Cd shoot concentrations. In addition, the differences observed between PL22 and I16 could reinforce previous suggestions of divergent strategies for metals detoxification developing in the two metallicolous populations.

Interactions of cadmium and zinc in high zinc tolerant native species Andropogon gayanus cultivated in hydroponics: growth endpoints, metal bioaccumulation, and ultrastructural analysis

Cadmium (Cd) and zinc (Zn) toxicity causes physiological disorders and harms plants, interfering with the rehabilitation of areas affected by mining activities. This study evaluated how the exposure to Zn and/or Cd affects the growth of native andropogon grass (Andropogon gayanus Kunth) plants originally found in areas contaminated with Cd and/or Zn due to zinc mining activities. Plants were cultivated for 7 weeks in a nutrient solution treated with Zn (142.3-854.0 μM) or Cd (0.9-13.3 μM) separately or combined with a molar ratio of 64:1 (Zn:Cd). A control treatment was grown in a complete Hoagland and Arnon solution (without Cd). Plant height, stem diameter, internode length, dry weight, Cd and Zn concentration, and accumulation in shoots/roots, as well as ultrastructure of roots and leaves were analyzed at the end of the experiment. The root dry weight was not significantly affected by the addition of the metals. Moreover, Zn provided higher shoot dry weight (up to 160%) relative to control. Andropogon grass tolerated both metals better separately than when applied together. Transmission electron microscopy analyses showed modifications such as vesiculation and vacuolation in the ultrastructure of andropogon tissues by Cd and/or Zn. The andropogon grass was tolerant to the doses tested, evidencing that it has potential for recovering areas contaminated with Zn and/or Cd.

Different strategies of Cd tolerance and accumulation in Arabidopsis halleri and Arabidopsis arenosa

Pseudometallophytes are commonly used to study the evolution of metal tolerance and accumulation traits in plants. Within the Arabidopsis genus, the adaptation of Arabidopsis halleri to metalliferous soils has been widely studied, which is not the case for the closely related species Arabidopsis arenosa. We performed an in-depth physiological comparison between the A. halleri and A. arenosa populations from the same polluted site, together with the geographically close non-metallicolous (NM) populations of both species. The ionomes, growth, photosynthetic parameters and pigment content were characterized in the plants that were growing on their native site and in a hydroponic culture under Cd treatments. In situ, the metallicolous (M) populations of both species hyperaccumulated Cd and Zn. The NM population of A. halleri hyperaccumulated Cd and Zn while the NM A. arenosa did not. In the hydroponic experiments, the NM populations of both species accumulated more Cd in their shoots than the M populations. Our research suggests that the two Arabidopsis species evolved different strategies of adaptation to extreme metallic environments that involve fine regulation of metal homeostasis, adjustment of the photosynthetic apparatus and accumulation of flavonols and anthocyanins.

Responses of the species complex Fallopia × bohemica to single-metal contaminations to Cd, Cr or Zn: growth traits, metal accumulation and secondary metabolism

Plant responses to heavy metals and their storage constitute a crucial step to understand the environmental impacts of metallic trace elements (MTEs). In controlled experiments, we previously demonstrated the tolerance and resilience of Japanese knotweed to soil artificial polymetallic contamination. Using the same experimental design, we tested here the effect of three individual MTEs on Fallopia × bohemica performance traits. Rhizome fragments from three different sites (considered as distinct morphotypes) were grown in a greenhouse for 1 month on a prairial soil artificially contaminated with either Cd, Cr (VI) or Zn at concentrations corresponding to relatively highly polluted soils. Our results confirmed the high tolerance of Bohemian knotweed to metal stress, though, plant response to MTE pollution was dependant on MTE identity. Bohemian knotweed was stimulated by Cr (VI) (increased root and aerial masses), did not display any measurable change in performance traits under Cd at the high dose of 10 mg kg^-1, and uptook all MTEs in its rhizome, but only Zn was transferred to its aerial parts. We also highlighted changes in root secondary metabolism that were more accentuated with Zn, including the increase of anthraquinone, stilbene and biphenyl derivatives. These results compared to multi-contamination experiments previously published suggest complex interactions between metals and plant, depending principally on metal identity and also suggest a potential role of soil microbes in the interactions.

Combined use of companion planting and PGPR for the assisted phytoextraction of trace metals (Zn, Pb, Cd)

Biomass production and metal accumulation in plant tissue (bioconcentration) are two critical factors limiting the phytoextraction rate. Metal translocation to aboveground organs should be accounted for as the third most important factor, as harvesting of the plant roots is usually economically disadvantageous. These three parameters could be potentially increased with the use of companion planting, a well-known agricultural technique, and inoculation with plant growth-promoting bacteria (PGPB). The aim of the study was to determine whether intercropping and inoculation with endophytic PGPB (Burkholderia phytofirmans PsJN^T) can increase the efficiency of phytoextraction of Zn, Pb, and Cd. The study was conducted on Brassica juncea (L.) Czern. "Małopolska" grown in a monoculture or co-planted with Zea mays L. "Codimon" and Medicago sativa L. "Sanditi." Results show that companion planting and inoculation with rhizobacteria can increase the efficiency of metal phytoextraction, mainly by increasing the yield of dry biomass and the survival rate of plants grown on contaminated soil. We have shown that the simultaneous planting of B. juncea with M. sativa and inoculation with PGPB were the most efficient variants of assisted phytoextraction reaching a recovery of 95% Zn, 90% Cd, and on average about 160% Pb compared with control B. juncea plants grown in monoculture.

Protein lysine methylation contributes to modulating the response of sensitive and tolerant Arabidopsis species to cadmium stress

The mechanisms underlying the response and adaptation of plants to excess of trace elements are not fully described. Here, we analysed the importance of protein lysine methylation for plants to cope with cadmium. We analysed the effect of cadmium on lysine-methylated proteins and protein lysine methyltransferases (KMTs) in two cadmium-sensitive species, Arabidopsis thaliana and A. lyrata, and in three populations of A. halleri with contrasting cadmium accumulation and tolerance traits. We showed that some proteins are differentially methylated at lysine residues in response to Cd and that a few genes coding KMTs are regulated by cadmium. Also, we showed that 9 out of 23 A. thaliana mutants disrupted in KMT genes have a tolerance to cadmium that is significantly different from that of wild-type seedlings. We further characterized two of these mutants, one was knocked out in the calmodulin lysine methyltransferase gene and displayed increased tolerance to cadmium, and the other was interrupted in a KMT gene of unknown function and showed a decreased capacity to cope with cadmium. Together, our results showed that lysine methylation of non-histone proteins is impacted by cadmium and that several methylation events are important for modulating the response of Arabidopsis plants to cadmium stress.

Comparative Study of Growth, Cadmium Accumulation and Tolerance of Three Chickpea (Cicer arietinum L.) Cultivars

Trace metals (TM) contamination is a severe problem in the environment and produced an adverse effect on the productivity of crops. Cadmium (Cd) is a TM ranked seven among the top 20 pollutants due to its high toxicity and solubility in water, taken up by the plants and affects their growth and metabolism. In this study, we evaluated the growth, Cd accumulation and tolerance capacities of three chickpea (Cicer arietinum L.) cultivars (NC234 (NC2), ICCV89310 (IC8) and ICCV89323-B (IC8-B)), subjected to two Cd concentrations (25 and 50 µM) in hydroponic culture. The toxicity of Cd reduced the plant height and fresh and dry biomass in all cultivars. The maximum reduction was observed at 50 µM of Cd. Compared with IC8-B, cultivars IC8 and NC2 exhibited better performance with high growth, biomass, root to shoot (R/S) ratio and water content under high Cd stress. To measure the accumulation of Cd in root and shoot, an inductively coupled plasma optical emission spectrometer (ICP-OES) was used. IC8 and NC2 had comparatively high Cd tolerance and accumulation ability (> 100 µg g^-1 dry weight), with IC8 being more tolerant and accumulated higher Cd in shoot than NC2, while cultivar IC8-B was sensitive. Root accumulated more Cd than shoot in a dose-dependent manner. The bioconcentration factors (BCF) and bioaccumulation coefficients (BAC) were far higher than one (> 1) and increased with an increase in Cd concentrations, while the translocation factor (TF) was less than one (< 1), suggesting that all the three cultivars were unable to transfer Cd from the root to the shoot efficiently. Our results indicated that IC8 and NC2 proved to be resistant, while IC8-B showed sensitivity when exposed to high Cd stress (50 µM).

SaHsfA4c From Sedum alfredii Hance Enhances Cadmium Tolerance by Regulating ROS-Scavenger Activities and Heat Shock Proteins Expression

The heat shock transcription factor (Hsf) family, an important member in plant stress response, affects cadmium (Cd) tolerance in plants. In this study, we identified and functionally characterized a transcript of the Hsf A4 subgroup from Sedum alfredii. Designated as SaHsfA4c, the open reading frame was 1,302 bp long and encoded a putative protein of 433 amino acids containing a complete DNA-binding domain (DBD). Heterologous expression of SaHsfA4c in yeast enhanced Cd stress tolerance and accumulation, whereas expression of the alternatively spliced transcript InSaHsfA4c which contained an intron and harbored an incomplete DBD, resulted in relatively poor Cd stress tolerance and low Cd accumulation in transgenic yeast. The function of SaHsfA4c under Cd stress was characterized in transgenic Arabidopsis and non-hyperaccumulation ecotype S. alfredii. SaHsfA4c was able to rescue the Cd sensitivity of the Arabidopsis athsfa4c mutant. SaHsfA4c reduced reactive oxygen species (ROS) accumulation and increased the expression of ROS-scavenging enzyme genes and Hsps in transgenic lines. The present results suggest that SaHsfA4c increases plant resistance to stress by up-regulating the activities of ROS-scavenging enzyme and the expression of Hsps.

Urban soil phytomanagement for Zn and Cd in situ removal, greening, and Zn-rich biomass production taking care of snail exposure

The phytoextraction potential of Arabidopsis halleri (L.) O'Kane & Al Shehbaz and Salix viminalis L. to partially remove Zn and Cd in soil was investigated. In an urban field site, a very short rotation coppice of willows was implemented, and growth parameters were monitored for 3 years. A. halleri was cultivated in the same site with or without fertilizer to improve biomass yield and/or Zn and Cd aerial part concentrations. Effects of harvest and co-cultivation on these two parameters were measured. To determine if willows and A. halleri leaves were risky in case of consumption by a herbivorous invertebrate like the landsnail Cantareus aspersus, metal concentrations of snails fed with Zn- and Cd-enriched and low enriched leaves were compared. Willows and A. halleri grew well on the metal-contaminated soil (1.7 and 616 mg kg^-1 Cd and Zn, respectively). The A. halleri Zn foliar concentration reached the Zn hyperaccumulation threshold (> 10,000 mg kg^-1 DW) in the presence of NPK fertilizer and although the soil was alkaline (pH > 8.2). Cd concentration increased with harvest and fertilizer. Cd and Zn foliar concentrations of willows were far above baseline values. Laboratory snails exposure revealed that willow leaves ingestion caused a moderate increase of Cd, Pb, and Zn bioaccumulation in snails compared to the one caused by A. halleri ingestion. The soil and plant metal concentrations were reflected by field snail biomonitoring. This study confirmed the interest of selecting A. halleri and willows to partially remove Zn and Cd in the soil and emphasized their potential usefulness in greening urban contaminated area and producing raw materials for green chemistry while paying attention to the environmental pollutant transfer.

Genetic mapping and transcriptional profiling of phytoremediation and heavy metals responsive genes in sorghum

Decreasing freshwater supply has led to the consideration of treated wastewater as an alternative source for agricultural irrigation. However, the higher content of heavy metals restricts their use in agricultural purposes. In the current study, a set of 181 SSR markers were used to perform association mapping in a structured sorghum population consisting of 107 accessions evaluated under tap-water and heavy-metals-containing water irrigation conditions. Significant differences between optimal and stressed growing conditions were shown in all evaluated phenotypic traits. Association mapping revealed 14 significant associations between 12 SSR markers and heavy metals stress indices of phenotypic traits (R² = 11.54-30.85%). BLASTP annotation of QTLs genomic sequences identified 102 gene homologs, of which 19 are known to be implicated in phytoremediation and heavy metals tolerance. All nineteen genes exhibit differential expression patterns in heavy metals tolerant and susceptible sorghum accessions, and their transcriptional levels were highly elevated under heavy metals stress, indicating a possible functional association among these genes and provides strong evidence for their role in phytoremediation and heavy metal stress tolerance. Clustering and expression patterns of pentatricopeptide repeat and zinc finger protein genes suggest crucial roles of those genes in phytoremediation and heavy metals tolerance.

SpHMA1 is a chloroplast cadmium exporter protecting photochemical reactions in the Cd hyperaccumulator Sedum plumbizincicola

Sedum plumbizincicola is able to hyperaccumulate cadmium (Cd), a nonessential and highly toxic metal, in the above-ground tissues, but the mechanisms for its Cd hypertolerance are not fully understood. Here, we show that the heavy metal ATPase 1 (SpHMA1) of S. plumbizincicola plays an important role in chloroplast Cd detoxification. Compared with the HMA1 ortholog in the Cd nonhyperaccumulating ecotype of Sedum alfredii, the expression of SpHMA1 in the leaves of S. plumbizincicola was >200 times higher. Heterologous expression of SpHMA1 in Saccharomyces cerevisiae increased Cd sensitivity and Cd transport activity in the yeast cells. The SpHMA1 protein was localized to the chloroplast envelope. SpHMA1 RNA interference transgenic plants and CRISPR/Cas9-induced mutant lines showed significantly increased Cd accumulation in the chloroplasts compared with wild-type plants. Chlorophyll fluorescence imaging analysis revealed that the photosystem II of SpHMA1 knockdown and knockout lines suffered from a much higher degree of Cd toxicity than wild type. Taken together, these results suggest that SpHMA1 functions as a chloroplast Cd exporter and protects photosynthesis by preventing Cd accumulation in the chloroplast in S. plumbizincicola and hyperexpression of SpHMA1 is an important component contributing to Cd hypertolerance in S. plumbizincicola.

Variability of trace element distribution in Noccaea spp., Arabidopsis spp., and Thlaspi arvense leaves: the role of plant species and element accumulation ability

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was applied for the determination of Cd and Zn distributions within the leaves of Cd- and Zn-hyperaccumulating plants, Noccaea caerulescens, N. praecox, and Arabidopsis halleri, in contrast to nonaccumulator species, Thlaspi arvense and A. thaliana. The elemental mapping of the selected leaf area was accomplished via line scans with a 110-μm-diameter laser beam at a 37-μm s^-1 scan speed and repetition rate of 10 Hz. The lines were spaced 180 μm apart and ablated at an energy density of 2 J cm^-2. The elemental imaging clearly confirmed that Cd was predominantly distributed within the parenchyma of the T. arvense, whereas in the Noccaea spp. and A. halleri, the highest intensity Cd signal was observed in the veins of the leaves. For Zn, higher intensities were observed in the veins for all the plant species except for A. thaliana. Close relationships between Zn and Ca were identified for the Noccaea spp. leaves. These relationships were not confirmed for A. halleri. Significant correlations were also proved between the Cd and Zn distribution in A. halleri, but not for the Noccaea spp. For both T. arvense and A. thaliana, no relevant significant relationship for the interpretation of the results was observed. Thus, the LA-ICP-MS imaging is proved as a relevant technique for the description and understanding of the elements in hyperaccumulating or highly accumulating plant species, although its sensitivity for the natural element contents in nonaccumulator plant species is still insufficient.

Toxic Effects of Cd and Zn on the Photosynthetic Apparatus of the Arabidopsis halleri and Arabidopsis arenosa Pseudo-Metallophytes

Hyperaccumulation and hypertolerance of Trace Metal Elements (TME) like Cd and Zn are highly variable in pseudo-metallophytes species. In this study we compared the impact of high Cd or Zn concentration on the photosynthetic apparatus of the Arabidopsis arenosa and Arabidopsis halleri pseudo-metallophytes growing on the same contaminated site in Piekary Slaskie in southern Poland. Plants were grown in hydroponic culture for 6 weeks, and then treated with 1.0 mM Cd or 5.0 mM Zn for 5 days. Chlorophyll a fluorescence and pigment content were measured after 0, 1, 2, 3, 4, and 5 days in plants grown in control and exposed to Cd or Zn treatments. Moreover, the effect of TME excess on the level of oxidative stress and gas-exchange parameters were investigated. In both plant species, exposure to high Cd or Zn induced a decrease in chlorophyll and an increase in anthocyanin contents in leaves compared to the control condition. After 5 days Cd treatment, energy absorbance, trapped energy flux and the percentage of active reaction centers decreased in both species. However, the dissipated energy flux in the leaves of A. arenosa was smaller than in A. halleri. Zn treatment had more toxic effect than Cd on electron transport in A. halleri compared with A. arenosa. A. arenosa plants treated with Zn excess did not react as strongly as in the Cd treatment and a decrease only in electron transport flux and percentage of active reaction centers compared with control was observed. The two species showed contrasting Cd and Zn accumulation. Cd concentration was almost 3-fold higher in A. arenosa leaves than in A. halleri. On the opposite, A. halleri leaves contained 3-fold higher Zn concentration than A. arenosa. In short, our results showed that the two Arabidopsis metallicolous populations are resistant to high Cd or Zn concentration, however, the photosynthetic apparatus responded differently to the toxic effects.

CAX1 suppresses Cd-induced generation of reactive oxygen species in Arabidopsis halleri

The molecular analysis of metal hyperaccumulation in species such as Arabidopsis halleri offers the chance to gain insights into metal homeostasis and into the evolution of adaptation to extreme habitats. A prerequisite of metal hyperaccumulation is metal hypertolerance. Genetic analysis of a backcross population derived from Arabidopsis lyrata × A. halleri crosses revealed three quantitative trait loci for Cd hypertolerance. A candidate gene for Cdtol2 is AhCAX1, encoding a vacuolar Ca²⁺ /H⁺ antiporter. We developed a method for the transformation of vegetatively propagated A. halleri plants and generated AhCAX1-silenced lines. Upon Cd²⁺ exposure, several-fold higher accumulation of reactive oxygen species (ROS) was detectable in roots of AhCAX1-silenced plants. In accordance with the dependence of Cdtol2 on external Ca²⁺ concentration, this phenotype was exclusively observed in low Ca²⁺ conditions. The effects of external Ca²⁺ on Cd accumulation cannot explain the phenotype as they were not influenced by the genotype. Our data strongly support the hypothesis that higher expression of CAX1 in A. halleri relative to other Arabidopsis species represents a Cd hypertolerance factor. We propose a function of AhCAX1 in preventing a positive feedback loop of Cd-elicited ROS production triggering further Ca²⁺ -dependent ROS accumulation.

bHLH104 confers tolerance to cadmium stress in Arabidopsis thaliana

Cd is a non-essential heavy metal that is toxic to both plants and animals. Here, we reveal that the transcription factor bHLH104 positively regulates Cd tolerance in Arabidopsis thaliana. We show that Fe deficiency-responsive genes were induced by Cd treatment, and that their upregulation was suppressed in bhlh104 loss-of-function mutants, but enhanced upon overexpression of bHLH104. Correspondingly, the bhlh104 mutants displayed sensitivity to Cd stress, whereas plants overexpressing bHLH104 exhibited enhanced Cd tolerance. Further analysis suggested that bHLH104 positively regulates four heavy metal detoxification-associated genes, IREG2, MTP3, HMA3 and NAS4, which play roles in Cd sequestration and tolerance. The bHLH104 overexpression plants accumulated high levels of Cd in the root but low levels of Cd in the shoot, which might contribute to the Cd tolerance in those lines. The present study thus points to bHLH104 as a potentially useful tool for genetic engineering of plants with enhanced Cd tolerance.

Modification of gold surface by electrosynthesized mono aza crown ether substituted catechol-terminated alkane dithiol and its application as a new electrochemical sensor for trace detection of cadmium ions

Among the toxic metals, cadmium is a very dangerous pollutant because it can extremely damage organs in humans and animals. This toxic metal is introduced into water from different industries such as metal plating, batteries, and alloys. Cadmium bioaccumulates in vital organs and unlike organic pollutants does not show any biological degradation. In this study, an electroactive self-assembled monolayer (SAM) was developed by covalent attachment of a novel mono aza-crown ether substituted catechol-terminated hexane dithiol onto the gold surface. The electrochemical behavior of the fabricated SAM electrode was investigated using voltammetry techniques and electrochemical impedance spectroscopy (EIS). The obtained results from voltammetric experiments revealed that the crown ether moiety of SAM forms a selective complex with cadmium ion. Under optimal conditions, Cd²⁺ could be detected in the range of 15 μM to 65 μM with a detection limit of 4.5 μM. Selectivity measurements reveal that the sensor is specific for Cd²⁺ even in the presence of high concentrations of other metal ions. The proposed sensor was applied to the determination of cadmium ion in water samples with high sensitivity and good selectivity.

Studies on lead and cadmium toxicity in Dianthus carthusianorum calamine ecotype cultivated in vitro

Information on metallophytes during reclamation of land contaminated with heavy metals is sparse. We investigated the response of D. carthusianorum calamine ecotype to Pb and Cd stress. We focused on in vitro selection of tolerant plant material for direct use in chemically degraded areas. Shoot cultures were treated with various concentrations of Pb or Cd ions. Plantlet status was estimated as micropropagation efficiency, growth tolerance index (GTI) and through physiological analysis. Moreover, determination of plant Pb, Cd and other elements was performed. The application of Pb(NO₃ )₂ resulted in stronger growth inhibition than application of CdCl₂ . In the presence of Pb ions, a reduction was observed of both, the micropropagation coefficient to 1.1-1.8 and the GTI to 48%. In contrast, Cd ions had a positive influence on tested cultures, expressed as an increase of GTI up to 243% on medium enriched with 1.0 μm CdCl₂ . Moreover, photosynthetic pigment content in shoots cultivated on media with CdCl₂ was higher than in control treatment. The adaptation to Cd was associated with decreased accumulation of phenols in the order: 0.0 μm > 1.0 μm > 3.0 μm > 5.5 μm CdCl₂ . It seems that high tolerance to Cd is related to K uptake, which is involved in antioxidant defence. This work presents an innovative approach to the impact of Cd ions on plant growth and suggests a potential biological role of this metal in species from metalliferous areas.

Contrasting cadmium resistance strategies in two metallicolous populations of Arabidopsis halleri

While cadmium (Cd) tolerance is a constitutive trait in the Arabidopsis halleri species, Cd accumulation is highly variable. Recent adaptation to anthropogenic metal stress has occurred independently within the genetic units of A. halleri and the evolution of different mechanisms involved in Cd tolerance and accumulation has been suggested. To gain a better understanding of the mechanisms underlying Cd tolerance and accumulation in A. halleri, ionomic inductively coupled plasma mass spectrometry (ICP-MS), transcriptomic (RNA sequencing) and metabolomic (high-performance liquid chromatography-mass spectrometry) profiles were analysed in two A. halleri metallicolous populations from different genetic units (PL22 from Poland and I16 from Italy). The PL22 and I16 populations were both hypertolerant to Cd, but PL22 hyperaccumulated Cd while I16 behaved as an excluder both in situ and when grown hydroponically. The observed hyperaccumulator vs excluder behaviours were paralleled by large differences in the expression profiles of transporter genes. Flavonoid-related transcripts and metabolites were strikingly more abundant in PL22 than in I16 shoots. The role of novel A. halleri candidate genes possibly involved in Cd hyperaccumulation or exclusion was supported by the study of corresponding A. thaliana knockout mutants. Taken together, our results are suggestive of the evolution of divergent strategies for Cd uptake, transport and detoxification in different genetic units of A. halleri.

Cadmium associates with oxalate in calcium oxalate crystals and competes with calcium for translocation to stems in the cadmium bioindicator Gomphrena claussenii

Cd binds to oxalate crystals, where it replaces Ca in the vacuoles of a bioindicator plant Gomphrena clausenii.

Heavy metal ATPase 3 (HMA3) confers cadmium hypertolerance on the cadmium/zinc hyperaccumulator Sedum plumbizincicola

Cadmium (Cd) is highly toxic to most organisms, but some rare plant species can hyperaccumulate Cd in aboveground tissues without suffering from toxicity. The mechanism underlying Cd detoxification by hyperaccumulators is interesting but unclear. Here, the heavy metal ATPase 3 (SpHMA3) gene responsible for Cd detoxification was isolated from the Cd/zinc (Zn) hyperaccumulator Sedum plumbizincicola. RNA interference (RNAi)-mediated silencing and overexpression of SpHMA3 were induced to investigate its physiological functions in S. plumbizincicola and a nonhyperaccumulating ecotype of Sedum alfredii. Heterologous expression of SpHMA3 in Saccharomyces cerevisiae showed Cd-specific transport activity. SpHMA3 was highly expressed in the shoots and the protein was localized to the tonoplast. The SpHMA3-RNAi lines were hypersensitive to Cd but not to Zn, with the growth of shoots and young leaves being severely inhibited by Cd. Overexpressing SpHMA3 in the nonhyperaccumulating ecotype of S. alfredii greatly increased its tolerance to and accumulation of Cd, but not Zn. These results indicate that elevated expression of the tonoplast-localized SpHMA3 in the shoots plays an essential role in Cd detoxification, which contributes to the maintenance of the normal growth of young leaves of S. plumbizincicola in Cd-contaminated soils.

A Genetic Transformation Method for Cadmium Hyperaccumulator Sedum plumbizincicola and Non-hyperaccumulating Ecotype of Sedum alfredii

The present study demonstrates the development of an Agrobacterium-mediated genetic transformation method for species of the Sedum genus, which includes the Cd/Zn hyperaccumulator Sedum plumbizincicola and the non-hyperaccumulating ecotype of S. alfredii. Multiple shoots were induced from stem nodes of two Sedum plants using Murashige and Skoog (MS) medium containing 0.1 mg/L cytokinin 6-benzyladenine (6-BA) and 1.0 mg/L auxin 1-naphthaleneacetic acid (NAA). The shoot primordia were used as direct targets for Agrobacterium infection. Selection on hygromycin was highly effective in generating Agrobacterium-transformed explants. This callus-free procedure allowed us to obtain transgenic plantlets after rooting hygromycin-resistant shoots on phytohormone-free MS medium containing the antibiotic. The presence and expression of the reporter genes gusA and GFP in transgenic plants were confirmed by a real-time polymerase chain reaction, histochemical GUS assays, and confocal microscopy. This reliable method for genetic transformation of Sedum plants will help us to understand gene functions and the molecular mechanisms underlying Cd hypertolerance and hyperaccumulation in these species.

Comparative Assessment of Response to Cadmium in Heavy Metal-Tolerant Shrubs Cultured In Vitro

Two species of Pb-adapted shrubs, Alyssum montanum and Daphne jasminea, were evaluated in vitro for their tolerance to elevated concentrations of cadmium. Shoot cultures were treated with 0.5, 2.5, and 5.0 μM CdCl₂ for 16 weeks and analyzed for their organogenic response, biomass accretion, pigment content, and macronutrient status. Cadmium accumulation and its root-to-shoot translocation were also determined. In both species, rooted microplantlets, suitable for acclimatization, were obtained in the presence of Cd applied as selection agent. In A. montanum, low and moderate dose of Cd stimulated multiplication, rooting, and biomass production. Growth tolerance index (GTI) in Cd-treated shoots ranged from 120 to 215%, while in the roots 51-202%. In turn, in Cd-treated D. jasminea proliferation and rooting were inhibited, and GTI for shoots decreased with increasing doses of Cd. However, roots exposed to Cd had higher biomass accretion. Both species accumulated Cd in developed organs, and its content increased with increasing CdCl₂ dose. Interestingly, D. jasminea accumulated higher amounts of Cd in the roots than A. montanum and immobilized this metal in the root system. On the contrary, A. montanum translocated some part of accumulated Cd to the shoots, but with low efficiency. In the presence of Cd, A. montanum maintained macronutrient homeostasis and synthesized higher amounts of phytosynthetic pigments in the shoots. D. jasminea accumulated root biomass, immobilized Cd, and restricted its translocation at the expense of nutrient balance. Considering remediation potential, A. montanum could be exploited in phytoextraction, while D. jasminea in phytostabilization of polluted substrate.

Cadmium-zinc accumulation and photosystem II responses of Noccaea caerulescens to Cd and Zn exposure

A population of the metallophyte Noccaea (Thlaspi) caerulescens originating from a Zn-enriched area at Røros Copper Mine (Norway) was studied. N. caerulescens tolerance to accumulate Cd and Zn was evaluated in hydroponic experiments by chlorophyll fluorescence imaging analysis. In the field-collected N. caerulescens mother plants, Zn shoot concentrations were above Zn hyperaccumulation threshold while, in hydroponic experiments under 40-μM Cd exposure, shoot Cd concentrations were clearly above Cd hyperaccumulation threshold. Cadmium ions and, to a less extent, Zn were mainly retained in the roots. Exposure to Cd enhanced Zn translocation to the shoot, while decreased significant total Ca²⁺ uptake, suggesting that Cd uptake occurs through Ca²⁺ transporters. Nevertheless, it increased Ca²⁺ translocation to the leaf, possibly for photoprotection of photosystem II (PSII). Exposure to 800 μM Zn or 40 μM Cd resulted in increased Fe³⁺ uptake suggesting that in N. caerulescens, Cd uptake does not take place through the pathway of Fe³⁺ uptake and that conditions that lead to Cd and Zn accumulation in plants may also favor Fe accumulation. Despite the significant high toxicity levels of Zn and Cd in leaves, under Zn and Cd exposure, respectively, the allocation of absorbed light energy at PSII did not differ compared to controls. The results showed that N. caerulescens keep Cd and Zn concentrations in the mesophyll cells in non-toxic forms for PSII and that the increased Ca and Fe accumulation in leaves alleviates the toxicity effects. Chlorophyll fluorescence imaging revealed that PSII of N. caerulescens resisted better the phytotoxic effects of 20 times higher Zn than Cd exposure concentration. Overall, it is concluded that the use of chlorophyll fluorescence imaging constitutes a promising basis for investigating heavy metal tolerance of plants.

Calcium Deficiency Triggers Phloem Remobilization of Cadmium in a Hyperaccumulating Species

Understanding cadmium (Cd) accumulation in plants is critical for the development of plant-based strategies for soil remediation and crop safety. Sedum alfredii is a nonbrassica plant species known to hyperaccumulate Cd. The characteristics of Cd uptake, distribution, and retranslocation affected by the Ca status were investigated at cellular levels in S. alfredii Low Ca supply significantly increased Cd contents in shoots of S. alfredii, particularly in the young leaves. Micro x-ray fluorescence images confirmed that sequestration of Cd was greatly enhanced in the young leaves under Ca deficiency stress, with a significant amount of Cd localized in mesophyll cells, compared to the young leaves supplied with high Ca levels. Cd influx into protoplasts isolated from young leaves was significantly inhibited by the addition of Ca channel inhibitors, but not by pre-exposure to Ca deficiency. In stems, the Cd signal in vascular systems under low Ca levels was 10-fold higher than in those treated with higher Ca levels. A detailed investigation of vascular bundles revealed that an extremely high Cd signal induced by low Ca supply occurred in the phloem tissues, but not in the xylem tissues. Transfer of Cd pretreated plants to nutrient solutions at different Ca levels confirmed that a much higher amount of Cd was reallocated to the new growth tissues under low Ca stress compared to plants supplied with sufficient Ca. These results suggest that Ca deficiency triggered a highly efficient phloem remobilization of Cd in S. alfredii and subsequently enhanced Cd accumulation in its young leaves.

Fate of cadmium in the rhizosphere of Arabidopsis halleri grown in a contaminated dredged sediment

In regions impacted by mining and smelting activities, dredged sediments are often contaminated with metals. Phytotechnologies could be used for their management, but more knowledge on the speciation of metals in the sediment and on their fate after colonization by plant roots is needed. This work was focused on a dredged sediment from the Scarpe river (North of France), contaminated with Zn and Cd. Zn, Cd hyperaccumulating plants Arabidopsis halleri from metallicolous and non-metallicolous origin were grown on the sediment for five months in a pot experiment. The nature and extent of the modifications in Cd speciation with or without plant were determined by electron microscopy, micro X-ray fluorescence and bulk and micro X-ray absorption spectroscopy. In addition, changes in Cd exchangeable and bioavailable pools were evaluated, and Cd content in leachates was measured. Finally, Cd plant uptake and plant growth parameters were monitored. In the original sediment, Cd was present as a mixed Zn, Cd, Fe sulfide. After five months, although pots still contained reduced sulfur, Cd-bearing sulfides were totally oxidized in vegetated pots, whereas a minor fraction (8%) was still present in non-vegetated ones. Secondary species included Cd bound to O-containing groups of organic matter and Cd phosphates. Cd exchangeability and bioavailability were relatively low and did not increase during changes in Cd speciation, suggesting that Cd released by sulfide oxidation was readily taken up with strong interactions with organic matter and phosphate ligands. Thus, the composition of the sediment, the oxic conditions and the rhizospheric activity (regardless of the plant origin) created favorable conditions for Cd stabilization. However, it should be kept in mind that returning to anoxic conditions may change Cd speciation, so the species formed cannot be considered as stable on the long term.

Resolution of Brassicaceae Phylogeny Using Nuclear Genes Uncovers Nested Radiations and Supports Convergent Morphological Evolution

Brassicaceae is one of the most diverse and economically valuable angiosperm families with widely cultivated vegetable crops and scientifically important model plants, such as Arabidopsis thaliana. The evolutionary history, ecological, morphological, and genetic diversity, and abundant resources and knowledge of Brassicaceae make it an excellent model family for evolutionary studies. Recent phylogenetic analyses of the family revealed three major lineages (I, II, and III), but relationships among and within these lineages remain largely unclear. Here, we present a highly supported phylogeny with six major clades using nuclear markers from newly sequenced transcriptomes of 32 Brassicaceae species and large data sets from additional taxa for a total of 55 species spanning 29 out of 51 tribes. Clade A consisting of Lineage I and Macropodium nivale is sister to combined Clade B (with Lineage II and others) and a new Clade C. The ABC clade is sister to Clade D with species previously weakly associated with Lineage II and Clade E (Lineage III) is sister to the ABCD clade. Clade F (the tribe Aethionemeae) is sister to the remainder of the entire family. Molecular clock estimation reveals an early radiation of major clades near or shortly after the Eocene–Oligocene boundary and subsequent nested divergences of several tribes of the previously polytomous Expanded Lineage II. Reconstruction of ancestral morphological states during the Brassicaceae evolution indicates prevalent parallel (convergent) evolution of several traits over deep times across the entire family. These results form a foundation for future evolutionary analyses of structures and functions across Brassicaceae.

The N-terminal degenerated metal-binding domain is involved in the heavy metal transport activity of TaHMA2

We identified key residues of TaHMA2, and the N- and C-terminal regions of the protein have different roles in its transport function when heterologously expressed in yeast. TaHMA2, a P1B-type ATPase from wheat (Triticum aestivum L.), plays an important role in heavy metal homeostasis in plants. A previous study showed that overexpressing TaHMA2 in rice (Oryza sativa L.), Arabidopsis thaliana, or tobacco (Nicotiana tabacum L.) resulted in various responses to heavy metals. Here, we report the heterologous expression of TaHMA2 in the yeast Saccharomyces cerevisiae. TaHMA2 expression increased the yeast's sensitivity to Cd, but not to Zn, Pb or Co, and increased Cd accumulation was concurrently observed. The eGFP-TaHMA2 fusion protein was localized to the plasma membrane and showed a discontinuous pattern. Mutagenesis of the cysteine and glutamate residues in the N-terminal metal-binding domain (N-MBD) impaired the function of TaHMA2. Deletion of most of the C terminus (TaHMA2ΔC, 712-1003) partially abolished the protein's function, whereas deletion of the N terminus (TaHMA2ΔN, 2-699) completely abolished Cd sensitivity. These data suggest that cysteine and glutamate residues are important for the metal-binding/translocation function of TaHMA2. Additional studies are needed to further understand the selectivity of TaHMA2 in planta.

Can ligand addition to soil enhance Cd phytoextraction? A mechanistic model study

Phytoextraction is a potential method for cleaning Cd-polluted soils. Ligand addition to soil is expected to enhance Cd phytoextraction. However, experimental results show that this addition has contradictory effects on plant Cd uptake. A mechanistic model simulating the reaction kinetics (adsorption on solid phase, complexation in solution), transport (convection, diffusion) and root absorption (symplastic, apoplastic) of Cd and its complexes in soil was developed. This was used to calculate plant Cd uptake with and without ligand addition in a great number of combinations of soil, ligand and plant characteristics, varying the parameters within defined domains. Ligand addition generally strongly reduced hydrated Cd (Cd(2+)) concentration in soil solution through Cd complexation. Dissociation of Cd complex ([Formula: see text]) could not compensate for this reduction, which greatly lowered Cd(2+) symplastic uptake by roots. The apoplastic uptake of [Formula: see text] was not sufficient to compensate for the decrease in symplastic uptake. This explained why in the majority of the cases, ligand addition resulted in the reduction of the simulated Cd phytoextraction. A few results showed an enhanced phytoextraction in very particular conditions (strong plant transpiration with high apoplastic Cd uptake capacity), but this enhancement was very limited, making chelant-enhanced phytoextraction poorly efficient for Cd.

Cadmium-induced hydrogen sulfide synthesis is involved in cadmium tolerance in Medicago sativa by reestablishment of reduced (homo)glutathione and reactive oxygen species homeostases

Until now, physiological mechanisms and downstream targets responsible for the cadmium (Cd) tolerance mediated by endogenous hydrogen sulfide (H₂S) have been elusive. To address this gap, a combination of pharmacological, histochemical, biochemical and molecular approaches was applied. The perturbation of reduced (homo)glutathione homeostasis and increased H₂S production as well as the activation of two H₂S-synthetic enzymes activities, including _L-cysteine desulfhydrase (LCD) and _D-cysteine desulfhydrase (DCD), in alfalfa seedling roots were early responses to the exposure of Cd. The application of H₂S donor sodium hydrosulfide (NaHS), not only mimicked intracellular H₂S production triggered by Cd, but also alleviated Cd toxicity in a H₂S-dependent fashion. By contrast, the inhibition of H₂S production caused by the application of its synthetic inhibitor blocked NaHS-induced Cd tolerance, and destroyed reduced (homo)glutathione and reactive oxygen species (ROS) homeostases. Above mentioned inhibitory responses were further rescued by exogenously applied glutathione (GSH). Meanwhile, NaHS responses were sensitive to a (homo)glutathione synthetic inhibitor, but reversed by the cotreatment with GSH. The possible involvement of cyclic AMP (cAMP) signaling in NaHS responses was also suggested. In summary, LCD/DCD-mediated H₂S might be an important signaling molecule in the enhancement of Cd toxicity in alfalfa seedlings mainly by governing reduced (homo)glutathione and ROS homeostases.

A comprehensive set of transcript sequences of the heavy metal hyperaccumulator Noccaea caerulescens

Noccaea caerulescens is an extremophile plant species belonging to the Brassicaceae family. It has adapted to grow on soils containing high, normally toxic, concentrations of metals such as nickel, zinc, and cadmium. Next to being extremely tolerant to these metals, it is one of the few species known to hyperaccumulate these metals to extremely high concentrations in their aboveground biomass. In order to provide additional molecular resources for this model metal hyperaccumulator species to study and understand the mechanism of adaptation to heavy metal exposure, we aimed to provide a comprehensive database of transcript sequences for N. caerulescens. In this study, 23,830 transcript sequences (isotigs) with an average length of 1025 bp were determined for roots, shoots and inflorescences of N. caerulescens accession "Ganges" by Roche GS-FLEX 454 pyrosequencing. These isotigs were grouped into 20,378 isogroups, representing potential genes. This is a large expansion of the existing N. caerulescens transcriptome set consisting of 3705 unigenes. When translated and compared to a Brassicaceae proteome set, 22,232 (93.2%) of the N. caerulescens isotigs (corresponding to 19,191 isogroups) had a significant match and could be annotated accordingly. Of the remaining sequences, 98 isotigs resembled non-plant sequences and 1386 had no significant similarity to any sequence in the GenBank database. Among the annotated set there were many isotigs with similarity to metal homeostasis genes or genes for glucosinolate biosynthesis. Only for transcripts similar to Metallothionein3 (MT3), clear evidence for an additional copy was found. This comprehensive set of transcripts is expected to further contribute to the discovery of mechanisms used by N. caerulescens to adapt to heavy metal exposure.