Strategies for Cd accumulation in Dittrichia viscosa (L.) Greuter: role of the cell wall, non-protein thiols and organic acids

The Role of Low-Molecular-Weight Organic Acids in Metal Homeostasis in Plants

Low-molecular-weight organic acids (LMWOAs) are essential O-containing metal-binding ligands involved in maintaining metal homeostasis, various metabolic processes, and plant responses to biotic and abiotic stress. Malate, citrate, and oxalate play a crucial role in metal detoxification and transport throughout the plant. This review provides a comparative analysis of the accumulation of LMWOAs in excluders, which store metals mainly in roots, and hyperaccumulators, which accumulate metals mainly in shoots. Modern concepts of the mechanisms of LMWOA secretion by the roots of excluders and hyperaccumulators are summarized, and the formation of various metal complexes with LMWOAs in the vacuole and conducting tissues, playing an important role in the mechanisms of metal detoxification and transport, is discussed. Molecular mechanisms of transport of LMWOAs and their complexes with metals across cell membranes are reviewed. It is discussed whether different endogenous levels of LMWOAs in plants determine their metal tolerance. While playing an important role in maintaining metal homeostasis, LMWOAs apparently make a minor contribution to the mechanisms of metal hyperaccumulation, which is associated mainly with root exudates increasing metal bioavailability and enhanced xylem loading of LMWOAs. The studies of metal-binding compounds may also contribute to the development of approaches used in biofortification, phytoremediation, and phytomining.

SpCTP3 from the hyperaccumulator Sedum plumbizincicola positively regulates cadmium tolerance by interacting with SpMDH1

Cadmium (Cd) is a heavy metal pollutant mainly originating from the discharge of industrial sewage, irrigation with contaminated water, and the use of fertilizers. The phytoremediation of Cd polluted soil depends on the identification of the associated genes in hyperaccumulators. Here, a novel Cd tolerance gene (SpCTP3) was identified in hyperaccumulator Sedum plumbizincicola. The results of Cd2+ binding and thermodynamic analyses, revealed the CXXC motif in SpCTP3 functions is a Cd2+ binding site. A mutated CXXC motif decreased binding to Cd by 59.93%. The subcellular localization analysis suggested that SpCTP3 is primarily a cytoplasmic protein. Additionally, the SpCTP3-overexpressing (OE) plants were more tolerant to Cd and accumulated more Cd than wild-type Sedum alfredii (NHE-WT). The Cd concentrations in the cytoplasm of root and leaf cells were significantly higher (53.75% and 71.87%, respectively) in SpCTP3-OE plants than in NHE-WT. Furthermore, malic acid levels increased and decreased in SpCTP3-OE and SpCTP3-RNAi plants, respectively. Moreover, SpCTP3 interacted with malate dehydrogenase 1 (MDH1). Thus, SpCTP3 helps regulate the subcellular distribution of Cd and increases Cd accumulation when it is overexpressed in plants, ultimately Cd tolerance through its interaction with SpMDH1. This study provides new insights relevant to improving the Cd uptake by Sedum plumbizincicola.

Selenium-molybdenum interactions reduce chromium toxicity in Nicotiana tabacum L. by promoting chromium chelation on the cell wall

Chromium (Cr) is a hazardous heavy metal that negatively affects animals and plants. The micronutrients selenium (Se) and molybdenum (Mo) have been widely shown to alleviate heavy metal toxicity in plants. However, the molecular mechanism of Cr chelation on the cell wall by combined treatment with Se and Mo has not been reported. Therefore, this study aimed to explore the effects of Se-Mo interactions on the subcellular distribution of Cr (50 µM) and on cell wall composition, structure, functional groups and Cr content, in addition to performing a comprehensive analysis of the transcriptome. Our results showed that the cell walls of shoots and roots accumulated 51.0% and 65.0% of the Cr, respectively. Furthermore, pectin in the cell wall bound 69.5%/90.2% of the Cr in the shoots/roots. Se-Mo interactions upregulated the expression levels of related genes encoding galacturonosyltransferase (GAUT), UTP-glucose-1-phosphate uridylyltransferase (UGP), and UDP-glucose-4-epimerase (GALE), involved in polysaccharide biosynthesis, thereby increasing pectin and cellulose levels. Moreover, combined treatment with Se and Mo increased the lignin content and cell wall thickness by upregulating the expression levels of genes encoding cinnamyl alcohol dehydrogenase (CAD), peroxidase (POX) and phenylalanine amino-lyase (PAL), involved in lignin biosynthesis. Fourier-transform infrared (FTIR) spectroscopy results showed that Se + Mo treatment (in combination) increased the number of carboxylic acid groups (-COOH) groups, thereby enhancing the Cr chelation ability. The results not only elucidate the molecular mechanism of action of Se-Mo interactions in mitigating Cr toxicity but also provide new insights for phytoremediation and food safety.

Application of green synthesized bimetallic nZVI-Cu nanoparticle as a sustainable alternative to chemical fertilizers to enhance growth and photosynthetic efficiency of rice seedlings

Application of nanomaterials in agriculture has been extensively explored over the past decade leading to a wide ambit of nanoparticle-based agrochemicals. Metallic nanoparticles consisting of plant macro- and micro-nutrients have been used as nutritional supplements for plants through soil amendments, foliar sprays, or seed treatment. However, most of these studies emphasize monometallic nanoparticles which limit the range of usage and effectivity of such nanoparticles (NPs). Hence, we have employed a bimetallic nanoparticle (BNP) consisting of two different micro-nutrients (Cu & Fe) in rice plants to test its efficacy in terms of growth and photosynthesis. Several experiments were designed to assess growth (root-shoot length, relative water content) and photosynthetic parameters (pigment content, relative expression of rbcS, rbcL & ChlGetc.). To determine whether the treatment induced any oxidative stress or structural anomalies within the plant cells, histochemical staining, anti-oxidant enzyme activities, FTIR, and SEM micrographs were undertaken. Results indicated that foliar application of 5 mg L-1 BNP increased vigor and photosynthetic efficiency whereas 10 mg L-1 concentration induced oxidative stress to some extent. Furthermore, the BNP treatment did not perturb the structural integrity of the exposed plant parts and also did not induce any cytotoxicity. Application of BNPs in agriculture has not been explored extensively to date and this study is one of the first reports that not only documents the effectivity of Cu-Fe BNP but also critically explores the safety of its usage on rice plants making it a useful lead to design new BNPs and explore their efficacy.

The Fragile culm19 (FC19) mutation largely improves plant lodging resistance, biomass saccharification, and cadmium resistance by remodeling cell walls in rice

Cell wall is essential for plant upright growth, biomass saccharification, and stress resistance. Although cell wall modification is suggested as an effective means to increase biomass saccharification, it is a challenge to maintain normal plant growth with improved mechanical strength and stress resistance. Here, we reported two independent fragile culm mutants, fc19-1 and fc19-2, resulting from novel mutations of OsIRX10, produced by the CRISPR/Cas9 system. Compared to wild-type, the two mutants exhibited reduced contents of xylose, hemicellulose, and cellulose, and increased arabinose and lignin without significant alteration in levels of pectin and uronic acids. Despite brittleness, the mutants displayed increased breaking force, leading to improved lodging resistance. Furthermore, the altered cell wall and increased biomass porosity in fc19 largely increased biomass saccharification. Notably, the mutants showed enhanced cadmium (Cd) resistance with lower Cd accumulation in roots and shoots. The FC19 mutation impacts transcriptional levels of key genes contributing to Cd uptake, sequestration, and translocation. Moreover, transcriptome analysis revealed that the FC19 mutation resulted in alterations of genes mainly involved in carbohydrate and phenylpropanoid metabolism. Therefore, a hypothetic model was proposed to elucidate that the FC19 mutation-mediated cell wall remodeling leads to improvements in lodging resistance, biomass saccharification, and Cd resistance.

Root cell wall remodeling: A way for exopolysaccharides to mitigate cadmium toxicity in rice seedling

Exopolysaccharides (EPS) are macromolecules with environment beneficial properties. Currently, numerous studies focus on the absorption of heavy metals by EPS, but less attention has been paid to the effects of EPS on the plants. This study explored the effects of EPS from Lactobacillus plantarum LPC-1 on the structure and function of cell walls in rice seedling roots under cadmium (Cd) stress. The results showed that EPS could regulate the remodeling process of the cell walls of rice roots. EPS affects the synthesis efficiency and the content of the substances that made up the cell wall, and thus plays an essential role in limiting the uptake and transport of Cd in rice root. Furthermore, EPS could induce plant resistance to heavy metals by regulating the lignin biosynthesis pathway in rice roots. Finally, the cell wall remodeling induced by EPS likely contributes to plant stress responses by activating the reactive oxygen species (ROS) signaling.

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.

Growth, physiological, and temperature characteristics in chinese cabbage pakchoi as affected by Cd- stressed conditions and identifying its main controlling factors using PLS model

BACKGROUND

Although hormesis induced by heavy metals is a well-known phenomenon, the involved biological mechanisms are not fully understood. Cadmium (Cd) is a prevalent heavy metal in the environment. Exposure of Cd, via intake or consumption of Cd-contaminated air or food, poses a huge threat to human health. Chinese cabbage pakchoi (Brassica chinensis L.) is widely planted and consumed as a popular vegetable in China. Therefore, studying the response of Chinese cabbage pakchoi to Cd- stressed conditions is critical to assess whether cabbage can accumulate Cd and serve as an important Cd exposure pathway to human beings. In this study, we investigated the influence of Cd stress on growth, photosynthetic physiology, antioxidant enzyme activities, nutritional quality, anatomical structure, and canopy temperature in Chinese cabbage pakchoi. A partial least squares (PLS) model was used to quantify the relationship between physical and chemical indicators with Cd accumulation in cabbage, and identify the main controlling factors.

RESULTS

Results showed that Cd stress significantly inhibited cabbage's growth and development. When Cd stress was increased, the phenotypic indicators were significantly reduced. Meanwhile, Cd stress significantly enhanced the oxidative stress response of cabbage, such as the activities of catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), and the content of malondialdehyde (MDA) in leaves. Such a change tended to increase fenestrated tissues' thickness but decrease the thickness of leaf and spongy tissues. Moreover, Cd stress significantly increased soluble sugar, protein, and vitamin C contents in leaves as well as the temperature in the plant canopy. The PLS model analysis showed that the studied phenotypic and physicochemical indicators had good relationships with Cd accumulation in roots, shoots, and the whole plant of cabbage, with high coefficient of determination (R²) values of 0.891, 0.811, and 0.845, and low relative percent deviation (RPD) values of 3.052, 2.317, and 2.557, respectively. Furthermore, through analyzing each parameter's variable importance for projection (VIP) value, the SOD activity was identified as a key factor for indicating Cd accumulation in cabbage. Meanwhile, the effects of CAT on Cd accumulation in cabbage and the canopy mean temperature were also high.

CONCLUSION

Cd stress has significant inhibitory effects and can cause damage cabbage's growth and development, and the SOD activity may serve as a key factor to indicate Cd uptake and accumulation in cabbage.

The Role of Hemicellulose in Cadmium Tolerance in Ramie (Boehmeria nivea (L.) Gaud.)

Ramie cell walls play an important role in cadmium (Cd) detoxification. However, the Cd binding capacity of the cell wall components and the cell wall compositions among ramie species remains unclear. Therefore, this study compared two ramie populations (‘Dazhuhuangbaima’ (low-Cd-accumulating population) and ‘Zhongzhu 1’ (high-Cd-accumulating population)) with different Cd enrichment characteristics. The two ramie populations were treated with 0, 25, and 75 mg kg⁻¹ Cd for 30 days; then, their root length, plant height, biomass, Cd enrichment in the organs, subcellular Cd distribution, Cd content in the cell wall polysaccharides, and hemicellulose content were determined. The root length, plant height, biomass, and Cd enrichment in all organs were significantly higher (p ≤ 0.05) in ‘Zhongzhu 1’ than in ‘Dazhuhuangbaima’ under Cd stress. In addition, the subcellular Cd distribution analysis revealed that Cd was mainly found in the cell wall in both ramie populations. Among the cell wall fractions, Cd was mainly bound to the hemicelluloses, with 60.38–73.10% and 50.05–64.45% Cd accumulating in the ‘Zhongzhu 1’ and ‘Dazhuhuangbaima’ cell wall hemicelluloses, respectively. However, the Cd concentration in the ‘Zhongzhu 1’ hemicellulose was significantly higher (p ≤ 0.05) than that in the ‘Dazhuhuangbaima’ hemicellulose. Hemicellulose content analysis further revealed that the hemicellulose concentration increased with the Cd concentration in both populations, but it was significantly higher (p ≤ 0.05) in ‘Zhongzhu 1’ than in ‘Dazhuhuangbaima’ across all Cd treatments. Thus, ramie copes under Cd stress by increasing the hemicellulose content in the cell wall. The findings in this study confirm that hemicellulose is the main enrichment site for Cd in ramie. It also provides a theoretical basis for Cd enrichment breeding in ramie.

Bacterial inoculant-assisted phytoremediation affects trace element uptake and metabolite content in Salix atrocinerea

Natural plant-associated microorganisms are of critical importance to plant growth and survival in field conditions under toxic concentrations of trace elements (TE) and these plant-microbial processes can be harnessed to enhance phytoremediation. The total bacterial diversity from grey willow (Salix atrocinerea) on a brownfield heavily-polluted with lead (Pb) and arsenic (As) was studied through pyrosequencing. Culturable bacteria were isolated and in vitro tested for plant growth-promotion (PGP) traits, arsenic (As) tolerance and impact on As speciation. Two of the most promising bacterial strains - the root endophyte Pantoea sp. AV62 and the rhizospheric strain Rhodococcus erythropolis AV96 - were inoculated in field to S. atrocinerea. This bioaugmentation resulted in higher As and Pb concentrations in both, roots and leaves of bacterial-inoculated plants as compared to non-inoculated plants. In consequence, bacterial bioaugmentation also affected parameters related to plant growth, oxidative stress, the levels of phytochelatins and phenylpropanoids, together with the differential expression of genes related to these tolerance mechanisms to TE in leaves. This study extends our understanding about plant-bacterial interactions and provides a solid basis for further bioaugmentation studies aiming to improve TE phytoremediation efficiency and predictability in the field.

Cd absorption characteristics of Suaeda salsa under different sediment burial and exogenous Cd input conditions in the Yellow River estuary, China

Suaeda salsa (L.) Pall., a typical halophyte plant in the Yellow River estuary, has high enrichment capacity for heavy metals. However, few studies have investigated the Cd absorption characteristics of S. salsa under different sediment burial and exogenous Cd input conditions, especially following the water-sediment regulation scheme (WSRS), which brought sediment burial and exogenous substances to the estuary. So, we established a greenhouse pot culture experiment with four sediment burial depths (0 cm, 3 cm, 6 cm, and 12 cm) and exogenous Cd input levels (0 mg·kg-1, 0.5 mg·kg-1, 1.0 mg·kg-1, and 1.5 mg·kg-1) and analyzed the leaf, stem, root, and total biomass; leaf, stem, and root Cd content; and storage, sediment Cd content, accumulation factor, root/leaf (R/L), root/stem (R/S), and stem/leaf (S/L) ratios to study the Cd absorption characteristics of S. salsa under the different sediment burial and exogenous Cd input. Results showed that high Cd content in roots, stems, and leaves was harmful to S. salsa growth, and then led to a decrease in biomass (characterized by stem, leaf, and total biomass). Suaeda salsa exhibited a survival strategy to deal with Cd toxicity, which involved the roots absorbing Cd from the sediment and storing it in stems and leaves (stem and leaf Cd content peaked at 0.5 mg·kg-1 Cd input) at low Cd input, whereas roots stored more Cd and reduced Cd transport to stems and leaves at high Cd input. Therefore, we observed the maximum value of leaf (500.63 ± 19.15 g·m-2), stem (648.22 ± 50.08 g·m-2), and total biomass (1246.92 ± 55.49 g·m-2) in the treatment with 1.5 mg·kg-1 Cd input and 3-cm sediment depth due to the Cd content in leaves and stems being relatively low. The accumulation factors of leaves, stems, and roots varied (0.39-0.99, 0.19-2.58, and 0.80-20.45, respectively), and most of the accumulation factors for roots and leaves and the R/L and R/S ratios were >1, which indicated that S. salsa had high enrichment levels of Cd, which mostly accumulated in the roots. Shallow or moderate burial depth was beneficial to S. salsa growth, but sediment burial was not beneficial to Cd absorption because the sum of leaf, stem, and root Cd storage was higher at 0-cm depth compared with the other depths. Variance analysis showed that the influence of Cd input on leaf, stem, root, and sediment Cd content and stem and root Cd storage was significant (P < 0.05), whereas sediment burial, interaction of sediment burial and Cd input on Cd content, storage, and biomass were not significant (P > 0.05). Therefore, we concluded that more attention should be paid to the control of sediment burial and heavy metal input, especially during the WSRS, in the Yellow River estuary.

Molecular and Biochemical Insights Into Early Responses of Hemp to Cd and Zn Exposure and the Potential Effect of Si on Stress Response

With the intensification of human activities, plants are more frequently exposed to heavy metals (HM). Zinc (Zn) and cadmium (Cd) are frequently and simultaneously found in contaminated soils, including agronomic soils contaminated by the atmospheric fallout near smelters. The fiber crop Cannabis sativa L. is a suitable alternative to food crops for crop cultivation on these soils. In this study, Cd (20 μM) and Zn (100 μM) were shown to induce comparable growth inhibition in C. sativa. To devise agricultural strategies aimed at improving crop yield, the effect of silicon (Si; 2 mM) on the stress tolerance of plants was considered. Targeted gene expression and proteomic analysis were performed on leaves and roots after 1 week of treatment. Both Cd- and Zn-stimulated genes involved in proline biosynthesis [pyrroline-5-carboxylate reductase (P5CR)] and phenylpropanoid pathway [phenylalanine ammonia-lyase (PAL)] but Cd also specifically increased the expression of PCS1-1 involved in phytochelatin (PC) synthesis. Si exposure influences the expression of numerous genes in a contrasting way in Cd- and Zn-exposed plants. At the leaf level, the accumulation of 122 proteins was affected by Cd, whereas 47 proteins were affected by Zn: only 16 proteins were affected by both Cd and Zn. The number of proteins affected due to Si exposure (27) alone was by far lower, and 12 were not modified by heavy metal treatment while no common protein seemed to be modified by both CdSi and ZnSi treatment. It is concluded that Cd and Zn had a clear different impact on plant metabolism and that Si confers a specific physiological status to stressed plants, with quite distinct impacts on hemp proteome depending on the considered heavy metal.

Cell wall modification induced by an arbuscular mycorrhizal fungus enhanced cadmium fixation in rice root

The chemistry of root cell wall of rice could be changed by inoculation of arbuscular mycorrhizal fungi (AMF). Hydroponic experiments were conducted to investigate the roles of such changes on cadmium (Cd) uptake and distribution in rice. Results showed that inoculation of AM fungus Rhizophagus intraradices (RI) significantly enhanced (p < 0.05) shoot biomass, plant height and root length of rice, and decreased Cd concentration in shoot and root under Cd stress. Moreover, Cd in root was mainly found in pectin and hemicellulose 1 (HC1) components of root cell wall. Inoculation of RI increased the levels of pectin, HC1 and lignin content, accompanied by the increments of L-phenylalanine ammonia-lyase (PAL) and pectin methylesterase (PME) activities. Results of Fourier transform infrared spectroscopy further showed that more hydroxyl and carboxyl groups in root cell wall were observed in mycorrhizal treatment, compared with control. This study demonstrates that cell wall components could be the locations for Cd fixation, which reduced Cd transportation from root to shoot. Inoculation of AMF may remodel root cell wall biosynthesis and affect the characteristics of Cd fixation. The entering and fixing mechanisms should be further studied.

Evaluation of cytotoxic and antioxidant potential of Dittrichia viscosa (L.) Greuter used in traditional medicine

ETHNOPHARMACOLOGICAL RELEVANCE

Dittrichia viscosa (L.) Greuter ("Sarı ot, Yapışkan andız otu" in Turkish) is a medicinal plant that has been traditionally used in the Mediterranean area. This plant is used by the local population for the treatment of cancer. Investigation of their biological activities is therefore very important to be supported by scientific basis for traditional use.

AIMS OF THE STUDY

In this study, it is aimed to assess the phytochemical composition, in vitro antioxidant, cytotoxic, and antiproliferative activities of the aqueous and ethanolic extracts obtained from the aerial parts (stems, leaves, flowers) of D. viscosa, collected from two sites in Turkey (Istanbul and Marmaris) against breast and prostate tumor cell lines.

MATERIALS AND METHODS

Validated methods were used to evaluate the in vitro antioxidant capacity (DPPH, ABTS, CUPRAC), cytotoxicity (Cell Viability Assay), antiproliferative (Apoptosis assay), and phytochemical compositions. The nepetin (N), 3-O-methylquercetin (Q), and hispidulin (H) in the extracts of D. viscosa were quantified by HPLC and LC-HRMS. Furthermore, in order to control the standards of benefiting from the plant in a healthy way, the contents of some heavy metals were also assessed by ICP-OES in the plant and soil samples as well as the species soil's physical and chemical characteristics.

RESULTS

We have found that heavy metal accumulation in the soil does not exceed the allowable limit value except for the nickel. The results showed that ethanol extraction is an efficient strategy to get NQH molecules with a higher content compared with other extraction techniques. However, using the same extraction method revealed that the amount of NQH molecules in the samples of two different regions were variable. The results suggested that all extracts had a high amount of total phenolic content (12.354-22.184 μg GAE/mg) and total flavonoid content (4.442-17.263 μg QE/g). In the antioxidant assay according to the DPPH method, the aqueous ethanol extracts (IC₅₀; 21.00 μg/mL) showed stronger antioxidant activity than BHT. A significant reduction in cell viability was particularly observed in MDA-MB-231 cells, which were sensitive to ethanolic extracts in Istanbul (12-22%) and in Marmaris (14-15%), while PC3 cell lines were also more sensitive to extracts of the aqueous in Istanbul (16%) and the decoction in Marmaris (12%) after 72 h. Especially, it was observed that Marmaris and Istanbul samples induced the toxicity against PC3 cells.

CONCLUSION

The study supports the medicinal use of D. viscosa as a potential anticancer against breast and prostate cancer cells in vitro and underlines the immense therapeutic potential of the plant.

Cellular and subcellular distribution and factors influencing the accumulation of atmospheric Hg in Tillandsia usneoides leaves

Atmospheric Hg is a highly toxic heavy metal with bioaccumulative properties. However, relatively few studies have focused on the distribution of Hg in cellular and subcellular structures of plants and factors influencing its accumulation. In this study, we selected Tillandsia usneoides, which is a widely used bioindicator for Hg, to analyze the concentration of Hg in different cells (foliar trichomes, epidermal cells, mesophyll cells, and vascular bundle cells), different subcellular structures (cell wall, cell membrane, vacuoles, and organelles) and different cell wall components (pectin, hemicellulose 1, and hemicellulose 2). It was determined that Hg was present in different types of cells, but there was no significant difference, suggesting that atmospheric Hg circulates dynamically in the surface and internal structural cells of T. usneoides leaves. Subcellular analysis showed that as Hg concentration increased, more Hg accumulated in the vacuoles and cell wall through the compartmentalization mechanism. Hemicellulose had the highest content of Hg, indicating that it is the primary Hg-binding component of the cell wall. The FTIR analysis results showed that after the Hg treatment, the cell wall -OH and COO- absorption peaks changed most significantly, indicating that these functional groups play a vital role in the Hg accumulation process.

Alleviation mechanisms of metal(loid) stress in plants by silicon: a review

Silicon (Si), although not considered as an essential element for plants in general, can ameliorate the phytotoxicity induced by excess metal(loid)s whether non-essential (e.g. Cd, Pb, Cr, Al, As, and Sb) or essential (e.g. Cu, Ni, and Zn). The Si-enhanced resistance allowing plants to cope with this type of abiotic stress has been developed at multiple levels in plants. Restriction of root uptake and immobilization of metal(loid)s in the rhizosphere by Si is probably one of the first defence mechanism. Further, retention of elements in the root apoplasm might enhance the resistance and vigour of plants. At the cellular level, the formation of insoluble complexes between Si and metal(loid)s and their storage within cell walls help plants to decrease available element concentration and restrict symplasmic uptake. Moreover, Si influences the oxidative status of plants by modifying the activity of various antioxidants, improves membrane stability, and acts on gene expression, although its exact role in these processes is still not well understood. This review focuses on all currently known plant-based mechanisms related to Si supply and involved in amelioration of stress caused by excess metal(loid)s.

Effect of cornstalk biochar on phytoremediation of Cd-contaminated soil by Beta vulgaris var. cicla L

Cadmium (Cd) contamination is the most common and extensive heavy metal pollution in the farmland of China. Phytoremediation is considered as a promising measure for Cd-contaminated soil remediation, but the remediation efficiency still needs to be enhanced. Biochar as an effective amendment medium is widely manufactured and studied for the soil remediation of heavy metals. In this study, a greenhouse pot trial was conducted to investigate the effects of cornstalk biochar on Cd accumulation of Beta vulgaris var. cicla L. (Beta vulgaris) in Cd contaminated soil. The Cd availability, speciation and nutrients in soil, biomass and Cd chemical forms in the Beta vulgaris root were studied to explore the mechanism that how the cornstalk biochar promoted Cd accumulation in Beta vulgaris. Biochar amendment reduced the DTPA-extractable Cd concentration and stimulated the growth of root. Compared to the Beta vulgaris without biochar treatment, the results of 5% biochar amendment showed that the root dry weight of Beta vulgaris increased to 267%, Cd accumulation in Beta vulgaris increased to 206% and the Cd concentration in leaves and roots increased by 36% and 52%, respectively. Additionally, after 5% biochar was applied to soil, the total content of organic matter-bound Cd and residual Cd increased by 38%, while the content of Fe-Mn oxides-bound Cd decreased by 40%. Meanwhile, Cd may mainly bind to the root cell wall and the ratio of NaCl-extracted Cd to HAc-extracted Cd increased to 166% with 5% biochar amendment. According to our study, Cd in soil can be removed by Beta vulgaris and phytoremediation efficiency can be improved with biochar amendment. The combination of phytoremediation and biochar amendment is a promising strategy for the Cd-contaminated soil remediation.

Boron inhibits cadmium uptake in wheat (Triticum aestivum) by regulating gene expression

Various nutrients (Mg, Zn, Fe, Mn, Si, etc.) can supress cadmium (Cd) uptake and alleviate Cd toxicity, but the mechanisms are not the same. In this study, the molecular mechanism governing the effects of boron (B) on uptake of Cd in hydroponically grown wheat was characterized. As compared to control (0 μM Cd), B concentration per plant decreased by 22% and 29% under 5 μM Cd and 50 μM Cd treatment respectively. In addition, B application decreased Cd concentration and accumulation in whole wheat. Correlation analysis of different elements show that there was a highly negative correlation between concentrations of B and Cd (r = -0.854 with significant correlation) in wheat. Additionally, 16,543 differentially expressed genes (DEGs) (7666 up- and 8877 down-regulated) were detected between 0 and 5 μM Cd treatments in wheat roots by transcriptome sequencing. Gene ontology functional category and Kyoto encyclopedia of genes and genomes pathway analyses indicated that the DEGs were involved in biological process, cellular component, and molecular function. Five highly homologous genes to Cd transporters were identified; these genes were involved in metal ion binding, transmembrane ion transport, and protein transport. According to the qRT-PCR results, expression of all these genes was down-regulated in the 462 μM of B treatment compared with the 46.2 μM of B treatment regardless of the Cd treatments (0.5 or 5 μM Cd). These results suggest that B is an inhibitor of Cd uptake, and the down-regulation of five highly homologous genes could be associated with decreased uptake of Cd after B application.

Microalgal Metallothioneins and Phytochelatins and Their Potential Use in Bioremediation

The persistence of heavy metals (HMs) in the environment causes adverse effects to all living organisms; HMs accumulate along the food chain affecting different levels of biological organizations, from cells to tissues. HMs enter cells through transporter proteins and can bind to enzymes and nucleic acids interfering with their functioning. Strategies used by microalgae to minimize HM toxicity include the biosynthesis of metal-binding peptides that chelate metal cations inhibiting their activity. Metal-binding peptides include genetically encoded metallothioneins (MTs) and enzymatically produced phytochelatins (PCs). A number of techniques, including genetic engineering, focus on increasing the biosynthesis of MTs and PCs in microalgae. The present review reports the current knowledge on microalgal MTs and PCs and describes the state of art of their use for HM bioremediation and other putative biotechnological applications, also emphasizing on techniques aimed at increasing the cellular concentrations of MTs and PCs. In spite of the broad metabolic and chemical diversity of microalgae that are currently receiving increasing attention by biotechnological research, knowledge on MTs and PCs from these organisms is still limited to date.

Ecological types and bioindicator macrophyte species of pollution of riparian vegetation of Oued Lârbaa in Taza City of Morocco

The object of our study is devoted to the Spermatophyta of the wetlands of Oued Lârbaa, the main river of the city of Taza, Morocco, and which is under strong anthropic pressure. Our work involved a floristic inventory, to define ecological types and groups of dominant plants in relation to seasonal factors and types of pollution, explaining the meaning of their presence. For this purpose, floristic sampling was carried out along the Oued during the dry periods (2017 and 2018) and the wet period (2018). A total of 66 plant species belonging to 54 genera and 30 families were identified, including 44 species during the wet period (2018) and 27 species during the dry periods (2017 and 2018). This difference is due to the favorable conditions for vegetation development during the period of precipitation and to the increase in pollution rates during the dry season. The inventoried flora shows the dominance of 10 ecological types characterized by the following plants: Cynodon dactylon, Arundo donax, Olea europaea, and Tamarix gallica (common between the two periods); Dittrichia viscosa, Visnaga daucoides, Typha angustifolia, and Ricinus communis (during the dry periods of 2017 and 2018); and Juncus maritimus and Populus nigra (during the wet period of 2018). The ecological types identified in this work decrease from the dry season to the rainy season, while specific richness increases. These dominant plants are all considered as bioindicators of the presence of heavy metals.

Ecotype-Specific Pathways of Reactive Oxygen Species Deactivation in Facultative Metallophyte Silene Vulgaris (Moench) Garcke Treated with Heavy Metals

This research aimed to indicate mechanisms involved in protection against the imbalanced generation of reactive oxygen species (ROS) during heavy metals (HMs) exposition of Silene vulgaris ecotypes with different levels of metal tolerance. Specimens of non-metallicolous (NM), calamine (CAL), and serpentine (SER) ecotypes were treated in vitro with Zn, Pb, and Cd ions applied simultaneously in concentrations that reflected their contents in natural habitats of the CAL ecotype (1× HMs) and 2.5- or 5.0-times higher than the first one. Our findings confirmed the sensitivity of the NM ecotype and revealed that the SER ecotype was not fully adapted to the HM mixture, since intensified lipid peroxidation, ultrastructural alternations, and decline in photosynthetic pigments' content were ascertained under HM treatment. These changes resulted from insufficient antioxidant defense mechanisms based only on ascorbate peroxidase (APX) activity assisted (depending on HMs concentration) by glutathione-S-transferase (GST) and peroxidase activity at pH 6.8 in the NM ecotype or by GST and guaiacol-type peroxidase in the SER one. In turn, CAL specimens showed a hormetic reaction to 1× HMs, which manifested by both increased accumulation of pigments and most non-enzymatic antioxidants and enhanced activity of catalase and enzymes from the peroxidase family (with the exception of APX). Interestingly, no changes in superoxide dismutase activity were noticed in metallicolous ecotypes. To sum up, the ROS scavenging pathways in S. vulgaris relied on antioxidants specific to the respective ecotypes, however the synthesis of polyphenols was proved to be a universal reaction to HMs.

Effect of pH and citric acid on the growth, arsenic accumulation, and phytochelatin synthesis in Eupatorium cannabinum L., a promising plant for phytostabilization

Heavy-metal contamination of soils has increased in the last decades due to anthropogenic and industrial activities. Arsenic is one of the pollutants that is commonly found in industrial soils and is toxic for both plants and humans. The pH of the soil or the culture medium is one of the most important factors that interferes with the bioavailability of this metalloid to the plant. The addition of chelating agents, such as citric acid (CA), can increase the absorption of As by plants. Therefore, the objective of this work is to study the effect of the pH and the exogenous addition of citric acid on the growth, As accumulation, and thiol compounds in Eupatorium cannabinum; this plant grows naturally in contaminated soils in Asturias, Spain, and has a potential use in phytoremediation. The results showed that E. cannabinum was able to tolerate As stress even at extreme pH values and accumulated a high amounts of As in its roots, which makes it a promising species for the phytostabilization of soils polluted with this metalloid. An addition of 20 mg CA L^-1 led to increased biomass and As accumulation at acidic pH. In order to determine if thiolic compounds, such as phytochelatins, are involved in As accumulation and detoxification in E. cannabinum, we analyzed the synthesis of these compounds in the presence and absence of As and/or citric acid. Our results suggest that these thiolic compounds play a major role in As detoxification, since the presence of CA as a chelating agent reduced the amount of thiols necessary to cope with the toxicity caused by As.

Effects of Rhizophagus clarus and biochar on growth, photosynthesis, nutrients, and cadmium (Cd) concentration of maize (Zea mays) grown in Cd-spiked soil

Cadmium (Cd) toxicity in agricultural crops is a widespread problem. Little is known about biochar and arbuscular mycorrhizal fungi (AMF) effect on Cd concentration in maize plant either applied separately or in combination. Current study was performed to demonstrate effects of biochar and Rhizophagus clarus on plant growth, photosynthesis activity, nutrients (P, Ca, Mg, Fe, Cu, and Mn), and Cd concentration in maize grown in Cd-spiked soil. The alkaline soil was spiked by Cd factor at three levels: 0 (Cd 0), 5 (Cd 5), and 10 (Cd 10) mg/kg; biochar factor at two levels: 0 and 1%; and mycorrhizal inoculum factor at two levels: MF0 and MF1 (R. clraus). Plants were harvested after 70 days of seed germination, and various morphological and physiological parameters, as well as elemental concentration and root colonization, were recorded. Addition of biochar increased plant biomass by 21% (Cd 5) and 93% (Cd 10), MF1 enhanced by 53% (Cd 0) and 69% (Cd 10), while biochar + MF1 enhanced dry plant biomass by 70% (Cd 0) and 94% (Cd 10). Results showed maximum increase of 94% (Cd 10) in plant biomass was observed in Cd-spiked soil. Root colonization decreased proportionally by increasing Cd concentration and at Cd 10, colonization was 36.7% and 31.7% for MF1 and biochar + MF1 treatments, respectively. Besides that, addition of biochar enhanced root attributes (root length, volume, and surface area) by 34-58% compared to control in Cd 10. The MF1 increased these attributes by 11-78% while biochar + MF1 enhanced by 32-61% in Cd-spiked soil. However, biochar + MF1 neutralized Cd stress in maize plant for gaseous attributes (assimilation rate, transpiration rate, intercellular CO₂, and stomatal conductance). The MF1 enhanced Cd concentration in plant as it was 3.32 mg/kg in Cd 5 and 6.73 mg/kg in Cd 10 treatments while addition of biochar phytostabilized Cd and reduced its concentration in plants by 2.0 mg/kg in Cd 5 and 4.27 mg/kg in Cd 10. The biochar + MF1 had 2.9 mg/kg and 4.8 mg/kg Cd concentration in Cd 5 and Cd 10 plants, respectively. Phosphorus concentration was augmented in shoots (up to 26%) and roots (up to 20%) of maize plant in biochar-amended soil than control plants. In biochar + MF1, concentration of P was 1.01% and 0.73% in Cd 5 and Cd 10, respectively. It is concluded that biochar + MF1 treatment enhances plant biomass while addition of sole biochar reduced Cd uptake, slightly indifferent to earlier treatment.

Dual Role of Metallic Trace Elements in Stress Biology-From Negative to Beneficial Impact on Plants

Heavy metals are an interesting group of trace elements (TEs). Some of them are minutely required for normal plant growth and development, while others have unknown biological actions. They may cause injury when they are applied in an elevated concentration, regardless of the importance for the plant functioning. On the other hand, their application may help to alleviate various abiotic stresses. In this review, both the deleterious and beneficial effects of metallic trace elements from their uptake by roots and leaves, through toxicity, up to the regulation of physiological and molecular mechanisms that are associated with plant protection against stress conditions have been briefly discussed. We have highlighted the involvement of metallic ions in mitigating oxidative stress by the activation of various antioxidant enzymes and emphasized the phenomenon of low-dose stimulation that is caused by non-essential, potentially poisonous elements called hormesis, which is recently one of the most studied issues. Finally, we have described the evolutionary consequences of long-term exposure to metallic elements, resulting in the development of unique assemblages of vegetation, classified as metallophytes, which constitute excellent model systems for research on metal accumulation and tolerance. Taken together, the paper can provide a novel insight into the toxicity concept, since both dose- and genotype-dependent response to the presence of metallic trace elements has been comprehensively explained.

Nickel uptake and distribution in Agropyron cristatum L. in the presence of pyrene

PAHs affect the uptake of heavy metal by plants. The uptake pathway, distribution and detoxification of nickel (Ni) in Agropyron cristatum L. (A. cristatum) were investigated in the presence of pyrene in this study. Most of Ni was adsorbed on the cell wall in the insoluble phosphate (57.31-72.18%) form and pectate and protein integrated (38.27-38.98%) form. Ni was transferred to the organelle (from 37.84% to 40.52%) in the presence of pyrene. The concentration of Ni in A. cristatum decreased by 27.42%; it was affected by the ATP production inhibitor and 29.49% by the P-type ATPase inhibitor. The results indicated that the uptake of Ni related closely to the synthesis and decomposition of ATP and was an active uptake process. Contents of phytochelatins (PCs) in A. cristatum in Ni contaminated soils increased by 19.97%, and an additional 4.13% increase occurred in the presence of pyrene when compared to single Ni contamination. The content of malic acid in A. cristatum was the highest for 262.78 mg g^-1 in shoots and 46.81 mg g^-1 in roots with Ni contamination. Besides, acetic acid in shoots and roots increased by 40.25% and 102.63% with Ni contamination, and by 61.59% and 185.71% with Ni-pyrene co-contamination. This study preliminarily explored the inhibitory mechanism of pyrene on plant uptake of Ni.

Effect of cadmium on young plants of Virola surinamensis

The steady increase in cadmium (Cd) levels in the environment from anthropogenic actions has contributed to environmental degradation. Virola surinamensis is a forest species that has desirable characteristics such as deep and dense roots, relatively rapid growth and high biomass production to remedy contaminated environments by Cd. The aim of this study was to assess the physiological responses and the phytoextraction and tolerance capacity of young plants of V. surinamensis submitted to Cd concentrations. The experimental design was a completely randomized design with five Cd concentrations (0, 15, 30, 45 and 60 mg L^-1) for 60 days. Leaf water potential (Ψ_pd), stomatal conductance (gs) and transpiration (E) reduced in plants exposed to Cd. Lower values of maximum photochemical efficiency of photosystem II (Fv/Fm), electron transport rate (ETR) and photochemical quenching coefficient (qP) were accompanied by reduction of photosynthesis (A) with increasing concentrations of Cd, although the non-photochemical quenching coefficient (NPQ), and intercellular CO₂ concentration (Ci) showed increase. Instantaneous water-use efficiency (A/E), net photosynthesis to intercellular CO₂ concentration ratio (A/Ci) and total chlorophyll (Chl) reduced with increasing levels of Cd. Cadmium concentrations increased in different plant tissues (root > stem > leaf). The tolerance index (TI) indicated that V. surinamensis presented medium and high tolerance to Cd. The results of bioconcentration factor (BCF) and translocation factor (TF) showed low plant efficacy in Cd phytoextraction and suggest that V. surinamensis may be promising for phytostabilization of Cd.

ROS management is mediated by ascorbate-glutathione-α-tocopherol triad in co-ordination with secondary metabolic pathway under cadmium stress in Withania somnifera

Being static, plants are frequently exposed to various essential and non-essential heavy metals from the surroundings. This exposure results in considerable ROS generation leading to oxidative stress, the primary response of the plants under heavy metal stress. Withania somnifera is a reputed Indian medicinal plant in Ayurveda, having various pharmacological activities due to the presence of withanolides. The present study deals with the understanding endurance of oxidative stress caused by heavy metal exposure and its management through antioxidant partners in synchronization with secondary metabolites in W. somnifera. The quantitative assessment of enzymatic/non-enzymatic antioxidants revealed significant participation of ascorbate-glutathione-α-tocopherol triad in ROS management. Higher activities of glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) resulted in glutathione and ascorbate accumulation. In addition, superoxide dismutase (SOD), glutathione peroxidase (GPX) and peroxidase (POD) were contributed considerably in ROS homeostasis maintenance. In-situ localization and assays related to ROS generation/scavenging revealed key management of ROS status under Cd stress. Higher antioxidative and reducing power activity attributed to the tolerance capability to the plant. Increased expression of withanolide biosynthetic pathway genes such as WsHMGR, WsDXS, WsDXR and WsCAS correlated with enhanced withanolides. The present study indicated the crucial role of the ascorbate-glutathione-α-tocopherol triad in co-ordination with withanolide biosynthesis in affording the oxidative stress, possibly through a cross-talk between the antioxidant machinery and secondary metabolite biosynthesis. The knowledge may be useful in providing the guidelines for developing abiotic stress resistance in plants using conventional and molecular approaches.

Effects of soil amendments on the growth response and phytoextraction capability of a willow variety (S. viminalis × S. schwerinii × S. dasyclados) grown in contaminated soils

This study was conducted to evaluate the effects of lime and bisphosphonates (BPs) such as N10O chelate amendment on the growth, physiological and biochemical parameters, and phytoextraction potential of the willow variety Klara (Salix viminalis × S. schwerinii × S. dasyclados) grown in soils heavily contaminated with copper (Cu), nickel (Ni) and zinc (Zn). The plants were irrigated with tap or processed water (mine wastewater). The results suggest that the combined effects of the contaminated soil and processed water inhibited growth parameters, gas exchange parameters and chlorophyll fluorescence (Fv/Fm) values. In contrast, malondialdehyde (MDA) content, organic acids, total phenolic and total flavonoid contents, and the accumulation of metals/metalloids in the plant tissues were increased compared to the control. When the soil was supplemented with lime and N10O; growth, physiological, biochemical parameters, and resistance capacity were significantly higher compared to unamended soil treatments, especially in the contaminated soil treatments. The combined lime‒ and N10O‒amended soil treatment produced higher growth rates, resistance capacity, photosynthesis rates and phytoextraction efficiency levels relative to either the lime‒amended or the N10O‒amended soil treatments. This study provides practical evidence of the efficient chelate‒assisted phytoextraction capability of Klara and highlights its potential as a viable and inexpensive novel approach for in situ remediation of Cu‒, Ni‒ and Zn‒contaminated soils and mine wastewaters.

Distribution and chemical forms of cadmium in Coptis chinensis Franch. determined by laser ablation ICP-MS, cell fractionation, and sequential extraction

Coptis chinensis Franch., is a widely used medicinal plant in China. This plant is often contaminated by cadmium (Cd) and render health risk to human consumers. Understanding distribution of Cd and its chemical forms is important to evaluate accumulation of the metal and its detoxification mechanisms in this plant. Since few studies have focused on this aspect, we used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to spatially locate Cd in rhizome cross-sections, and ICP-MS to analyze the Cd subcellular distribution and the chemical forms of Cd in different tissues. Rhizome bioimaging results showed that Cd was distributed predominantly within the periderm, cortex, pith, and root trace vascular bundle. The LA-ICP-MS results suggested that Ca²⁺ channels might be a pathway for Cd entry into the plant. Subcellular distribution data indicated that most of Cd was associated with the cell wall (41.8-77.1%) and the soluble fraction (14.4-52.7%) in all tissues. Analysis of chemical forms revealed that majority Cd existed in less mobile and less toxic forms in all tissues, and P could convert to insoluble phosphate with Cd to moderate Cd toxicity. The new understanding of Cd accumulation and detoxification might provide novel strategies for reducing the levels of Cd in C. chinensis Franch., thereby mitigating its potential transfer to humans and providing a theoretical basis for evaluating the Cd status in other medicinal plants. Further, our findings might provide a basis for establishing a reasonable Cd limit level of traditional Chinese medicinal materials.

Sodium Hydrosulfide Mitigates Cadmium Toxicity by Promoting Cadmium Retention and Inhibiting Its Translocation from Roots to Shoots in Brassica napus

The association between hydrogen sulfide (H₂S) and cell wall composition with regard to the mitigation of cadmium (Cd) toxicity in Brassica napus L. was investigated. Cd caused growth retardation, leaf chlorosis, and decreased endogenous H₂S content in Brassica napus roots. Stimulating l-cysteine desulfhydrase (LCD)-mediated H₂S production with H₂S releaser (NaHS) markedly improved plant growth, reduced Cd content in stems and leaves, and rescued Cd-induced chlorosis. Furthermore, increased Cd retention was observed in root cell walls, indicating that NaHS reduced Cd movement from the roots to upper-plant parts. Exogenous NaHS also significantly increased the content of pectin and the activity of pectin methylesterase in cell walls of roots, thereby increasing Cd retention in pectin fractions. However, intensification of H₂S barely affected hemicellulose content under Cd stress. Intensified H₂S signal, therefore, alleviates Cd toxicity in Brassica napus by increasing pectin content and its demethylation, increasing Cd fixation in cell walls, and reducing root-to-shoot Cd translocation.

Comparative transcriptome analysis revealed key factors for differential cadmium transport and retention in roots of two contrasting peanut cultivars

BACKGROUND

Peanut is the world's fourth largest oilseed crop that exhibits wide cultivar variations in cadmium (Cd) accumulation. To establish the mechanisms of Cd distribution and accumulation in peanut plants, eight cDNA libraries from the roots of two contrasting cultivars, Fenghua 1 (low-Cd cultivar) and Silihong (high-Cd cultivar), were constructed and sequenced by RNA-sequencing. The expression patterns of 16 candidate DEGs were validated by RT-qPCR analysis.

RESULTS

A total of 75,634 genes including 71,349 known genes and 4484 novel genes were identified in eight cDNA libraries, among which 6798 genes were found to be Cd-responsive DEGs and/or DEGs between these two cultivars. Interestingly, 183 DEGs encoding ion transport related proteins and 260 DEGs encoding cell wall related proteins were identified. Among these DEGs, nine metal transporter genes (PDR1, ABCC4 and ABCC15, IRT1, ZIP1, ZIP11, YSL7, DTX43 and MTP4) and nine cell wall related genes (PEs, PGIPs, GTs, XYT12 CYP450s, LACs, 4CL2, C4H and CASP5) showed higher expression in Fenghua 1 than in Silihong.

CONCLUSIONS

Both the metal transporters and cell wall modification might be responsible for the difference in Cd accumulation and translocation between Fenghua 1 and Silihong. These findings would be useful for further functional analysis, and reveal the molecular mechanism responsible for genotype difference in Cd accumulation.

Ralstonia eutropha Q2-8 reduces wheat plant above-ground tissue cadmium and arsenic uptake and increases the expression of the plant root cell wall organization and biosynthesis-related proteins

In this study, the molecular mechanisms involved in Ralstonia eutropha Q2-8-induced increased biomass and reduced cadmium (Cd) and arsenic (As) uptake in wheat plants (Triticum aestivum cv. Yangmai 16) were investigated in growth chambers. Strain Q2-8 significantly increased plant biomass (22-75%) without and with Cd (5 μM) + As (10 μM) stress and reduced plant above-ground tissue Cd (37%) and As (34%) contents compared to those in the controls. Strain Q2-8 significantly increased the proportions of Cd and As in wheat root cell walls. Under Cd and As stress, 109 root proteins were differentially expressed among which those involved in metabolisms, stress and defence, and energy were dominant in the presence of strain Q2-8. Furthermore, energy-, defence-, and cell wall biosynthesis-related proteins were found to be up-regulated. Notably, differentially expressed cell wall biosynthesis-related proteins in roots were only found in bacteria-inoculated plants under Cd and As stress. The results suggest that strain Q2-8 can alleviate Cd and As toxicity to wheat plant seedlings and reduce above-ground tissue Cd and As uptake by increasing the efficiency of root energy metabolism, defence, and cell wall biosynthesis under Cd and As stress.

Effect of inoculation with arbuscular mycorrhizal fungi and blanching on the bioaccessibility of heavy metals in water spinach (Ipomoea aquatica Forsk.)

A plant's tolerance to heavy metals (HMs) and its detoxification mechanisms are associated with the subcellular distribution of HMs and their chemical forms. In this study, water spinach (Ipomoea aquatica Forsk.) was grown in two soils contaminated with a single HM (cadmium, Cd) or combined HMs (Cd and nickel, Ni). Inoculation of arbuscular mycorrizal fungi (AMF) was conducted to increase the accumulation of phosphorus (P) in plants. One major exception was to decrease the migration and accumulation of HMs in edible parts by the formation of P-HM complexes. The effects of blanching and simulated digestion on bioaccessibility were also assessed. The experimental results showed that the water spinach species used in this study had a high capacity to accumulate HMs. AMF treatment improved water spinach growth and decreased the accumulation of Ni but not that of Cd. Soluble and inorganic Cd and Ni were the major subcellular fractions and chemical forms in water spinach; these two HMs also exhibited higher migration capacities in comparison to chromium (Cr). Relative to raw tissues, 45-84% of Cd, Cr, and Ni were leached after blanching. Approximately 32-55%, 16-50%, and 27-40% of Cd, Cr, and Ni, respectively, were bioaccessible and could be metabolized by in vitro digestive fluids.

The differences of cell wall in roots between two contrasting soybean cultivars exposed to cadmium at young seedlings

The plant root cell wall (CW) is the first structure that comes into contact with extracellular cadmium (Cd), and it plays an important role in the absorption, immobilization, and translocation of Cd in the roots. However, the differences in the cell wall components between Cd-tolerant and Cd-sensitive cultivars are unclear. A hydroponic experiment was carried out to investigate the differences in the concentrations of Cd, total sugars, and uronic acid in pectin, hemicellulose 1, hemicellulose 2, cellulose, and lignin, as well as pectin methylesterase enzyme activity (PME) in the roots of two soybean cultivars that differ with respect to Cd tolerance exposed to 0 and 23 μM Cd treatments. The bound forms of Cd in the roots were found to differ between the two soybean genotypes; 50.2% of the Cd in the root cell wall accumulates in the pectin in the highly Cd-tolerant and low Cd-accumulating cultivar HX3, while 50.6% of the root cell wall Cd accumulates in cellulose in the Cd-sensitive and high Cd-accumulating cultivar BX10. The total sugar and uronic acid concentrations of the cell wall components increased in response to Cd stress, while the concentrations of total sugars and uronic acid in BX10 were higher than in HX3 (except for hemicellulose 1). Increased demethylation of pectin may be the main reason that Cd is mainly concentrated in the primary wall in HX3, because the PME activity was higher in HX3 than it was in BX10 under Cd treatment. Furthermore, BX10 had a higher lignin concentration after Cd treatment, and showed the same change in cellulose. Cd in the root cell wall of BX10 was fixed in the secondary cell wall, which may be a result of the coupling to cellulose and lignin. In conclusion, root cell walls in soybean cultivars that differ in Cd tolerance may possess different mechanisms to prevent Cd from entering cells, and the sequestration of Cd in different cell wall components may determine the differences in Cd tolerance between the two genotypes.

Sodium chloride decreases cadmium accumulation and changes the response of metabolites to cadmium stress in the halophyte Carpobrotus rossii

BACKGROUND AND AIMS

Salinity affects the bioavailability of cadmium (Cd) in soils and Cd accumulation in plants, but the associated mechanisms remain unclear. This study aimed to assess the metabolic response to NaCl and Cd and the relationship between metabolites and Cd accumulation in the halophyte Carpobrotus rossii, which has potential for Cd phytoextraction.

METHODS

Plants were grown in nutrient solution with 0-400 mm NaCl in the presence of 5 or 15 µm Cd, with varied or constant solution Cd2+ activity. Plant growth and Cd uptake were measured, and the accumulation of peptides, and organic and amino acids in plant tissues were assessed.

KEY RESULTS

The addition of NaCl to Cd-containing solutions improved plant growth along with 70-87 % less shoot Cd accumulation, resulting from decreases in Cd root uptake and root-to-shoot translocation irrespective of Cd2+ activity in solutions. Moreover, Cd exposure increased the concentration of phytochelatins, which correlated positively with Cd concentrations in plants regardless of NaCl addition. In comparison, Cd inhibited the synthesis of organic acids in shoots and roots in the absence of NaCl, but increased it in shoots in the presence of NaCl. While Cd increased the concentrations of amino acids in plant shoots, the effect of NaCl on the synthesis of amino acids was inconsistent.

CONCLUSIONS

Our data provide the first evidence that NaCl decreased Cd shoot accumulation in C. rossii by decreasing Cd root uptake and root-to-shoot translocation even under constant Cd2+ activity. The present study also supports the important role of peptides and organic acids, particular of phytochelatins, in Cd tolerance and accumulation although the changes of those metabolites was not the main reason for the decreased Cd accumulation.

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

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

Comparative transcriptome combined with morpho-physiological analyses revealed key factors for differential cadmium accumulation in two contrasting sweet sorghum genotypes

Cadmium (Cd) is a widespread soil contaminant threatening human health. As an ideal energy plant, sweet sorghum (Sorghum bicolor (L.) Moench) has great potential in phytoremediation of Cd-polluted soils, although the molecular mechanisms are largely unknown. In this study, key factors responsible for differential Cd accumulation between two contrasting sweet sorghum genotypes (high-Cd accumulation one H18, and low-Cd accumulation one L69) were investigated. H18 exhibited a much higher ability of Cd uptake and translocation than L69. Furthermore, Cd uptake through symplasmic pathway and Cd concentrations in xylem sap were both higher in H18 than those in L69. Root anatomy observation found the endodermal apoplasmic barriers were much stronger in L69, which may restrict the Cd loading into xylem. The molecular mechanisms underlying these morpho-physiological traits were further dissected by comparative transcriptome analysis. Many genes involved in cell wall modification and heavy metal transport were found to be Cd-responsive DEGs and/or DEGs between these two genotypes. KEGG pathway analysis found phenylpropanoid biosynthesis pathway was over-represented, indicating this pathway may play important roles in differential Cd accumulation between two genotypes. Based on these results, a schematic representation of main processes involved in differential Cd uptake and translocation in H18 and L69 is proposed, which suggests that higher Cd accumulation in H18 depends on a multilevel coordination of efficient Cd uptake and transport, including efficient root uptake and xylem loading, less root cell wall binding, and weaker endodermal apoplasmic barriers.

Differential Tolerance to Lead and Cadmium of Micropropagated Gypsophila fastigiata Ecotype

In vitro techniques may provide a suitable tool for effective propagation and conservation of plant species representing various ecological niches. The elaboration of such protocols is also prerequisite for selection of heavy-metal-tolerant plant material that could be afterwards used for restoration or remediation of polluted sites. In this study, culture protocol for Gypsophila fastigiata propagation was developed. The highest multiplication coefficient, which reached 6.5, and the best growth parameters were obtained on modified MS medium supplemented with 1.0 mg L-1 2iP and 0.2 mg L-1 IAA. The obtained cultures were treated with different concentrations of lead nitrate (0.1, 0.5, and 1.0 mM Pb(NO3)2) or cadmium chloride (0.5, 2.5, and 5.0 μM CdCl2). The growth parameters, photosynthetic pigments, and phenolic compound content were examined in order to evaluate whether tested metal salts can have an adverse impact on studied culture. It was ascertained that Pb ions induced growth disturbances and contributed to shoot wither. On the contrary, the proliferative shoot cultures were established on media containing Cd ions and the multiplication coefficients and shoot length increased on all media enriched with CdCl2. Chlorophylls and carotenoid contents were negatively affected by application of 5.0 μM of cadmium; nevertheless, in shoots treated with 2.5 μM CdCl2, increased accumulation of photosynthetic pigments occurred and their amount was similar to untreated culture. Adaptation to Cd was associated with stimulation of phenolic compound synthesis. Hence, we have reported on unambiguous positive result of in vitro selection procedure to obtain vigorous shoot culture tolerant to cadmium.

Metal accumulation and detoxification mechanisms in mycorrhizal Betula pubescens

Metal detoxification in plants is a complex process that involves different mechanisms, such as the retention of metals to the cell wall and their chelation and subsequent compartmentalization in plant vacuoles. In order to identify the mechanisms involved in metal accumulation and tolerance in Betula pubescens, as well as the role of mycorrhization in these processes, mycorrhizal and non-mycorrhizal plants were grown in two industrial soils with contrasting concentrations of heavy metals. Mycorrhization increased metal uptake at low metal concentrations in the soil and reduced it at high metal concentrations, which led to an enhanced growth and biomass production of the host when growing in the most polluted soil. Our results suggest that the sequestration on the cell wall is the main detoxification mechanism in white birch exposed to acute chronic metal-stress, while phytochelatins play a role mitigating metal toxicity inside the cells. Given its high Mn and Zn root-to-shoot translocation rate, Betula pubescens is a very promising species for the phytoremediation of soils polluted with these metals.

Reciprocal Interactions between Cadmium-Induced Cell Wall Responses and Oxidative Stress in Plants

Cadmium (Cd) pollution renders many soils across the world unsuited or unsafe for food- or feed-orientated agriculture. The main mechanism of Cd phytotoxicity is the induction of oxidative stress, amongst others through the depletion of glutathione. Oxidative stress can damage lipids, proteins, and nucleic acids, leading to growth inhibition or even cell death. The plant cell has a variety of tools to defend itself against Cd stress. First and foremost, cell walls might prevent Cd from entering and damaging the protoplast. Both the primary and secondary cell wall have an array of defensive mechanisms that can be adapted to cope with Cd. Pectin, which contains most of the negative charges within the primary cell wall, can sequester Cd very effectively. In the secondary cell wall, lignification can serve to immobilize Cd and create a tougher barrier for entry. Changes in cell wall composition are, however, dependent on nutrients and conversely might affect their uptake. Additionally, the role of ascorbate (AsA) as most important apoplastic antioxidant is of considerable interest, due to the fact that oxidative stress is a major mechanism underlying Cd toxicity, and that AsA biosynthesis shares several links with cell wall construction. In this review, modifications of the plant cell wall in response to Cd exposure are discussed. Focus lies on pectin in the primary cell wall, lignification in the secondary cell wall and the importance of AsA in the apoplast. Regarding lignification, we attempt to answer the question whether increased lignification is merely a consequence of Cd toxicity, or rather an elicited defense response. We propose a model for lignification as defense response, with a central role for hydrogen peroxide as substrate and signaling molecule.

Comparative study of Cd uptake and tolerance of two Italian ryegrass (Lolium multiflorum) cultivars

Cadmium (Cd) is one of the most toxic heavy metals and is difficult to be removed from contaminated soil and water. Italian ryegrass (Lolium multiflorum), as an energy crop, exhibits a valuable potential to develop Cd polluted sites due to its use as a biofuel rather than as food and forage. Previously, via a screening for Cd-tolerant ryegrass, the two most extreme cultivars (IdyII and Harukaze) with high and low Cd tolerance during seed germination, respectively, were selected. However, the underlying mechanism for Cd tolerance was not well investigated. In this study, we comparatively investigated the growth, physiological responses, and Cd uptake and translocation of IdyII and Harukaze when the seedlings were exposed to a Cd (0–100 μM) solution for 12 days. As expected, excess Cd inhibited seedling growth and was accompanied by an accumulation of malondialdehyde (MDA) and reduced photosynthetic pigments in both cultivars. The effects of Cd on the uptake and translocation of other nutrient elements (Zn, Fe, Mn and Mg) were dependent on Cd concentrations, cultivars, plant tissues and elements. Compared with Harukaze, IdyII exhibited better performance with less MDA and higher pigment content. Furthermore, IdyII was less efficient in Cd uptake and translocation compared to Harukaze, which might be explained by the higher non-protein thiols content in its roots. Taken together, our data indicate that IdyII is more tolerant than Harukaze, which partially resulted from the differences in Cd uptake and translocation.

Physiological mechanisms of a wetland plant (Echinodorus osiris Rataj) to cadmium detoxification

Physiological responses of Echinodorus osiris Rataj plant under cadmium (Cd) stress (5 and 15 mg L^-1) were studied by researching the change of non-enzymatic antioxidants and the exudation of root organic acids. There was a significant increase of ascorbic acid, glutathione, and non-protein thiols in the plant, and the increment was much obvious in roots than that in leaves with increased Cd stress. The accumulation of Cd was associated with mitochondrial structural damages in roots, while the organelle structure, such as chloroplast, in leaves remains intact. In exudates collected from the plants in the treatment with 15 mg L^-1 Cd, oxalate, citric, and succinic acids responded intensively than other organic acids.

Mycorrhization protects Betula pubescens Ehr. from metal-induced oxidative stress increasing its tolerance to grow in an industrial polluted soil

In recent years, the use of woody plants in phytoremediation has gained popularity due to their high biomass production and their association with mycorrhizal fungi, which can improve their survival and development rates under stress conditions. In this study, mycorrhized and non-mycorrhized white birch plants (Betula pubescens Ehr.) were grown in control and a metal-polluted industrial soil. After 60days of culture, plant growth and metal accumulation, the content of photosynthetic pigments and oxidative-stress markers, as well as the enzymatic activities and gene expressions of antioxidant enzymes were measured. According to our results, mycorrhized birch plants grown in control soil showed an increased activity and gene expression of catalase and ascorbate peroxidase, along with hydrogen peroxide overproduction, which could support the importance of the reactive oxygen species as signaling molecules in the regulation of plant-fungus interactions. Additionally, in polluted soil mycorrhized plants had higher biomass but lower metal accumulation, probably because the symbiotic fungus acted as a barrier to the entrance of metals into the host plants. This behavior led to mitigation in the oxidative challenge, reduced hydrogen peroxide content and diminished activities of the antioxidant enzymes in comparison to non-mycorrhized plants.

Exogenous malic acid alleviates cadmium toxicity in Miscanthus sacchariflorus through enhancing photosynthetic capacity and restraining ROS accumulation

Malic acid (MA) plays an important role in the regulation of plant growth, stomatal aperture, nutrition elements homeostasis and toxic metals tolerance. However, little is known about the effects of exogenous MA on physiological and biochemical responses to toxic metals in plants. To measure the alleviation roles of exogenous MA against cadmium (Cd), we determined the effects of MA on plant growth, net photosynthetic rate (Pn), reactive oxygen species (ROS) accumulation and the activities of anti-oxidant enzymes in the leaves of Miscanthus sacchariflorus (M. sacchariflorus) under Cd stress. The Cd exposure alone significantly inhibited plant growth and Pn, but increased the accumulation of ROS even though the anti-oxidant enzymes were markedly activated in the leaves of M. sacchariflorus. Treatment with MA significantly enhanced plant growth and decreased Cd accumulation accompanied by increasing Pn under Cd stress as compared to Cd stress alone, especially when treatment with high concentration of MA (200μM) was used. In addition, Cd and MA indicated synergistic effects by further increasing the activities and genes expression of partial anti-oxidant enzymes, thus resulting in higher glutathione accumulation and reduction of ROS production. The results showed that application of MA alleviated Cd-induced phytotoxicity and oxidant damage through the regulation of both enzymatic and non-enzymatic anti-oxidants under Cd stress in M. sacchariflorus.

Photosynthesis and growth response of maize (Zea mays L.) hybrids exposed to cadmium stress

Cadmium (Cd) is a biologically non-essential heavy metal while the cultivation of Cd-tolerant varieties/hybrids (V) seems the most promising strategy for remediation of Cd-contaminated soils. For this, 24-day-old seedlings of seven maize hybrids, DKC 65-25, DKC 61-25, DKC 919, 23-T-16, 32-B-33, 31-P-41, and Syn hybrid, were grown in hydroponic conditions for 21 additional days in various Cd concentrations (0, 5, 10, and 15 μM). Effects of variety, Cd, and their interaction were highly significant (p ≤ 0.05) for studied plant agronomic and physiological traits except the V × Cd interaction for leaf chlorophyll content, root-shoot length, and root dry weight. The Cd accumulation in root and shoot increased gradually with increasing Cd treatments while copper (Cu), zinc (Zn), and manganese (Mn) uptake was decreased in all hybrids. The reduction in root and shoot biomass and Cd uptake was lower in 32-B-33 and 23-T-16 compared to other hybrids. The highest accumulation of Cu, Zn, and Mn was observed in 32-B-33, DK C65-25, and 31-P-41, respectively. The differential uptake and accumulation of Cd by maize hybrids may be useful in selection and breeding for Cd-tolerant genotypes.

Inoculation with endophytic Bacillus megaterium H3 increases Cd phytostabilization and alleviates Cd toxicity to hybrid pennisetum in Cd-contaminated aquatic environments

A hydroponic culture experiment was performed to investigate the effects of endophytic Bacillus megaterium H3 on the plant biomass, Cd accumulation and tolerance of hybrid pennisetum, and the mechanisms involved in the different levels of Cd-contaminated aquatic environments. Strain H3 significantly increased the plant growth (ranging from 13 to 71 %) and total Cd uptake (ranging from 41 to 160 %) but decreased Cd translocation factors of hybrid pennisetum treated with 0-20 μM Cd compared with the controls. Furthermore, most of Cd (71-77 %) was accumulated in the roots of the bacterial-inoculated hybrid pennisetum. Notably, strain H3 could significantly increase the production of oxalic and propanedioic acids (ranging from 18 to 188 %) but decrease the production of phytochelatins of hybrid pennisetum compared to the controls under different levels of Cd stress. The live bacterial-induced increase in organic acid production and decrease in phytochelatins production by hybrid pennisetum might be responsible for the increased plant growth, root Cd accumulation, and Cd toxicity alleviation of the plant under different levels of Cd stress. The results highlight that hybrid pennisetum plus endophytic B. megaterium H3 may be utilized for biomass production and Cd phytostabilization of the plant in the different levels of Cd-contaminated aquatic environments.

Impact of cadmium stress on two maize hybrids

Some physiological parameters and composition of the root cell walls of two maize hybrids (monocots), the sensitive Novania and the tolerant Almansa were studied after treatment with cadmium cations. After 10 days of Cd²⁺ treatment (1 × 10^-5 M and 5 × 10^-5 M), plant growth inhibition, in the sensitive hybrid in particular, as well as a certain alteration in root structure and pigment content were observed. The Cd²⁺ accumulation was ten times higher in the roots than in the shoots. Chemical analyses and atomic absorption spectroscopy proved that Cd²⁺ modified the composition of the root cell walls by a significant increase in the content of alkali-soluble polysaccharide fractions, particularly in the tolerant hybrid. An increase in the content of phenolic compounds, mainly in the tolerant hybrid, and a decrease in protein content were observed in the presence of Cd²⁺ in the alkali fractions. The results indicate that the changes in the cell wall polysaccharide fractions and their proportion to lignin and cellulose are obviously involved in the tolerance and/or defence against Cd²⁺ of the maize hybrids studied.

Physiological responses of the hybrid larch (Larix × eurolepis Henry) to cadmium exposure and distribution of cadmium in plantlets

Phytoextraction of Cd is a growing biotechnology although we currently know few Cd hyperaccumulators, i.e., plant species able to accumulate at least 0.1 mg Cd g(-1) dry weight in aerial organs. Owing their deep root system and high biomass, trees are more and more preferred to herbaceous species for phytoextraction. Assuming that conifers could be relevant models under cold climates, we investigated cadmium tolerance of the hybrid larch Larix × eurolepis Henry (Larix decidua × Larix kaempferi) and the efficiency of this species to store this metal. In vitro grown larches were chosen in order to reduce time of exposure and to more rapidly evaluate their potential efficiency to accumulate Cd. One-month-old plantlets were exposed for 2 and 4 weeks to 250 and 500 μM Cd. Results showed that they tolerated a 4-week exposure to 250 μM Cd, whereas the content of photosynthetic pigment strongly dropped in plantlets growing in the presence of 500 μM Cd. In the presence of 250 μM Cd, shoot growth slightly decreased but photosynthetic pigment and total soluble carbohydrate contents were not modified and no lipid peroxidation was detected. In addition, these plantlets accumulated proline, particularly in shoots (two to three times more than control). In roots, Cd concentration in the intracellular fraction was always higher than in the cell wall fraction contrary to shoots where Cd concentration in the cell wall fraction increased with time and Cd concentration in the medium. In shoots, Cd concentration was lower than in roots with a ratio of 0.2 after 4 weeks of exposure but stayed around 0.2 mg g(-1) dry weight, thus a value higher than the threshold requested for Cd hyperaccumulators. Hybrid larch would thus be a relevant candidate for field test of Cd phytoextraction.

Growth and physiological responses of submerged plant Vallisneria natans to water column ammonia nitrogen and sediment copper

Background. The decline of submerged plant populations due to high heavy metal (e.g., Cu) levels in sediments and ammonia nitrogen (ammonia-N) accumulation in the freshwater column has become a significant global problem. Previous studies have evaluated the effect of ammonia-N on submerged macrophytes, but few have focused on the influence of sediment Cu on submerged macrophytes and their combined effects.

Methods. In this paper, we selected three levels of ammonia-N (0, 3, and 6 mg L⁻¹) and sediment Cu (25.75 ± 6.02 as the control, 125.75 ± 6.02, and 225.75 ± 6.02 mg kg⁻¹), to investigate the influence of sediment Cu and ammonia-N on submerged Vallisneria natans. We measured the relative growth rate (RGR), above- and below- ground biomass, chlorophyll, non-protein thiol (NP-SH), and free proline.

Results and Discussion. The below-ground biomass of V. natans decreased with increasing Cu sediment levels, suggesting that excessive sediment Cu can result in significant damage to the root of V. natans. Similarly, the above-ground biomass significantly decreased with increasing ammonia-N concentrations, indicating that excessive water ammonia-N can cause significant toxicity to the leaf of V. natans. In addition, high ammonia-N levels place a greater stress on submerged plants than sediment Cu, which is indicated by the decline of RGR and chlorophyll, and the increase of (NP-SH) and free proline. Furthermore, high sediment Cu causes ammonia-N to impose greater injury on submerged plants, and higher sediment Cu levels (Cu ≥ 125.75 mg kg⁻¹) led to the tolerant values of ammonia-N for V. natans decreasing from 6 to 3 mg L⁻¹. This study suggests that high sediment Cu restricts the growth of plants and intensifies ammonia-N damage to V. natans.