Pb Tolerance Research Articles

Lead tolerance and accumulation in Hirschfeldia incana, a Mediterranean Brassicaceae from metalliferous mine spoils.

Lead is a heavy metal of particular concern with respect to environmental quality and health. The lack of plant species that accumulate and tolerate Pb is a limiting factor to understand the molecular mechanisms involved in Pb tolerance. In this study we identified Hirschfeldia incana, a Brassicaceae collected from metalliferous mine spoils in Morocco, as a Pb accumulator plant. H. incana exhibited high Pb accumulation in mine soils and in hydroponic cultures. Major Pb accumulation occurred in the roots and a part of Pb translocated from the roots to the shoots, even to the siliques. These findings demonstrated that H. incana is a Pb accumulator species. The expression of several candidate genes after Pb-exposure was measured by quantitative PCR and two of them, HiHMA4 and HiMT2a, coding respectively for a P1B-type ATPase and a metallothionein, were particularly induced by Pb-exposure in both roots and leaves. The functional characterization of HiHMA4 and HiMT2a was achieved using Arabidopsis T-DNA insertional mutants. Pb content and primary root growth analysis confirmed the role of these two genes in Pb tolerance and accumulation. H. incana could be considered as a good experimental model to identify genes involved in lead tolerance and accumulation in plants.

Different strategies to achieve Pb-tolerance by the two Trebouxia algae coexisting in the lichen Ramalina farinacea

Lichen thalli are permeable to airborne substances, including heavy metals, which are harmful to cell metabolism. Ramalina farinacea shows a moderate tolerance to Pb. This lichen comprises two Trebouxia phycobionts, provisionally referred to as TR1 and TR9, with distinct physiological responses to acute oxidative stress. Thus, there is a more severe decay in photosynthesis and photosynthetic pigments in TR1 than in TR9. Similarly, under oxidative stress, antioxidant enzymes and HSP70 protein decrease in TR1 but increase in TR9. Since Pb toxicity is associated with increased ROS formation, we hypothesized greater Pb tolerance in this phycobiont. Accordingly, the aim of the present study was to characterize the physiological differences in the responses of TR1 and TR9 to Pb exposure. Liquid cultures of isolated phycobionts were incubated for 7 days in the presence of Pb(NO3)2. Thereafter, extracellular and intracellular Pb accumulation, photosynthetic pigments, and photosynthesis (as modulated chlorophyll fluorescence) were analyzed along with the antioxidant enzymes glutathione reductase (GR), superoxide dismutase (SOD), ascorbate peroxidase (APx), and catalase (CAT), and the stress-related protein HSP70. Pb uptake increased with the amount of supplied Pb in both algae. However, while significantly more metal was immobilized extracellularly by TR9, the amount of intracellular Pb accumulation was three times higher in TR1. In neither of the phycobionts were significant effects on photosynthetic pigments or photosynthetic electron transport observed. While under control conditions GR, SOD, and APx levels were significantly higher in TR1 than in TR9, only in the latter were these enzymes induced by Pb. This resulted in quantitatively similar antioxidant activities in the two algae when exposed to Pb. In conclusion, the phycobionts of R. farinacea make use of two different strategies against stress, in which the integration of distinct anatomical and physiological features affords similar levels of Pb tolerance.

Genome expression profile analysis reveals important transcripts in maize roots responding to the stress of heavy metal Pb

Lead (Pb) has become one of the most abundant heavy metal pollutants of the environment. With its large biomass, maize could be an important object for studying the phytoremediation of Pb-contaminated soil. In our previous research, we screened 19 inbred lines of maize for Pb concentration, and line 178 was identified to be a hyperaccumulator for Pb in both the roots and aboveground parts. To identify important genes and metabolic pathways related to Pb accumulation and tolerance, line 178 was underwent genome expression profile under Pb stress and a control (CK). A total of approximately 11 million cDNA tags were sequenced and 4 665 539 and 4 936 038 clean tags were obtained from the libraries of the test and CK, respectively. In comparison to CK, 2379 and 1832 genes were identified up- or downregulated, respectively, more than fivefolds under Pb stress. Interestingly, all the genes were related to cellular processes and signaling, information storage and processing or metabolism functions. Particularly, the genes involved in posttranslational modification, protein turnover and chaperones; signal transduction, carbohydrate transport and metabolism; and lipid transport and metabolism significantly changed under the treatment. In addition, seven pathways including ribosome, photosynthesis, and carbon fixation were affected significantly, with 118, 12, 34, 21, 18, 72 and 43 differentially expressed genes involved. The significant upregulation of the ribosome pathway may reveal an important secret for Pb tolerance of line 178. And the sharp increase of laccase transcripts and metal ion transporters were suggested to account in part for Pb hyperaccumulation in the line.

Lead tolerance and phytoremediation potential of Brazilian leguminous tree species at the seedling stage

A greenhouse pot experiment was conducted to evaluate the potential of three Brazilian leguminous woody species, Mimosa caesalpiniaefolia, Erythrina speciosa and Schizolobium parahyba, for the revegetation of lead- (Pb-) contaminated areas. The response of seedlings to increasing Pb concentrations (0, 250, 500 and 1000 mg kg−1) in the soil was studied. In addition to Pb accumulation and translocation, the following parameters were assessed: chlorophyll, nitrate, ammonia, lipid peroxidation (MDA) and free amino acid content; seedling growth; and nitrogenase activity. No differences were observed in the germination of woody species seeds sown in soils with or without Pb addition. M. caesalpiniaefolia did not show visual symptoms of Pb toxicity, while the other two species demonstrated stress symptoms, including reduced shoot biomass yield, leaf area and height. Biochemical analyses of plant tissues revealed markedly different responses to increasing Pb concentrations, such as changes in foliar soluble amino acid composition in S. parahyba; changes in ammonia and nitrate content in E. speciosa, M. caesalpiniaefolia and S. parahyba; and changes in MDA content in S. parahyba. The levels of chlorophyll a and b and carotenoid were affected in the species studied. For the Nitrogen-fixing (N2-fixing) species E. speciosa, an increase of Pb in the soil affected nodule formation and growth, which led to reduced nitrogenase activity in seedlings. The concentration of Pb in shoots and roots increased with the Pb concentration in soil. However, most of the Pb absorbed accumulated in the roots, and only a small fraction was translocated to aboveground parts. These findings were confirmed by the low bioconcentration factor (BCF) and translocation factor (TF) values for the three species. The tolerance index (TI) values suggested that M. caesalpiniaefolia, a N2-fixing tree, was the species that was most tolerant to high Pb concentrations in soil, while E. speciosa and S. parahyba showed moderate tolerance. Of the three Brazilian native woody species studied, M. caesalpiniaefolia was found to have the highest Pb tolerance and phytostabilisation potential in Pb-contaminated soils.

Pollution-induced community tolerance of freshwater biofilms: measuring heterotrophic tolerance to Pb using an enzymatic toxicity test

This study aims at investigating the impacts of Pb on freshwater biofilms with a pollution-induced community tolerance (PICT) approach using a recently developed short-term toxicity test based on β-glucosidase activity to measure biofilms' tolerance to Pb. We first investigated more closely the influence of the total suspended solids (TSS) concentrations of biofilm suspensions used for short-term toxicity tests performed to assess Pb tolerance. The Pb EC(50) values of four dilutions of the same biofilm suspension increased with their TSS concentrations. TSS-normalization allowed to obtain a unique measure of Pb tolerance, thus confirming that TSS-normalization of EC(50) values is a good means to estimate biofilm tolerance to Pb. The experiment was repeated with three different biofilm samples collected at different sites and dates. Second, biofilms were exposed to Pb (0, 1, 10 and 100 μg/L) for 3 weeks in microcosms to assess the impacts of Pb exposure on the communities. An increase in Pb tolerance was observed for the biofilm exposed to 100 μg/L. Automated Ribosomal Intergenic Spacer Analysis revealed modifications of bacterial and eukaryotic community structure with Pb exposure. Moreover, exposure to 100 μg/L Pb also led to an increase in Zn tolerance but not Cu tolerance. This study shows that tolerance acquisition to Pb can be detected after exposure to environmental concentrations of Pb using a PICT methodology and normalized EC(50) values as measures of Pb tolerance.

Evaluation of the ability of Nerium oleander L. to remediate Pb-contaminated soils

In the present paper we studied the accumulation of Pb in the species Nerium oleander, found growing in a mining area (Riotinto, SW Spain). Soil samples were taken around plants to characterize Pb contamination, and plant samples were collected in different sites in order to determine Pb content in leaves, stems and roots. Additionally, a hydroponic experiment was performed to determine critical Pb concentrations for plant growth and investigate the Pb tolerance strategy. Plants were exposed to different Pb concentrations (0, 20, 40, 80, 100μM) during 30days. We measured growth rates, water content (%), photosynthetic pigments (chlorophyll a, b, and carotenoids), and biochemical parameters (MAD, CAT, POD). The median total Pb concentration in soils where N. oleander grows was approximately 270mg/kg, but the plant accumulated only 2mg/kg in leaves, 13mg/kg in the inner part of the roots and 26mg/kg in the root bark (median values). Bioaccumulation (leaf/soil ratio) and translocation factor (leaf/root ratio) were lower than 1, indicating that the species did not translocate Pb to the aerial parts, which may be useful with regard to phytostabilizing Pb traits together with the ornamental flowers. Lead treatment inhibited plant growth and increased malondialdehyde (MDA) content in leaves, and at the higher Pb concentration (100μM), toxicity symptoms occurred.

Physiological responses and tolerance mechanisms to Pb in two xerophils: Salsola passerina Bunge and Chenopodium album L.

Lead (Pb) has great toxicity to human beings and other livings. Although there are varied ways to rehabilitate the Pb contaminated area, phytoremediation of Pb pollution in arid lands is still a difficult task, it is therefore urgent to find and identify Pb tolerant plants in arid areas. The physiological responses and tolerance mechanisms to Pb stress (expressed as the Pb concentration, e.g., 0, 50, 150, 300, 600, 800, 1000mg/L) were investigated for the xerophils Salsola passerina Bunge and Chenopodium album L. Results indicated that S. passerina exhibited higher Pb tolerance than Ch. album in terms of the seed germination rate, bio-activities of SOD and POD, and lower MDA production. There were two ways for S. passerina to reduce Pb toxicity in organism level, e.g., cell wall precipitation and state transfer of free Pb into anchorage. These findings demonstrate that S. passerina is a Pb tolerant species and may have potential application in phytoremediation of Pb contaminated arid lands.

Pb-induced ultrastructural alterations and subcellular localization of Pb in two species of Lespedeza by TEM-coupled electron energy loss spectroscopy

Transmission electron microscopy (TEM) coupled electron energy loss spectroscopy (EELS) was conducted in the present study, in order to explore mechanisms involved in Pb tolerance and detoxification in two species of Lespedeza (Fabaceae). Both Lespedeza chinensis and Lespedeza davidii have been recently shown to be suitable for chelate-assisted Pb phytoremediation. Ultrastructural alterations, Pb localization and speciation in leaf and root cells of both species were investigated after Pb exposure. Additionally, the influence of EDTA which facilitates Pb translocation in plants was investigated. In general, deleterious effects were more pronounced in root cells of both species than in leaf cells. Similar ultrastructural alterations such as vacuolization and dilation of the nuclear envelopes were induced in leaf and root cells of both species by Pb treatment. The effects were intensified with the addition of EDTA. Autophagy occurred only in leaf cells of L. davidii after Pb and Pb plus EDTA treatment, and in root cells of both species after Pb treatment. Moreover, electron-dense deposits with variable shapes such as acicular, granulated, punctuated and amorphous, were present in cell walls, intercellular spaces, cytoplasm and vacuoles of root cells of both species after Pb and Pb plus EDTA treatment. These deposits contained Pb as identified via the Pb-M 4,5 edge by EELS in a 120 kV TEM. As phosphorus was always measured at the sites of Pb deposition, it is likely that Pb in Lespedeza is stored as lead phosphate particularly in cell walls and vacuoles. The occasional presence of S and Ca in Pb depositions may point to additional other Pb compounds. Our studies suggest that Pb is transported both in apoplastic and symplastic pathways, independent of the presence or absence of EDTA. Different possible detoxification strategies such as sequestration in cell walls or compartmentalization in vacuoles exist both in L. chinensis and L. davidii. Only minor differences in ultrastructural alterations and Pb localization were observed between two species.

A comparison of lead accumulation and tolerance among heavy metal hyperaccumulating and non-hyperaccumulating metallophytes

The aim of the present study was to compare lead accumulation and tolerance among heavy metal hyperaccumulating and non-hyperaccumulating metallophytes. To this purpose, we compared Pb tolerance and accumulation in hydroponics among calamine and non-calamine populations of Silene vulgaris, Noccaea caerulescens, and Matthiola flavida. We established the effects of Ca on Pb tolerance and accumulation in M. flavida, and measured exchangeable soil Pb and Ca at two calamine sites. Results revealed that calamine populations of S. vulgaris and N. caerulescens were Pb hypertolerant, but the calamine M. flavida population was not. Pb hyperaccumulation capacity was exclusively found in one of the calamine N. caerulescens populations. 1) Pb hypertolerance is sometimes lacking in metallophyte populations from strongly Pb-enriched soil, probably due to a relatively high level of exchangeable soil Ca, 2) Ca effectively counteracts Pb uptake and Pb toxicity, 3) The tendency to hyperaccumulate Pb is a population-specific phenomenon in N. caerulescens, 4) Pb hypertolerance in N. caerulescens is not necessarily associated with a tendency to hyperaccumulate Pb, 5) apparent natural Pb hyperaccumulation in M. flavida is not reproducible in hydroponics, probably due to the absence of air-born contamination in laboratory experiments.

Tolerância e potencial fitorremediador de Stizolobium aterrimum associada ao fungo micorrízico arbuscular Glomus etunicatum em solo contaminado por chumbo

A poluição do solo com metais pesados tem aumentado significativamente nos últimos anos, devido à ação antrópica. Diversas técnicas podem ser utilizadas para reverter ou minimizar a condição de contaminação do solo, porém muitas delas são prejudiciais ao solo. Uma alternativa é a utilização da fitorremediação, já que as plantas possuem a capacidade de absorver elementos do solo e, dessa maneira, promover sua descontaminação com teores excessivos de metais e outros elementos potencialmente tóxicos. A associação de plantas com fungos micorrízicos arbusculares (FMAs) pode influenciar a absorção desses elementos. Com o objetivo de avaliar o potencial fitorremediador de plantas de Stizolobium aterrimum associadas ou não a FMAs em solos com concentrações crescentes de Pb, realizou-se um experimento sob condições de casa de vegetação, em esquema fatorial 4 x 2. Os tratamentos consistiram na adição de quatro doses de Pb (0, 250, 500 e 1.000 mg dm-3) e da inoculação ou não de FMA. Os resultados mostraram que a planta foi tolerante ao Pb nas doses utilizadas. A associação com FMA não influenciou a absorção de Pb pela planta. No entanto, a micorrização influenciou a fixação biológica de N2, observando-se maior atividade da enzima nitrogenase em plantas micorrizadas. Apesar dos bons resultados obtidos com relação à tolerância dessa planta ao Pb, mais estudos precisam ser realizados acerca da absorção desse elemento, principalmente em solo multicontaminado, que é a realidade encontrada em sistemas poluídos.

Lead (Pb)-Induced Regulation of Growth, Photosynthesis, and Mineral Nutrition in Maize (Zea mays L.) Plants at Early Growth Stages

The phytotoxic effects of lead (Pb) on seed germinability, seedling growth, photosynthetic performance, and nutrient accumulation (K(+) and Cu(2+)) in two maize genotypes (EV-1098 and EV-77) treated with varying levels of PbSO(4) (0.01, 0.1, and 1.0 mg L(-1)) were appraised in this study. In the seed germination experiment, lead stress significantly reduced seed germination percentage and index, plumule and radicle lengths as well as fresh and dry weights in both genotypes. In the second experiment, lengths and fresh and dry weights of shoots and roots decreased due to Pb in both genotypes with increase in plant age. Higher Pb levels also decreased photosynthetic rate (A), water use efficiency (A/E), and intrinsic water use efficiency (A/g(s)), but increased transpiration rate (E) and C(i)/C(a) ratio as a result of increase in stomatal conductance (g(s)). The concentrations of K(+) and Cu(2+) decreased in root, stem, and leaves of both genotypes, which could be a direct consequence of multifold increase in Pb concentration in these tissues. Overall, cv. EV-1098 had better Pb tolerance potential than EV-77 because the former genotype showed less reduction in seed germinability parameters, photosynthetic performance, and K(+) and Cu(2+) accumulation in shoot and root under lead stress.

Selected Morphological Characteristics, Lead Uptake and Phytochelatin Synthesis by Coffeeweed (Sesbania exaltata Raf.) Grown in Elevated Levels of Lead-Contaminated Soil

Remediation of lead-contaminated soil is significant due to the inherent toxicity of lead (Pb), and the quantity of Pb discharged into the soil. One of the most cost-effective and environmentally sound technologies for the cleanup of metal-contaminated soils is through the use of plants. While much is known about the ecological evolution of metal tolerance in plants, the physiological, biochemical, and genetic mechanisms of tolerance is not well understood in the majority of resistant ecotypes such as the legume, Sesbania exaltata Raf. This study was therefore conducted to determine the morphological and physiological characteristics of Sesbania that had been grown in Pb-contaminated soil, and to assess phytochelatin synthesis as a way of elucidating its relative Pb tolerance. Sesbania plants were grown in the greenhouse and exposed to various levels of Pb: 0, 1000, and 2000 mg Pb/kg soil. Plants were harvested after 6, 8, and 10 weeks of growth and morphological characteristics (e.g., root and shoot biomass, root length, number of root nodules, shoot height, number of leaves, number of flowers, number and length of pods) were recorded. Generally, there were no statistical differences in morphological characteristics among the treatments. Further, no discernible phytotoxic symptoms, such as chlorosis, wilting, or necrotic lesions, in neither roots nor shoots were observed. We concluded that while Sesbania did not fit the model of a hyperaccumulator, the plant was, nonetheless, tolerant to elevated Pb levels. Our assessment for phytochelatin synthesis as a tolerance mechanism was inconclusive and further investigations of tolerance mechanisms are warranted.

Elevated expression of TcHMA3 plays a key role in the extreme Cd tolerance in a Cd‐hyperaccumulating ecotype of Thlaspi caerulescens

Cadmium (Cd) is a highly toxic heavy metal for plants, but several unique Cd-hyperaccumulating plant species are able to accumulate this metal to extraordinary concentrations in the aboveground tissues without showing any toxic symptoms. However, the molecular mechanisms underlying this hypertolerance to Cd are poorly understood. Here we have isolated and functionally characterized an allelic gene, TcHMA3 (heavy metal ATPase 3) from two ecotypes (Ganges and Prayon) of Thlaspi caerulescens contrasting in Cd accumulation and tolerance. The TcHMA3 alleles from the higher (Ganges) and lower Cd-accumulating ecotype (Prayon) share 97.8% identity, and encode a P(1B)-type ATPase. There were no differences in the expression pattern, cell-specificity of protein localization and transport substrate-specificity of TcHMA3 between the two ecotypes. Both alleles were characterized by constitutive expression in the shoot and root, a tonoplast localization of the protein in all leaf cells and specific transport activity for Cd. The only difference between the two ecotypes was the expression level of TcHMA3: Ganges showed a sevenfold higher expression than Prayon, partly caused by a higher copy number. Furthermore, the expression level and localization of TcHMA3 were different from AtHMA3 expression in Arabidopsis. Overexpression of TcHMA3 in Arabidopsis significantly enhanced tolerance to Cd and slightly increased tolerance to Zn, but did not change Co or Pb tolerance. These results indicate that TcHMA3 is a tonoplast-localized transporter highly specific for Cd, which is responsible for sequestration of Cd into the leaf vacuoles, and that a higher expression of this gene is required for Cd hypertolerance in the Cd-hyperaccumulating ecotype of T. caerulescens.

Effects of Lead and EDTA-Assisted Lead on Biomass, Lead Uptake and Mineral Nutrients in Lespedeza chinensis and Lespedeza davidii

The ability of Lespedeza chinensis G. Don and Lespedeza davidii France of the family Fabaceae for Pb tolerance and accumulation in the presence and absence of the chelating agent EDTA was studied. Plants were cultured in soils containing different concentrations of Pb and Pb plus EDTA for 75 days. In the absence of EDTA, Pb supplement had no negative influence on plant growth of both species, however, stimulated the increase of shoot and root biomass of L. chinensis and L. davidii. Correspondingly, Pb accumulation in plants was low, and most of Pb was accumulated in roots, even though high doses of Pb had been supplied in soils. In the presence of EDTA, the Pb accumulation in all parts of both species was promoted, especially the translocation from roots to leaves. Regarding the influences of Pb and Pb plus EDTA treatment on the mineral nutrients, such as Mg, K, Ca, Fe, Mn, Cu and Zn in plants, Pb supplement resulted in both increase and decrease of mineral nutrients in L. chinensis and L. davidii in the absence of EDTA. In the presence of EDTA, the accumulation of mineral nutrients in plants was facilitated with the exception of Mn. The present results indicate that L. chinensis and L. davidii are Pb tolerant. Both plants can be used as potential species for chelate-assisted Pb phytoremediation. L. davidii has greater potential for phytoremediation of Pb than L. chinensis.

Identification of Calotropis procera L. as a potential phytoaccumulator of heavy metals from contaminated soils in Urban North Central India

Lead and cadmium levels were monitored in soil at fifteen urban (riverbank, roadside, industrial and residential) sites in the north central part of India. Calotropis procera, a hardy xerophytic plant was identified and selected for remedial potential as it was seen growing well at all sites. Root and leaf samples were collected simultaneously with soil samples to assess the characteristics of accumulation and tolerance of Pb and Cd in C. procera. Chlorophyll and phenological studies were undertaken to investigate the health of plants. The overall trend of Pb and Cd content in soil and plant samples was in the order Industrial>Roadside>Riverbank>Residential. The highest uptake of both the metals was observed in plants from industrial sites. Sites with more anthropogenic disturbance like vehicular and machinery exhausts exhibited reduced chlorophyll levels, stunted growth as well as a delayed, shortened reproductive phase. The ratios of Pb in leaves to Pb in soil were in the range of 0.60–1.37; while similar ratios of Cd were in the range of 1.25–1.83. Highly significant correlation coefficients were determined between concentrations of Pb and Cd in the samples with R2 values 0.839 for soil, 0.802 for leaf and 0.819 for root samples. The strong correlation between the degree of contamination and concentrations of Pb and Cd in plant samples identifies C. procera as an effective heavy metal remediator of contaminated lands coupled with environmental stress.

Expression of the Phytochelatin Synthase TaPCS1 in Transgenic Aspen, Insight into the Problems and Qualities in Phytoremediation of Pb

The response of Populus tremula × tremuloides cv. Etrepole transgenic lines expressing the phytochelatin synthase TaPCS1 for Pb tolerance and accumulation was studied. In a hydroponic experiment, the concentrations of Pb in plants did not differ significantly between any of the transgenic lines assayed and the wild type (wt) plants, with any of the Pb solutions tested. However, total biomass and Pb accumulation were significantly higher in transgenic lines (PTa3, PTa5, PTa10) than in the control (wt) line when the plants were grown in solutions containing 0.75 and 1.5 mM Pb. The PTa3 and PTa5 lines accumulated 1.7 times more Pb than the wt plants. A concentration of 3.0 mM Pb was found to be toxic for both transgenic and wt plants. Biomass production was higher in transgenic lines PTa3 and PTa5 than in the wt plants growing in M4 mining soil, accumulating more Pb and Zn than in the wt plants. When the plant material was grown in soil M15, none of the parameters differed significantly between the transgenic and wt plants. The different response in soils M4 and M15 indicated that the physicochemical properties of the soil play a determinant role in the phytoremediation potential.

Pb-induced cellular defense system in the root meristematic cells of Allium sativum L

BackgroundElectron microscopy (EM) techniques enable identification of the main accumulations of lead (Pb) in cells and cellular organelles and observations of changes in cell ultrastructure. Although there is extensive literature relating to studies on the influence of heavy metals on plants, Pb tolerance strategies of plants have not yet been fully explained. Allium sativum L. is a potential plant for absorption and accumulation of heavy metals. In previous investigations the effects of different concentrations (10-5 to 10-3 M) of Pb were investigated in A. sativum, indicating a significant inhibitory effect on shoot and root growth at 10-3 to 10-4 M Pb. In the present study, we used EM and cytochemistry to investigate ultrastructural alterations, identify the synthesis and distribution of cysteine-rich proteins induced by Pb and explain the possible mechanisms of the Pb-induced cellular defense system in A. sativum.ResultsAfter 1 h of Pb treatment, dictyosomes were accompanied by numerous vesicles within cytoplasm. The endoplasm reticulum (ER) with swollen cisternae was arranged along the cell wall after 2 h. Some flattened cisternae were broken up into small closed vesicles and the nuclear envelope was generally more dilated after 4 h. During 24-36 h, phenomena appeared such as high vacuolization of cytoplasm and electron-dense granules in cell walls, vacuoles, cytoplasm and mitochondrial membranes. Other changes included mitochondrial swelling and loss of cristae, and vacuolization of ER and dictyosomes during 48-72 h. In the Pb-treatment groups, silver grains were observed in cell walls and in cytoplasm, suggesting the Gomori-Swift reaction can indirectly evaluate the Pb effects on plant cells.ConclusionsCell walls can immobilize some Pb ions. Cysteine-rich proteins in cell walls were confirmed by the Gomori-Swift reaction. The morphological alterations in plasma membrane, dictyosomes and ER reflect the features of detoxification and tolerance under Pb stress. Vacuoles are ultimately one of main storage sites of Pb. Root meristematic cells of A. sativum exposed to lower Pb have a rapid and effective defense system, but with the increased level of Pb in the cytosol, cells were seriously injured.

Salicylic Acid-Involved in Lead Tolerance Associated with the Maintenance of Reducing Conditions in Mice

Lead (Pb(2+)) intoxication may initiate many disorders in human and animals. In a previous study, we demonstrate that application of salicylic acid protects nuclear DNA integrity against Pb(2+) stress in mice. In this study, further evidence showed that the exogenous salicylic acid-involved in Pb(2+) tolerance in mice was associated with the maintenance of reducing conditions in cells, as revealed by an increased antioxidative enzyme activity, elevated reduced glutathione content and ratio to oxidised form of glutathione, and decreased lipid peroxidation. In addition, the presence of a 30 kD protein was tightly linked to the Pb(2+) detoxification.

Induction of Lead‐Binding Phytochelatins in Vetiver Grass [Vetiveria zizanioides (L.)

Elevated lead (Pb) concentrations in residential houseyards around house walls painted with Pb-based pigments pose serious human health risks, especially to children. Vetiver grass (Vetiveria zizanioides L.) has shown promise for use in in situ Pb phytoremediation efforts. However, little is known about the biochemical mechanisms responsible for the observed high Pb tolerance by vetiver. We hypothesized that vetiver exposure to Pb induced the synthesis of phytochelatins (PC(n)) and the formation of Pb-PC(n) complexes, alleviating the phytotoxic effects of free Pb ions. Our main objective was to identify PC(n) and Pb-PC(n) complexes in root and shoot compartments of vetiver grass using high-performance liquid chromatography coupled to electrospray mass spectrometry (HPLC-ES-MS). After 7 d of exposure to Pb, vetiver accumulated up to 3000 mg Pb kg(-1) in shoot tissues, but much higher Pb concentrations were measured in root ( approximately 20,000 mg kg(-1)), without phytotoxic symptoms. Scanning electron micrographs showed Pb deposition in the vascular tissues of root and shoot, suggesting Pb translocation to shoot. Collision-induced dissociation analyses in MS/ MS mode during HPLC-ES-MS analysis allowed for the confirmation of four unique PC(n) (n = 1-4) based on their respective amino acid sequence. The high tolerance of vetiver grass to Pb was attributed to the formation of PC(n) and Pb-PC(n) complexes within the plant tissues, using ES-MS and Pb mass isotopic patterns. These data illustrate the mechanism of high Pb tolerance by vetiver grass, suggesting its potential usefulness for the remediation of Pb-contaminated residential sites.

Analysis of phytochelatin complexes in the lead tolerant vetiver grass [ Vetiveria zizanioides (L.)] using liquid chromatography and mass spectrometry

Ethylenediamene tetraacetic acid (EDTA) has been used to mobilize soil lead (Pb) and enhance plant uptake for phytoremediation. Chelant bound Pb is considered less toxic compared to free Pb ions and hence might induce less stress on plants. Characterization of possible Pb complexes with phytochelatins (PC n , metal-binding peptides) and EDTA in plant tissues will enhance our understanding of Pb tolerance mechanisms. In a previous study, we showed that vetiver grass ( Vetiveria zizanioides L.) can accumulate up to 19,800 and 3350 mg Pb kg −1 dry weight in root and shoot tissues, respectively; in a hydroponics set-up. Following the basic incubation study, a greenhouse experiment was conducted to elucidate the efficiency of vetiver grass (with or without EDTA) in remediating Pb-contaminated soils from actual residential sites where Pb-based paints were used. The levels of total thiols, PC n , and catalase (an antioxidant enzyme) were measured in vetiver root and shoot following chelant-assisted phytostabilization. In the presence of 15 mM kg −1 EDTA, vetiver accumulated 4460 and 480 mg Pb kg −1 dry root and shoot tissue, respectively; that are 15- and 24-fold higher compared to those in untreated controls. Despite higher Pb concentrations in the plant tissues, the amount of total thiols and catalase activity in EDTA treated vetiver tissues was comparable to chelant unamended controls, indicating lowered Pb toxicity by chelation with EDTA. The identification of glutathione (referred as PC 1) ( m/ z 308.2), along with chelated complexes like Pb–EDTA ( m/ z 498.8) and PC 1–Pb–EDTA ( m/ z 805.3) in vetiver root tissue using electrospray tandem mass spectrometry (ES-MS) highlights the possible role of such species towards Pb tolerance in vetiver grass.