Phytoremediation of cadmium-contaminated soils by young Douglas fir trees: effects of cadmium exposure on cell wall composition

The adaptation of root cell wall pectin to copper toxicity in two citrus species differing in copper tolerance: remodeling and responding

Citrus species are prone to suffer from copper (Cu) toxicity because of improper application of Cu-based agrochemicals. Copper immobilization mediated by pectin methylesterase (PME) in the root cell wall (CW) is effective for Cu detoxification. However, the underlying mechanisms of the structural modification and stress responses of citrus root CW pectin to Cu toxicity have been less discussed. In the present study, seedlings of 'Shatian pummelo' (Citrus grandis L. Osbeck) and 'Xuegan' (Citrus sinensis L. Osbeck), which differ in Cu tolerance, were irrigated with nutrient solution containing 0.5 (as control), 100, 300 or 500 μM Cu for 18 weeks in sandy culture or 24 h in hydroponics. At the end of treatments in the 18-week sandy culture, Cu toxicity on CW pectin content, Cu distribution, degree of pectin methylesterification (DPM) and the PME enzyme activity were discussed. At the genome-wide level, PME gene family was identified from the two citrus species, and qRT-PCR array of citrus PMEs under control and 300 μM Cu stress for 18 weeks were performed to screen the Cu-responsive PME genes. Moreover, the candidate genes that responded to Cu toxicity were further examined within 24 h. The results showed that Cu toxicity increased the root CW pectin content. The root CW pectin under Cu toxicity was remodeled by upregulation of the expression of the Cu-responsive PME genes followed by increasing PME activity, which mainly promoted low methylesterased pectin level and the Cu content on root CW pectin. Compared with C. sinensis, C. grandis root CW had a lower DPM and higher Cu content on the Cu-stressed root CW pectin, contributing to its higher Cu tolerance. Our present study provided theoretical evidence for root CW pectin remodeling in response to Cu toxicity of citrus species.

Hydrogen peroxide mediates cadmium accumulation in the root of a high cadmium-accumulating rice (Oryza sativa L.) line

Hydrogen peroxide (H₂O₂) is a vital signaling molecule in response to cadmium (Cd) stress in plants. However, the role of H₂O₂ on Cd accumulation in root of different Cd-accumulating rice lines remains unclear. Exogenous H₂O₂ and 4-hydroxy-TEMPO (H₂O₂ scavenger) were applied to investigate the physiological and molecular mechanisms of H₂O₂ on Cd accumulation in the root of a high Cd-accumulating rice line Lu527-8 through hydroponic experiments. Interestingly, it was found Cd concentration in the root of Lu527-8 increased significantly when exposed to exogenous H₂O₂, while reduced significantly when exposed to 4-hydroxy-TEMPO under Cd stress, proving the role of H₂O₂ in regulating Cd accumulation in Lu527-8. Lu527-8 showed more Cd and H₂O₂ accumulation in the roots, along with more Cd accumulation in cell wall and soluble fraction, than the normal rice line Lu527-4. In particular, more pectin accumulation, especially low demethylated pectin, was observed in the root of Lu527-8 when exposed to exogenous H₂O₂ under Cd stress, resulting in more negative functional groups with greater capacity to binding Cd in the root cell wall of Lu527-8. It indicated that H₂O₂-induced cell wall modification and vacuolar compartmentalization contributes greatly to more Cd accumulation in the root of the high Cd-accumulating rice line.

Hydrogen peroxide contributes to cadmium binding on root cell wall pectin of cadmium-safe rice line (Oryza sativa L.)

Cell wall pectin is essential for cadmium (Cd) accumulation in rice roots and hydrogen peroxide (H₂O₂) plays an important role as a signaling molecule in cell wall modification. The role of H₂O₂ in Cd binding in cell wall pectin is unclear. D62B, a Cd-safe rice line, was found to show a greater Cd binding capacity in the root cell wall than a high Cd-accumulating rice line of Wujin4B. In this study, we further investigated the mechanism of the role of H₂O₂ in Cd binding in root cell wall pectin of D62B compared with Wujin4B. Cd treatment significantly increased the H₂O₂ concentration and pectin methyl esterase (PME) activity in the roots of D62B and Wujin4B by 22.45-42.44% and 12.15-15.07%, respectively. The H₂O₂ concentration and PME activity significantly decreased in the roots of both rice lines when H₂O₂ was scavenged by 4-hydroxy-Tempo. The PME activity of D62B was higher than that of Wujin4B. The concentrations of high and low methyl-esterified pectin in the roots of D62B significantly increased when exposed to Cd alone but significantly decreased when exposed to Cd and exogenous 4-hydroxy-Tempo. No significant difference was detected in Wujin4B. Exogenous 4-hydroxy-Tempo significantly decreased the Cd concentration in the cell wall pectin in both rice lines. The modification of H₂O₂ in Cd binding was further explored by adding H₂O₂. The maximum Cd adsorption amounts on the root cell walls of both rice lines were improved by exogenous H₂O₂·H₂O₂ treatment significantly influenced the relative peak area of the main functional groups (hydroxyl, carboxyl), and the groups intensely shifted after Cd adsorption in the root cell wall of D62B, while there was no significant difference in Wujin4B. In conclusion, Cd stress stimulated the production of H₂O₂, thus promoting pectin biosynthesis and demethylation and releasing relative functional groups involved in Cd binding on cell wall pectin, which is beneficial for Cd retention in the roots of Cd-safe rice line.

Phytoremediation as an effective tool to handle emerging contaminants

Phytoremediation is a process which effectively uses plants as a tool to remove, detoxify or immobilize contaminants. It has been an eco-friendly and cost-effective technique to clean contaminated environments. The contaminants from various sources have caused an irreversible damage to all the biotic factors in the biosphere. Bioremediation has become an indispensable strategy in reclaiming or rehabilitating the environment that was damaged by the contaminants. The process of bioremediation has been extensively used for the past few decades to neutralize toxic contaminants, but the results have not been satisfactory due to the lack of cost-effectiveness, production of byproducts that are toxic and requirement of large landscape. Phytoremediation helps in treating chemical pollutants on two broad categories namely, emerging organic pollutants (EOPs) and emerging inorganic pollutants (EIOPs) under in situ conditions. The EOPs are produced from pharmaceutical, chemical and synthetic polymer industries, which have potential to pollute water and soil environments. Similarly, EIOPs are generated during mining operations, transportations and industries involved in urban development. Among the EIOPs, it has been noticed that there is pollution due to heavy metals, radioactive waste production and electronic waste in urban centers. Moreover, in recent times phytoremediation has been recognized as a feasible method to treat biological contaminants. Since remediation of soil and water is very important to preserve natural habitats and ecosystems, it is necessary to devise new strategies in using plants as a tool for remediation. In this review, we focus on recent advancements in phytoremediation strategies that could be utilized to mitigate the adverse effects of emerging contaminants without affecting the environment.

Kosteletzkya pentacarpos: A Potential Halophyte Candidate for Phytoremediation in the Meta(loid)s Polluted Saline Soils

Kosteletzkya pentacarpos (L.) Ledebour is a perennial facultative halophyte species from the Malvacea family that grows in coastal areas with high amounts of salt. The tolerance of K. pentacarpos to the high concentration of salt (0.5–1.5% salinity range of coastal saline land) has been widely studied for decades. Nowadays, with the dramatic development of the economy and urbanization, in addition to the salt, the accumulation of mate(loid)s in coastal soil is increasing, which is threatening the survival of halophyte species as well as the balance of wetland ecosystems. Recently, the capacity of K. pentacarpos to cope with either single heavy metal stress or a combination of multiple meta(loid) toxicities was studied. Hence, this review focused on summarizing the physiological and biochemical behaviors of K. pentacarpos that has been simultaneously exposed to the combination of several meta(loid) toxicities. How the salt accumulated by K. pentacarpos impacts the response to meta(loid) stress was discussed. We conclude that as a potential candidate for phytoremediation, K. pentacarpos was able to cope with various environmental constrains such as multiple meta(loid) stresses due to its relative tolerance to meta(loid) toxicity.

The predominant role of pectin in binding Cd in the root cell wall of a high Cd accumulating rice line (Oryza sativa L.)

Cell wall (CW) plays an important role in Cd accumulation in roots of metal-tolerant plants, including rice. The role of CW polysaccharides, especially pectin, in binding Cd in roots of a high Cd accumulating (HA) rice line of Lu527-8 and a non-high Cd accumulating (NHA) rice line of Lu527-4 was investigated in this study. About 59%-63% of Cd in roots of the two rice lines was bound to CWs, indicating that CW was the main site for Cd accumulation in roots of the two rice lines. Cd adsorbed on the root CWs of the HA was 1.1-1.2 times more than that of the NHA, demonstrating the root CWs of the HA showed greater Cd binding ability. Cd exposure induced more Cd accumulation in pectin and hemicellulose in the HA. In particular, up to 65% of Cd accumulation in root CWs of the HA was observed in pectin. The removal of pectin lead to a 50% decrease for the amounts of Cd adsorption on root CWs of the HA, indicating that pectin was the major binding site for Cd in root CWs of the HA. The HA showed greater pectin methylesterase activities, resulting in lower degree of pectin methylesterification along with more low-methylesterified pectins in root CWs than the NHA. The more accumulation of low-methylesterified pectins in CWs induced by Cd contributed greatly to the high Cd accumulation in roots of the HA rice line of Lu527-8.

Pivotal role for root cell wall polysaccharides in cultivar-dependent cadmium accumulation in Brassica chinensis L

Polysaccharides are the main components of plant cell walls in which they make an important contribution to cadmium (Cd) fixation. However, knowledge regarding the role of root cell wall polysaccharides in Cd accumulation in low-Cd cultivars is limited. Here, we compared the differences in root cell wall polysaccharides between two cultivars of Brassica chinensis L. (pakchoi) with different Cd accumulation abilities. A hydroponic experiment was conducted using low- (Huajun 2) and high-Cd (Hanlv) pakchoi cultivars. We investigated Cd subcellular distribution and Cd accumulation in cell wall polysaccharides and examined polysaccharide modifications in root cell walls by Fourier transform infrared spectroscopy. A Cd adsorption kinetics experiment was conducted to examine the connection between Cd-induced polysaccharide modifications and Cd fixation by cell walls. Amounts of Cd were significantly higher and more Cd was bound to cell walls in the roots of Huajun 2 than in those of Hanlv. These results indicated that the greater Cd retention capacity of the root cell wall in Huajun 2 accounted for the low Cd accumulation in the shoot. Up to 79.4% and 32.1% of cell-wall-bound Cd was found in the pectin and hemicellulose 1, respectively, and higher amounts of Cd were found in these cell wall components of Huajun 2 than in those of Hanlv. Exposure to Cd significantly increased amounts of pectin and hemicellulose 1 in both pakchoi cultivars, but the pectin levels were significantly higher in Huajun 2 than in Hanlv. Huajun 2 had higher pectin methylesterase (PME) activity and a lower degree of pectin methyl-esterification (DM) than Hanlv, although Cd treatments resulted in increased PME activity and decreased DM in both cultivars. The higher Cd treatment (44.5 μM) resulted in enhanced Cd-binding capacity in root cell walls of the two cultivars with higher Cd adsorption levels in the root cell wall of Huajun 2. These results indicate that differences in the amount of cell wall polysaccharide and DM play key roles in establishing the genotypic differences underlying Cd accumulation in pakchoi. These findings conduce to a better understanding of the physiological mechanisms underlying low Cd accumulation in pakchoi and the breeding of new, low-Cd pakchoi cultivars.

Tolerance of Douglas Fir Somatic Plantlets to Aluminum Stress: Biological, Cytological, and Mineral Studies

Aluminum (Al) is well known as a potent inhibitor of plant growth and development. It is notably present in soils in the soluble and bioavailable form Al³⁺ when the soil pH drops below 5. This situation is frequent, especially in softwood forests when litter decomposition is slow. In the present work, we studied the effects of Al³⁺ on the growth and development of Douglas fir plantlets. Somatic plantlets, regenerated via somatic embryogenesis, were grown in vitro on media supplemented with different concentrations of aluminum chloride (AlCl₃): 0 µM, 200 µM, 500 µM. and 1 mM. We show that a concentration of 500 µM AlCl3 in medium significantly reduced root elongation (−21.8%), as well as stem growth (−14.6%). Also, a 25% reduction in dry mass of the plantlets was observed in presence of a concentration of 200 µM of AlCl3. Histological analysis of root tissues revealed significant damage, especially in conducting vessels. In addition, mineral cation content of plantlets was disturbed under Al exposure. More particularly, the Mg and K contents of needles and the Ca content of stems and needles were significantly reduced in presence of a concentration of 500 µM AlCl3 in the culture medium (−35.6%, −33.5%, −24%, and −34% respectively). However, all these damages appeared at relatively high Al concentrations when compared with other herbaceous species. This study shed light on the ability of Douglas fir in vitro plantlets to cope with the acid-driven toxicity of Al.

Tolerance and remedial potential of trees submitted to atrazine and sulfentrazone in the rhizosphere

Residual herbicides are important for agricultural production but they may be lost underground. Trees capable of removing or degrading these products are interesting to the agro system. The objective was to evaluate the tolerance and remedial potential of trees in soil contaminated by atrazine and sulfentrazone. The experiment was arranged in a 7 × 3 factorial scheme. Factor one was composed of Inga edulis Mart., Myrsine gardneriana A.DC., Schizolobium parahyba (Vell.) Blake, Toona ciliata M. Roem., Trichilia hirta L. and Triplaris americana L. Factor two consisted of monthly solutions of atrazine (1000 g ha^-1), sulfentrazone (150 g ha^-1) and water only (control), applied through subgrade irrigation. The following parameters were evaluated: visual intoxication, plant growth, and biomass accumulation. Cucumber biomass was used as an indicator of herbicide residues in soil. Symptoms of intoxication were found only in S. parahyba and T. americana. Growth and biomass of the species were not affected by herbicides, except for T. americana. The herbicides provided higher biomass for T. hirta. Saplings of I. edulis, M. gardneriana, S. parahyba, T. ciliate, and T. hirta tolerate atrazine and sulfentrazone. Triplaris americana is sensitive to sulfentrazone. Inga edulis decreased sulfentrazone residues in the soil.

Using tree leaves and barks collected from contaminated and uncontaminated areas as indicators of air metallic pollution

The aim of this study was to show the usefulness of woody species in assessing air pollution by cadmium, copper, lead, and zinc. For this, washed leaves, unwashed leaves, and barks of a coniferous species (Cupressus sempervirens var. fastigiata) and a broadleaved one (Eucalyptus cladocalyx F. Muell) were analyzed for cadmium, copper, lead and zinc contents. A transect sampling approach was carried out during spring 2016. Fifty stations were selected along an intensive traffic road, and three in a control area. The results showed that the highest concentrations were recorded in barks for copper (21.86 µg/g, 23.33 µg/g) and lead (14.53 µg/g, 63.33 µg/g), and in unwashed leaves for cadmium (0.57 µg/g, 1.19 µg/g) and zinc (48.94 µg/g, 47.6 µg/g) for E. cladocalyx F. Muell, and C. sempervirens var. fastigiata, respectively. Lead and zinc are the most accumulated metals compared to cadmium and copper in all samples. The studied metal contents in urban area were significantly higher than that of the control one. This represents that traffic road has influenced the metals contamination of the surrounding area. Results of the bioconcentration factor (BCF) for both species indicate that the studied species could be used as biomonitors to identify ecological problems and to predict effect on wildlife habitats. The highest values of metal accumulation index (MAI) indicate the effectiveness of these trees for controlling the air metals in the polluted areas. Result shows too that the studied species could be used for phytoextraction of heavy metals from the polluted soils and/or air.

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.

Miscanthus accessions distinctively accumulate cadmium for largely enhanced biomass enzymatic saccharification by increasing hemicellulose and pectin and reducing cellulose CrI and DP

In this study, total eight distinct Miscanthus accessions were collected from the cadmium (Cd)-supplied soil pots, and mild alkali pretreatments (0.5%, 1% NaOH) were then performed to enhance biomass enzymatic saccharification. Due to large Cd accumulation, all Miscanthus accessions showed significantly reduced cellulose levels and features (CrI, DP) with much increased hemicellulose and pectin contents in the mature stems. Under mild alkali pretreatments, all Miscanthus samples exhibited largely increased hexoses yields released from enzymatic hydrolysis, and one desirable accession had an almost complete biomass saccharification with the hexoses yield at 100% (% cellulose). Notably, the biomass residues remained from enzymatic hydrolysis upon 1% NaOH pretreatment could absorb 73-96% Cd (% of total), suggesting an applicable approach for Cd phyto-remediation. Hence, a hypothetic model was proposed to elucidate that the enhanced biomass saccharification should be mainly due to much reduced cellulose CrI and DP in the Cd-accumulated Miscanthus accessions.

Prediction of the structure of a novel amylopectin-based Cd-associated molecule in the stem of common reed grown in the presence of Cd

We previously found a novel Cd-associated molecule with an apparent molecular weight of 10–50 kDa in common reeds grown in the presence of Cd. The partial structure of this molecule was predicted by enzymatic digestion to release Cd from a trace amount that had been partially purified from the cell sap. The major component was branched α-glucan, whereas a peptide, β-1,4 glucan, and mannose were found as minor components. Uronic acids appeared to provide functional groups that bind Cd.

Pectinous cell wall thickenings formation - A common defense strategy of plants to cope with Pb

Lead, one of the most abundant and hazardous trace metals affecting living organisms, has been commonly detected in plant cell walls including some tolerant plants, mining ecotypes and hyperaccumulators. We have previously shown that in tip growing Funaria sp. protonemata cell wall is remodeled in response to lead by formation of thickenings rich in low-methylesterified pectins (pectin epitope JIM5 - JIM5-P) able to bind metal ions, which accumulate large amounts of Pb. Hence, it leads to the increase of cell wall capacity for Pb compartmentalization. Here we show that diverse plant species belonging to different phyla (Arabidopsis, hybrid aspen, star duckweed), form similar cell wall thickenings in response to Pb. These thickenings are formed in tip growing cells such as the root hairs, and in diffuse growing cells such as meristematic and root cap columella cells of root apices in hybrid aspen and Arabidopsis and in mesophyll cells in star duckweed fronds. Notably, all analyzed cell wall thickenings were abundant in JIM5-P and accumulated high amounts of Pb. In addition, the co-localization of JIM5-P and Pb commonly occurred in these cells. Hence, cell wall thickenings formed the extra compartment for Pb accumulation. In this way plant cells increased cell wall capacity for compartmentalization of this toxic metal, protecting protoplast from its toxicity. As cell wall thickenings occurred in diverse plant species and cell types differing in the type of growth we may conclude that pectinous cell wall thickenings formation is a widespread defense strategy of plants to cope with Pb. Moreover, detection of natural defense strategy, increasing plant cell walls capacity for metal accumulation, reveals a promising direction for enhancing plant efficiency in phytoremediation.

Douglas fir (pseudotsuga menziesii) plantlets responses to as, PB, and sb-contaminated soils from former mines

Phytoremediation of metalloids by conifers is not widely studied although they may be relevant for several contaminated sites, especially those located in cold areas and sometimes under dry climates. Here, seeds of Douglas fir were sown in greenhouse on three soils collected in two French former mines: a gold mine (soils L1 and L2) and a lead and silver mine (soil P). These soils are highly contaminated by Pb, As, and Sb at different concentrations. Plants were harvested after ten weeks. Growth parameters, primary metabolite content, and shoot and root ionomes were determined. Douglas firs grown on the soils L1 and P had a lower biomass than controls and a higher oxidation status whereas those grown on the soil L2 exhibited a more developed root system and only slight modifications of carbon and nitrogen nutrition. Based on trace element (TE) concentrations in shoots and roots and their translocation factor (TF), Douglas fir could be a relevant candidate for As phytoextraction (0.8 g. kg(-1) dry weight in shoots and a TF of 1.1) and may be used to phytostabilize Pb and Sb (8.8 g and 127 mg. kg(-1) in roots for Pb and Sb, respectively, and TF lower than 0.1).

Target or barrier? The cell wall of early- and later-diverging plants vs cadmium toxicity: differences in the response mechanisms

Increasing industrialization and urbanization result in emission of pollutants in the environment including toxic heavy metals, as cadmium and lead. Among the different heavy metals contaminating the environment, cadmium raises great concern, as it is ecotoxic and as such can heavily impact ecosystems. The cell wall is the first structure of plant cells to come in contact with heavy metals. Its composition, characterized by proteins, polysaccharides and in some instances lignin and other phenolic compounds, confers the ability to bind non-covalently and/or covalently heavy metals via functional groups. A strong body of evidence in the literature has shown the role of the cell wall in heavy metal response: it sequesters heavy metals, but at the same time its synthesis and composition can be severely affected. The present review analyzes the dual property of plant cell walls, i.e., barrier and target of heavy metals, by taking Cd toxicity as example. Following a summary of the known physiological and biochemical responses of plants to Cd, the review compares the wall-related mechanisms in early- and later-diverging land plants, by considering the diversity in cell wall composition. By doing so, common as well as unique response mechanisms to metal/cadmium toxicity are identified among plant phyla and discussed. After discussing the role of hyperaccumulators' cell walls as a particular case, the review concludes by considering important aspects for plant engineering.