Leaf chlorosis in oilseed rape plants (Brassica napus) grown on cadmium-polluted soil: causes and consequences for photosynthesis and growth

Heavy metal hyperaccumulating plants: how and why do they do it? And what makes them so interesting?

The term "hyperaccumulator" describes a number of plants that belong to distantly related families, but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without suffering phytotoxic effects. Three basic hallmarks distinguish hyperaccumulators from related non-hyperaccumulating taxa: a strongly enhanced rate of heavy metal uptake, a faster root-to-shoot translocation and a greater ability to detoxify and sequester heavy metals in leaves. An interesting breakthrough that has emerged from comparative physiological and molecular analyses of hyperaccumulators and related non-hyperaccumulators is that most key steps of hyperaccumulation rely on different regulation and expression of genes found in both kinds of plants. In particular, a determinant role in driving the uptake, translocation to leaves and, finally, sequestration in vacuoles or cell walls of great amounts of heavy metals, is played in hyperaccumulators by constitutive overexpression of genes encoding transmembrane transporters, such as members of ZIP, HMA, MATE, YSL and MTP families. Among the hypotheses proposed to explain the function of hyperaccumulation, most evidence has supported the "elemental defence" hypothesis, which states that plants hyperaccumulate heavy metals as a defence mechanism against natural enemies, such as herbivores. According to the more recent hypothesis of "joint effects", heavy metals can operate in concert with organic defensive compounds leading to enhanced plant defence overall. Heavy metal contaminated soils pose an increasing problem to human and animal health. Using plants that hyperaccumulate specific metals in cleanup efforts appeared over the last 20 years. Metal accumulating species can be used for phytoremediation (removal of contaminant from soils) or phytomining (growing plants to harvest the metals). In addition, as many of the metals that can be hyperaccumulated are also essential nutrients, food fortification and phytoremediation might be considered two sides of the same coin. An overview of literature discussing the phytoremediation capacity of hyperaccumulators to clean up soils contaminated with heavy metals and the possibility of using these plants in phytomining is presented.

Effect of cadmium-contaminated soils on dry matter yield and mineral composition of raya (Brassica juncea) and spinach (Spinacia oleracea)

Raya (Brassica juncea) and spinach (Spinacia oleracea), grown as leafy vegetables, are known to accumulate large amounts of heavy metals in their shoots and roots because of their high biomass and root proliferation. In a pot experiment, a sandy loam soil was polluted with cadmium (Cd) at rates of 0, 5, 10, 20, 40 and 80 mg kg−1 soil to assess the accumulation pattern and its effect on the dry matter yield and mineral composition of these vegetables. There was a decrease in dry matter yield due to the phytotoxic effect of Cd. The rate of Cd application at which a significant decline in root and shoot dry matter yield occurred varied depending on the vegetable. It was 20 mg Cd kg−1 soil in the shoots for both crops. However, the roots of raya were found to be more tolerant of Cd toxicity than those of spinach, as is evident from the fact that a significant decline in dry matter yield occurred at 20 and 10 mg Cd kg−1 soil, respectively. Since no visual toxic symptoms were observed on the leaves of raya in any of the treatments, it is clear that the metal may accumulate in this vegetable without visual evidence of its presence. However, at application levels beyond 40 mg kg−1 soil, toxicity symptoms, in the form of interveinal chlorosis of the leaf lamina followed by necrosis and leaf rolling, were clearly evident in the case of spinach. The reduction in root and shoot growth corresponded with the amounts of extractable Cd in the soils. The total content of Cd in the crops increased gradually as the rate of applied Cd rose and the roots accumulated much higher amounts than the shoots. The relationship of Cd with Zn and Fe was synergistic in both roots and shoots at the lower rates, but antagonistic at higher Cd application rates for both the crops, while in the case of Mn and Cu, the relationship was negative and antagonistic.

Cadmium accumulation and tolerance of two safflower cultivars in relation to photosynthesis and antioxidative enzymes

To investigate the effects of cadmium (Cd) on photosynthetic and antioxidant activities of safflower (Carthamus tinctorius L.) plants, two cultivars (Yuming and New safflower No. 4) were used for long-term pot experiment, under 0, 25, 50 or 100 mg Cd kg(-1) (DW) soil conditions. The results showed that there is a large amount of Cd (148.6-277.2 mg kg(-1)) accumulated in the shoot of safflower, indicating this species might be a potential Cd accumulator. Exposure to 25-100 mg Cd kg(-1) soil decreased the net photosynthetic rate by 25.6%-48.9% for New safflower No. 4, and 16.7%-57.3% for Yuming, respectively. The inhibition of photosynthesis might result from the limitation of stomatal conductance, reduction in photosynthetic pigment, and destruction of photosynthetic apparatus caused by Cd stress. Cd caused an enhancement of malondialdehyde (MDA), an increase in activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and a decrease in catalase (CAT) activity for both cultivars. It seems that SOD and APX accounted for the scavenging of oxidant stress in safflower cultivars. The physiological response of safflower plants to Cd stress was cultivar- and dose-dependent. New safflower No. 4 exhibited high photosynthetic performance at high Cd stress, which may be contributed by high intercellular CO(2) concentration, APX activity and Car/Chl ratio. In contrast, Yuming is more tolerant to Cd toxicity at low Cd level, in which an efficient antioxidant system is involved.

Iron stabilizes thylakoid protein-pigment complexes in Indian mustard during Cd-phytoremediation as revealed by BN-SDS-PAGE and ESI-MS/MS

Two-dimensional BN-SDS-PAGE, ESI-MS/MS and electron microscopy (EM) were used to study the role of iron (Fe) under cadmium (Cd) stress in retention of thylakoidal multiprotein complexes (MPCs) and chloroplast ultrastructure of Indian mustard, a moderate hyperaccumulator plant. Mustard was grown hydroponically with or without iron for 17 days and then exposed to CdCl2 for 3 days. Fe deficiency led to an increase in oxidative stress and damage to chloroplast/thylakoids accompanied by a decrease in chlorophyll content; exposure of plants to Cd further enhanced the oxidative stress and Cd accumulation (more in -Fe plants). However, the presence of iron aided plants in the suppression of oxidative stress and retention of chloroplasts and chlorophylls under Cd stress. Proteomic analyses by 2D BN-SDS-PAGE and mass spectrometry showed that Fe deficiency considerably decreased the amount of LHCII trimer, ATPase-F1 portion, cyt b6/f and RuBisCO. No or less reduction, was observed for PSI(RCI+LHCI), the PSII-core monomer, and the PSII subcomplex, while an increase in the LHCII monomer was noted. Under iron deficiency, Cd proved to be very deleterious to MPCs, except for the PSII subcomplex, the LHCII monomer and free proteins which were increased. Iron proved to be very protective in retaining almost all the complexes. MPCs showed greater susceptibility to Cd than Fe deficiency, mainly at the level of RuBisCO and cyt b6/f; an increase in the amount of the PSII subcomplex, LHCII monomer and free proteins indicates differences in the mechanisms affected by Fe deficiency and Cd stress when compared to Fe-fed plants. This study furthers our understanding of the sites actually damaged in MPCs under Fe deficiency and Cd stress. A role emerges for iron in the protection of MPCs and, hence, of the chloroplast. The present study also indicates the importance of iron for efficient phytoextraction/phytoremediation.

Comparative analysis of proteomic changes in contrasting flax cultivars upon cadmium exposure

Cadmium (Cd) is classified as a serious pollutant due to its high toxicity, high carcinogenicity, and widespread presence in the environment. Phytoremediation represents an effective low-cost approach for removing pollutants from contaminated soils, and a crop with significant phytoremediation potential is flax. However, significant differences in Cd accumulation and tolerance were previously found among commercial flax cultivars. Notably, cv. Jitka showed substantially higher tolerance to elevated Cd levels in soil and plant tissues than cv. Tábor. Here, significant changes in the expression of 14 proteins (related to disease/defense, metabolism, protein destination and storage, signal transduction, energy and cell structure) were detected by image and mass spectrometric analysis of two-dimensionally separated proteins extracted from Cd-treated cell suspension cultures derived from these contrasting cultivars. Further, two proteins, ferritin and glutamine synthetase (a key enzyme in glutathione biosynthesis), were only up-regulated by Cd in cv. Jitka, indicating that Cd tolerance mechanisms in this cultivar may include maintenance of low Cd levels at sensitive sites by ferritin and low-molecular weight thiol peptides binding Cd. The identified changes could facilitate marker-assisted breeding for Cd tolerance and the development of transgenic flax lines with enhanced Cd tolerance and accumulation capacities for phytoremediating Cd-contaminated soils.

Brassinosteroids protect Lycopersicon esculentum from cadmium toxicity applied as shotgun approach

Surface-sterilized seeds of two tomato cultivars (cv. K-25 and Sarvodya) were soaked in 100 microM CdCl(2) for 8 h (shotgun approach). The resulting 59-day-old seedlings were sprayed with 10(-8) M of 28-homobrassinolide (HBL) or 24-epibrassinolide (EBL) to their foliage. Both cultivars showed significantly different response to Cd stress. Cadmium severely restricted the growth, photosynthetic efficiency, and activity of nitrate reductase (E.C. 1.6.6.1) and carbonic anhydrase (E.C. 4.2.1.1) in Sarvodya as compared to K-25. However, the activities of antioxidative enzymes were significantly higher in K-25. This result may be considered an indication of better tolerance of the K-25 cultivars to Cd stress. Moreover, the spray of both the brassinosteroids (HBL/EBL) were found very effective in neutralizing the adverse effects generated by metals that reflect in better photosynthetic performance by the cultivars. An interesting aspect of this study is that HBL or EBL spray caused a further increase in proline content and antioxidative enzyme activities, which were already enhanced by Cd stress. This effect of brassinosteroids (HBL/EBL) was more pronounced in K-25 than in Sarvodya, representing the tolerance and adoptable behavior of K-25.

Cadmium tolerance of carbon assimilation enzymes and chloroplast in Zn/Cd hyperaccumulator Picris divaricata

To better understand the photosynthesis under stress, the effect of cadmium on carbon assimilation and chloroplast ultrastructure of a newly found Zn/Cd hyperaccumulator Picris divaricata in China was investigated in solution culture. The shoot and root Cd concentrations increased with increase in Cd supply, reaching maxima of 1109 and 5604mgkg(-1) dry weight at 75microM Cd, respectively. As Cd supply to P. divaricata increased, the shoot and root dry weight, leaf water content (except 75microM Cd), concentrations of chlorophyll a and b, chlorophyll a/b ratio and the concentration of carotenoids were not depressed at high Cd. However, the stomatal conductance, transpiration rate, net photosynthetic rate and intercellular CO(2) concentration were significantly affected when the Cd concentration reached 10, 10, 25 and 75microM, respectively. Meanwhile, carbonic anhydrase (CA; EC 4.2.1.1) activity and Rubisco (EC 4.1.1.39) content reached maxima in the presence of 50 and 5microM Cd, respectively. In addition, CA activity correlated positively with shoot Cd in plants treated with Cd at a range of 0-50microM. Moreover, the activities of NADP(+)-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13), Rubisco and fructose-1, 6-bisphosphatase (EC 3.1.3.11) were not significantly suppressed by increased Cd supply. Although the mesophyll cell size was reduced, chloroplast ultrastructure remained intact at the highest Cd treatment. Our finding revealed that P. divaricata chloroplast and the enzymes of carbon assimilation tolerate high levels of Cd, demonstrating its potential in possible application in phytoremediation.

Variability of zinc tolerance among and within populations of the pseudometallophyte species Arabidopsis halleri and possible role of directional selection

We estimated the level of quantitative polymorphism for zinc (Zn) tolerance in neighboring metallicolous and nonmetallicolous populations of Arabidopsis halleri and tested the hypothesis that divergent selection has shaped this polymorphism. A short-term hydroponic test was used to capture the quantitative polymorphism present between edaphic types, among and within populations. We measured six morphological and physiological traits on shoots and roots to estimate the response of A. halleri to Zn. In order to assess the adaptive value of Zn tolerance polymorphism, we compared differentiation of quantitative traits with that of molecular markers. Zinc tolerance of metallicolous populations was, on average, higher than that of nonmetallicolous populations according to the morphological and physiological traits measured. Phenotypic variability within edaphic types was very high and mainly explained by polymorphism among individuals within populations. Genetic differentiation for photosystem II yield of leaves (a measure of photosynthetic efficiency) was greater than the differentiation for microsatellite and thus, probably shaped by divergent selection. Overall, these results suggest that, in the sampled populations, Zn tolerance has been increased in metallicolous populations through selection on standing genetic variation within local nonmetallicolous ancestral populations.

Rapid quantitative assessment of visible injury to vegetation and visual amenity effects of fluoride air pollution

Quantitative measures of visible injury are proposed for the protection of the aesthetic acceptability and health of ecosystems. Visible indications of air pollutant injury symptoms can be assessed rapidly and economically over large areas of mixed species such as native ecosystems. Reliable indication requires close attention to the criteria for assessment, species selection, and the influence of other environmental conditions on plant response to a pollutant. The estimation of fluoride-induced visible injury in dicotyledonous species may require techniques that are more varied than the measurement of necrosis in linear-leaved monocotyledons and conifers. A scheme is described for quantitative estimates of necrosis, chlorosis and deformation of leaves using an approximately geometric series of injury categories that permits rapid and sufficiently consistent determination and recognises degrees of aesthetic offence associated with foliar injury to plants.

Effect of selenium on characteristics of rape chloroplasts modified by cadmium

Selenium appears to be an important protective agent that decreases cadmium-induced toxic effects in animals and plants. The aim of these studies was to investigate the changes of properties of chloroplast membranes obtained from Cd-treated rape seedlings caused by Se additions. Chloroplasts were isolated from leaves of 3-week-old rape plants cultured on Murashige-Skoog media supplied with 2 microM Na(2)SeO(4) and/or 400 microM CdCl(2) under in vitro conditions. The following physicochemical characteristics of chloroplasts were chosen as indicators of Se-effects: average size, zeta potential, ultrastructure, lipid and fatty acid composition and fluidity of envelope membrane. The results suggest that Se can partly counterbalance the destructive effects of Cd. This protective action led to an increase of chloroplast size reduced by Cd treatment and rebuilt, to some extent, the chloroplast ultrastructure. Lipid and fatty acid composition of chloroplast envelopes modified by Cd showed a decrease in digalactosyl-diacylglycerol content and an increase of content of monogalactosyl-diacylglycerol and phospholipid fractions, as well as an increase of fatty acid saturation of all lipids studied. The change in fatty acid saturation correlated well with a decrease of membrane fluidity and with a diminishing of absolute values of zeta potential. The presence of selenium in cultured media caused a partial reversal of the detected changes, which was especially visible in properties related to the hydrophobic part of an envelope, i.e. fatty acid saturation and fluidity.

A comparative analysis of fatty acid composition of root and shoot lipids in Zea mays under copper and cadmium stress

A comparative analysis of fatty acid composition was conducted in maize (Zea mays L.) under copper and cadmium stress. The unsaturation level (double-bond index) of phosphatidylethanolamine (PE) and digalactosyldiacylglycerol (DGDG) was increased in response to both metal treatments, whereas the phosphatidylinositol (PI), the phosphatidylcholine (PC) showed no significant changes. The Cu-treated roots showed a marked increase (about 2-fold) in the phospholipid (PL) content, while the Cd-treated roots showed a slight but insignificant increase. The steryl lipid SL/PL ratio was markedly decreased in response to Cu stress, and therefore, may indicate an activated phospholipid biosynthesis and turnover, in response to damage caused by Cu stress. The double bond indices of chloroplastic lipids: phosphatidylglycerol (PG), monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) revealed a similar but not identical pattern of change. The PG and MGDG contents in shoots were markedly decreased under Cu (by 53 and 48%) and Cd (by 78 and 65%) stress. The increase in the malondialdehyde (MDA) content in roots induced by both metals indicates lipid peroxidation. Generally, in the presence of Cu fatty acid composition was markedly modulated but to lesser extent under Cd stress. These results suggest that changes in the fatty acid composition under Cu and Cd stress conditions are metal-specific and may therefore result in differential metal tolerance.

Iron-induced oxidative stress in a macrophyte: a chemometric approach

Iron-induced oxidative stress in plants of Bacopa monnieri L., a macrophyte with medicinal value, was investigated using the chemometric approach. Cluster analysis (CA) rendered two distinct clusters of roots and shoots. Discriminant analysis (DA) identified discriminating variables (NP-SH and APX) between the root and shoot tissues. Principal component analysis (PCA) results suggested that protein, superoxide dismutase (SOD), ascorbic acid, proline, and Fe uptake are dominant in root tissues, whereas malondialdehyde (MDA), guaiacol peroxidase (POD), cysteine, and non-protein thiol (NP-SH) in shoot of the stress plant. Discriminant partial-least squares (DPLS) results further confirmed that SOD and ascorbic acid contents dominated in root tissues, while NP-SH, cysteine, POD, ascorbate peroxidase (APX), and MDA in shoot. MDA and NP-SH were identified as most pronounced variables in plant during the highest exposure time. The chemometric approach allowed for the interpretation of the induced biochemical changes in plant tissues exposed to iron.

Effects of cadmium on photosynthetic oxygen evolution from single stomata in Brassica juncea (L.) Czern

Scanning electrochemical microscopy (SECM) was utilized to investigate photosynthetic oxygen evolution from single stomata in leaves of live Brassica juncea (L.) Czern cultured in nutrient solution to which 0.2 or 0.01 mM CdC12 had been added. The bulk leaf surface serves as an insulator normally; therefore, a typical negative feedback was observed on the probe approach curves (PACs) when the probe approached epidermal cells. When the probe tip approached an open stoma, a higher tip current was detected due to the O2 release from this stoma. Thus, SECM can be used to map the O2 concentration profile near the leaf surface and study stomatal complex structure size and density. The oxygen release from single stomata was also analyzed by comparison of experimental PACs with those simulated by COMSOL multiphysics software (version 3.4). In addition to an increase in the stomatal complex size and a decrease in the complex density, the Cd accumulation caused up to a 26% decrease in photosynthetic rate determined at the level of a single stoma. The O2 evolution was also monitored by recording the tip current vs time when a tip sat above the center of a stoma. Periodic peaks in O2 release-time curves were observed, varying from 400 to 1600 s. The opening and closing activities of single stomata were also imaged by SECM.

The role of phytochelatins and antioxidants in tolerance to Cd accumulation in Brassica juncea L

A hydroponics experiment using Indian mustard (Brassica juncea L.) was conducted to investigate the effect of different concentrations (10-160 microM) of cadmium (Cd) and a fixed concentration (500 microM) of ethylene diamine tetra acetic acid (EDTA) on Cd accumulation and its toxicity for 14 and 28 days (d). The results showed that Cd alone and Cd+EDTA increased total dry biomass production, photosynthetic pigments and total protein content of B. juncea up to 160 microM with respect to control for 14d (hormesis effect). Further, on treatment with Cd at 160 microM for 28d, dry biomass of root and shoot, total protein content and total chlorophyll decreased up to 73%, 58%, 67% and 53% respectively, while in the case of Cd+EDTA, the decrease in the above parameters was 38%, 50%, 57% and 46% with respect to their control. It was observed that the maximum Cd accumulation after 28d in the root and shoot was 1925 and 977 mg kg(-1) dry weight (dw), respectively, while in the case of Cd+EDTA it was 1013 and 2316 mg kg(-1)dw, respectively. Levels of phytochelatins (PCs), glutathione reductase (GR; EC 1.6.4.2), non-protein thiols (NP-SH) and glutathione (GSH) were monitored as plants primary and secondary metal detoxifying responses. Glutathione reductase showed three-fold increased activity for Cd and 2.2-fold for Cd+EDTA at 160 microM after 14d followed by decreased activity after 28d with respect to control. Maximum synthesis of PCs was found at 10 microM of Cd exposure followed by a gradual decline after 28d. This may be correlated with reduced level of GSH, probably due to reduced GR activity, resulting in enhanced oxidative stress as also proved by phenotypic changes in plants such as browning of roots and yellowing of leaves. Thus, the capacity of B. juncea to accumulate and tolerate high concentrations of Cd, through enhanced level of PCs, GSH, NP-SH and GR suggests its applicability for phytoremediation.

Proteomics applied on plant abiotic stresses: role of heat shock proteins (HSP)

The most crucial function of plant cell is to respond against stress induced for self-defence. This defence is brought about by alteration in the pattern of gene expression: qualitative and quantitative changes in proteins are the result, leading to modulation of certain metabolic and defensive pathways. Abiotic stresses usually cause protein dysfunction. They have an ability to alter the levels of a number of proteins which may be soluble or structural in nature. Nowadays, in higher plants high-throughput protein identification has been made possible along with improved protein extraction, purification protocols and the development of genomic sequence databases for peptide mass matches. Thus, recent proteome analysis performed in the vegetal Kingdom has provided new dimensions to assess the changes in protein types and their expression levels under abiotic stress. As reported in this review, specific and novel proteins, protein-protein interactions and post-translational modifications have been identified, which play a role in signal transduction, anti-oxidative defence, anti-freezing, heat shock, metal binding etc. However, beside specific proteins production, plants respond to various stresses in a similar manner by producing heat shock proteins (HSPs), indicating a similarity in the plant's adaptive mechanisms; in plants, more than in animals, HSPs protect cells against many stresses. A relationship between ROS and HSP also seems to exist, corroborating the hypothesis that during the course of evolution, plants were able to achieve a high degree of control over ROS toxicity and are now using ROS as signalling molecules to induce HSPs.

Phytochelatin systhesis and cadmium uptake of Brassica napus

Soil contamination with cadmium (Cd) poses risk to human health. Metal hyperaccumulator plants play an important role in phytoextraction of heavy metals from such contaminated sites. Accumulation of Cd and its influence on the induction of phytochelatins in Brassica napus was investigated. Brassica napus plants were grown in nutrient culture with 1 and 5 microM Cd for 10 days. The biomass negatively correlates with Cd concentration in the nutrient solution and the reduction in dry weight was significantly higher for the root than the shoot. Cadmium accumulation positively correlates with the Cd concentration in the nutrient solution and the Cd accumulation in root is significantly higher than the shoot. High-performance liquid chromatography (HPLC) analysis revealed the induction of PC2, PC3 and PC4 in response to Cd in B. napus and their concentrations vary with the Cd level in the external solution. In 1 microM Cd treated plants; PC2 was the dominant thiol fraction in the root, followed by PC3 and PC4, whereas in the shoot, PC3 is the dominant species followed by PC4 and PC2. In 5 microM Cd treated plants, the concentration of both PC3 and PC4 are higher than that of PC2 in the roots. In the shoot, the concentration of PC3 and PC4 was higher than the PC2 irrespective of the quantity of Cd uptake, implying that the detoxification of Cd involves higher molecular weight thiol complexes in the shoot. Considering the high aboveground biomass and Cd accumulation in the shoot, B. napus can be a potential candidate for the phytoextraction of Cd.

Cross tolerance to heavy-metal and cold-induced photoinhibiton in leaves of Pisum sativum acclimated to low temperature

Under high light intensity, low temperatures as well as heavy metals induce photoinhibition of PSII and oxidative stress in leaves. Since cold acclimation of leaves ameliorates their capacity of antioxidative defence, cross tolerance between cold-induced and heavy metal-induced photoinhibition was investigated in pea leaves grown at either 22 °C or 6 °C. The experimental conditions were chosen to induce a uniform level of short-term photoinhibition at low temperature or in the presence of CuSO4 or CdCl2 in leaves grown at 22 °C. Under all conditions photoinhibition of PSII was lower in cold-acclimated (6°C-grown) than in non-acclimated (22°C-grown) pea leaves. In darkness PSII was not affected by all treatments. Other parameters like catalase activity, chlorophyll content and metabolite contents were most sensitive to CuSO4, but less affected by CdCl2 and low temperature treatments. Strong oxidation of ascorbate and concomitant loss of catalase activity showed the enhanced oxidative stress in CuSO4-treated leaves. Generally, all measured parameters were less affected in cold-acclimated leaves than in non-acclimated leaves under all experimental conditions. Cold-acclimated pea leaves contained higher levels of ascorbate and particularly of glutathione and a higher capacity to keep the primary electron acceptor of PSII more oxidised. Incubation with heavy metals caused a nearly complete loss of reduced glutathione. It is suggested that reduced glutathione served as a source for phytochelatin synthesis. The extraordinarily high glutathione content in cold-acclimated pea leaves might therefore increase their ability to chelate heavy metals and thus to protect leaves from heavy-metal induced damage.

Normalisation of real-time RT-PCR gene expression measurements in Arabidopsis thaliana exposed to increased metal concentrations

Accurate quantification by real-time RT-PCR relies on normalisation of the measured gene expression data. Normalisation with multiple reference genes is becoming the standard, but the best reference genes for gene expression studies within one organism may depend on the applied treatments or the organs and tissues studied. Ideally, reference genes should be evaluated in all experimental systems. A number of candidate reference genes for Arabidopsis have been proposed, which can be used as a starting point to evaluate their expression stability in individual experimental systems by available computer algorithms like geNorm and NormFinder. Using this approach, we identified the best three reference genes from a set of ten candidates, which included three traditional "housekeeping" genes, for normalisation of gene expression when roots and leaves of Arabidopsis thaliana are exposed to cadmium (Cd) and copper (Cu). The expression stabilities of AT5G15710 (F-box protein), AT2G28390 (SAND family protein) and AT5G08290 (mitosis protein YLS8) were the highest when considering the effect to the roots and shoots of Cd and Cu treatments. Even though the effect of Cd and excess Cu on the plants is very different, the same best reference genes were identified when considering Cd or Cu treatments separately. This suggests that these three genes may also be suitable when studying the gene expression after exposure of Arabidopsis thaliana to increased concentrations of other metals.

Vitamin E is essential for the tolerance of Arabidopsis thaliana to metal-induced oxidative stress

Arabidopsis (Arabidopsis thaliana) plants were grown in a hydroponic culture system for 7 to 14 d in the absence or presence of 75 microM Cd or 75 microM Cu. The Cu treatment resulted in visual leaf symptoms, together with anthocyanin accumulation and loss of turgor. Pronounced lipid peroxidation, which was detected by autoluminescence imaging and malondialdehyde titration, was observed in Cu-treated leaves. The Cd treatment also resulted in loss of leaf pigments but lipid peroxidation and oxidative stress were less pronounced than in the leaves exposed to Cu. Analysis of low-molecular-weight chloroplast and cytosolic antioxidants (ascorbate, glutathione, tocopherols, carotenoids) and antioxidant enzymes (thiol-based reductases and peroxidases) revealed relatively few responses to metal exposure. However, there was a marked increase in vitamin E (alpha-tocopherol) in response to Cd and Cu treatments. Ascorbate increased significantly in Cu-exposed leaves. Other antioxidants either remained stable or decreased in response to metal stress. Transcripts encoding enzymes of the vitamin E biosynthetic pathway were increased in response to metal exposure. In particular, VTE2 mRNA was enhanced in Cu- and Cd-treated plants, while VTE5 and hydroxylpyruvate dioxygenase (HPPD) mRNAs were only up-regulated in Cd-treated plants. Consistent increases in HPPD transcripts and protein were observed. The vitamin E-deficient (vte1) mutant exhibited an enhanced sensitivity towards both metals relative to the wild-type (WT) control. Unlike the vte1 mutants, which showed enhanced lipid peroxidation and oxidative stress in the presence of Cu or Cd, the ascorbate-deficient (vtc2) mutant showed WT responses to metal exposure. Taken together, these results demonstrate that vitamin E plays a crucial role in the tolerance of Arabidopsis to oxidative stress induced by heavy metals such as Cu and Cd.

Protective effect of selenium in Broccoli (Brassica oleracea) plants subjected to cadmium exposure

The protective effect of selenium against the cadmium-induced oxidative effect in broccoli ( Brassica oleracea) plants was studied. Plants grown in hydroponic culture were supplied with selenium [as Se(IV)] and cadmium [as Cd(II)], individually or simultaneously. Cadmium accumulation in roots was noticeably higher than in the aerial parts of the plants, and this effect was even more acute when selenium was simultaneously added. Cadmium phytotoxicity was evidenced by an increase in the malondialdehyde (MDA) concentration in the roots and a decrease of photosynthetic pigment and tocopherol concentration in the aerial parts of the plant. The simultaneous addition of selenium alleviated cadmium-induced stress in the roots after 40 days of exposition. In the leaves, a more remarkable decrease of tocopherol and chlorophyll concentration was observed in the cadmium-enriched plants after 10 days of exposure. The results provided evidence that selenium supplementation helps the plant to minimize the cadmium oxidant effect. Tocopherol concentration in broccoli fruit of cadmium-supplied plants was not affected in comparison to control. However, the proportion of alpha-tocopherol increases with the addition of selenium. This response is important not only for the protective effect against oxidative damage in the plant but also in terms of human nutrition.

Cadmium induced oxidative stress influence on glutathione metabolic genes of Camellia sinensis (L.) O. Kuntze

Glutathione, a tripeptide with sulfhydryl (-SH) group is a very crucial compound primarily involved in redox balance maintenance of the cellular environment. In this study, we monitored the influence of Cd exposure on the transcript levels of glutathione metabolic genes in bud tissues, the youngest leaf, of Camellia sinensis L. In addition, some physiochemical parameters were also studied. Cd exposure decreased chlorophyll and protein contents, while increase was observed in lipid peroxidation upon Cd treatments. These changes were found to be concentration and duration dependent, indicating the occurrence of oxidative stress upon Cd exposure. The transcript levels of glutathione biosynthetic genes viz. gamma-glutamylcysteine synthetase (gamma-ECS) and glutathione synthetase (GSHS) increased upon Cd exposure. Furthermore, transcript levels of glutathione reductase (GR), an enzyme involved in reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH), also showed upregulation on Cd exposure. However, the transcript levels of glutathione-S-transferase (GST), an enzyme involved in forming metal-GSH complex and help in sequestration of high levels of metal ions to vacuole, did not show any change on Cd treatment. This study document that Cd exposure induces oxidative stress in Camellia sinensis and the upregulation in transcript levels of glutathione metabolic genes except GST have suggested the role of these enzymes in the protection of plants from high level Cd exposure.

Oxidative stress and DNA damages induced by cadmium accumulation

Experimental evidence shows that cadmium (Cd) could induce oxidative stress and then causes DNA damage in animal cells, however, whether such effect exists in plants is still unclear. In the present study, Vicia faba plants was exposed to 5 and 10 mg/L Cd for 4 d to investigate the distribution of Cd in plant, the metal effects on the cell lipids, antioxidative enzymes and DNA damages in leaves. Cd induced an increase in Cd concentrations in plants. An enhanced level of lipid peroxidation in leaves and an enhanced concentration of H2O2 in root tissues suggested that Cd caused oxidative stress in Vicia faba. Compared with control, Cd-induced enhancement in superoxide dismutase activity was more significant at 5 mg/L than at 10 mg/L in leaves, by contrast, catalase and peroxidase activities were significantly suppressed by Cd addition. DNA damage was detected by neutral/neutral, alkaline/neutral and alkaline/alkaline Comet assay. Increased levels of DNA damages induced by Cd occurred with reference to oxidative stress in leaves, therefore, oxidative stress induced by Cd accumulation in plants contributed to DNA damages and was likely an important mechanism of Cd-phytotoxicity in Vicia faba plants.

Evidence for PSII donor-side damage and photoinhibition induced by cadmium treatment on rice (Oryza sativa L.)

The effects of cadmium (from 7.5 to 75 microM) on chloroplasts of rice were studied at the structural and biochemical level. Loss of pigments, reduction of thylakoids and decrease in oxygen evolution and Fv/Fm ratio occur in leaves following cadmium treatment. However, the amount of photosystem II reaction center proteins and that of its light harvesting complex is not affected, indicating that cadmium does not adversely influence the structural organization of this photosystem. In thylakoids isolated from cadmium-treated plants a loss in the capability to reduce 2,6-dichlorophenolindophenol is observed, which is partially restored if diphenylcarbazide is used as an electron donor, indicating that cadmium affects water splitting activity. In thylakoids isolated from control plants and treated with cadmium, diphenylcarbazide preserves most of the photosystem II activity lost after incubation with cadmium; most of the S(2) multiline electron paramagnetic resonance signal from the manganese cluster is lost, whereas the TyrD(+) and other signals are retained. Light-induced photosystem II damage, in vitro, is promoted by Cd-treatment as deduced from the mobility shift of the D1 protein observed by immunoblot.

Phytochelatin synthesis and response of antioxidants during cadmium stress in Bacopa monnieri L

The phytotoxicity imposed by cadmium (Cd) and its detoxifying responses of Bacopa monnieri L. have been investigated. Effect on biomass, photosynthetic pigments and protein level were evaluated as gross effect, while lipid peroxidation and electrolyte leakage reflected oxidative stress. Induction of phytochelatins and enzymatic and non-enzymatic antioxidants were monitored as plants primary and secondary metal detoxifying responses, respectively. Plants accumulated substantial amount of Cd in different plant parts (root, stem and leaf), the maximum being in roots (9240.11 microg g(-1) dw after 7 d at 100 microM). Cadmium induced oxidative stress, which was indicated by increase in lipid peroxidation and electrical conductivity with increase in metal concentration and exposure duration. Photosynthetic pigments showed progressive decline while protein showed slight increase at lower concentrations. Enzymes viz., superoxide dismutase (SOD, EC 1.15.1.1), guaiacol peroxidase (GPX, EC 1.11.1.7) ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) showed stimulation except catalase (CAT, EC 1.11.1.6) which showed declining trend. Initially, an enhanced level of cysteine, glutathione and non-protein thiols was observed, which depleted with increase in exposure concentration and duration. Phytochelatins induced significantly at 10 microM Cd in roots and at 50 microM Cd in leaves. The phytochelatins decreased in roots at 50 microM Cd, which may be correlated with reduced level of GSH, probably due to reduced GR activity, which exerted increased oxidative stress as also evident by the phenotypic changes in the plant like browning of roots and slight yellowing of leaves. Thus, besides synthesis of phytochelatins, availability of GSH and concerted activity of GR seem to play a central role for Bacopa plants to combat oxidative stress caused by metal and to detoxify it. Plants ability to accumulate and tolerate high amount of Cd through enhanced level of PCs and various antioxidants suggest it to be a suitable candidate for phytoremediation.

Cadmium-induced plant stress investigated by scanning electrochemical microscopy

In vivo oxygen evolution above single stomata in Brassica juncea has been used to investigate, for the first time, the effect of Cd-induced stress as imaged by scanning electrochemical microscopy (SECM). SECM images showed a clear stomatal structure-a pore, whose aperture is modulated by two guard cells, serving as the conduit for the oxygen produced. Lower stomatal density and larger stoma size were found in plants treated with 0.2 mM CdCl2 compared with control plants. Either the introduction of Cd caused a slower cell replication in the plane of the epidermis, hence fewer stomata, and/or the number of open stomata was reduced when plants were under Cd-stress. Oxygen evolution above individual stomatal complexes in Cd-treated plants was lower than that from control plants, as determined from the electrochemical current above the middle of each stoma. All guard cells under illumination were swollen, indicating that the stomata were open in both control and treated plants. Thus, decreased oxygen evolution in response to Cd cannot be attributed to simple closing of the stomata, but to a lower photosynthetic yield. SECM provides an excellent tool for monitoring the effects of Cd on photosynthetic activity at the scale of individual stomata.

The plant MT1 metallothioneins are stabilized by binding cadmiums and are required for cadmium tolerance and accumulation

The small Arabidopsis genome contains nine metallothionein-like (MT) sequences with classic, cysteine-rich domains separated by spacer sequences, quite unlike the small conserved MT families found vertebrate genomes. Phylogenetic analysis revealed four ancient and divergent classes of plant MTs that predate the monocot-dicot divergence. A distinct cysteine spacing pattern suggested differential metal ion specificity for each class. The in vivo stability of representatives of the four classes of plant MT proteins and a mouse MT2 control expressed in E. coli were enhanced by cadmium (Cd). Particular MTs were also stabilized by arsenic (As), copper (Cu), and or zinc (Zn). To understand why plants have such a diversity of MT sequences, the Arabidopsis MT1 class, comprised of three genes, MT1a, MT1b, and MT1c, was characterized in more detail in plants. MT1 family transcripts were knocked down to less than 5-10% of wild-type levels in Arabidopsis by expression of a RNA interference (RNAi) construct. The MT1 knockdown plant lines were all hypersensitive to Cd and accumulated several fold lower levels of As, Cd, and Zn than wildtype, while Cu and Fe levels were unaffected. The ancient class of MT1 protein sequences may be preserved in plant genomes, because it has distinct metal-binding properties, confers tolerance to cadmium, and can assist with zinc homeostasis.

Off-line separation and determination of rare earth elements associated with chloroplast pigments of hyperaccumulator Dicranopteris dichotoma by normal-phase liquid chromatography and ICP?MS

An off-line normal-phase liquid chromatography-ICP-MS method has been used for separation and determination of the rare earth elements (REE) associated with chloroplast pigments of Dicranopteris dichotoma. The stability of REE-bound pigments was tested, and almost no destruction of REE-bound pigments occurred during the so-called normal-phase liquid chromatography. The accumulated free REE ions on the microcrystalline cellulose column were cleaned by elution with 5 mmol L(-1) 2-ethylhexyl hydrogen 2-ethylhexylphosphonate (P507), to avoid exchange of these free ions with metals from the pigments. When these precautions were taken, the method was applied to the study of REE-bound pigments in D. dichotoma. ICP-MS results showed REE were present in chlorophylls and lutein, although REE concentrations in carotene and pheophytin were both below procedural blank levels. By careful analysis of the eluate fractions containing chlorophyll a it was found that REE-bound chlorophyll a in D. dichotoma was slightly enriched in the fractions with relatively short retention time. Results indicated that the retention time of REE-bound chlorophyll a might be slightly less than that of magnesium chlorophyll a, and REE-bound chlorophylls might be of relatively low polarity in comparison with magnesium bound chlorophylls. This phenomenon could be explained by the special double-decker sandwich-structure of REE-bound chlorophylls, as was reported by us and other authors. On the basis of these results we preferred to consider that REE can replace magnesium in chlorophyll a of D. dichotoma, and that the role of REE-bound chlorophylls in photosynthesis cannot be neglected. These data might be useful for understanding of both the properties of REE-bound pigments and the effect of REE on plant photosynthesis.

Responses induced by high concentration of cadmium in Phragmites australis roots

Cadmium is an important environmental pollutant with high toxicity to plants. We report the effects of high-dose Cd (100 micro M for 21 days) on the root apparatus of Phragmites australis plants, which are characterized by elevated water detoxification capacity and widely used in phytoremediation programmes. The examination of root sections by light and electron microscopy failed to reveal any significant cadmium-induced structural or ultra-structural modifications. However, histochemical localization of Cd disclosed accumulation of the metal in the parenchyma cells below the exodermis. Phytochelatins (PC) are thiol-rich peptides whose synthesis is induced by a range of metals. Our results indicate that total PC production increases after exposure to Cd, which suggests a pivotal role for phytochelatins in the sequestration of metal. Cd treatment also induced lignin deposition and marked stimulation of root antioxidant systems, suggesting that, because of its ability to adopt different strategies against the harmful effects of cadmium, Phragmites australis is a plant with high detoxification potential.

Interaction of cadmium with glutathione and photosynthesis in developing leaves and chloroplasts of Phragmites australis (Cav.) Trin. ex Steudel

We investigated how the presence of cadmium (Cd) at the emergence of Phragmites australis Trin. (Cav.) ex Steudel plants from rhizomes interacted with leaf and chloroplast physiological and biochemical processes. About 8.5 nmol Cd mg-1 chlorophyll was found in leaves, and 0.83 nmol Cd mg-1 chlorophyll was found in chloroplasts of plants treated with 50 microm Cd. As a result, a 30% loss of chlorophyll was measured concomitantly with a comparable percentage reduction in light-saturated photosynthesis. Rubisco content and activity were lowered by 10% and 60%, respectively. Antioxidant activity was stimulated by Cd treatment and was associated with an increase in the glutathione and pyridine pools, and with a larger pool of reduced glutathione. It is suggested that the glutathione pool and its predominance in the reduced state protected the activity of many key photosynthetic enzymes against the thiophilic binding of Cd. Chloroplast ultrastructure was not significantly altered with 50 microm treatment and the efficiency of photosystem II, measured as the fluorescence ratio Fv/Fm, remained high because F0 and Fm were proportionally decreased. In plants treated with 100 microm Cd, all effects were exacerbated, but Fv/Fm remained close to that of control leaves and the glutathione and pyridine nucleotides pools were lowered. The results suggest that glutathione exerted a direct important protective role on photosynthesis in the presence of Cd.

Heavy metal toxicity: cadmium permeates through calcium channels and disturbs the plant water status

Because plant wilting has been described as a consequence of cadmium (Cd2+) toxicity, we investigate Cd2+ effects on plant water losses, gas exchanges and stomatal behaviour in Arabidopsis thaliana L. Effects of 1-week Cd2+ application in hydroponic condition (CdCl2 10-100 micro m) were analyzed. A 10- micro m Cd2+ concentration had no significant effect on the plant-water relationship and carbon assimilation. At higher Cd2+ concentrations, a Cd2+ -dependent decrease in leaf conductance and CO2 uptake was observed despite the photosynthetic apparatus appeared not to be affected as probed by fluorescence measurements. In epidermal strip bioassays, nanomolar Cd2+ concentrations reduced stomatal opening under light in A. thaliana, Vicia faba and Commelina communis. Application of 5 micro m ABA limited the root-to-shoot translocation of cadmium. However, the Cd2+-induced stomatal closure was likely ABA-independent, since a 5-day treatment with 50 micro m Cd2+ did not affect the plant relative water content. Additionally, a similar Cd2+-induced stomatal closure was observed in the ABA insensitive mutant abi1-1. Interestingly, this mutant displayed a higher transpiration rate than the wild type but did not accumulate more Cd2+, arguing that Cd2+ uptake is not dependent only on the transpiration flow. Application of putative calcium channels inhibitors suppressed the inhibitory effect of Cd2+ in epidermal strip experiments, suggesting that Cd2+ could enter the guard cell through calcium channels. Patch-clamp studies with V. faba guard cell protoplasts showed that plasma membrane K+ channels were insensitive to external Cd2+ application whereas Ca2+ channels were found permeable to Cd2+. In conclusion, we propose that Cd2+ affects guard cell regulation in an ABA-independent manner by entering the cytosol via Ca2+ channels.