Overexpression of bacterial γ-glutamylcysteine synthetase mediates changes in cadmium influx, allocation and detoxification in poplar

Genome-Wide Identification and Characterization of Four Gene Families Putatively Involved in Cadmium Uptake, Translocation and Sequestration in Mulberry

The zinc-regulated transporters, iron-regulated transporter-like proteins (ZIPs), the natural resistance and macrophage proteins (NRAMP), the heavy metal ATPases (HMAs) and the metal tolerance or transporter proteins (MTPs) families are involved in cadmium (Cd) uptake, translocation and sequestration in plants. Mulberry (Morus L.), one of the most ecologically and economically important (as a food plant for silkworm production) genera of perennial trees, exhibits excellent potential for remediating Cd-contaminated soils. However, there is no detailed information about the genes involved in Cd²⁺ transport in mulberry. In this study, we identified 31 genes based on a genome-wide analysis of the Morus notabilis genome database. According to bioinformatics analysis, the four transporter gene families in Morus were distributed in each group of the phylogenetic tree, and the gene exon/intron structure and protein motif structure were similar among members of the same group. Subcellular localization software predicted that these transporters were mainly distributed in the plasma membrane and the vacuolar membrane, with members of the same group exhibiting similar subcellular locations. Most of the gene promoters contained abiotic stress-related cis-elements. The expression patterns of these genes in different organs were determined, and the patterns identified, allowing the categorization of these genes into four groups. Under low or high-Cd²⁺ concentrations (30 μM or 100 μM, respectively), the transcriptional regulation of the 31 genes in root, stem and leaf tissues of M. alba seedlings differed with regard to tissue and time of peak expression. Heterologous expression of MaNRAMP1, MaHMA3, MaZIP4, and MaIRT1 in Saccharomyces cerevisiae increased the sensitivity of yeast to Cd, suggested that these transporters had Cd transport activity. Subcellular localization experiment showed that the four transporters were localized to the plasma membrane of yeast and tobacco. These results provide the basis for further understanding of the Cd tolerance mechanism in Morus, which can be exploited in Cd phytoremediation.

Exogenous glutathione enhances cadmium accumulation and alleviates its toxicity in Populus × canescens

Glutathione (GSH) plays an important role in cadmium (Cd) tolerance in woody plants, but the underlying mechanisms remain largely unknown. To elucidate the physiological and transcriptional regulation mechanisms of GSH-mediated Cd tolerance in woody plants, we exposed Populus × canescens (Ait.) Smith saplings to either 0 or 75 μM Cd together with one of three external GSH levels. Glutathione treatments include buthionine sulfoximine (BSO, an inhibitor of GSH biosynthesis), no external GSH and exogenous GSH. External GSH resulted in higher Cd2+ uptake rate in the roots, greater Cd amount in poplars, lower Cd-induced H2O2 levels in the roots, and higher contents of endogenous GSH in Cd-treated roots and leaves. Furthermore, external GSH led to upregulated transcript levels of several genes including zinc/iron regulated transporter related protein 6.2 (ZIP6.2) and natural resistance-associated macrophage protein 1.3 (NRAMP1.3), which probably take part in Cd uptake, glutathione synthetase 2 (GS2) implicated in Cd detoxification, metal tolerance protein 1 (MTP1) and ATP-binding cassette transporter C3 (ABCC3) involved in Cd vacuolar accumulation in the roots, γ-glutamylcysteine synthetase (ECS) and phytochelatin synthetase family protein 1 (PCS1) involved in Cd detoxification, and oligopeptide transporter 7 (OPT7) probably implicated in Cd detoxification in the leaves of Cd-exposed P. × canescens. In contrast, BSO often displayed the opposite effects on Cd-triggered physiological and transcriptional regulation responses in poplars. These results suggest that exogenous GSH can enhance Cd accumulation and alleviate its toxicity in poplars. This is probably attributed to external-GSH-induced higher net Cd2+ influx in the roots, greater Cd accumulation in aerial parts, stronger scavenging of reactive oxygen species, and transcriptional overexpression of several genes involved in Cd uptake, detoxification and accumulation.

Toxic effects of cadmium on tall fescue and different responses of the photosynthetic activities in the photosystem electron donor and acceptor sides

Tall fescue (Festuca arundinacea Schreb) is a turf grass species which is widely used for rhizoremediation of organic contaminants and shows notable prospects in heavy metal phytoremediation. In this study, different concentrations of cadmium ion (Cd2+) were applied to study toxic effects of Cd2+ and responses of tall fescue by soilless culture. Tall fescue showed comparable high tolerance to Cd2+ as Indian mustard (Brassica juncea L.). Additionally, the treatment with high concentration of Cd2+ leaded to decreased chlorophyll contents, production of reactive oxygen species (ROS) and lipid peroxidation, as well as damage of cell membrane, necrosis and apoptosis in tall fescue roots, and toxicity of Cd2+ on physiologic properties of tall fescue has been well discussed. Moreover, in photosystem II electron donor side, electron transport from oxygen evolution complex (OEC) to Yz residue of D1 protein was inhibited under high Cd2+ treatments, which may be due to the Cd2+ induced ROS production and the replacement of Ca2+ in the core of OEC. In electron acceptor side, electron transport efficiency from quinone B to photosystem I acceptors increased under high Cd2+ treatments, which may be an important response for plants against Cd2+ toxicity and its mechanism needs our further study.

Improved stress tolerance and productivity in transgenic rice plants constitutively expressing the Oryza sativa glutathione synthetase OsGS under paddy field conditions

Reactive oxygen species, which increase under various environmental stresses, have deleterious effects on plants. An important antioxidant, glutathione, is used to detoxify reactive oxygen species in plant cells and is mainly produced by two enzymes: gamma-glutamylcysteine synthetase (γ-ECS) and glutathione synthetase (GS). To evaluate the functional roles of the glutathione synthetase gene (OsGS) in rice, we generated four independent transgenic rice plants (TG1-TG4) that overexpressed OsGS under the control of the constitutively expressed OsCc1 promoter. When grown under natural paddy field conditions, the TG rice plants exhibited greater growth development, higher chlorophyll content, and higher GSH/GSSH ratios than control wild-type (WT) rice plants. Subsequently, the TG rice plants enhanced redox homeostasis by preventing hydroperoxide-mediated membrane damage, which improved their adaptation to environmental stresses. As a result, TG rice plants improved rice grain yield and total biomass following increases in panicle number and number of spikelets per panicle, despite differences in climate during the cultivation periods of 2014 and 2015. Overall, our results indicate that OsGS overexpression improved redox homeostasis by enhancing the glutathione pool, which resulted in greater tolerance to environmental stresses in the paddy fields.

Enhanced metal tolerance correlates with heterotypic variation in SpMTL, a metallothionein-like protein from the hyperaccumulator Sedum plumbizincicola

Mechanistic insight into metal hyperaccumulation is largely restricted to Brassicaceae plants; therefore, it is of great importance to obtain corresponding knowledge from system outside the Brassicaceae. Here, we constructed and screened a cDNA library of the Cd/Zn hyperaccumulator Sedum plumbizincicola and identified a novel metallothionein-like protein encoding gene SpMTL. SpMTL showed functional similarity to other known MT proteins and also to its orthologues from non-hyperaccumulators. However, three additional cysteine residues were observed in SpMTL and appeared to be hyperaccumulator specific. Removal of these three residues significantly decreased its ability to tolerate Cd and the stoichiometry of Cd against SpMTL (molar ratio of Cd/SpMTL) to a level comparable to those of Cd/SaMTL and Cd/SeMTL in the corresponding non-hyperaccumulating relatives. SpMTL expressed in S. plumbizincicola roots at a much higher level than those of its orthologues in the non-hyperaccumulator roots. Interestingly, a positive correlation was observed between transcript levels of SpMTL in roots and Cd accumulation in leaves. Taking these results together, we propose that elevated transcript levels and heterotypic variation in protein sequences of SpMTL might contribute to the trait of Cd hyperaccumulation and hypertolerance in S. plumbizincicola.

Sulfur Protects Pakchoi (Brassica chinensis L.) Seedlings against Cadmium Stress by Regulating Ascorbate-Glutathione Metabolism

Cadmium (Cd) pollution in food chains pose a potential health risk for humans. Sulfur (S) is a significant macronutrient that plays a significant role in the regulation of plant responses to diverse biotic and abiotic stresses. However, no information is currently available about the impact of S application on ascorbate-glutathione metabolism (ASA-GSH cycle) of Pakchoi plants under Cd stress. The two previously identified genotypes, namely, Aikangqing (a Cd-tolerant cultivar) and Qibaoqing (a Cd-sensitive cultivar), were utilized to investigate the role of S to mitigate Cd toxicity in Pakchoi plants under different Cd regimes. Results showed that Cd stress inhibited plant growth and induced oxidative stress. Exogenous application of S significantly increased the tolerance of Pakchoi seedlings suffering from Cd stress. This effect was demonstrated by increased growth parameters; stimulated activities of the antioxidant enzymes and upregulated genes involved in the ASA-GSH cycle and S assimilation; and by the enhanced ASA, GSH, phytochelatins, and nonprotein thiol production. This study shows that applying S nutrition can mitigate Cd toxicity in Pakchoi plants which has the potential in assisting the development of breeding strategies aimed at limiting Cd phytoaccumulation and decreasing Cd hazards in the food chain.

Over-expression of tobacco UBC1 encoding a ubiquitin-conjugating enzyme increases cadmium tolerance by activating the 20S/26S proteasome and by decreasing Cd accumulation and oxidative stress in tobacco (Nicotiana tabacum)

Ubiquitin (Ub)-conjugating enzyme (UBC, E2) receives Ub from Ub-activating enzyme (E1) and transfers it to target proteins, thereby playing a key role in Ub/26S proteasome-dependent proteolysis. UBC has been reported to be involved in tolerating abiotic stress in plants, including drought, salt, osmotic and water stresses. To isolate the genes involved in Cd tolerance, we transformed WT (wild-type) yeast Y800 with a tobacco cDNA expression library and isolated a tobacco cDNA, NtUBC1 (Ub-conjugating enzyme), that enhances cadmium tolerance. When NtUBC1 was over-expressed in tobacco, cadmium tolerance was enhanced, but the Cd level was decreased. Interestingly, 20S proteasome activity was increased and ubiquitinated protein levels were diminished in response to cadmium in NtUBC1 tobacco. By contrast, proteasome activity was decreased and ubiquitinated protein levels were slightly enhanced by Cd treatment in control tobacco, which is sensitive to Cd. Moreover, the oxidative stress level was induced to a lesser extent by Cd in NtUBC1 tobacco compared with control plants, which is ascribed to the higher activity of antioxidant enzymes in NtUBC1 tobacco. In addition, NtUBC1 tobacco displayed a reduced accumulation of Cd compared with the control, likely due to the higher expression of CAX3 (Ca2+/H+ exchanger) and the lower expression of IRT1 (iron-responsive transporter 1) and HMA-A and -B (heavy metal ATPase). In contrast, atubc1 and atubc1atubc2 Arabidopsis exhibited lower Cd tolerance and proteasome activity than WT. In conclusion, NtUBC1 expression promotes cadmium tolerance likely by removing cadmium-damaged proteins via Ub/26S proteasome-dependent proteolysis or the Ub-independent 20S proteasome and by diminishing oxidative stress through the activation of antioxidant enzymes and decreasing Cd accumulation due to higher CAX3 and lower IRT1 and HMA-A/B expression in response to 50 µM Cd challenge for 3 weeks.

Integration of Cadmium Accumulation, Subcellular Distribution, and Physiological Responses to Understand Cadmium Tolerance in Apple Rootstocks

Cadmium (Cd) is a nonessential and highly toxic element causing agricultural problems. However, little information is available about the variation in Cd tolerance among apple rootstocks and its underlying physiological regulation mechanisms. This study investigated Cd accumulation, subcellular distribution, and chemical forms as well as physiological changes among four apple rootstocks exposed to either 0 or 300 μM CdCl2. The results showed that variations in Cd tolerance existed among these rootstocks. Cd exposure caused decline in photosynthesis, chlorophyll and biomass in four apple rootstocks, which was less pronounced in M. baccata, indicating its higher Cd tolerance. This finding was corroborated with higher Cd tolerance indexes (TIs) of the whole plant in M. baccata than those in the other three apple rootstocks. Among the four apple rootstocks, M. baccata displayed the lowest Cd concentrations in roots, wood, and leaves, the smallest total Cd amounts as well as the lowest BCF. In apple rootstocks, it was found that to immobilize Cd in cell wall and soluble fraction (most likely in vacuole) and to convert it into pectate- or protein- integrated forms and undissolved Cd phosphate forms may be the primary strategies to reduce Cd mobility and toxicity. The physiological changes including ROS, carbohydrates and antioxidants were in line with the variations of Cd tolerance among four apple rootstocks. In comparison with the other three apple rootstocks, M. baccata had lower concentrations of ROS in roots and bark, H2O2 in roots and leaves and MDA in roots, wood and bark, but higher concentrations of soluble sugars in bark and starch in roots and leaves, and enhanced antioxidants. These results indicate that M. baccata are more tolerant to Cd stress than the other three apple rootstocks under the current experiment conditions, which is probably related to Cd accumulation, subcellular partitioning and chemical forms of Cd and well-coordinated antioxidant defense mechanisms.

Non-enhanced phytoextraction of cadmium, zinc, and lead by high-yielding crops

Heavy metal soil contamination from mining and smelting has been reported in several regions around the world, and phytoextraction, using plants to accumulate risk elements in aboveground harvestable organs, is a useful method of substantially reducing this contamination. In our 3-year experiment, we tested the hypothesis that phytoextraction can be successful in local soil conditions without external fertilizer input. The phytoextraction efficiency of 15 high-yielding crop species was assessed in a field experiment performed at the Litavka River alluvium in the Příbram region of Czechia. This area is heavily polluted by Cd, Zn, and Pb from smelter installations which also polluted the river water and flood sediments. Heavy metal concentrations were analyzed in the herbaceous plants' aboveground and belowground biomass and in woody plants' leaves and branches. The highest Cd and Zn mean concentrations in the aboveground biomass were recorded in Salix x fragilis L. (10.14 and 343 mg kg^-1 in twigs and 16.74 and 1188 mg kg^-1 in leaves, respectively). The heavy metal content in woody plants was significantly higher in leaves than in twigs. In addition, Malva verticillata L. had the highest Cd, Pb, and Zn concentrations in herbaceous species (6.26, 12.44, and 207 mg kg^-1, respectively). The calculated heavy metal removal capacities in this study proved high phytoextraction efficiency in woody species; especially for Salix × fragilis L. In other tested plants, Sorghum bicolor L., Helianthus tuberosus L., Miscanthus sinensis Andersson, and Phalaris arundinacea L. species are also recommended for phytoextraction.

NADPH oxidases differentially regulate ROS metabolism and nutrient uptake under cadmium toxicity

The role of NADPH oxidases under cadmium (Cd) toxicity was studied using Arabidopsis thaliana mutants AtrbohC, AtrbohD and AtrbohF, which were grown under hydroponic conditions with 25 and 100 μM Cd for 1 and 5 days. Cadmium reduced the growth of leaves in WT, AtrbohC and D, but not in AtrbohF. A time-dependent increase in H₂ O₂ and lipid peroxidation was observed in all genotypes, with AtrbohC showing the smallest increase. An opposite behaviour was observed with NO accumulation. Cadmium increased catalase activity in WT plants and decreased it in Atrboh mutants, while glutathione reductase and glycolate oxidase activities increased in Atrboh mutants, and superoxide dismutases were down-regulated in AtrbohC. The GSH/GSSG and ASA/DHA couples were also affected by the treatment, principally in AtrbohC and AtrbohF, respectively. Cadmium translocation to the leaves was severely reduced in Atrboh mutants after 1 day of treatment and even after 5 days in AtrbohF. Similar results were observed for S, P, Ca, Zn and Fe accumulation, while an opposite trend was observed for K accumulation, except in AtrbohF. Thus, under Cd stress, RBOHs differentially regulate ROS metabolism, redox homeostasis and nutrient balance and could be of potential interest in biotechnology for the phytoremediation of polluted soils.

Functional components of the bacterial CzcCBA efflux system reduce cadmium uptake and accumulation in transgenic tobacco plants

Cadmium (Cd) is a toxic trace element released into the environment by industrial and agricultural practices, threatening the health of plants and contaminating the food/feed chain. Biotechnology can be used to develop plant varieties with a higher capacity for Cd accumulation (for use in phytoremediation programs) or a lower capacity for Cd accumulation (to reduce Cd levels in food and feed). Here we generated transgenic tobacco plants expressing components of the Pseudomonas putida CzcCBA efflux system. Plants were transformed with combinations of the CzcC, CzcB and CzcA genes, and the impact on Cd mobilization was analysed. Plants expressing PpCzcC showed no differences in Cd accumulation, whereas those expressing PpCzcB or PpCzcA accumulated less Cd in the shoots, but more Cd in the roots. Plants expressing both PpCzcB and PpCzcA accumulated less Cd in the shoots and roots compared to controls, whereas plants expressing all three genes showed a significant reduction in Cd levels only in shoots. These results show that components of the CzcCBA system can be expressed in plants and may be useful for developing plants with a reduced capacity to accumulate Cd in the shoots, potentially reducing the toxicity of food/feed crops cultivated in Cd-contaminated soils.

Comparative transcriptomic analysis reveals the roles of overlapping heat-/drought-responsive genes in poplars exposed to high temperature and drought

High temperature (HT) and drought are both critical factors that constrain tree growth and survival under global climate change, but it is surprising that the transcriptomic reprogramming and physiological relays involved in the response to HT and/or drought remain unknown in woody plants. Thus, Populus simonii saplings were exposed to either ambient temperature or HT combined with sufficient watering or drought. RNA-sequencing analysis showed that a large number of genes were differentially expressed in poplar roots and leaves in response to HT and/or desiccation, but only a small number of these genes were identified as overlapping heat-/drought-responsive genes that are mainly involved in RNA regulation, transport, hormone metabolism, and stress. Furthermore, the overlapping heat-/drought-responsive genes were co-expressed and formed hierarchical genetic regulatory networks under each condition compared. HT-/drought-induced transcriptomic reprogramming is linked to physiological relays in poplar roots and leaves. For instance, HT- and/or drought-induced abscisic acid accumulation and decreases in auxin and other phytohormones corresponded well with the differential expression of a few genes involved in hormone metabolism. These results suggest that overlapping heat-/drought-responsive genes will play key roles in the transcriptional and physiological reconfiguration of poplars to HT and/or drought under future climatic scenarios.

Comparative of Quercus spp. and Salix spp. for phytoremediation of Pb/Zn mine tailings

A pot experiment was conducted to evaluate the feasibility of using tree seedlings for the phytoremediation of lead/zinc (Pb/Zn) mine tailings. Seedlings of three Quercus spp. (Q. shumardii, Q. phellos, and Q. virginiana) and rooted cuttings of two Salix spp. (S. matsudana and S. integra) were transplanted into pots containing 50 and 100 % Pb/Zn mine tailings to evaluate their tolerance of heavy metals. The five species showed different tolerance levels to the Pb/Zn tailings treatments. Q. virginiana was highly tolerant to heavy metals and grew normally in the Pb/Zn tailings. The root systems showed marked differences between the Quercus spp. and Salix spp., indicating that different mechanisms operated to confer tolerance of heavy metals. The maximum efficiency of photosystem II photochemistry value of the five species showed no differences among the treatments, except for Q. shumardii. All species showed low metal translocation factors (TFs). However, S. integra had significantly higher TF values for Zn (1.42-2.18) and cadmium (1.03-1.45) than did the other species. In this respect, Q. virginiana showed the highest tolerance and a low TF, implying that it is a candidate for phytostabilization of mine tailings in southern China. S. integra may be useful for phytoextraction of tailings in temperate regions.

Paxillus involutus-Facilitated Cd2+ Influx through Plasma Membrane Ca2+-Permeable Channels Is Stimulated by H2O2 and H+-ATPase in Ectomycorrhizal Populus × canescens under Cadmium Stress

Using a Non-invasive Micro-test Technique, flux profiles of Cd2+, Ca2+, and H+ were investigated in axenically grown cultures of two strains of Paxillus involutus (MAJ and NAU), ectomycorrhizae formed by these fungi with the woody Cd2+-hyperaccumulator, Populus × canescens, and non-mycorrhizal (NM) roots. The influx of Cd2+ increased in fungal mycelia, NM and ectomycorrhizal (EM) roots upon a 40-min shock, after short-term (ST, 24 h), or long-term (LT, 7 days) exposure to a hydroponic environment of 50 μM CdCl2. Cd2+ treatments (shock, ST, and LT) decreased Ca2+ influx in NM and EM roots but led to an enhanced influx of Ca2+ in axenically grown EM cultures of the two P. involutus isolates. The susceptibility of Cd2+ flux to typical Ca2+ channel blockers (LaCl3, GdCl3, verapamil, and TEA) in fungal mycelia and poplar roots indicated that the Cd2+ entry occurred mainly through Ca2+-permeable channels in the plasma membrane (PM). Cd2+ treatment resulted in H2O2 production. H2O2 exposure accelerated the entry of Cd2+ and Ca2+ in NM and EM roots. Cd2+ further stimulated H+ pumping activity benefiting NM and EM roots to maintain an acidic environment, which favored the entry of Cd2+ across the PM. A scavenger of reactive oxygen species, DMTU, and an inhibitor of PM H+-ATPase, orthovanadate, decreased Ca2+ and Cd2+ influx in NM and EM roots, suggesting that the entry of Cd2+ through Ca2+-permeable channels is stimulated by H2O2 and H+ pumps. Compared to NM roots, EM roots exhibited higher Cd2+-fluxes under shock, ST, and LT Cd2+ treatments. We conclude that ectomycorrhizal P. × canescens roots retained a pronounced H2O2 production and a high H+-pumping activity, which activated PM Ca2+ channels and thus facilitated a high influx of Cd2+ under Cd2+ stress.

Exogenous Glutathione Enhances Mercury Tolerance by Inhibiting Mercury Entry into Plant Cells

Despite the increasing understanding of the crucial roles of glutathione (GSH) in cellular defense against heavy metal stress as well as oxidative stress, little is known about the functional role of exogenous GSH in mercury (Hg) tolerance in plants. Here, we provide compelling evidence that GSH contributes to Hg tolerance in diverse plants. Exogenous GSH did not mitigate the toxicity of cadmium (Cd), copper (Cu), or zinc (Zn), whereas application of exogenous GSH significantly promoted Hg tolerance during seed germination and seedling growth of Arabidopsis thaliana, tobacco, and pepper. By contrast, addition of buthionine sulfoximine, an inhibitor of GSH biosynthesis, severely retarded seed germination and seedling growth of the plants in the presence of Hg. The effect of exogenous GSH on Hg specific tolerance was also evident in the presence of other heavy metals, such as Cd, Cu, and Zn, together with Hg. GSH treatment significantly decreased H₂O₂ and O₂^- levels and lipid peroxidation, but increased chlorophyll content in the presence of Hg. Importantly, GSH treatment resulted in significantly less accumulation of Hg in Arabidopsis plants, and thin layer chromatography and nuclear magnetic resonance analysis revealed that GSH had much stronger binding affinity to Hg than to Cd, Cu, or Zn, suggesting that tight binding of GSH to Hg impedes Hg uptake, leading to low Hg accumulation in plant cells. Collectively, the present findings reveal that GSH is a potent molecule capable of conferring Hg tolerance by inhibiting Hg accumulation in plants.

The apoplasmic pathway via the root apex and lateral roots contributes to Cd hyperaccumulation in the hyperaccumulator Sedum alfredii

Although the significance of apoplasmic barriers in roots with regards to the uptake of toxic elements is generally known, the contribution of apoplasmic bypasses (ABs) to cadmium (Cd) hyperaccumulation is little understood. Here, we employed a combination of stable isotopic tracer techniques, an ABs tracer, hydraulic measurements, suberin lamellae staining, metabolic inhibitors, and antitranspirants to investigate and quantify the impact of the ABs on translocation of Cd to the xylem in roots of a hyperaccumulating (H) ecotype and a non-hyperaccumulating (NH) ecotype of Sedum alfredii. In the H ecotype, the Cd content in the xylem sap was proportional to hydrostatic pressure, which was attributed to pressure-driven flow via the ABs. The contribution of the ABs to Cd transportation to the xylem was dependent on the Cd concentration applied to the H ecotype (up to 37% at the highest concentration used). Cd-treated H ecotype roots showed significantly higher hydraulic conductance compared with the NH ecotype (76 vs 52 × 10–8 m s–1MPa–1), which is in accordance with less extensive suberization due to reduced expression of suberin-related genes. The main entry sites of apoplasmically transported Cd were localized in the root apexes and lateral roots of the H ecotype, where suberin lamellae were not well developed. These findings highlight the significance of the apoplasmic bypass in Cd hyperaccumulation in hyperaccumulating ecotypes of S. alfredii.

Metal phytoremediation: General strategies, genetically modified plants and applications in metal nanoparticle contamination

The accumulation of metals in different environmental compartments poses a risk to both the environment and biota health. In particular, the continuous increase of these elements in soil ecosystems is a major worldwide concern. Phytoremediation has been gaining more attention in this regard. This approach takes advantage of the unique and selective uptake capabilities of plant root systems, and applies these natural processes alongside the translocation, bioaccumulation, and contaminant degradation abilities of the entire plant and, although it is a relatively recent technology, beginning in the 90's, it is already considered a green alternative solution to the problem of metal pollution, with great potential. This review focuses on phytoremediation of metals from soil, sludge, wastewater and water, the different strategies applied, the biological and physico-chemical processes involved and the advantages and limitations of each strategy. Special note is given to the use of transgenic species and phytoremediation of metallic nanoparticles.

Sexual competition affects biomass partitioning, carbon-nutrient balance, Cd allocation and ultrastructure of Populus cathayana females and males exposed to Cd stress

Although increasing attention has been paid to plant adaptation to soil heavy metal contamination, competition and neighbor effects have been largely overlooked, especially in dioecious plants. In this study, we investigated growth as well as biochemical and ultrastructural responses of Populus cathayana Rehder females and males to cadmium (Cd) stress under different sexual competition patterns. The results showed that competition significantly affects biomass partitioning, photosynthetic capacity, leaf and root ultrastructure, Cd accumulation, the contents of polyphenols, and structural and nonstructural carbohydrates. Compared with single-sex cultivation, plants of opposite sexes exposed to sexual competition accumulated more Cd in tissues and their growth was more strongly inhibited, indicating enhanced Cd toxicity under sexual competition. Under intrasexual competition, females showed greater Cd accumulation, more serious damage at the ultrastructural level and greater reduction in physiological activity than under intersexual competition, while males performed better under intrasexual competition than under intersexual competition. Males improved the female microenvironment by greater Cd uptake and lower resource consumption under intersexual competition. These results demonstrate that the sex of neighbor plants and competition affect sexual differences in growth and in key physiological processes under Cd stress. The asymmetry of sexual competition highlighted here might regulate population structure, and spatial segregation and phytoremediation potential of both sexes in P. cathayana growing in heavy metal-contaminated soils.

Iron plaque decreases cadmium accumulation in Oryza sativa L. and serves as a source of iron

Cadmium (Cd) contamination occurs in paddy soils; hence it is necessary to reduce Cd content of rice. Application and mode of action of ferrous sulphate in minimizing Cd in rice was monitored in the present study. Pot culture with Indian rice variety Swarna (MTU 7029) was maintained in Cd-spiked soil containing ferrous sulphates, which is expected to reduce Cd accumulation in rice. Responses in rhizosphere pH, root surface, metal accumulation in plant and molecular physiological processes were monitored. Iron plaque was induced on root surfaces after FeSO4 application and the amount of Fe in plaque reduced with increases in Cd in the soil. Rhizosphere pH decreased during plaque formation and became more acidic due to secretion of organic acids from the roots under Cd treatment. Moreover, iron chelate reductase activity increased with Cd treatment, but in the absence of Cd, activity of this enzyme increased in plaque-induced plants. Cd treatment caused expression of OsYSL18, whereas OsYSL15 was expressed only in roots without iron plaque. Fe content of plants increased during plaque formation, which protected plants from Cd-induced Fe deficiency and metal toxicity. This was corroborated with increased biomass, chlorophyll content and quantum efficiency of photo-synthesis among plaque-induced plants. We conclude that ferrous sulphate-induced iron plaque prevents Cd accumulation and Fe deficiency in rice. Iron released from plaque via organic acid mediated dissolution during Cd stress.

Lead uptake increases drought tolerance of wild type and transgenic poplar (Populus tremula x P. alba) overexpressing gsh 1

Growth and development of plants largely depends on their adaptation ability in a changing climate. This is particularly true on heavy metal contaminated soils, but the interaction of heavy metal stress and climate on plant performance has not been intensively investigated. The aim of the present study was to elucidate if transgenic poplars (Populus tremula x P. alba) with enhanced glutathione content possess an enhanced tolerance to drought and lead (Pb) exposure (single and in combination) and if they are good candidates for phytoremediation of Pb contaminated soil. Lead exposure reduced growth and biomass accumulation only in above-ground tissue of wild type poplar, although most of lead accumulated in the roots. Drought caused a decline of the water content rather than reduced biomass production, while Pb counteracted this decline in the combined exposure. Apparently, metals such as Pb possess a protective function against drought, because they interact with abscisic acid dependent stomatal closure. Lead exposure decreased while drought increased glutathione content in leaves of both plant types. Lead accumulation was higher in the roots of transgenic plants, presumably as a result of chelation by glutathione. Water deprivation enhanced Pb accumulation in the roots, but Pb was subject to leakage out of the roots after re-watering. Transgenic plants showed better adaptation under mild drought plus Pb exposure partially due to improved glutathione synthesis. However, the transgenic plants cannot be considered as a good candidate for phytoremediation of Pb, due to its small translocation to the shoots and its leakage out of the roots upon re-watering.

Sex-specific responses of Populus deltoides to Glomus intraradices colonization and Cd pollution

The positive effects of arbuscular mycorrhizal fungi (AM) on the survival, growth and physiology of plants under various stress conditions have been widely recognized. However, whether sex-dependent susceptibility to AM colonization exists, which can induce a differential tolerance between the sexes to stress conditions, is still unclear. In this study, we investigated the effects of Glomus intraradices on Cd-stressed males and females of Populus deltoides (spiked with 10 mg Cd per kg dry substrate) in terms of morphology, physiology, biochemistry, ultrastructure, and toxin storage and translocation. Exposure to Cd promoted the colonization by G. intraradices in males, but not in females. Generally, females suffered more impairments than males in response to Cd stress, reflected by leaf symptoms, the extent of lipid peroxidation, and integrity of the cellular ultrastructure, whether they were inoculated or not. Inoculation with G. intraradices alleviated the phytotoxic effects of Cd in females by stimulating antioxidant enzymes, decreasing levels of reactive oxygen species (ROS) and restricting Cd transfer to the shoots. In contrast, these beneficial effects induced by AM were not detected in mycorrhizal males compared to non-mycorrhizal males, based on thiobarbituric acid-reactive substances (TBARS) and cellular ultrastructure. Inoculation with AM promoted Cd accumulation in males but not in females, and caused the sequestration of more toxic Cd in the root systems in both sexes. Therefore, our results suggest that inoculated males of P. deltoides are suitable candidates for phytostabilization in Cd-polluted soils, due to their higher accumulation ability and greater tolerance relative to inoculated females.

Physiological and transcriptional regulation in poplar roots and leaves during acclimation to high temperature and drought

To elucidate the physiological and transcriptional regulatory mechanisms that underlie the responses of poplars to high temperature (HT) and/or drought in woody plants, we exposed Populus alba × Populus tremula var. glandulosa saplings to ambient temperature (AT) or HT under 80 or 40% field capacities (FC), or no watering. HT increased the foliar total carbon (C) concentrations, and foliar δ(13) C and δ(18) O. HT triggered heat stress signaling via increasing levels of abscisic acid (ABA) and indole-3-acetic acid (IAA) in poplar roots and leaves. After perception of HT, poplars initiated osmotic adjustment by increasing foliar sucrose and root galactose levels. In agreement with the HT-induced heat stress and the changes in the levels of ABA and carbohydrates, we detected increased transcript levels of HSP18 and HSP21, as well as NCED3 in the roots and leaves, and the sugar transporter gene STP14 in the roots. Compared with AT, drought induced greater enhancement of foliar δ(13) C and δ(18) O in poplars at HT. Similarly, drought caused greater stimulation of the ABA and foliar glucose levels in poplars at HT than at AT. Correspondingly, desiccation led to greater increases in the mRNA levels of HSP18, HSP21, NCED3, STP14 and INT1 in poplar roots at HT than at AT. These results suggest that HT has detrimental effects on physiological processes and it induces the transcriptional regulation of key genes involved in heat stress responses, ABA biosynthesis and sugar transport and HT can cause greater changes in drought-induced physiological and transcriptional responses in poplar roots and leaves.

Influence of low calcium availability on cadmium uptake and translocation in a fast-growing shrub and a metal-accumulating herb

Calcium (Ca) and the toxic heavy metal cadmium (Cd) are antagonistic ions competing for uptake in plants when they co-occur in soil solutions, and high Ca concentrations can reduce the uptake of Cd in plants. However, less is known about the effects of low Ca bioavailability on Cd uptake and translocation in plants. We hypothesized that low Ca availability would enhance Cd uptake and translocation in Sesbania sesban, a fast-growing shrub potentially useful for Cd removal from contaminated soils, and Brassica juncea, a well-known Cd-hyperaccumulator. The two species were grown under controlled conditions for 21 days in hydroponic nutrient solutions with either 0.2 or 2 mM Ca and 0 or 50 µM Cd in a 2 × 2 factorial experimental design. Both species had a lower relative growth rate, final root length and shoot and root biomasses at 50 µM Cd compared with growth without Cd. The negative effects of Cd on both species were exacerbated at low Ca availability. Brassica juncea had higher root Cd concentrations than S. sesban, but the shoot Cd concentrations did not differ between the two species. The low Ca concentration enhanced the uptake of Cd in the roots of both species, but Cd translocation to the shoots was low, especially at 0.2 mM Ca. We conclude that the low Ca concentration enhanced the uptake of Cd into roots of S. sesban and B. juncea and increased the phytotoxicity of Cd. The translocation of Cd to the shoots of the two species was, however, lower at 0.2 mM than at 2 mM Ca, implying that Cd removal from polluted soil cannot simply be increased by adjusting ion concentrations.

Challenging synergistic activity of poplar-bacteria association for the Cd phytostabilization

The synergistic activity between plants and microorganisms may contribute to the implementation of proactive management strategies in the stabilization of contaminated sites, although heavy metals, such as cadmium (Cd), are potentially toxic to them. The aim of this study was to evaluate the degree of tolerance to Cd contamination (supplying twice 40 mg kg(-1) of Cd) in poplar cuttings [clone I-214, P. × euramericana (Dode) Guinier] inoculated or not with two concentrations of Serratia marcescens strain (1 × 10(7) CFU/g and 2 × 10(7) CFU/g of potting mix). The response of the plant-bacteria system to excess Cd was investigated with special reference to the structural traits of plants and the functional efficiency of bacteria. Bacterial colonization and substrate components were previously assessed in order to define the best solution for formulating the experimental plant growth media. The tested plant-bacteria association, especially when bacteria were provided in double concentration, stimulated specific tolerance mechanisms to Cd through the promotion of the poplar growth. Inoculated plants produced larger leaves and increased stem diameter, while roots grew longer and wider in Cd-treated plants. The effect of bacterial inoculum on plant growth traits and metal partitioning in plant organs was assessed in order to define the potential of this poplar clone to be a suitable candidate for phytostabilization of Cd-contaminated soil. The final effect of the inoculation with bacteria, which alleviated the metal load and Cd phytotoxicity due to their bioaccumulation ability, suggests promising phytostabilization potential of these plant-bacteria associations.

Subcellular cadmium distribution and antioxidant enzymatic activities in the leaves of two castor (Ricinus communis L.) cultivars exhibit differences in Cd accumulation

The aims of this study were: (1) the study of cadmium (Cd) accumulation and toxicity in different castor cultivars (Ricinus communis L.); (2) to investigate changes in antioxidant enzymatic activities and the subcellular distribution of Cd in young and old leaves from two different castor cultivars, after exposure to two different Cd concentrations, and explore the underlying mechanism of Cd detoxification focusing on antioxidant enzymes and subcellular compartmentalization. The Cd concentration, toxicity, and subcellular distribution, as well as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities were measured in Zibo-3 and Zibo-9 cultivars after exposure to two different concentrations of Cd (2mg/L and 5mg/L) for 10 days. This research revealed Cd accumulation characteristics in castor are root>stem>young leaf>old leaf. Castor tolerance was Cd dose exposure and the cultivars themselves dependent. Investigation of subcellular Cd partitioning showed that Cd accumulated mainly in the heat stable protein (HSP) and cellular debris fractions, followed by the Cd rich granule (MRG), heat denatured protein (HDP), and organelle fractions. With increasing Cd concentration in nutrient solution, the decreased detoxified fractions (BDM) and the increased Cd-sensitive fractions (MSF) in young leaves may indicate the increased Cd toxicity in castor cultivars. The BDM-Cd fractions or MSF-Cd in old leaves may be linked with Cd tolerance of different cultivars of castor. The antioxidant enzymes that govern Cd detoxification were not found to be active in leaves. Taken together, these results indicate Cd tolerance and toxicity in castor can be explained by subcellular partitioning.

Architectural and biochemical changes in embryonic tissues of maize under cadmium toxicity

Heavy metals greatly alter plant morphology and architecture, however detailed mechanisms of such changes are not fully explored. Two experiments were conducted to investigate the influence of cadmium (CdCl2 · 2.5H2 O) on some germination, morphological, biochemical and histological characteristics of developing embryonic tissue of maize. In the first experiment, maize seeds were germinated in increasing levels of CdCl2 (200-2000 μm) in sand and measurements were taken of changes in germination and seedling development attributes. Based on these parameters, 1000 μM CdCl2 was chosen for detailed biochemical and histological measurements. In the second experiment, seeds were germinated in Petri dishes and supplied with 0 (control) or 1000 μM CdCl2 (Cd-treated). Radicle, plumule, coleoptile and coleorhiza were measured for biochemical and histological changes. The highest amount of Cd was in the coleorhiza and radicle. Free proline, soluble sugars, anthocyanin, soluble phenolics, ascorbic acid, H2 O2 and MDA were significantly higher in coleorhizae, followed by the coleoptile, radicle and plumule. Although the radicle and coleorhiza were relatively poor targets of Cd than the other tissues, Cd stress reduced cortical cell size and vascular tissues, and deformed xylem and phloem parenchyma in all plant parts. In conclusion, the main reason for reduced germination was the influence of Cd on architecture of the coleorhiza and coleoptile, which was the result of oxidative stress and other physiological changes taking place in these tissues.

Effects of selenium and silicon on enhancing antioxidative capacity in ramie (Boehmeria nivea (L.) Gaud.) under cadmium stress

Hydroponic experiments were performed to investigate the ameliorating effects and mitigation mechanisms of selenium and silicon on Cd toxicity in Boehmeria nivea (L.) Gaud. Metal accumulation, chlorophyll content, activities of antioxidant enzymes, and antioxidant contents in ramie were evaluated. The results revealed that cadmium was mainly accumulated in the roots of plants rather than in the aerial parts. Additionally, under 5 mg L(-1) Cd stress, both Se (1 μmol L(-1)) and Si (1 mmol L(-1)) treatments decreased the Cd concentrations in plants. Besides, the treatments also inhibited the translocation ability of Cd from roots to the aboveground parts, which might be related to the decline of generation of reactive oxygen species (ROS). The application of Se and/or Si ameliorated Cd toxicity via stimulating the activities of antioxidant enzymes such as superoxide dismutase (SOD), guaiacol peroxidase (POD), and ascorbate peroxidase (APX), which resulted in the significant decrease of the contents of malondialdialdehyde (MDA) and hydrogen peroxide (H2O2) in ramie leaves. In addition, the content of nonenzymatic antioxidant such as glutathione (GSH) was increased significantly through the addition of selenite and silicate. Also, ascorbate (AsA) and vitamin E played a crucial role in scavenging excess ROS within plants. On the whole, appropriate doses of Se and Si were found to benefit plant growth and enhance the ability of ramie to alleviate Cd-induced stress. Moerover, the effects of combination of Se and Si appeared to be more superior compared to addition separately in response to Cd stress.