Arabidopsis thaliana phytochelatin synthase 2 is constitutively active in vivo and can rescue the growth defect of the PCS1-deficient cad1-3 mutant on Cd-contaminated soil

Phytochelatins Bind Zn(II) with Micro- to Picomolar Affinities without the Formation of Binuclear Complexes, Exhibiting Zinc Buffering and Muffling Rather than Storing Functions

Phytochelatins (PCs) are poly-Cys peptides containing a repeating γ-Glu-Cys motif synthesized in plants, algae, certain fungi, and worms by PC synthase from reduced glutathione. It has been shown that an excess of toxic metal ions induces their biosynthesis and that they are responsible for the detoxification process. Little is known about their participation in essential metal binding under nontoxic, basal conditions under which PC synthase is active. This study presents spectroscopic and thermodynamic interactions with the PC2-PC5 series, mainly focusing on the relations between Zn(II) complex stability and cellular Zn(II) availability. The investigations employed mass spectrometry, UV-vis spectroscopy, potentiometry, competition assays with zinc probes, and isothermal titration calorimetry (ITC). All peptides form ZnL complexes, while ZnL2 was found only for PC2, containing two to four sulfur donors in the coordination sphere. Binuclear species typical of Cd(II)-PC complexes are not formed in the case of Zn(II). Results demonstrate that the affinity for Zn(II) increases linearly from PC2 to PC4, ranging from micro- to low-picomolar. Further elongation does not significantly increase the stability. Stability elevation is driven mainly by entropic factors related to the chelate effect and conformational restriction rather than enthalpic factors related to the increasing number of sulfur donors. The affinity of the investigated PCs falls within the range of exchangeable Zn(II) concentrations (hundreds of pM) observed in plants, supporting for the first time a role of PCs both in buffering and in muffling cytosolic Zn(II) concentrations under normal conditions, not exposed to zinc excess, where short PCs have been identified in numerous studies. Furthermore, we found that Cd(II)-PC complexes demonstrate significantly higher metal capacities due to the formation of polynuclear species, which are lacking for Zn(II), supporting the role of PCs in Cd(II) storage (detoxification) and Zn(II) buffering and muffling. Our results on phytochelatins' coordination chemistry and thermodynamics are important for zinc biology and understanding the molecular basis of cadmium toxicity, leaving room for future studies.

In Planta Arsenic Thiolation in Rice and Arabidopsis thaliana

Inorganic and methylated thioarsenates have recently been reported to form in paddy soil pore waters and accumulate in rice grains. Among them, dimethylmonothioarsenate (DMMTA) is particularly relevant because of its high cytotoxicity and potential misidentification as nonregulated dimethylarsenate (DMA). Studying DMMTA uptake and flag leaf, grain, and husk accumulation in rice plants during grain filling, substantial dethiolation to DMA was observed with only 8.0 ± 0.1, 9.1 ± 0.6, and 1.4 ± 0.2% DMMTA remaining, respectively. More surprisingly, similar shares of DMMTA were observed in control experiments with DMA, indicating in planta DMA thiolation. Exposure of different rice seedling varieties to not only DMA but also to arsenite and monomethylarsenate (MMA) revealed in planta thiolation as a common process in rice. Up to 35 ± 7% DMA thiolation was further observed in the shoots and roots of the model plant Arabidopsis thaliana. Parameters determining the ratio and kinetics of thiolation versus dethiolation are unknown, yet, but less DMA thiolation in glutathione-deficient mutants compared to wild-type plants suggested glutathione concentration as one potential parameter. Our results demonstrate that pore water is not the only source for thioarsenates in rice grains and that especially the currently nonregulated DMA needs to be monitored as a potential precursor of DMMTA formation inside rice plants.

Arabidopsis transcription factor WRKY45 confers cadmium tolerance via activating PCS1 and PCS2 expression

Cadmium (Cd) has long been recognized as toxic pollutant to crops worldwide. The biosynthesis of glutathione-dependent phytochelatin (PC) plays crucial roles in the detoxification of Cd in plants. However, its regulatory mechanism remains elusive. Here, we revealed that Arabidopsis transcription factor WRKY45 confers Cd tolerance via promoting the expression of PC synthesis-related genes PCS1 and PCS2, respectively. Firstly, we found that Cd stress induces the transcript levels of WRKY45 and its protein abundance. Accordingly, in contrast to wild type Col-0, the increased sensitivity to Cd is observed in wrky45 mutant, while overexpressing WRKY45 plants are more tolerant to Cd. Secondly, quantitative real-time PCR revealed that the expression of AtPCS1 and AtPCS2 is stimulated in overexpressing WRKY45 plants, but decreased in wrky45 mutant. Thirdly, WRKY45 promotes the expression of PCS1 and PCS2, electrophoresis mobility shift assay analysis uncovered that WRKY45 directly binds to the W-box cis-element of PCS2 promoter. Lastly, the overexpression of WRKY45 in Col-0 leads to more accumulation of PCs in Arabidopsis, and the overexpression of PCS1 or PCS2 in wrky45 mutant plants rescues the phenotypes induced by Cd stress. In conclusion, our results show that AtWRKY45 positively regulates Cd tolerance in Arabidopsis via activating PCS1 and PCS2 expression.

Effect of phosphate on arsenic species uptake in plants under hydroponic conditions

Monothioarsenate (MTA) is a newly discovered arsenic (As) compound that can be formed under reduced sulfur conditions, mainly in paddy soil pore waters. It is structurally similar to arsenate As(V) and inorganic phosphate (Pi), which is taken up through phosphate transporters. Due to the similarity between As(V) and Pi, As(V) enters into plants instead of Pi. The important role played by phytochelatin (PC), glutathione (GSH), and the PC-vacuolar transporters ABCC1 and ABCC2 under As stress in plants is well known. However, the plant uptake and mechanisms surrounding MTA still have not been completely addressed. This investigation was divided in two stages: first, several hydroponic assays were set up to establish the sensibility-tolerance of wild-type Arabidopsis thaliana (accession Columbia-0, Col-0). Then Col-0 was used as a control plant to evaluate the effects of As(V) or MTA in (PC)-deficient mutant (cad1-3), glutathione biosynthesis mutant (cad2), and PC transport (abcc1-2). The inhibitory concentration (IC50) root length was calculated for both As species. According to the results, both arsenic species (As(V) and MTA) exhibited high toxicity for the genotypes evaluated. This could mean that these mechanisms play a constitutive role in MTA detoxification. Second, for the Pi-MTA and As(V)-Pi competition assays, a series of experiments on hydroponic seedlings of A. thaliana were carried out using Col-0 and a pht1;1. The plants were grown under increasing Pi concentrations (10 μM, 0.1 mM, or 1 mM) at 10 μM As(V) or 50 μM MTA. The total As concentration in the roots was significantly lower in plants exposed to MTA, there being less As content in the pht1;1 mutant at the lowest Pi concentrations tested compared with the As(V)/Pi treatments. In addition, a higher rate of As translocation from the roots to the shoots under MTA was observed in comparison to the As(V)-treatments.

Regulatory mechanisms of sulfur metabolism affecting tolerance and accumulation of toxic trace metals and metalloids in plants

Soil contamination with trace metals and metalloids can cause toxicity to plants and threaten food safety and human health. Plants have evolved sophisticated mechanisms to cope with excess trace metals and metalloids in soils, including chelation and vacuolar sequestration. Sulfur-containing compounds, such as glutathione and phytochelatins, play a crucial role in their detoxification, and sulfur uptake and assimilation are regulated in response to the stress of toxic trace metals and metalloids. This review focuses on the multi-level connections between sulfur homeostasis in plants and responses to such stresses, especially those imposed by arsenic and cadmium. We consider recent progress in understanding the regulation of biosynthesis of glutathione and phytochelatins and of the sensing mechanism of sulfur homeostasis for tolerance of trace metals and metalloids in plants. We also discuss the roles of glutathione and phytochelatins in controlling the accumulation and distribution of arsenic and cadmium in plants, and possible strategies for manipulating sulfur metabolism to limit their accumulation in food crops.

The Effect of Cadmium on Plants in Terms of the Response of Gene Expression Level and Activity

Cadmium (Cd) is a heavy metal that can cause damage to living organisms at different levels. Even at low concentrations, Cd can be toxic to plants, causing harm at multiple levels. As they are unable to move away from areas contaminated by Cd, plants have developed various defence mechanisms to protect themselves. Hyperaccumulators, which can accumulate and detoxify heavy metals more efficiently, are highly valued by scientists studying plant accumulation and detoxification mechanisms, as they provide a promising source of genes for developing plants suitable for phytoremediation techniques. So far, several genes have been identified as being upregulated when plants are exposed to Cd. These genes include genes encoding transcription factors such as iron-regulated transporter-like protein (ZIP), natural resistance associated macrophage protein (NRAMP) gene family, genes encoding phytochelatin synthases (PCs), superoxide dismutase (SOD) genes, heavy metal ATPase (HMA), cation diffusion facilitator gene family (CDF), Cd resistance gene family (PCR), ATP-binding cassette transporter gene family (ABC), the precursor 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and precursor 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) multigene family are also influenced. Thanks to advances in omics sciences and transcriptome analysis, we are gaining more insights into the genes involved in Cd stress response. Recent studies have also shown that Cd can affect the expression of genes related to antioxidant enzymes, hormonal pathways, and energy metabolism.

The Effect of Cadmium Tolerant Plant Growth Promoting Rhizobacteria on Plant Growth Promotion and Phytoremediation: A Review

Cadmium (Cd) is a heavy metal of considerable toxicity with destructive impacts on plants, microbes and environments. Its toxicity is due to mishandling and manual hazards in plants and is primarily observed within the soil to cause decline of plants and microbial activity inside the rhizosphere. Cadmium accumulation in crops and the probability of Cd entering the food chain are grave for public health in the worldwide. Cadmium toxicity leads to depletion in seed germination, initial seedling growth, plant biomass, chlorosis, necrosis, hindrance of photosynthetic machinery and other physiological and biological activities in plants. Cadmium triggers the reactive oxygen species (ROS) that influences gene mutation and DNA damage that affects the cell cycle and cell division. Cd toxicity altered the levels of phenolic compounds, carbohydrates, glycine betaine, proline and organic acids in crops. Under stress conditions, the plant growth promoting rhizobacteria (PGPR) have various properties such as enzymatic activities, plant growth hormones production, phosphate solubilization, siderophores production and chelating agents that help the plants tolerate against Cd stress and also increase phenolic compound levels and osmolytes. Hence, this review highlights the crucial role of cadmium tolerant PGPR for crop production, declining metal phytoavailability and enhancing morphological and physiological boundaries of plants under stress conditions. It could be an environment friendly and cost effective technology under sustainable crop production.

Nitrate enhances cadmium accumulation through modulating sulfur metabolism in sweet sorghum

Sweet sorghum deploys tremendous potential for phytoremediation of cadmium (Cd)-polluted soils. Nitrate increases Cd accumulation in sweet sorghum, but the mechanism underlying this is still elusive. Sulfur-containing metabolites have been corroborated to play important roles in Cd tolerance in plants. Thus, whether sulfur metabolism contributed to nitrate-increased Cd accumulation in sweet sorghum was investigated in the present study. Two-way ANOVA analysis showed that most sulfur-containing metabolites concentrations and relevant enzymes activities were regulated by nitrate, Cd and interplay of nitrate and Cd. By using grey correlation analysis and Pearson correlation coefficient, Cd accumulation in shoots as affected by nitrate was also mainly ascribed to sulfur metabolism. ATP sulfurylase (ATPS) activities and non-protein thiol (NPT) concentrations in leaves were the two prominent factors that positively correlated with Cd accumulation in shoots. Excess nitrate elevated ATPS activities in leaves which contributed to increased NPT and phytochelatins (PCs) concentrations in leaves. Nitrate enhanced Cd accumulation in shoots of sweet sorghum under a low level of Cd treatment. Intriguingly, Cd accumulation in shoots of sweet sorghum was similar between a low level and a high level of Cd treatment. Principal Components Analysis (PCA) based on 34 parameters failed to separate the low Cd treatment from the high Cd treatment either, suggesting sweet sorghum is exclusively suitable for phytoremediation of slight Cd-polluted arable lands. Taken together, enhanced Cd accumulation in shoots of sweet sorghum by excess nitrate application is closely correlated with sulfur metabolism containing elevated ATPS activities, NPT and PCs concentrations in leaves.

Phytochelatins: Sulfur-Containing Metal(loid)-Chelating Ligands in Plants

Phytochelatins (PCs) are small cysteine-rich peptides capable of binding metal(loid)s via SH-groups. Although the biosynthesis of PCs can be induced in vivo by various metal(loid)s, PCs are mainly involved in the detoxification of cadmium and arsenic (III), as well as mercury, zinc, lead, and copper ions, which have high affinities for S-containing ligands. The present review provides a comprehensive account of the recent data on PC biosynthesis, structure, and role in metal(loid) transport and sequestration in the vacuoles of plant cells. A comparative analysis of PC accumulation in hyperaccumulator plants, which accumulate metal(loid)s in their shoots, and in the excluders, which accumulate metal(loid)s in their roots, investigates the question of whether the endogenous PC concentration determines a plant's tolerance to metal(loid)s. Summarizing the available data, it can be concluded that PCs are not involved in metal(loid) hyperaccumulation machinery, though they play a key role in metal(loid) homeostasis. Unraveling the physiological role of metal(loid)-binding ligands is a fundamental problem of modern molecular biology, plant physiology, ionomics, and toxicology, and is important for the development of technologies used in phytoremediation, biofortification, and phytomining.

Elemental mercury accumulation in Eichhornia crassipes (Mart.) Solms-Laubach

The aquatic macrophyte Eichhornia crassipes has great potential for the control of Hg pollution in the environment. The aim of this study was to investigate the capability of E. crassipes to accumulate elemental mercury (Hg⁰). The plants were exposed for 30 days to 5, 10, 20, 40, and 80 mg of Hg⁰ in a 1-L Hoagland medium with the Hg⁰ settled at the bottom of the flask. The roots of the plants did not touch the mercury during the treatment. After exposure, the total Hg (T-Hg) concentrations in the roots, leaves, and stems were measured using a direct mercury (Hg) analyzer. The highest concentrations were found at 80 mg Hg⁰ treatment in the roots, leaves, and stems, in that order. The translocation factor indicated a poor capability of Hg to translocate from the roots to the shoots. The relative growth and the root-length inhibition measurements showed that the differences between Hg⁰ treatments were not significant. In addition, the treatments negatively affected the chlorophyll concentration. The carotenoid content was found to be significantly different at 20 and 40 mg of Hg⁰ in 1 L. Regarding the carbonyl index in root proteins, significant differences compared to control were found at the highest Hg treatment. Based on these results, it was shown that E. crassipes is able to take up elemental Hg from Hoagland medium. However, the Hg⁰ treatments did not show a strong stress-response activation mechanism in the evaluated plant tissues.

Molecular mechanism of Cu metal and drought stress resistance triggered by Porostereum spadiceum AGH786 in Solanum lycopersicum L

Rapid industrialization and global warming have threatened the plants with multiple abiotic stresses, such as heavy metals and drought stress. For crop cultivation, the conventional approach of cleaning the soils by excavation is very costly and not feasible for large scale. Establishing toxin-free and drought-resistant crops is a major challenge in the environment under natural and anthropogenic pressure. In the past decades, copper contamination of agricultural land has become an emerging concern. For dry land reclamation, several new strategies, including bioremediation (phytoremediation and microbial remediation), have been used. Owing to the potential of Cu hyperaccumulators, the current project aims to enhance the drought tolerance and the phytoremediation potential of Solanum lycopersicum L. with the inoculation of copper and 12% polyethylene glycol (PEG)-induced drought stress-tolerant endophytic fungus Porostereum spadiceum AGH786 under the combined stress of copper heavy metal and PEG-induced drought stress. When S. lycopersicum L. was watered with individual stress of copper (Cu) concentration (400 ppm) in the form of copper sulfate (CuSO₄.5H₂O), 12% PEG-induced drought stress and the combined stress of both negatively affected the growth attributes, hormonal, metabolic, and antioxidant potential, compared with control. However, the multistress-resistant AGH786 endophytic fungus ameliorated the multistress tolerance response in S. lycopersicum L. by positively affecting the growth attributes, hormonal, metabolic, and antioxidant potential, and by restricting the root-to-shoot translocation of Cu and inducing its sequestration in the root tissues of affected plants. AGH786-associated plants exhibited a reduction in the severity of copper (Cu) and drought stress, with higher levels of SlCOPT (Cu transporters) and SlMT (metallothionine) gene expressions in root and shoot tissues, indicating that AGH786 contributed to resistance to copper metal toxicity and drought stress in the host S. lycopersicum L.

Exogenously Applied Sodium Nitroprusside Mitigates Lead Toxicity in Rice by Regulating Antioxidants and Metal Stress-Related Transcripts

Sustainable agriculture is increasingly being put in danger by environmental contamination with dangerous heavy metals (HMs), especially lead (Pb). Plants have developed a sophisticated mechanism for nitric oxide (NO) production and signaling to regulate hazardous effects of abiotic factors, including HMs. In the current study, we investigated the role of exogenously applied sodium nitroprusside (SNP, a nitric oxide (NO) donor) in ameliorating the toxic effects of lead (Pb) on rice. For this purpose, plants were subjected to 1.2 mM Pb alone and in combination with 100 µM SNP. We found that under 1.2 mM Pb stress conditions, the accumulation of oxidative stress markers, including hydrogen peroxide (H₂O₂) (37%), superoxide anion (O₂⁻) (28%), malondialdehyde (MDA) (33%), and electrolyte leakage (EL) (34%), was significantly reduced via the application of 100 µM SNP. On the other hand, under the said stress of Pb, the activity of the reactive oxygen species (ROS) scavengers such as polyphenol oxidase (PPO) (60%), peroxidase (POD) (28%), catalase (CAT) (26%), superoxide dismutase (SOD) (42%), and ascorbate peroxidase (APX) (58%) was significantly increased via the application of 100 µM SNP. In addition, the application of 100 µM SNP rescued agronomic traits such as plant height (24%), number of tillers per plant (40%), and visible green pigments (44%) when the plants were exposed to 1.2 mM Pb stress. Furthermore, after exposure to 1.2 mM Pb stress, the expression of the heavy-metal stress-related genes OsPCS1 (44%), OsPCS2 (74%), OsMTP1 (83%), OsMTP5 (53%), OsMT-I-1a (31%), and OsMT-I-1b (24%) was significantly enhanced via the application of 100 µM SNP. Overall, our research evaluates that exogenously applied 100 mM SNP protects rice plants from the oxidative damage brought on by 1.2 mM Pb stress by lowering oxidative stress markers, enhancing the antioxidant system and the transcript accumulation of HMs stress-related genes.

Transcription factor ANAC004 enhances Cd tolerance in Arabidopsis thaliana by regulating cell wall fixation, translocation and vacuolar detoxification of Cd, ABA accumulation and antioxidant capacity

Cadmium (Cd) is toxic to plants, which have evolved multiple strategies to cope with Cd stress. In this study, we identified a nucleus-localized NAC-type transcription factor, ANAC004, which is induced by Cd and involved in regulating Cd resistance in Arabidopsis. First, anac004 mutants exhibited Cd sensitive phenotype and accumulated more Cd (12-23% higher than wild type in roots and shoots); plants overexpressing ANAC004 showed the opposite phenotype and with lower Cd accumulation. Second, ANAC004 enhanced Cd fixation in cell wall hemicellulose, thus reducing Cd²⁺ influx into root cells. Third, ANAC004 was involved in the process of vacuolar Cd compartmentalization by regulating the genes associated with Cd detoxification (PCS1/2, NAS4, ABCC1/2/3, MTP1/3, IREG2 and NRAMP3/4). Fourth, ANAC004 reduced root-to-shoot Cd translocation through down-regulated Cd translocation-related genes (HMA2 and HMA4). Finally, the expression of genes related to ABA synthesis (AAO3, MCSU, and NCED3) and the activities of antioxidant enzymes (SOD, POD and CAT) were all reduced in anac004 mutants, leading to reduced levels of endogenous ABA and increased accumulation of reactive oxygen species (O₂^.- and H₂O₂) and MDA, which ultimately weakened resistance to Cd. Our results suggest that ANAC004 decreases Cd accumulation in Arabidopsis through enhancing cell wall Cd immobilization, increasing vacuolar Cd detoxification, and inhibiting Cd translocation, thus improving Cd resistance, processes that might be mediated by ABA signaling and antioxidant defense systems.

Mesophyll specific expression of a bacterial mercury transporter-based vacuolar sequestration machinery sufficiently enhances mercury tolerance of Arabidopsis

We aimed to efficiently enhance plant Hg(II) tolerance by the transgenic approach utilizing a bacterial mercury transporter MerC, an Arabidopsis mesophyll specific promoter pRBCS1A, and a vacuolar membrane targeting syntaxin AtVAM3/SYP22. We generated two independent homozygous Arabidopsis pRBCS1A-TCV lines expressing mT-Sapphire-MerC-AtVAM3 under the control of pRBCS1A. Quantitative RT-PCR showed that the transgene was expressed specifically in shoots of pRBCS1A-TCV lines. Confocal analyses further demonstrated the leaf mesophyll specific expression of mT-Sapphire-MerC-AtVAM3. Confocal observation of the protoplast derived from the F1 plants of the pRBCS1A-TCV line and the tonoplast marker line p35S-GFP-δTIP showed the tonoplast colocalization of mT-Sapphire-MerC-AtVAM3 and GFP-δTIP. These results clearly demonstrated that mT-Sapphire-MerC-AtVAM3 expression in Arabidopsis is spatially regulated as designed at the transcript and the membrane trafficking levels. We then examined the Hg(II) tolerance of the pRBCS1A-TCV lines as well as the p35S-driven MerC-AtVAM3 expressing line p35S-CV under the various Hg(II) stress conditions. Short-term (12 d) Hg(II) treatment indicated the enhanced Hg(II) tolerance of both pRBCS1A-TCV and p35S-CV lines. The longer (3 weeks) Hg(II) treatment highlighted the better shoot growth of the transgenic plants compared to the wild-type Col-0 and the pRBCS1A-TCV lines were more tolerant to Hg(II) stress than the p35S-CV line. These results suggest that mesophyll-specific expression of MerC-AtVAM3 is sufficient or even better to enhance the Arabidopsis Hg(II) tolerance. The Hg accumulation in roots and shoots did not differ between the wild-type Col-0 and the MerC-AtVAM3 expressing plants, suggesting that the boosted Hg(II) tolerance of the transgenic lines would be attributed to vacuolar Hg-sequestration by the tonoplast-localized MerC. Further perspectives of the MerC-based plant engineering are also discussed.

Dimethylmonothioarsenate Is Highly Toxic for Plants and Readily Translocated to Shoots

Arsenic is one of the most relevant environmental pollutants and human health threats. Several arsenic species occur in soil pore waters. Recently, it was discovered that these include inorganic and organic thioarsenates. Among the latter, dimethylmonothioarsenate (DMMTA) is of particular concern because in mammalian cells, its toxicity was found to exceed even that of arsenite. We investigated DMMTA toxicity for plants in experiments with Arabidopsis thaliana and indeed observed stronger growth inhibition than with arsenite. DMMTA caused a specific, localized deformation of root epidermal cells. Toxicity mechanisms apparently differ from those of arsenite since no accumulation of reactive oxygen species was observed in DMMTA-exposed root tips. Also, there was no contribution of the phytochelatin pathway to the DMMTA detoxification as indicated by exposure experiments with respective mutants and thiol profiling. RNA-seq analysis found strong transcriptome changes dominated by stress-responsive genes. DMMTA was taken up more efficiently than the methylated oxyarsenate dimethylarsenate and highly mobile within plants as revealed by speciation analysis. Shoots showed clear indications of DMMTA toxicity such as anthocyanin accumulation and a decrease in chlorophyll and carotenoid levels. The toxicity and efficient translocation of DMMTA within plants raise important food safety issues.

Phytochelatin-mediated metal detoxification pathway is crucial for an organomercurial phenylmercury tolerance in Arabidopsis

An organomercurial phenylmercury activates AtPCS1, an enzyme known for detoxification of inorganic metal(loid) ions in Arabidopsis and the induced metal-chelating peptides phytochelatins are essential for detoxification of phenylmercury. Small thiol-rich peptides phytochelatins (PCs) and their synthases (PCSs) are crucial for plants to mitigate the stress derived from various metal(loid) ions in their inorganic form including inorganic mercury [Hg(II)]. However, the possible roles of the PC/PCS system in organic mercury detoxification in plants remain elusive. We found that an organomercury phenylmercury (PheHg) induced PC synthesis in Arabidopsis thaliana plants as Hg(II), whereas methylmercury did not. The analyses of AtPCS1 mutant plants and in vitro assays using the AtPCS1-recombinant protein demonstrated that AtPCS1, the major PCS in A. thaliana, was responsible for the PheHg-responsive PC synthesis. AtPCS1 mutants cad1-3 and cad1-6, and the double mutant of PC-metal(loid) complex transporters AtABCC1 and AtABCC2 showed enhanced sensitivity to PheHg as well as to Hg(II). The hypersensitivity of cad1-3 to PheHg stress was complemented by the own-promoter-driven expression of AtPCS1-GFP. The confocal microscopy of the complementation lines showed that the AtPCS1-GFP was preferentially expressed in epidermal cells of the mature and elongation zones, and the outer-most layer of the lateral root cap cells in the meristematic zone. Moreover, in vitro PC-metal binding assay demonstrated that binding affinity between PC and PheHg was comparable to Hg(II). However, plant ionomic profiles, as well as root morphology under PheHg and Hg(II) stress, were divergent. These results suggest that PheHg phytotoxicity is different from Hg(II), but AtPCS1-mediated PC synthesis, complex formation, and vacuolar sequestration by AtABCC1 and AtABCC2 are similarly functional for both PheHg and Hg(II) detoxification in root surficial cell types.

Heterologous Expression of the Phytochelatin Synthase CaPCS2 from Chlamydomonas acidophila and Its Effect on Different Stress Factors in Escherichia coli

Phytochelatins (PCs) are cysteine-rich small peptides, enzymatically synthesized from reduced glutathione (GSH) by cytosolic enzyme phytochelatin synthase (PCS). The open reading frame (ORF) of the phytochelatin synthase CaPCS2 gene from the microalgae Chlamydomonas acidophila was heterologously expressed in Escherichia coli strain DH5α, to analyze its role in protection against various abiotic agents that cause cellular stress. The transformed E. coli strain showed increased tolerance to exposure to different heavy metals (HMs) and arsenic (As), as well as to acidic pH and exposure to UVB, salt, or perchlorate. In addition to metal detoxification activity, new functions have also been reported for PCS and PCs. According to the results obtained in this work, the heterologous expression of CaPCS2 in E. coli provides protection against oxidative stress produced by metals and exposure to different ROS-inducing agents. However, the function of this PCS is not related to HM bioaccumulation.

Cytosolic phosphofructokinases are important for sugar homeostasis in leaves of Arabidopsis thaliana

BACKGROUND AND AIMS

ATP-dependent phosphofructokinases (PFKs) catalyse phosphorylation of the carbon-1 position of fructose-6-phosphate, to form fructose-1,6-bisphosphate. In the cytosol, this is considered a key step in channelling carbon into glycolysis. Arabidopsis thaliana has seven genes encoding PFK isoforms, two chloroplastic and five cytosolic. This study focuses on the four major cytosolic isoforms of PFK in vegetative tissues of A. thaliana.

METHODS

We isolated homozygous knockout individual mutants (pfk1, pfk3, pfk6 and pfk7) and two double mutants (pfk1/7 and pfk3/6), and characterized their growth and metabolic phenotypes.

KEY RESULTS

In contrast to single mutants and the double mutant pfk3/6 for the hypoxia-responsive isoforms, the double mutant pfk1/7 had reduced PFK activity and showed a clear visual and metabolic phenotype with reduced shoot growth, early flowering and elevated hexose levels. This mutant also has an altered ratio of short/long aliphatic glucosinolates and an altered root-shoot distribution. Surprisingly, this mutant does not show any major changes in short-term carbon flux and in levels of hexose-phosphates.

CONCLUSIONS

We conclude that the two isoforms PFK1 and PFK7 are important for sugar homeostasis in leaf metabolism and apparently in source-sink relationships in A. thaliana, while PFK3 and PFK6 only play a minor role under normal growth conditions.

A transcriptomic view of cadmium retention in roots of cadmium-safe rice line (Oryza sativa L.)

A better understanding of the mechanisms controlling cadmium (Cd) accumulation in rice will benefit the development of strategies to minimize Cd accumulation in grains. A Cd-safe rice line designated D62B accumulated less than 0.2 mg Cd kg^-1 in brown rice due to its strong capacity for Cd retention in roots. Here transcriptomic was used to clarify the underlying mechanisms of Cd response in roots of D62B compared with a high Cd-accumulating line (Wujin4B). There were 777, 1058 differentially expressed genes (DEGs) in D62B and Wujin4B, respectively, when exposed to Cd. The functions of DEGs were clearly line-specific. Cell wall biosynthesis responded more intensively to Cd stress in D62B, facilitating Cd restriction. Meanwhile, more glutathione (GSH) and phytochelatins synthesized in D62B with the upregulation of sulphur and GSH metabolism. Besides, membrane proteins played critical roles in Cd response in D62B, whereas 18 terms involved in regulation were enriched in Wujin4B. Exogenous GSH further induced the expression of genes related to GSH metabolism and cell wall biosynthesis, leading to the retention of more Cd. Great responsiveness of cell wall biosynthesis and GSH metabolism could be considered the most important specific mechanisms for Cd retention in the roots of Cd-safe rice line.

The critical role of the shoot base in inhibiting cadmium transport from root to shoot in a cadmium-safe rice line (Oryza sativa L.)

Cadmium (Cd) is harmful to rice and human, thus screening and understanding the mechanism of Cd-safe rice lines, which accumulate little Cd in brown rice, is necessary. D62B was screened as a Cd-safe rice line with low Cd translocation from roots to shoots, and there must be a switch restricting Cd transport from roots to shoots. Here we found that shoot base played the role as switch. Cd concentration in the shoot base of D62B was 1.57 times higher compared with a high Cd-accumulating rice line (Wujin4B) and lower Cd translocation under Cd stress. Glutathione (GSH) and phytochelatins (PCs) were important in this process. GSH and PCs concentrations in the shoot bases of D62B were 1.01- 1.83 times higher than Wujin4B as well as the glutathione S-transferase (GST) and phytochelatin synthase (PCS) concentrations, keeping in consistent with up-regulation of the genes OsGST and OsPCS1. PCs synthesis was further promoted by exogenous GSH. Our results prove the role of shoot bases as switch for restricting Cd transport in D62B due to its great potential for GSH and PCs biosynthesis, and thereby Cd chelation. This could be considered a key mechanism for low Cd accumulation in brown rice of the Cd-safe rice line.

The Phosphofructokinase Isoform AtPFK5 Is a Novel Target of Plastidic Thioredoxin-f-Dependent Redox Regulation

The chloroplast primary metabolism is of central importance for plant growth and performance. Therefore, it is tightly regulated in order to adequately respond to multiple environmental conditions. A major fluctuation that plants experience each day is the change between day and night, i.e., the change between assimilation and dissimilation. Among other mechanisms, thioredoxin-mediated redox regulation is an important component of the regulation of plastid-localized metabolic enzymes. While assimilatory processes such as the Calvin–Benson cycle are activated under illumination, i.e., under reducing conditions, carbohydrate degradation is switched off during the day. Previous analyses have identified enzymes of the oxidative pentose phosphate pathway to be inactivated by reduction through thioredoxins. In this work, we present evidence that an enzyme of the plastidic glycolysis, the phosphofructokinase isoform AtPFK5, is also inactivated through reduction by thioredoxins, namely by thioredoxin-f. With the help of chemical oxidation, mutant analyses and further experiments, the highly conserved motif CXDXXC in AtPFK5 was identified as the target sequence for this regulatory mechanism. However, knocking out this isoform in plants had only very mild effects on plant growth and performance, indicating that the complex primary metabolism in plants can overcome a lack in AtPFK5 activity.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Characterization of a phytochelatin synthase gene from Ipomoea pes-caprae involved in cadmium tolerance and accumulation in yeast and plants

Phytochelatin synthases (PCSs) play a crucial part in heavy metal tolerance in plants via the synthesis of phytochelatins (PCs), which can chelate heavy metals (HMs) in the vacuole and decrease cell damage. Plant PCSs are commonly designated as key genes for phytoremediation. In this study, we identified a PCS gene (IpPCS1) from Ipomoea pes-caprae and investigated its role in regulating cadmium (Cd) tolerance and accumulation. The expression of a truncated IpPCS1t in yeast could complement the Cd-sensitive phenotype of the ycf1Δ mutant strain, as well as improve the Cd tolerance of the wild-type yeast strain, while promoting Cd accumulation in the yeast cells. The expression of IpPCS1 was induced in I. pes-caprae plants under Cd treatment. Compared with IpPCS1, the lack of a C-terminal in IpPCS1t did not affect its Cd tolerance, but might restrict the zinc (Zn) detoxification in yeast. The overexpression of IpPCS1t in Arabidopsis could improve the Cd tolerance slightly and had little impact on Cd accumulation in transgenic plant. Our results indicated that IpPCS1 has certain potential application value in Cd tolerance and detoxification, therefore provides a useful genetic resource for enhancing Cd tolerance and improving the Cd phytoremediation capacity of plants or organisms. In addition, our research is the first time to discover a new possible Cd activation site in the C-terminal of IpPCS1.

MYB4 transcription factor, a member of R2R3-subfamily of MYB domain protein, regulates cadmium tolerance via enhanced protection against oxidative damage and increases expression of PCS1 and MT1C in Arabidopsis

Here, we describe functional characterization of Arabidopsis thaliana MYB4 transcription factor, a member of R2R3-subfamily of MYB domain protein, in the regulation of Cd-stress tolerance in Arabidopsis. Transgenic Arabidopsis plants overexpressing MYB4 showed appreciable Cd tolerance than wild-type plants, while MYB4 loss of function mutant lines (atmyb4) showed increased sensitivity to Cd-stress. MYB4 overexpression lines showed strong activation of anti-oxidant defense components and increased Cd accumulation than wild-type and atmyb4 mutant lines under Cd-stress. MYB4 overexpression resulted in the coordinated activation of the expression of phytochelatin (PC) synthesis related genes and specifically enhanced the transcript abundance of phytochelatin synthase 1 (PCS1) and metallothionein 1C (MT1C) genes under Cd-stress. In contrast, atmyb4 mutant lines showed reduced Cd accumulation and compromised expression of PC-synthesis related genes. Electrophoretic gel mobility shift assays have demonstrated specific binding activity of recombinant AtMYB4 to the putative MYB4-binding motifs ACCAACCAA and GGTAGGT identified in the promoters of PCS1 and MT1C genes, respectively. Further analyses have revealed that MYB4 binds directly to PCS1 and MT1C promoters in vivo and positively regulates their transcriptional expression, suggesting that PCS1 and MT1C are the key targets of MYB4. Overall, our results have provided evidence that MYB4 regulates Cd-tolerance via the coordinated activity of improved anti-oxidant defense system and through the enhanced expression of PCS1 and MT1C under Cd-stress in Arabidopsis.

Selection of Agar Reagents for Medium Solidification Is a Critical Factor for Metal(loid) Sensitivity and Ionomic Profiles of Arabidopsis thaliana

For researchers in the plant metal field, the agar reagent used for the solid plate medium is a problematic factor because application of different agar types and even a different lot of the same agar type can mask the plant metal-related phenotypes and impair the reproducibility. In this study, we systematically assessed effects of different agar reagents on metal(loid) sensitivity and element accumulation of the Arabidopsis metal sensitive mutants. Three established mutants (cad1-3, cad1-6, and abcc1/2), and three different types of purified agar reagents (Type A, Type E, and Nacalai) with two independent batches for each reagent were subjected to the analyses. First, we found that element concentrations in the agar reagents largely varied among the agar types. Then the effects of agar reagents on the mutant metal(loid)-sensitivity were examined under As(III), Hg(II), Cd(II), and excess Zn(II) conditions. A significant variation of the mutant metal(loid)-sensitivity was observed among the different agar plates but the variation depended on the combination of metal(loid) stress and agar reagents. Briefly, the type-dependent variation was more evident under As(III) and Hg(II) than Cd(II) or excess Zn(II) conditions. A lot-dependent variation was also observed for Type A and Type E but not for Nacalai: hypersensitive phenotypes of cad1-3, cad1-6, and abcc1/2 under As(III) or Hg(II) treatments were diminished when different batches of the Type A or Type E agar types were used. We also found a significant variation of As and Hg accumulation in the wild-type and cad1-3. Plant As and Hg concentrations were remarkably higher and the difference between the genotypes was more evident when grown with Type A agar plates. We finally analyzed ionomic profiles in the plants exposed to As(III) stress. Agar-type specific ionomic changes in cad1-3 were more observed with the Type A plates than with the Nacalai plates. The presented results overall suggest that suitability of agar reagents for metal(loid)-related phenotyping depends on the experimental design, and an inappropriate selection of agar reagents can mask even very clear phenotypes of the established mutant like cad1-3. We also discuss perspectives on the agar problem in the plant metal study.

Transcriptome analysis reveals insight into molecular hydrogen-induced cadmium tolerance in alfalfa: the prominent role of sulfur and (homo)glutathione metabolism

BACKGROUND

Hydrogen gas (H2) is hypothesised to play a role in plants that are coping with stresses by regulating signal transduction and gene expression. Although the beneficial role of H2 in plant tolerance to cadmium (Cd) has been investigated previously, the corresponding mechanism has not been elucidated. In this report, the transcriptomes of alfalfa seedling roots under Cd and/or hydrogen-rich water (HRW) treatment were first analysed. Then, the sulfur metabolism pathways were focused on and further investigated by pharmacological and genetic approaches.

RESULTS

A total of 1968 differentially expressed genes (DEGs) in alfalfa seedling roots under Cd and/or HRW treatment were identified by RNA-Seq. The DEGs were classified into many clusters, including glutathione (GSH) metabolism, oxidative stress, and ATP-binding cassette (ABC) transporters. The results validated by RT-qPCR showed that the levels of relevant genes involved in sulfur metabolism were enhanced by HRW under Cd treatment, especially the genes involved in (homo)glutathione metabolism. Additional experiments carried out with a glutathione synthesis inhibitor and Arabidopsis thaliana cad2-1 mutant plants suggested the prominent role of glutathione in HRW-induced Cd tolerance. These results were in accordance with the effects of HRW on the contents of (homo)glutathione and (homo)phytochelatins and in alleviating oxidative stress under Cd stress. In addition, the HRW-induced alleviation of Cd toxicity might also be caused by a decrease in available Cd in seedling roots, achieved through ABC transporter-mediated secretion.

CONCLUSIONS

Taken together, the results of our study indicate that H2 regulated the expression of genes relevant to sulfur and glutathione metabolism and enhanced glutathione metabolism which resulted in Cd tolerance by activating antioxidation and Cd chelation. These results may help to elucidate the mechanism governing H2-induced Cd tolerance in alfalfa.

Negative regulation of cadmium tolerance in Arabidopsis by MMDH2

MMDH2 gene negatively regulates Cd tolerance by modulating reactive oxygen species (ROS) levels and the ROS-mediated signaling, thus, affecting the expression of PDR8. The molecular mechanism by which plants respond to stress caused by cadmium (Cd), one of the most toxic heavy metals to plants, is not well understood. Here, we show that MMDH2, a gene encoding mitochondrial malate dehydrogenase, is involved in Cd stress tolerance in Arabidopsis. The expression of MMDH2 was repressed by Cd stress. The mmdh2 knockdown mutants showed enhanced Cd tolerance, while the MMDH2-overexpressing lines were sensitive to Cd. Under normal and Cd stress conditions, lower H₂O₂ levels were detected in mmdh2 mutant plants than in wild-type plants. In contrast, higher H₂O₂ levels were found in MMDH2-overexpressing lines, and they were negatively correlated with malondialdehyde levels. In addition, the expression of the PDR8, a gene encoding a Cd efflux pump, increased and decreased in the mmdh2 mutant and MMDH2-overexpressing lines, in association with lower and higher Cd concentrations, respectively. These results suggest that the MMDH2 gene negatively regulates Cd tolerance by modulating reactive oxygen species (ROS) levels and the ROS-mediated signaling, thus, affecting the expression of PDR8.

Evolution and functional differentiation of recently diverged phytochelatin synthase genes from Arundo donax L

Phytochelatin synthases (PCSs) play pivotal roles in the detoxification of heavy metals and metalloids in plants; however, little information on the evolution of recently duplicated PCS genes in plant species is available. Here we characterize the evolution and functional differentiation of three PCS genes from the giant reed (Arundo donax L.), a biomass/bioenergy crop with remarkable resistance to cadmium and other heavy metals. Phylogenetic reconstruction with PCS genes from fully sequenced monocotyledonous genomes indicated that the three A. donax PCSs, namely AdPCS1-3, form a monophyletic clade. The AdPCS1-3 genes were expressed at low levels in many A. donax organs and displayed different levels of cadmium-responsive expression in roots. Overexpression of AdPCS1-3 in Arabidopsis thaliana and yeast reproduced the phenotype of functional PCS genes. Mass spectrometry analyses confirmed that AdPCS1-3 are all functional enzymes, but with significant differences in the amount of the phytochelatins synthesized. Moreover, heterogeneous evolutionary rates characterized the AdPCS1-3 genes, indicative of relaxed natural selection. These results highlight the elevated functional differentiation of A. donax PCS genes from both a transcriptional and an enzymatic point of view, providing evidence of the high evolvability of PCS genes and of plant responsiveness to heavy metal stress.

Metal ligands in micronutrient acquisition and homeostasis

Acquisition and homeostasis of micronutrients such as iron (Fe) and zinc (Zn) pose specific challenges. Poor solubility and high reactivity require controlled synthesis and supply of ligands to complex these metals extracellularly and intracellularly. Cytosolic labile pools represent only a minute fraction of the total cellular content. Several low-molecular-weight ligands are known in plants, including sulfur ligands (cysteine and peptides), nitrogen/oxygen ligands (S-adenosyl-l-methionine-derived molecules and histidine), and oxygen ligands (phenolics and organic acids). Some ligands are secreted into the extracellular space and influence the phytoavailability of metal ions. A second principal function is the intracellular buffering of micronutrients as well as the facilitation of long-distance transport in xylem and phloem. Furthermore, low-molecular-weight ligands are involved in the storage of metals, predominantly in vacuoles. A detailed molecular understanding is hampered by technical limitations, in particular the difficulty to detect and quantify cellular metal-ligand complexes. More, but still too little, is known about ligand synthesis and the transport across membranes, either with or without a complexed metal. Metal ligands have an immediate impact on human well-being. Engineering metal ligand synthesis and distribution in crops has tremendous potential to improve the nutritional quality of food and to tackle major human health risks.

Study of the contamination rate and change in growth features of lettuce (Lactuca sativa Linn.) in response to cadmium and a survey of its phytochelatin synthase gene

Crops can become contaminated when grown in soils containing heavy metals. Cadmium is a heavy metal that poses a significant health risk to humans. The purpose of this study was to evaluate the effect of cadmium on lettuce (Lactuca sativa Linn) and the contamination risk of lettuce grown in cadmium environments. The results showed that photosynthesis and growth parameters were significantly affected by cadmium. Lettuce has the ability to absorb large amounts of cadmium from the contaminated environment and so is a cadmium hyperaccumulator plant. The study showed that approximately 35% of the total absorbed cadmium is transmitted to aerial and edible parts of lettuce. This study was undertaken as lettuce has the ability to absorb and accumulate high levels of cadmium. There are however are no reports on the PCS gene and the potential for high cadmium accumulation in lettuce. The bioinformatics study revealed that lettuce has two phytochelatin synthase genes that produce 6 PCSs through splicing leading to the ability of lettuce to store high levels of cadmium. These six sequences although different in length have high similarity. Sequence structure, cellular location, three-dimensional structure, phylogeny and a comparison of their catalytic power were evaluated. The high accumulation of cadmium in lettuce and the presence of several PCSs contribute to the accumulation of cadmium in aerial tissues. The cultivation of lettuce in contaminated environments led us to evaluate suspected farms for the presence of cadmium in produce. Lettuce grown in industrial environments contaminated with cadmium can pose a serious threat to human health.

Overexpression of phytochelatin synthase AtPCS2 enhances salt tolerance in Arabidopsis thaliana

Phytochelatin synthase (PCS) is an enzyme that synthesizes phytochelatins, which are metal-binding peptides. Despite the important role of PCS in heavy metal detoxification or tolerance, the functional role of PCS with respect to other abiotic stresses remains largely unknown. In this study, we determined the function of Arabidopsis thaliana phytochelatin synthase 2 (AtPCS2) in the salt stress response. Expression of AtPCS2 was significantly increased in response to 100 and 200 mM NaCl treatment. AtPCS2-overexpressing transgenic Arabidopsis and tobacco plants displayed increased seed germination rates and seedling growth under high salt stress. In addition, transgenic Arabidopsis subjected to salt stress exhibited enhanced proline accumulation and reduced Na+/K+ ratios compared to wild type plants. Furthermore, decreased levels of hydrogen peroxide (H2O2) and lipid peroxidation were observed in transgenic Arabidopsis compared to wild type specimens. Salt stress greatly reduced transcript levels of CuSOD2, FeSOD2, CAT2, and GR2 in wild type but not transgenic Arabidopsis. Notably, levels of CAT3 in transgenic Arabidopsis were markedly increased upon salt stress, suggesting that low accumulation of H2O2 in transgenic Arabidopsis is partially achieved through induction of CAT. Collectively, these results suggest that AtPCS2 plays a positive role in seed germination and seedling growth under salt stress through a series of indirect effects that are likely involved in H2O2 scavenging, regulation of osmotic adjustment and ion homeostasis.

SCARECROW promoter-driven expression of a bacterial mercury transporter MerC in root endodermal cells enhances mercury accumulation in Arabidopsis shoots

Mercury accumulation in Arabidopsis shoots is accelerated by endodermis specific expression of fusion proteins of a bacterial mercury transporter MerC and a plant SNARE SYP121 under control of SCARECROW promoter. We previously demonstrated that the CaMV 35S RNA promoter (p35S)-driven ubiquitous expression of a bacterial mercury transporter MerC, fused with SYP121, an Arabidopsis SNARE protein increases mercury accumulation of Arabidopsis. To establish an improved fine-tuned mercury transport system in plants for phytoremediation, the present study generated and characterized transgenic Arabidopsis plants expressing MerC-SYP121 specifically in the root endodermis, which is a crucial cell type for root element uptake. We generated four independent transgenic Arabidopsis lines expressing a transgene encoding mCherry-MerC-SYP121 under the control of the endodermis-specific SCARECROW promoter (hereafter pSCR lines). Quantitative real-time PCR analysis showed that expression levels of the transgene in roots of the pSCR lines were 3-23% of the p35S driven-overexpressing line. Confocal microscopy analysis showed that mCherry-MerC-SYP121 was dominantly expressed in the endodermis of the meristematic zone as well as in the mature zone of the pSCR roots. Mercury accumulation in shoots of the pSCR lines exposed to inorganic mercury was overall higher than the wild-type and comparable to the p35S over-expressing line. These results suggest that endodermis-specific expression of the MerC-SYP121 fusion proteins in plant roots sufficiently enhances mercury uptake and accumulation into shoots, which would be an ideal phenotype for phytoremediation of mercury-contaminated environments.

Ectopic expression of a bacterial mercury transporter MerC in root epidermis for efficient mercury accumulation in shoots of Arabidopsis plants

For mercury phytoextraction, we previously demonstrated in Arabidopsis thaliana that a constitutive and ubiquitous promoter-driven expression of a bacterial mercury transporter MerC fused with SYP121, a plant SNARE for plasma membrane protein trafficking increases plant mercury accumulation. To advance regulation of ectopic expression of the bacterial transporter in the plant system, the present study examined whether merC-SYP121 expression driven by a root epidermis specific promoter (pEpi) is sufficient to enhance mercury accumulation in plant tissues. We generated five independent transgenic Arabidopsis plant lines (hereafter pEpi lines) expressing a transgene encoding MerC-SYP121 N-terminally tagged with a fluorescent protein mTRQ2 under the control of pEpi, a root epidermal promoter. Confocal microscopy analysis of the pEpi lines showed that mTRQ2-MerC-SYP121 was preferentially expressed in lateral root cap in the root meristematic zone and epidermal cells in the elongation zone of the roots. Mercury accumulation in shoots of the pEpi lines exposed to inorganic mercury was overall higher than the wild-type and comparable to the over-expressing line. The results suggest that cell-type specific expression of the bacterial transporter MerC in plant roots sufficiently enhances mercury accumulation in shoots, which could be a useful phenotype for improving efficiency of mercury phytoremediation.

WRKY12 represses GSH1 expression to negatively regulate cadmium tolerance in Arabidopsis

The WRKY transcription factor WRKY12 negatively regulates Cd tolerance in Arabidopsis via the glutathione-dependent phytochelatin synthesis pathway by directly targeting GSH1 and indirectly repressing phytochelatin synthesis-related gene expression. Cadmium (Cd) is a widespread pollutant toxic to plants. The glutathione (GSH)-dependent phytochelatin (PC) synthesis pathway plays key roles in Cd detoxification. However, its regulatory mechanism remains largely unknown. Here, we showed a previously unknown function of the WRKY transcription factor WRKY12 in the regulation of Cd tolerance by repressing the expression of PC synthesis-related genes. The expression of WRKY12 was inhibited by Cd stress. Enhanced Cd tolerance was observed in the WRKY12 loss-of-function mutants, whereas increased Cd sensitivity was found in the WRKY12-overexpressing plants. Overexpression and loss-of-function of WRKY12 were associated respectively with increased and decreased Cd accumulation by repressing or releasing the expression of the genes involved in the PC synthesis pathway. Transient expression assay showed that WRKY12 repressed the expression of GSH1, GSH2, PCS1, and PCS2. Further analysis indicated that WRKY12 could directly bind to the W-box of the promoter in GSH1 but not in GSH2, PCS1, and PCS2 in vivo. Together, our results suggest that WRKY12 directly targets GSH1 and indirectly represses PC synthesis-related gene expression to negatively regulate Cd accumulation and tolerance in Arabidopsis.

Wheat expansin gene TaEXPA2 is involved in conferring plant tolerance to Cd toxicity

Cadmium (Cd) is a severe and toxic heavy metal pollutant that affects plant growth and development. In this study, we found that the expression of an expansin gene, TaEXPA2, was upregulated in wheat leaves under CdCl2 toxicity. We characterized the involvement of TaEXPA2 in conferring Cd tolerance. Tobacco plants overexpressing TaEXPA2 showed higher germination rate, root elongation, and biomass accumulation compared to the wild-type (WT) plants upon CdCl2 treatment. The improved photosynthetic parameters and lesser cellular damage in transgenic plants exposed to Cd compared to that in the WT plants suggest that TaEXPA2 overexpression improves Cd tolerance in plants. Furthermore, we noticed that Cd was efficiently effluxed out of the cytoplasm in the transgenic plants owing to the enhanced activities of H+-ATPase, V-ATPase, and PPase, which helped in conferring Cd tolerance. Moreover, Cd concentration and ROS accumulation were lower in the transgenic plants than in WT plants as a consequence of enhanced antioxidant enzyme activities in the former. In addition, atexpa2, an Arabidopsis mutant, exhibited lower biomass and shorter primary root compared to its WT under Cd toxicity; however, the phenotype was recovered upon expression of TaEXPA2 in these mutants. Our results demonstrate that TaEXPA2 confers tolerance to Cd toxicity. The changed absorption/transportation of Cd and the antioxidative capacity may be involved in the improved tolerance of the transgenic plants with overexpression of TaEXPA2 to CdCl2 toxicity.

Genome-Wide Association Study of Cadmium Accumulation at the Seedling Stage in Rapeseed (Brassica napus L.)

Cadmium is a potentially toxic heavy metal to human health. Rapeseed (Brassica napus L.), a vegetable and oilseed crop, might also be a Cd hyperaccumulator, but there is little information on this trait in rapeseed. We evaluated Cd accumulation in different oilseed accessions and employed a genome-wide association study to identify quantitative trait loci (QTLs) related to Cd accumulation. A total of 419 B. napus accessions and inbred lines were genotyped with a 60K Illumina Infinium SNP array of Brassica. Wide genotypic variations in Cd concentration and translocation were found. Twenty-five QTLs integrated with 98 single-nucleotide polymorphisms (SNPs) located at 15 chromosomes were associated with Cd accumulation traits. These QTLs explained 3.49-7.57% of the phenotypic variation observed. Thirty-two candidate genes were identified in these genomic regions, and they were 0.33-497.97 kb away from the SNPs. We found orthologs of Arabidopsis thaliana located near the significant SNPs on the B. napus genome, including NRAMP6 (natural resistance-associated macrophage protein 6), IRT1 (iron-regulated transporter 1), CAD1 (cadmium-sensitive 1), and PCS2 (phytochelatin synthase 2). Of them, four candidate genes were verified by qRT-PCR, the expression levels of which were significantly higher after exposure to Cd than in the controls. Our results might facilitate the study of the genetic basis of Cd accumulation and the cloning of candidate Cd accumulation genes, which could be used to help reduce Cd levels in edible plant parts and/or create more efficient hyperaccumulators.

Identification of C-terminal Regions in Arabidopsis thaliana Phytochelatin Synthase 1 Specifically Involved in Activation by Arsenite

Phytochelatins (PCs) are major chelators of toxic elements including inorganic arsenic (As) in plant cells. Their synthesis confers tolerance and influences within-plant mobility. Previous studies had shown that various metal/metalloid ions differentially activate PC synthesis. Here we identified C-terminal parts involved in arsenite- [As(III)] dependent activation of AtPCS1, the primary Arabidopsis PC synthase. The T-DNA insertion in the AtPCS1 mutant cad1-6 causes a truncation in the C-terminal regulatory domain that differentially affects activation by cadmium (Cd) and zinc (Zn). Comparisons of cad1-6 with the AtPCS1 null mutant cad1-3 and the double mutant of tonoplast PC transporters abcc1/2 revealed As(III) hypersensitivity of cad1-6 equal to that of cad1-3. Both cad1-6 and cad1-3 showed increased As distribution to shoots compared with Col-0, whereas Zn accumulation in shoots was equally lower in cad1-6 and cad1-3. Supporting these phenotypes of cad1-6, PC accumulation in the As(III)-exposed plants were at trace level in both cad1-6 and cad1-3, suggesting that the truncated AtPCS1 of cad1-6 is defective in PCS activity in response to As(III). Analysis of a C-terminal deletion series of AtPCS1 using the PCS-deficient mutant of fission yeast suggested important regions within the C-terminal domain for As(III)-dependent PC synthesis, which were different from the regions previously suggested for Cd- or Zn-dependent activation. Interestingly, we identified a truncated variant more strongly activated than the wild-type protein. This variant could potentially be used as a tool to better restrict As mobility in plants.

Overexpression of a Functional Vicia sativa PCS1 Homolog Increases Cadmium Tolerance and Phytochelatins Synthesis in Arabidopsis

Phytochelatins (PCs) catalyzed by phytochelatin synthases (PCS) are important for the detoxification of metals in plants and other living organisms. In this study, we isolated a PCS gene (VsPCS1) from Vicia sativa and investigated its role in regulating cadmium (Cd) tolerance. Expression of VsPCS1 was induced in roots of V. sativa under Cd stress. Analysis of subcellular localization showed that VsPCS1 was localized in the cytoplasm of mesophyll protoplasts of V. sativa. Overexpression of VsPCS1 (35S::VsPCS1, in wild-type background) in Arabidopsis thaliana could complement the defects of Cd tolerance of AtPCS1-deficent mutant (atpcs1). Compared with atpcs1 mutants, 35S::VsPCS1/atpcs1 (in AtPCS1-deficent mutant background) transgenic plants significantly lowered Cd-fluorescence intensity in mesophyll cytoplasm, accompanied with enhanced Cd-fluorescence intensity in the vacuoles, demonstrating that the increased Cd tolerance may be attributed to the increased PC-based sequestration of Cd into the vacuole. Furthermore, overexpressing VsPCS1 could enhance the Cd tolerance in 35S::VsPCS1, but have no effect on Cd accumulation and distribution, showing the same level of Cd-fluorescence intensity between 35S::VsPCS1 and wild-type (WT) plants. Further analysis indicated this increased tolerance in 35S::VsPCS1 was possibly due to the increased PCs-chelated Cd in cytosol. Taken together, a functional PCS1 homolog from V. sativa was identified, which hold a strong catalyzed property for the synthesis of high-order PCs that retained Cd in the cytosol rather the vacuole. These findings enrich the original model of Cd detoxification mediated by PCS in higher plants.

A cadmium stress-responsive gene AtFC1 confers plant tolerance to cadmium toxicity

BACKGROUND

Non-essential trance metal such as cadmium (Cd) is toxic to plants. Although some plants have developed elaborate strategies to deal with absorbed Cd through multiple pathways, the regulatory mechanisms behind the Cd tolerance are not fully understood. Ferrochelatase-1 (FC1, EC4.99.1.1) is the terminal enzyme of heme biosynthesis, catalyzing insertion of ferrous ion into protoporphyrin IX. Recent studies have shown that FC1 is involved in several physiological processes. However, its biological function associated with plant abiotic stress response is poorly understood.

RESULTS

In this study, we showed that AtFC1 was transcriptionally activated by Cd exposure. AtFC1 overexpression (35S::FC1) lines accumulated more Cd and non-protein thiol compounds than wild-type, and conferred plant tolerance to Cd stress, with improved primary root elongation, biomass and chlorophyll (Chl) content, and low degree of oxidation associated with reduced H₂O₂, O^·2- and peroxides. In contrast, the AtFC1 loss of functional mutant fc1 showed sensitivity to Cd stress. Exogenous provision of heme, the product of AtFC1, partially rescued the Cd-induced toxic phenotype of fc1 mutants by improving the growth of seedlings, generation of glutathione (GSH) and phytochelatins (PCs), and GSH/PCs-synthesized gene expression (e.g. GSH1, GSH2, PCS1, and PCS2). To investigate the mechanism leading to the AtFC1 regulating Cd stress response in Arabidopsis, a transcriptome of fc1 mutant plants under Cd stress was profiled. Our data showed that disfunction of AtFC1 led to 913 genes specifically up-regulated and 522 genes down-regulated in fc1 mutants exposed to Cd. Some of the genes are involved in metal transporters, Cd-induced oxidative stress response, and detoxification.

CONCLUSION

These results indicate that AtFC1 would act as a positive regulator of plant tolerance to Cd stress. Our study will broaden our understanding of the role of FC1 in mediating plant response to Cd stress and provide a basis for further exploration of its downstream genes.

Phytochelatin Synthase has Contrasting Effects on Cadmium and Arsenic Accumulation in Rice Grains

Phytochelatin (PC) synthesis has been well demonstrated as a major metal tolerance mechanism in Arabidopsis thaliana, whereas its contribution to long-distance element transport especially in monocots remains elusive. Using rice as a cereal model, we examined physiological roles of Oryza sativa phytochelatin synthase 1 (OsPCS1) in the distribution and detoxification of arsenic (As) and cadmium (Cd), two toxic elements associated with major food safety concerns. First, we isolated four different transcript variants of OsPCS1 as well as one from OsPCS2. Quantitative real-time reverse transcription-PCR (RT-PCR) of each OsPCS transcript in rice seedlings suggested that expression of OsPCS1full, the longest OsPCS1 variant, was most abundant, followed by OsPCS2. Heterologous expression of OsPCS variants in PCS-deficient mutants of Schizosaccharomyces pombe and A. thaliana suggested that OsPCS1full possessed PCS activity in response to As(III) and Cd while the activity of other PCS variants was very low. To address physiological functions in toxic element tolerance and accumulation, two independent OsPCS1 mutant rice lines (a T-DNA and a Tos17 insertion line) were identified. The OsPCS1 mutants exhibited increased sensitivity to As(III) and Cd in hydroponic experiments, showing the importance of OsPCS1-dependent PC synthesis for rice As(III) and Cd tolerance. Elemental analyses of rice plants grown in soil with environmentally relevant As and Cd concentrations showed increased As accumulation and decreased Cd accumulation in grains of the T-DNA line. The Tos17 mutant also exhibited the reduced Cd accumulation phenotype. These contrasting effects on As and Cd distribution to grains suggest the existence of at least partially distinct PC-dependent pathways for As and Cd.

Thioarsenate Toxicity and Tolerance in the Model System Arabidopsis thaliana

Thioarsenates form from arsenite under sulfate-reducing conditions, e.g., in rice paddy soils, and are structural analogues of arsenate. Even though rice is one of the most important sources of human arsenic intake, nothing is published about uptake, toxicity, or tolerance of thioarsenates in plants. Experiments using the model system Arabidopsis thaliana showed that monothioarsenate is less toxic than arsenite, but more toxic than arsenate at concentrations ≥25 μM As, reflected in stronger seedling growth inhibition on agar plates. Despite higher toxicity, total As accumulation in roots was lower upon exposure to monothioarsenate compared to arsenate, and a higher root efflux was confirmed. Root-shoot translocation was higher for monothioarsenate than for arsenate. Compared to the wild type (Col-0), both arsenate and monothioarsenate induced higher toxicity in phytochelatin (PC)-deficient mutants (cad1-3) as well as in glutathione biosynthesis (cad2) and PC transport (abcc12) mutants, demonstrating the important role of the PC pathway, not only for arsenate, but also for monothioarsenate detoxification. In Col-0, monothioarsenate induced relatively higher accumulation of PCs than arsenate. The observed differences in plant uptake, toxicity, and tolerance of thioarsenate vs oxyarsenate show that studying the effects of As on plants should include experiments with thiolated As species.

Natural variation in Arabidopsis thaliana Cd responses and the detection of quantitative trait loci affecting Cd tolerance

Metal tolerance is often a result of metal storage or distribution. Thus, with the goal of advancing the molecular understanding of such metal homeostatic mechanisms, natural variation of metal tolerance in Arabidopsis thaliana was investigated. Substantial variation exists in tolerance of excess copper (Cu), zinc (Zn) and cadmium (Cd). Two accessions, Col-0 and Bur-0, and a recombinant inbred line (RIL) population derived from these parents were chosen for further analysis of Cd and Zn tolerance variation, which is evident at different plant ages in various experimental systems and appears to be genetically linked. Three QTLs, explaining in total nearly 50% of the variation in Cd tolerance, were mapped. The one obvious candidate gene in the mapped intervals, HMA3, is unlikely to contribute to the variation. In order to identify additional candidate genes the Cd responses of Col-0 and Bur-0 were compared at the transcriptome level. The sustained common Cd response of the two accessions was dominated by processes implicated in plant pathogen defense. Accession-specific differences suggested a more efficient activation of acclimative responses as underlying the higher Cd tolerance of Bur-0. The second hypothesis derived from the physiological characterization of the accessions is a reduced Cd accumulation in Bur-0.

Identification of alternatively spliced transcripts of rice phytochelatin synthase 2 gene OsPCS2 involved in mitigation of cadmium and arsenic stresses

The OsPCS2 exhibits root- and shoot-specific differential ratios of alternatively spliced transcripts in indica rice under Cd stress, and plays role in Cd and As stress tolerance and accumulation. Enzymatic activity of phytochelatin synthase (PCS) in plant produces phytochelatins, which help in sequestration of heavy metal(loid)s inside the cell vacuole to alleviate toxicity. Here we report that among the two PCS genes-OsPCS1 and OsPCS2 in indica rice (Oryza sativa) cultivar, the OsPCS2 produces an alternatively spliced OsPCS2b transcript that bears the unusual premature termination codon besides the canonically spliced OsPCS2a transcript. Root- and shoot-specific differential ratios of alternatively spliced OsPCS2a and OsPCS2b transcript expressions were observed under cadmium stress. Saccharomyces cerevisiae cells transformed with OsPCS2a exhibited increased cadmium (Cd) and arsenic (As) tolerance and accumulation, unlike the OsPCS2b transformed yeast cells. An intron-containing hairpin RNA-mediated gene silencing was carried out in endosperm-specific manner for efficient down-regulation of OsPCS genes in rice grains. Analysis of the transgenic rice lines grown under metal(loid) stress revealed almost complete absence of both OsPCS1 and OsPCS2 transcripts in the developing seeds coupled with the significant reduction in the content of Cd (~51%) and As (~35%) in grains compared with the non-transgenic plant. Taken together, the findings indicate towards a crucial role played by the tissue-specific alternative splicing and relative abundance of the OsPCS2 gene during heavy metal(loid) stress mitigation in rice plant.

Phytochelatin Synthesis Promotes Leaf Zn Accumulation of Arabidopsis thaliana Plants Grown in Soil with Adequate Zn Supply and is Essential for Survival on Zn-Contaminated Soil

Phytochelatin (PC) synthesis is essential for the detoxification of non-essential metals such as cadmium (Cd). In vitro experiments with Arabidopsis thaliana seedlings had indicated a contribution to zinc (Zn) tolerance as well. We addressed the physiological role of PC synthesis in Zn homeostasis of plants under more natural conditions. Growth responses, PC accumulation and leaf ionomes of wild-type and AtPCS1 mutant plants cultivated in different soils representing adequate Zn supply, Zn deficiency and Zn excess were analyzed. Growth on Zn-contaminated soil triggers PC synthesis and is strongly impaired in PC-deficient mutants. In fact, the contribution of AtPCS1 to tolerating Zn excess is comparable with that of the major Zn tolerance factor MTP1. For plants supplied with a normal level of Zn, a significant reduction in leaf Zn accumulation of AtPCS1 mutants was detected. In contrast, AtPCS1 mutants grown under Zn-limited conditions showed wild-type levels of Zn accumulation, suggesting the operation of distinct Zn translocation pathways. Contrasting phenotypes of the tested AtPCS1 mutant alleles upon growth in Zn- or Cd-contaminated soil indicated differential activation of PC synthesis by these metals. Experiments with truncated versions identified a part of the AtPCS1 protein required for the activation by Zn but not by Cd.

Expression of Caenorhabditis elegans PCS in the AtPCS1-deficient Arabidopsis thaliana cad1-3 mutant separates the metal tolerance and non-host resistance functions of phytochelatin synthases

Phytochelatin synthases (PCS) play key roles in plant metal tolerance. They synthesize small metal-binding peptides, phytochelatins, under conditions of metal excess. Respective mutants are strongly cadmium and arsenic hypersensitive. However, their ubiquitous presence and constitutive expression had long suggested a more general function of PCS besides metal detoxification. Indeed, phytochelatin synthase1 from Arabidopsis thaliana (AtPCS1) was later implicated in non-host resistance. The two different physiological functions may be attributable to the two distinct catalytic activities demonstrated for AtPCS1, that is the dipeptidyl transfer onto an acceptor molecule in phytochelatin synthesis, and the proteolytic deglycylation of glutathione conjugates. In order to test this hypothesis and to possibly separate the two biological roles, we expressed a phylogenetically distant PCS from Caenorhabditis elegans in an AtPCS1 mutant. We confirmed the involvement of AtPCS1 in non-host resistance by showing that plants lacking the functional gene develop a strong cell death phenotype when inoculated with the potato pathogen Phytophthora infestans. Furthermore, we found that the C. elegans gene rescues phytochelatin synthesis and cadmium tolerance, but not the defect in non-host resistance. This strongly suggests that the second enzymatic function of AtPCS1, which remains to be defined in detail, is underlying the plant immunity function.

Ethylene signalling is mediating the early cadmium-induced oxidative challenge in Arabidopsis thaliana

Cadmium (Cd) induces the generation of reactive oxygen species (ROS) and stimulates ethylene biosynthesis. The phytohormone ethylene is a regulator of many developmental and physiological plant processes as well as stress responses. Previous research indicated various links between ethylene signalling and oxidative stress. Our results support a correlation between the Cd-induced oxidative challenge and ethylene signalling in Arabidopsis thaliana leaves. The effects of 24 or 72 h exposure to 5 μM Cd on plant growth and several oxidative stress-related parameters were compared between wild-type (WT) and ethylene insensitive mutants (etr1-1, ein2-1, ein3-1). Cadmium-induced responses observed in WT plants were mainly affected in etr1-1 and ein2-1 mutants, of which the growth was less inhibited by Cd exposure as compared to WT and ein3-1 mutants. Both etr1-1 and ein2-1 showed a delayed response in the glutathione (GSH) metabolism, including GSH levels and transcript levels of GSH synthesising and recycling enzymes. Furthermore, the expression of different oxidative stress marker genes was significantly lower in Cd-exposed ein2-1 mutants, evidencing that ethylene signalling is involved in early responses to Cd stress. A model for the cross-talk between ethylene signalling and oxidative stress is proposed.

Specific mechanisms of tolerance to copper and cadmium are compromised by a limited concentration of glutathione in alfalfa plants

The induction of oxidative stress is a characteristic symptom of metal phytotoxicity and is counteracted by antioxidants such as glutathione (GSH) or homoglutathione (hGSH). The depletion of GSH│hGSH in fifteen-day-old alfalfa (Medicago sativa) plants pre-incubated with 1mM buthionine sulfoximine (BSO) affected antioxidant responses in a metal-specific manner under exposure to copper (Cu; 0, 6, 30 and 100μM) or cadmium (Cd; 0, 6 and 30μM) for 7 days. The phytotoxic symptoms observed with excess Cu were accompanied by an inhibition of root glutathione reductase (GR) activity, a response that was augmented in Cd-treated plants but reverted when combined with BSO. The synthesis of phytochelatins (PCs) was induced by Cd, whereas the biothiol concentration decreased in Cu-treated plants, which did not accumulate PCs. The depletion of GSH│hGSH by BSO also produced a strong induction of oxidative stress under excess Cu stress, primarily due to impaired GSH│hGSH-dependent redox homeostasis. In addition, the synthesis of PCs was required for Cd detoxification, apparently also determining the distribution of Cd in plants, as less metal was translocated to the shoots in BSO-incubated plants. Therefore, specific GSH│hGSH-associated mechanisms of tolerance were triggered by stress due to each metal.

MAN3 gene regulates cadmium tolerance through the glutathione-dependent pathway in Arabidopsis thaliana

Pollution of soil by the heavy metal cadmium (Cd) is a global environmental problem. The glutathione (GSH)-dependent phytochelatin (PC) synthesis pathway is one of the most important mechanisms contributing to Cd accumulation and tolerance. However, the regulation of this pathway is poorly understood. Here, we identified an Arabidopsis thaliana cadmium-tolerant dominant mutant xcd1-D (XVE system-induced cadmium-tolerance 1) and cloned XCD1 gene (previously called MAN3), which encodes an endo-β-mannanase. Overexpression of MAN3 led to enhanced Cd accumulation and tolerance, whereas loss-of-function of MAN3 resulted in decreased Cd accumulation and tolerance. In the presence of estradiol, enhanced Cd accumulation and tolerance in xcd1-D was associated with GSH-dependent, Cd-activated synthesis of PCs, which was correlated with coordinated activation of gene expression. Cd stress-induced expression of MAN3 and the consequently increased mannanase activity, led to increased mannose content in cell walls. Moreover, mannose treatment not only rescued the Cd-sensitive phenotype of the xcd1-2 mutant, but also improved the Cd tolerance of wild-type plants. Significantly, this mannose-mediated Cd accumulation and tolerance is dependent on GSH-dependent PC concentrations via coordinated control of expression of genes involved in PC synthesis. Our results suggest that MAN3 regulates the GSH-dependent PC synthesis pathway that contributes to Cd accumulation and tolerance in A. thaliana by coordinated control of gene expression.