Phytochelatins Research Articles

Advances in physiological mechanisms of selenium to improve heavy metal stress tolerance in plants.

Selenium (Se) is a metalloid mineral nutrient for human and animal health. Plants are the main foodstuff source of the Se intake of humans. For plants, the addition of an appropriate amount of Se could promotes growth and development, and improves the tolerance to environmental stress, especially stress from some of heavy metals (HM) stress, such as cadmium (Cd) and mercury (Hg). This paper mainly reviews and summarizes the physiological mechanism of Se in enhancing HM stress tolerance in plants. The antagonistic effect of Se on HM is a comprehensive effect that includes many physiological mechanisms. Se can promote the removal of excessive reactive oxygen species and reduce the oxidative damage of plant cells under HM elements stress. Se participates in the regulation of the transportation and distribution of HM ions in plants, and alleviates the damage caused by of HM stress. Moreover, Se combine with HM elements to form Se-HM complexes and promote the production of phytochelatins (PCs), thereby reducing the accumulation of HM ions in plants. Overall, Se plays an important role in plant response to HM stress, but current studies mainly focus on physiological mechanism, and further in-depth study on the molecular mechanism is essential to confirm the participation of Se in plant response to environmental stress. This review helps to comprehensively understand the physiological mechanism of Se in plant tolerance against to HM stress of plants, and provides important theoretical support for the practical application of Se in environmental remediation and agricultural development.

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

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

Understanding the mechanisms of zeolite in inhibiting Pb accumulation in different rice cultivars (Oryza sativa).

Zeolite is one of the potential passivating amendments for the immobilization of lead (Pb) in contaminated farmland soils. In this study, pot experiments were carried out to investigate the effects and the mechanisms of zeolite on Pb accumulation in two rice cultivars grown in a slightly Pb-contaminated soil. Results showed that Pb content in grains of Zheyou 18 (ZY-18) decreased by the addition of 6g zeolite kg-1 soil (E6), which can be attributed to the reduction in soil Pb availability, dissolved organic carbon (DOC), water-soluble iron (Fe) and manganese (Mn), and the transfer factor from soil to grain (TFsoil-grain). These reductions were mainly resulting from the significant increase in soil pH, glutathione (GSH), phytochelatins (PCs), and non-protein (NPT) content in rice root, and the decrease in soil redox potential (Eh), due to zeolite addition. Pb content in brown rice of DL-5 was not significantly affected with E6 treatment, whereas it was raised by applying 12g zeolite kg-1 soil (E12). The increase of Pb content of Donglian 5 (DL-5) grains with E12 treatment can be attributed to more Pb uptake by the root, higher Pb transfer factors (TFs) between various parts of rice, and significant decrease in GSH, PCs, and NPT contents in the root. It is concluded that a suitable rate of zeolite addition can immobilize Pb in slightly Pb-contaminated acidic soil. However, the final immobilization effect also depends on rice cultivars.

Exogenous Cysteine Improves Mercury Uptake and Tolerance in Arabidopsis by Regulating the Expression of Heavy Metal Chelators and Antioxidative Enzymes

Cysteine (Cys) is an essential amino acid component of the major heavy metal chelators, such as glutathione (GSH), metallothioneins (MTs), and phytochelatins (PCs), which are involved in the pathways of mercury (Hg) tolerance in plants. However, the mechanism through which Cys facilitates Hg tolerance in plants remains largely unclear. In this study, we investigated the effects of exogenous Cys on Hg uptake in the seedlings, roots, and shoots of Arabidopsis throughout 6 and 36 h of Hg exposure and on the regulation of Hg detoxification by heavy metal chelators and antioxidative enzymes. The results showed that exogenous Cys significantly improved Hg tolerance during the germination and seedling growth stages in Arabidopsis. Exogenous Cys significantly promoted Hg uptake in Arabidopsis roots by upregulating the expression of the Cys transporter gene AtLHT1, resulting in increased Hg accumulation in the roots and seedlings. In Arabidopsis seedlings, exogenous Cys further increased the Hg-induced glutathione synthase (GS1 and GS2) transcript levels, and the Hg and Hg + Cys treatments greatly upregulated MT3 expression after 36 h exposure. In the roots, MT3 was also significantly upregulated by treatment of 36 h of Hg or Hg + Cys. Notably, in the shoots, MT2a expression was rapidly induced (10-fold) in Hg presence and further markedly increased (20-fold) by exogenous Cys. Moreover, in the seedlings, exogenous Cys upregulated the transcripts of all superoxide dismutase (CuSOD1, CuSOD2, MnSOD1, FeSOD1, FeSOD2, and FeSOD3) within 6 h and subsequently increased the Hg-induced GR1 and GR2 transcript levels at 36 h, all of which could eliminate the promotion of reactive oxygen species production and cell damage caused by Hg. Additionally, exogenous Cys upregulated all the antioxidative genes rapidly in the roots and subsequently increased the expression of CuSOD1, CuSOD2, and MnSOD1 in the shoots. These results indicate that exogenous Cys regulates the transcript levels of heavy metal chelators and antioxidative enzymes differently in a time- and organ-specific manner under Hg stress. Taken together, our study elucidates the positive functional roles of exogenous Cys in the Hg uptake and tolerance mechanisms of Arabidopsis.

Low-molecular-weight organic acid-mediated tolerance and Pb accumulation in centipedegrass under Pb stress

Lead (Pb) is one of the most harmful, toxic pollutants to the ecological environment and humans. Centipedegrass, a fast-growing warm-season turfgrass, is excellent for Pb pollution remediation. Exogenous low-molecular-weight organic acid (LMWOA) treatment is a promising approach for assisted phytoremediation. However, the effects of this treatment on the tolerance and Pb accumulation of centipedegrass are unclear. This study investigated these effects on the physiological growth response and Pb accumulation distribution characteristics of centipedegrass. Applications of 400 μM citric acid (CA), malic acid (MA) and tartaric acid (TA) significantly reduced membrane lipid peroxidation levels of leaves and improved biomass production of Pb-stressed plants. These treatments mainly increased peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) activities and enhanced free protein (Pro), ascorbic acid (AsA) and phytochelatins (PCs) contents, ultimately improving the Pb tolerance of centipedegrass. Their promoting effects decreased as follows: TA>CA>MA. All the treatments decreased root Pb concentrations and increased stem and leaf Pb concentrations, thus increasing total Pb accumulation and TF values. MA had the best and worst effects on Pb accumulation and Pb transportation, respectively. CA had the best and worst effects on Pb transportation and Pb accumulation, respectively. TA exhibited strong effects on both Pb accumulation and transport. Furthermore, all treatments changed the subcellular Pb distribution patterns and distribution models of the chemical forms of Pb in each tissue. The root Pb concentration was more highly correlated with the Pb subcellular fraction distribution pattern, while the stem and leaf Pb concentrations were more highly correlated with the distribution models of the chemical forms of Pb. Overall, TA improved plant Pb tolerance best and promoted both Pb absorption and transportation well and is considered the best candidate for Pb-contaminated soil remediation with centipedegrass. This study provides a new idea for Pb-contaminated soil remediation with centipedegrass combined with LMWOAs.

Assessing the effects of nickel on, e.g., Medicago sativa L. nodules using multidisciplinary approach.

Industrial wastes and fertilizers can introduce excessive levels of nickel (Ni) into the environment, potentially causing threats to plants, animals, as well as human beings. However, the number of studies on the effects of Ni toxicity on nodules is fairly limited. To address this issue, the effects of increasing Ni concentration on alfalfa nodules were assessed at chemical, biochemical, and transcriptomic levels. For this purpose, plants were grown in soils supplied with Ni (control, 0 mg/kg; C1, 50 mg/kg; C2, 150 mg/kg; C3, 250 mg/kg; and C4, 500 mg/kg) for 90 days. Ni loads in leaves, roots, and nodules were monitored after the exposure period. A set of biochemical biomarkers of oxidative stress was determined in nodules including antioxidants and metal homeostasis as well as lipid peroxidation. Gene expression levels of the main targets involved in oxidative stress and metal homeostasis were assessed. Our data indicated a high concentration of Ni in leaves, roots, and nodules where values reached 25.64 ± 3.04 mg/kg, 83.23 ± 5.16 mg/kg, and 125.71 ± 4.53 mg/kg in dry weight, respectively. Moreover, a significant increase in nodule biomass was observed in plants exposed to C4 in comparison to control treatment and percentage increased by 63%. Then, lipid peroxidation increased with a rate of 95% in nodules exposed to C4. Enzymatic activities were enhanced remarkably, suggesting the occurrence of oxidative stress, with increased superoxide dismutase (SOD), glutathione reductase (GR), and ascorbate peroxidase (APX). Our results showed also a significant upregulation of SOD, GR and APX genes in nodules. Nodule homoglutathione (HGSH) levels increased with the different Ni concentrations, with a remarkable decrease of glutathione S-transferase (GST) activity and glutathione (GSH) content for the highest Ni concentration with 43% and 52% reduction, respectively. The phytochelatin (PC) and metallothionein (MT) concentrations increased in nodules, which implied the triggering of a cellular protection mechanism for coping with Ni toxicity. The results suggested that Ni promotes a drastic oxidative stress in alfalfa nodules, yet the expression of MT and PC to reduce Ni toxicity could be used as Ni stress bioindicators. Our findings provide new insights into the central role of alfalfa nodules in limiting the harmful effects of soil pollution. Therefore, nodules co-expressing antioxidant enzymes may have high phytoremediation potential.

Application of zinc oxide nanoparticles immobilizes the chromium uptake in rice plants by regulating the physiological, biochemical and cellular attributes.

Zinc oxide nano particles (ZnO NPs) have been employed as a novel strategy to regulate plant tolerance and alleviate heavy metal stress, but our scanty knowledge regarding the systematic role of ZnO NPs to ameliorate chromium (Cr) stress especially in rice necessitates an in-depth investigation. An experiment was performed to evaluate the effect of different concentrations of ZnO NPs (e.g., 0, 25, 50, 100mg/L) in ameliorating the Cr toxicity and accumulation in rice seedlings in hydroponic system. Our results demonstrated that Cr (100µM) severely inhibited the rice seedling growth, whereas exogenous treatment of ZnO NPs significantly alleviated Cr toxicity stress and promoted the plant growth. Moreover, application of ZnO NPs significantly augmented the germination energy, germination percentage, germination index, and vigor index. In addition, biomass accumulation, antioxidants (SOD, CAT, POD), nutrient acquisition (Zn, Fe) was also improved in ZnO NPs-treated plants, while the lipid peroxidation (MDA, H2O2), electrolyte leakage as well as Cr uptake and in-planta accumulation was significantly decreased. The burgeoning effects were more apparent at ZnO NPs (100mg/L) suggesting the optimum treatment to ameliorate Cr induced oxidative stress in rice plants. Furthermore, the treatment of ZnO NPs (100mg/L) reduced the level of endogenous abscisic acid (ABA) and stimulated the growth regulator hormones such as brassinosteroids (BRs) possibly linked with enhanced phytochelatins (PCs) levels. The ultrastructure analysis at cellular level of rice revealed that the application of 100mg/L ZnO NPs protected the chloroplast integrity and other cell organells via improvement in plant ionomics, antioxidant activities and down regulating Cr induced oxidative stress in rice plants. Conclusively,observations of the current study will be helpful in developing stratigies to decrease Cr contamination in food chain by employing ZnO NPs and to mitigate the drastic effects of Cr in plants for the sustainable crop growth.

Synergetic modulation of plant cadmium tolerance via MYB75-mediated ROS homeostasis and transcriptional regulation.

MYB75 enhances plant cadmium tolerance by mediating ROS homeostasis and cadmium tolerance-related genes expression. Cadmium (Cd) is a heavy metal with biological toxicity, which can be detoxified through chelation and compartmentation in plants. Transcriptional regulation mediates plant Cd tolerance by modulating these processes. However, the mechanism remains to be studied. Our results showed a previously unknown function of MYB75 transcription factor in the regulation of Cd tolerance. Cd exposure stimulates anthocyanin accumulation by raising MYB75 expression. Enhanced Cd tolerance was observed in the MYB75-overexpressing plants, whereas increased Cd sensitivity was found in the MYB75 loss-of-function mutants. Under Cd stress conditions, lower reactive oxygen species (ROS) levels were detected in MYB75-overexpressing plants than in wild type plants. In contrast, higher ROS levels were found in MYB75 loss-of-function mutants. Overexpression of MYB75 was associated with increased glutathione (GSH) and phytochelatin (PC) content under Cd exposure. Furthermore, the expression of Cd stress-related gene including ACBP2 and ABCC2 was elevated in MYB75-overexpressing plants, and this upregulation was mediated through the mechanism by which MYB75 directly bind to the promoter of ACBP2 and ABCC2. Our findings reveal an important role for MYB75 in the regulation of plant Cd tolerance via anthocyanin-mediated ROS homeostasis, and through upregulation of Cd stress-related gene at the transcriptional level.

Selective removal of Hg2+/As3+/5+ from water system using Suaeda maritima plant based bio-adsorbent hybrid electro-deionization process

Heavy metal pollution is a critical environmental issue to provide safe potable water. Here, we prepared bio-adsorbent (PC@PN) by blending of phytochelatins (PCs) (derived from Suaeda maritima (L.) Dumort plant) in poly(acrylonitrile) (PN). We report bio-adsorbent hybrid electro-deionization (EDI) for the selective removal of Hg2+/As3+/5+ in aqueous medium, and in-situ regeneration of adsorbent by using water-splitting process. EDI process effectively reduced the Hg2+/As3+/5+ concentration from 100 ppb to < 1.0 ppb, while Na+ (present in natural water system) was partial removed (500 ppm to 200–300 ppm). Interestingly, removal efficiency of Hg2+/As3+/5+ was close to 100%, while < 50% removal of Cd2+, and Pb2+ were observed under similar conditions. Quite low energy requirement (1.02–1.53 kWh/kg) corresponding to about 90% current efficiency, revealed the sustainability of the process. In addition, adsorbent exhibited high regeneration up to 60 cycles of experiments, without any significant performance deterioration. The EDI process with bio-adsorbent is promising for selective trace removal of ionic toxicants from the water system.

Structural and Biophysical Analysis of the Phytochelatin-Synthase-Like Enzyme from Nostoc sp. Shows That Its Protease Activity is Sensitive to the Redox State of the Substrate.

Phytochelatins (PCs) are nonribosomal thiol-rich oligopeptides synthetized from glutathione (GSH) in a γ-glutamylcysteinyl transpeptidation reaction catalyzed by PC synthases (PCSs). Ubiquitous in plant and present in some invertebrates, PCSs are involved in metal detoxification and homeostasis. The PCS-like enzyme from the cyanobacterium Nostoc sp. (NsPCS) is considered to be an evolutionary precursor enzyme of genuine PCSs because it shows sufficient sequence similarity for homology to the catalytic domain of the eukaryotic PCSs and shares the peptidase activity consisting in the deglycination of GSH. In this work, we investigate the catalytic mechanism of NsPCS by combining structural, spectroscopic, thermodynamic, and theoretical techniques. We report several crystal structures of NsPCS capturing different states of the catalyzed chemical reaction: (i) the structure of the wild-type enzyme (wt-NsPCS); (ii) the high-resolution structure of the γ-glutamyl-cysteine acyl-enzyme intermediate (acyl-NsPCS); and (iii) the structure of an inactive variant of NsPCS, with the catalytic cysteine mutated into serine (C70S-NsPCS). We characterize NsPCS as a relatively slow enzyme whose activity is sensitive to the redox state of the substrate. Namely, NsPCS is active with reduced glutathione (GSH), but is inhibited by oxidized glutathione (GSSG) because the cleavage product is not released from the enzyme. Our biophysical analysis led us to suggest that the biological function of NsPCS is being a part of a redox sensing system. In addition, we propose a mechanism how PCS-like enzymes may have evolved toward genuine PCS enzymes.

Influence of arsenate imposition on modulation of antioxidative defense network and its implication on thiol metabolism in some contrasting rice (Oryza sativa L.) cultivars.

Globally, many people have been suffering from arsenic poisoning. Arsenate (AsV) exposure to twelve rice cultivars caused growth retardation, triggered production of As-chelatin biopeptides and altered activities of antioxidants along with increase in ascorbate (AsA)-glutathione (GSH) contents as a protective measure. The effects were more conspicuous in cvs. Swarnadhan, Tulaipanji, Pusa basmati, Badshabhog, Tulsibhog and IR-20 to attenuate oxidative-overload mediated adversities. Contrastingly, in cvs. Bhutmuri, Kumargore, Binni, Vijaya, TN-1 and IR-64, effects were less conspicuous in terms of alterations in the said variables due to reduced generation of oxidative stress. Under As(V) imposition, the protective role of phytochelatins (PCs) were recorded where peaks height and levels of PCs (PC2, PC3 and PC4) were elevated significantly in the test seedlings with an endeavour to detoxify cells by sequestering arsenic-phytochelatin (As-PC) complex into vacuole that resulted in reprogramming of antioxidants network. Additionally, scatter plot correlation matrices, color-coded heat map analysis and regression slopes demonstrated varied adaptive responses of test cultivars, where cvs. Bhutmuri, Kumargore, Binni, Vijaya, TN-1 and IR-64 found tolerant against As(V) toxicity. Results were further justified by hierarchical clustering. These findings could help to grow identified tolerant rice cultivars in As-prone soil with sustainable growth and productivity after proper agricultural execution.

Responses of antioxidant enzymes and key resistant substances in perennial ryegrass (Lolium perenne L.) to cadmium and arsenic stresses

Cadmium (Cd) and arsenic (As) exist simultaneously in soil environment, which poses a serious threat to the safety of agricultural products and forage production. Four Perennial Ryegrass (Lolium perenne L.) cultivars with different accumulation characteristics (ʻNicaraguaʼ, ʻVenusʼ, ʻExcellentʼ and ʻMonroʼ) were selected as the material for pot experiment. The coupled responses of key components and related enzyme activities under combined stresses of Cd and As were investigated. key components contents include Non protein sulfhydryl (NPT), glutathione (GSH) and phytochelatins (PCs). The related enzyme includes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), γ-glutamylcysteine synthetase (γ-ECS), glutathione synthetase (GSS), phytochelatin synthetases (PCSase) and arsenate reductase (AR). The results showed that Cd contents of perennial ryegrass were higher than those of As contents with TFCd/As < 1. Cd and As contents in roots were in the higher proportion than those in shoots. Compared to control, POD activities increased by 2.72 folds under 120 mg kg−1 As treatment. The contents of PCs increased by 5.68 folds under 120 mg kg−1 As treatment. Under combined Cd and As stress, the MDA contents and antioxidant enzyme activities of ʻVenusʼ were higher than those of ʻNicaraguaʼ. ʻNicaraguaʼ, a high accumulation cultivar. Under the combined stresses of Cd and As, the enzyme activities and the key components were significantly correlated (P < 0.05) with the contents of Cd and As. The tolerance to Cd and As was improved with increase in GSH and PCs contents and γ-ECS, GSS, PCSase and AR activities. In conclusion, the antioxidant enzyme system and key resistant substances of perennial ryegrass have important and antagonistic effects on Cd and As stresses.

Ectopic expression γ-glutamylcysteine synthetase of Vicia sativa increased cadmium tolerance in Arabidopsis

Glutathione (GSH) is a multifunctional essential biothiol, and its metabolism is important for plant against toxic metals and metalloids. γ-Glutamylcysteine (γ-EC), which is catalyzed by γ-Glutamylcysteine synthetase (γ-ECS), is a rate-limiting intermediate in GSH synthesis. Here, a γ-ECS gene (Vsγ-ECS) from Vicia sativa was cloned, and its function in modulating Cd tolerance was studied. Vsγ-ECS is a chloroplast localization protein, and the expression of Vsγ-ECS was upregulated by Cd stress in root of V. sativa. Heterologous expression of Vsγ-ECS (35S::Vsγ-ECS) in Arabidopsis enhanced the Cd tolerance of plants through improved primary root length, fresh weight, chlorophyll content and low degree of oxidation associated with reduced H2O2 and lipid peroxidation. However, the Cd accumulation of Arabidopsis had no effect on Vsγ-ECS overexpression. Further analysis showed that the increased Cd tolerance in 35S::Vsγ-ECS was mainly due to the capacity of increasing GSH synthesis that improved Cd chelation by GSH and phytochelatins (PCs) and alleviated the oxidative stress caused by Cd stress. In summary, a γ-ECS was characterized from V. sativa, and it demonstrated a property for increasing GSH and PC synthesis to protect plants from Cd poisoning.

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

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

AMF modulated rhizospheric microbial enzyme activities and their impact on sulphur assimilation along with thiol metabolism in pigeonpea under Cd stress

Presence of cadmium (Cd) in the soil has a negative impact on soil enzymatic activities, which, thus, reduce growth of the inhabiting plant species. Symbiotic association between plants and arbuscular mycorrhizal fungi-AMF have potential to overcome Cd stress. Therefore, the present study investigated the role of different AMF species [Claroideoglomus claroideum-Cc, C. etunicatum-Ce, Funneliformis mosseae-Fm and Rhizoglomus intraradices-Ri] in imparting Cd tolerance to pigeonpea-Cajanus cajan (L.) Millsp. Plants. The experiment was a complete factorial combination of two factors comprising of Cd concentrations (0, 25, 50 mg kg−1) and AMF treatments (Control/Non-AM, Cc, Ce, Fm, Ri) with twelve replicates for each treatment. A negative correlation between Cd concentration and plant growth, along with reactive oxygen species (ROS) build up were recorded. All mycorrhizal species reduced Cd uptake and lowered oxidative burden with maximum benefits provided by + Ri inoculation, followed by + Fm and + Ce, with + Cc the least. These effects could be associated with their relative root colonizing (RC) abilities and glomalin production in the soil. Greater efficiency of Ri than other three species could be related to maximum soil enzymatic activities [urease (URE), phosphatases (PHAs), dehydrogenase (DHase), arylsulfatase (AS)], which, enhanced the bioavailability of nutrients (P, S, N, K) to the plants. Moreover, higher activities of S assimilating enzymes [ATP sulfurylase (ATPS), γ-Glutamylcysteine synthetase (γ-ECS)] led to greater synthesis of thiol compounds [glutathiones (GSH), phytochelatins (PCs), non-protein thiol (NP-SH)] which, further reduced Cd translocation from roots to shoots. The study indicated that pigeonpea was highly responsive to different AMF species, with host-Ri combination displaying the strongest potential to alleviate Cd stress.

Biological responses to heavy metal stress in the moss Leptodictyum riparium (Hedw.) Warnst

Leptodictyum riparium, a widely distributed aquatic moss, can both tolerate and accumulate very high concentrations of toxic heavy metals, with only slight apparent damage. Here we report the effects on photosynthetic yield, glutathione (GSH), phytochelatin (PCn) synthesis, nitrogen metabolism and cellular localization of molecules rich in SH groups in L. riparium exposed in vitro to heavy metals. We simulated the concentrations of Cu, Zn, Cd, Pb detected in Regi Lagni, Italy, one of the most contaminated freshwater sites in Southern Europe, in the laboratory to test how the moss responds to heavy metal contamination. There was a steady decrease of photosynthetic efficiency correlated with the heavy metal concentrations and ultrastructural organization. All PCn levels increased significantly as the concentration of heavy metals increased, while the GSH levels did not appear to be particularly affected. A significant increase of GDH and NADH-GOGAT activities increased with increasing heavy metal concentration. Immunoblotting analysis revealed an increase of the chl-GS2 while no significant increase was detected in the cyt-GS1. These results give insight into the molecular events underlying the metal-tolerance of the aquatic moss L. riparium exposed to environmental heavy metal concentrations.

Bioremediation of arsenic contamination from the environment: New approach to sustainable resource management

Present acceleration of Arsenic [As] exposure leads to severe health problems. Modern scientific approaches look towards potent bio-agents for the removal of such types of contaminations in sustainable ways. Microbes can potentially change the redox potential, solubility, pH by different complex reactions during bioremediation. There are many enzymes present in the microbial system which are involved in methylation such as As (V) reductase, monomethyl arsonic acid reductase, As (III) methyltransferase, and MMA (III) methyltransferase. On the other hand, microbes have As transformation ability and changed into different extractable forms with sulfide minerals such as arsenopyrite (FeAsS), enargite (Cu3AsS4) and realgar (As4S4). In some bacteria, the As-operon machinery thiol group bind with As, itdetoxifies its toxicity. Ars R gene and arsenic reductase enzyme (Ars C) play the key role in the reduction of As (V) to As (III) and detoxify by being transported outside of the cell by Ars AB As chemiosmotic efflux system. In fungi, As (V) is reduced to As (III) by the arsenate reductase and GSH glutathione converted into GSSH glutathione disulfide. In plants, As (III) conjugates with phytochelatin (PC) or GSH glutathione and accumulates in the vacuole or is converted into less toxic forms in the presence of arsenic reductase enzyme. This review focused on the potentiality and mechanisms of different microbes for As-detoxification in a sustainable manner.

Cadmium binding during leaf senescence in Festuca arundinacea: Promotion phytoextraction efficiency by harvesting dead leaves

Phytoextraction by harvesting dead leaves is a novel cadmium (Cd) phytoremediation strategy in tall fescue (Festuca arundinacea), which provides feasibility for the phytoremediation of Cd-polluted soils and cleaner food production. The highest Cd in dead leaves is the result of Cd accumulation during the process of leaf senescence. However, it is not known the mechanism of Cd accumulation during the leaf senescence, which limits the phytoextraction efficiency of this technology. In this study, we found that the contents of phytochelatins (PC), glutathione (GSH), and non-protein thiols (NPT) were increased during the process of leaf senescence and Cd stress significantly promoted PC, GSH, and NPT. Transcriptome analysis showed that the pathway of glutathione metabolism was significantly enriched in the senescent leaf under Cd stress. 19 genes encoding GST, enzymes catalyzing GSH-Cd binding, were up-regulated in the senescent leaf. The increases of PC, GSH, and NPT in the senescent leaf for Cd-binding could be from the pathways of the protein degradation rather than their synthesis, because genes encoding cysteine protease (catalyzes protein degradation) were significantly promoted, but both GSH synthetase (GS) and PC synthetase (PCS) did not show the significant changes between the young and senescent leaves. Our results indicated that Cd accumulation during the leaf senescence could be the result of the promotion of Cd-binding by PC, GSH, and NPT, which provide insights into the regulatory mechanism and further genetic engineering to promote the phytoextraction efficiency by harvesting dead leaves in tall fescue.

Potential use of a novel actinobacterial species to ameliorate tungsten nanoparticles induced oxidative damage in cereal crops

Tungsten nanoparticles (WNPs) could induce hazard impact on plant growth and development; however, no study investigated their phytotoxicity. On the other hand, plant growth-promoting bacteria (PGPB) can effectively reduce WNPs toxicity. To this end, Nocardiopsis sp. was isolated and employed to mitigate the phytotoxic effect of WNPs on three crops (wheat, barley, and oat). Soil contamination with WPNs induced the W accumulation in all tested crops, inhibited both growth and photosynthesis and induced oxidative damage. On the other hand, pre-inoculation with Nocardiopsis sp. significantly reduced W level in treated plants. Concomitantly, Nocardiopsis sp. strikingly mitigated the inhibitory effect of WNPs by augmenting both growth and reactive oxygen species (ROS) homeostasis. To cope with heavy metal stress, all the tested species orchestrated their antioxidant homeostasis through enhancing the production of antioxidant metabolites (e.g., phenolics, flavonoids and tocopherols) and elevated the activities of ROS-scavenging enzymes (e.g., APX, POX, CAT, as well as the enzymes involved in AsA/GSH cycle). Moreover, pre-inoculation with Nocardiopsis sp. improved the detoxification metabolism by enhancing the accumulation of phytochelatins (PCs), metallothionein (MTC) and glutathione-S-transferase (GST) in grasses grown in WNPs-contaminated soils. Overall, restrained ROS homeostasis and improved WNPs detoxification systems were the bases underlie the WNPs stress mitigating impact of Nocardiopsis sp treatment.

Foliar application of flavonoids (rutin) regulates phytoremediation efficiency of Amaranthus hypochondriacus L. by altering the permeability of cell membranes and immobilizing excess Cd in the cell wall

The gap between the current serious soil heavy metal (HM) contamination and the low efficiency of soil remediation threatens human health. The aim of this study was to propose a method to improve the efficiency of phytoremediation by exogenous rutin application and explain the potential mechanism. A series of rutin treatments were designed to evaluate the biomass, cadmium (Cd) accumulation and physiological and biochemical responses of Amaranthus hypochondriacus under different Cd stresses. The results showed a decline in cell membrane damage with rutin application, and more Cd ions were immobilized in the cell wall than in the vacuole, resulting in an increase in Cd tolerance in plants. The addition of rutin caused significant effects on the synthesis of glutathione (GSH), including the advancement of the conversion of GSH to phytochelatins (PCs). Among them, PC2 and PC3 in the leaves contributed the most to the high accumulation of Cd. Overall, the phytoremediation efficiency and phytoextraction amount of Amaranthus hypochondriacus with rutin application were improved maximumly by 219.48% and 260.00%, respectively. This study provides a constructive approach for improving the efficiency of phytoremediation by foliar application of flavonoids and contributes to the further development of soil remediation in Cd-contaminated fields.