Assessing the potential for zinc and cadmium phytoremediation with the hyperaccumulator Thlaspi caerulescens

Thlaspi caerulescens is a Zn and Cd hyperaccumulator, and has been tested for its phytoremediation potential. In this paper we examine the relationships between the concentrations of Zn and Cd in soil and in T. caerulescens shoots, and calculate the rates of Zn and Cd extraction from soil. Using published data from field surveys, field and pot experiments, we show that the concentrations of Zn and Cd in the shoots correlate with the concentrations of Zn and Cd in soils in a log-linear fashion over three orders of magnitude. There is little systematic difference between different populations of T. caerulescens in the relationship between soil and plant Zn concentrations. In contrast, populations from southern France are far superior to those from other regions in Cd accumulation. Bioaccumulation factors (plant to soil concentration ratio) for Zn and Cd decrease log-linearly with soil metal concentration. Model calculations show that phytoremediation using T. caerulescens is feasible when soil is only moderately contaminated with Zn and Cd, and the phytoremediation potential is better for Cd than for Zn if the populations from southern France are used. Recent progress in the understanding of the mechanisms of Zn and Cd uptake by T. caerulescens is also reviewed.     

Soil pH Effects on Uptake of Cd and Zn by Thlaspi caerulescens

For phytoextraction to be successful and viable in environmental remediation, strategies that can optimize plant uptake must be identified. Thlaspi caerulescens is an important hyperaccumulator of Cd and Zn, whether adjusting soil pH is an efficient way to enhance metal uptake by T. caerulescens must by clarified. This study used two soils differing in levels of Cd and Zn, which were adjusted to six different pH levels. Thlaspi caerulescens tissue metal concentrations and 0.1 M Sr(NO₃)₂ extractable soil metal concentrations were measured. The soluble metal form of both Cd and Zn was greatly increased with decreasing pH. Lowering pH significantly influenced plant metal uptake. For the high metal soil, highest plant biomass was at the lowest soil pH (4.74). The highest shoot metal concentration was at the second lowest pH (5.27). For low metal soil, due to low pH induced Al and Mn toxicity, both plant growth and metal uptake was greatest at intermediate pH levels. The extraordinary Cd phytoextraction ability of T. caerulescens was further demonstrated in this experiment. In the optimum pH treatments, Thlaspi caerulescens extracted 40% and 36% of total Cd in the low and high metal soils, respectively, with just one planting. Overall, decreasing pH is an effective strategy to enhance phytoextraction. But different soils had various responses to acidification treatment and a different optimum pH may exist. This pH should be identified to avoid unnecessarily extreme acidification of soils.     

Intraspecific variation of metal preference patterns for hyperaccumulation in Thlaspi caerulescens: evidence from binary metal exposures

Metal preferences with regard to accumulation were compared between populations of the heavy metal hyperaccumulator Thlaspi caerulescens, originating from calamine, serpentine and non-metalliferous soils. Plants were exposed for 3 weeks to factorial combinations of concentrations of different metals in binary mixture in hydroponics. The nature and degree of the interactions varied significantly between populations. In the calamine, non-Cd/Ni-hyperaccumulating population, La Calamine, there were no one-sided or mutual antagonistic interactions among the metals with regard to their accumulation in the plant. In three other populations capable of Cd and Ni hyperaccumulation, from calamine, serpentine and non-metalliferous soil respectively, there were one-sided or mutual antagonistic interactions between Cd and Zn, Cd and Ni, and Ni and Zn, possibly resulting from competition for transporters involved in uptake or plant-internal transport. Significant synergistic interactions, probably resulting from regulation of transporter expression, were also found, particularly in the La Calamine population. All the populations seemed to express a more or less Zn-specific high-affinity system. The serpentine and the non-metallicolous populations seemed to posses low-affinity systems with a preference for Cd and Zn over Ni, one of which may be responsible for the Ni hyperaccumulation of the serpentine population in its natural environment. The calamine population from Ganges also seemed to express a strongly Cd-specific high-affinity system which is in part responsible for the Cd-hyperaccumulation phenotype exhibited by this population in its natural environment.     

Natural variation in cadmium tolerance and its relationship to metal hyperaccumulation for seven populations of Thlaspi caerulescens from western Europe

Thlaspi caerulescens J. & C. Presl is a distinctive metallophyte of central and western Europe that almost invariably hyperaccumulates Zn to> 1.0% of shoot dry biomass in its natural habitats, and can hyperaccumulate Ni to> 0.1% when growing on serpentine soils. Populations from the Ganges region of southern France also have a remarkable ability to accumulate Cd in their shoots to concentrations well in excess of 0.01% without apparent toxicity symptoms. Because hyperaccumulation of Cd appears to be highly variable in this species, the relationship between Cd tolerance and metal accumulation was investigated for seven contrasting populations of T. caerulescens grown under controlled conditions in solution culture. The populations varied considerably in average plant biomass (3.1‐fold), shoot : root ratio (2.2‐fold), Cd hyperaccumulation (3.5‐fold), shoot : root Cd‐concentration ratio (3.1‐fold), and shoot Cd : Zn ratio (2.6‐fold), but the degree of hyperaccumulation of Cd and Zn were strongly correlated. Two populations from the Ganges region were distinct in exhibiting high degrees of both Cd tolerance and hyperaccumulation (one requiring 3 µM Cd for optimal growth), whereas across the other five populations there was an inverse relationship between Cd tolerance and hyperaccumulation, as has been noted previously for Zn. Metal hyperaccumulation was negatively correlated with shoot : root ratio, which could account quantitatively for the differences between populations in shoot Zn (but not Cd) concentrations. On exposure to 30 µM Cd, the two Ganges populations showed marked reductions in shoot Zn and Fe concentrations, although Cd accumulation was not inhibited by elevated Zn; in the other five populations, 30 µM Cd had little or no effect on Zn or Fe accumulation but markedly reduced shoot Ca concentration. These results support a proposal that Cd is taken up predominantly via a high‐affinity uptake system for Fe in the Ganges populations, but via a lower‐affinity pathway for Ca in other populations. Total shoot Cd accumulated per plant was much more closely related to population Cd tolerance than Cd hyperaccumulation, indicating that metal tolerance may be the more important selection criterion in developing lines with greatest phytoremediation potential.     

Phytoextraction of cadmium with Thlaspi caerulescens

The in situ phytoextraction of cadmium from soils can only be achieved using plants that are both tolerant to high Cd concentrations and able to extract sufficient amounts of the metal. However, very few plant species are capable of remediating Cd polluted soils in a reasonable time frame. This paper aims to show that the population of the hyperaccumulator Thlaspi caerulescens J. & C. Presl. from Viviez (south of France), which has a high Cd-accumulating capability, is an efficient tool to remove Cd from contaminated soils. Roots of T. caerulescensViviez proliferate in hot spots of metals in soils which is particularly advantageous because of heterogeneity of the distribution of metal in polluted soils. Isotopic techniques showed that plants from this population acquire Cd from the same pools as non-accumulating species, but that it was much more efficient than non-hyperaccumulators at removing the metal from the soil labile pool. This is due: to (i) a specific rooting strategy, and (ii) a high uptake rate resulting from the existence in this population of Cd-specific transport channels or carriers in the root membrane. Growth and overall extraction can be improved with appropriate N fertilisation, supplied either as mineral fertilisers or uncontaminated sewage sludge. Selecting bigger plants is possible from within a suitable Cd-accumulating population to improve the phytoextraction process. Growing the Cd-accumulating populations results in a reduction in the availability of Cd and Zn as shown with field and lysimeter experiments conducted for several years. As a result, on a practical aspect, Cd hyperaccumulating populations of T. caerulescens may be used as a tool to efficiently reduce the availability of Cd in soils, providing appropriate populations are used.     

Physiological responses to Cd and Zn in two Cd/Zn hyperaccumulating Thlaspi species

In a model hyperaccumulation study a Cd/Zn hyperaccumulator Thlaspi caerulescens accession Ganges and a recently reported Cd/Zn hyperaccumulator Thlaspi praecox grown in increasing Cd and Zn concentrations in the substrate and in field collected polluted soil were compared. Plant biomass, concentrations of Cd and Zn, total chlorophylls and anthocyanins, antioxidative stress parameters and activities of selected antioxidative enzymes were compared. Increasing Cd, but not Zn in the substrate resulted in the increase of biomass of roots and shoots of T. praecox and T. caerulescens. The two species hyperaccumulated Cd in the shoots to a similar extent, whereas T. caerulescens accumulated more Zn in the shoots than T. praecox. Cadmium amendment decreased total chlorophyll concentration and glutathione reductase activity, and increased non-protein thiols concentration only in T. praecox, suggesting that it is less tolerant to Cd than T. caerulescens. In the field-contaminated soil, T. caerulescens accumulated higher Cd concentrations; but as T. praecox produced higher biomass, both species have similar ability to extract Cd.     

Nickel and zinc accumulation capacities and tolerance to these metals in the excluder Thlaspi arvense and the hyperaccumulator Noccaea caerulescens

Representatives of Brassicaceae species—the hyperaccumulator Noccaea caerulescens F.K. Mey and the metal excluder Thlaspi arvense L.—were compared in terms of their ability to accumulate nickel (Ni) and zinc (Zn) and their tolerance to these metals. Four ecotypes of N. caerulescens were used: the ecotypes La Calamine (LC, Belgium) and Saint Felix de Palliéres (SF, France) grow naturally on calamine soils rich in Zn, Cd, and Pb; the ecotype Monte Prinzera (MP, Italy) originates from serpentine soils rich in Ni, Co, and Cr; and the ecotype Lellingen (LE, Luxembourg) inhabits non-metalliferous soils. The plants of N. caerulescens were grown for 8 weeks in a half-strength Hoagland solution supplemented with 25, 100, 200, 300, and 400 μM Ni(NO₃)₂ (ecotypes LC, SF, MP, LE) or 100, 200, 400, 800, and 1000 μM Zn(NO₃)₂ (ecotypes LC, SF, LE); the plants of T. arvense were grown in the presence of 10, 20, 25, and 30 μM Ni(NO₃)₂ or 40, 50, 60, 70, 80 μM Zn(NO₃)₂. The toxic effect of Ni and Zn was assessed from changes in dry matter of roots and shoots of treated plants compared to untreated. The content of metals in roots and shoots was determined by means of atomic absorption spectrophotometry. The Ni-accumulating capacity of N. caerulescens ecotypes increased in the order: LC < SF < LE < MP, and the Zn-accumulating capacity increased in the row: LC < SF < LE. In the hyperaccumulating plant N. caerulescens, the increments of biomass started to decrease at a lower metal content in roots than in shoots, whereas the opposite pattern was observed in the metal excluder T. arvense. Since T. arvense plants accumulated Ni and Zn in roots, whereas N. caerulescens accumulated these metals in shoots, one may assume that the greater sensitivity of root growth compared with shoots in N. caerulescens was determined by more effective mechanisms of metal detoxification in shoots. Conversely, the higher sensitivity of shoot growth compared to root growth in T. arvense was determined by more effective mechanisms of metal detoxification in roots. Being more tolerant to Ni and Zn than T. arvense plants, the N. caerulescens ecotypes differed substantially in terms of metal-accumulating capacity and their tolerance to heavy metals. The ecotype originating from non-metalliferous soils (LE) accumulated larger amounts of Zn, but was less tolerant compared with ecotypes growing naturally on calamine soils (SF and LC), whereas the ecotype occurring on serpentine soils (MP) exhibited a markedly greater tolerance to Ni, compared with other ecotypes examined, as well as the largest accumulation of this metal. The results indicate the existence of different mechanisms responsible for plant tolerance to Ni and Zn; the study of these mechanisms is a promising direction for future research.     

Effects of land irrigation with partially-treated wastewater on Frankia survival and infectivity

Phytoextraction of Cd by some populations of Thlaspi caerulescens which have the ability to co-hyperaccumulate Cd and Zn requires information about the distribution of both metals within the plant at the organ-level. This work was conducted to determine whether the distribution and solubility of Cd and Zn in Thlaspi caerulescens are affected by the age of plant and organ, and whether Cd and Zn have a common distribution in the plant in soils contaminated by both metals. A series of pot experiments were conducted where a Cd- and Zn-hyperaccumulating population was grown on soils contaminated by Cd and Zn. Temporal changes in metal concentration of roots and of shoots was recorded, along with the water and CaCl₂ solubility of metals in the plant organs. Also, leaves were grouped according to their age and their respective content of Cd and Zn was measured. Both metals were present at higher concentrations in leaves than in roots. The whole-plant content of Zn decreased with time while that of Cd increased or remained unchanged. At harvest, young leaves exhibited higher Cd concentration than older, but the reverse was true for Zn. Both metals were more soluble in dry leaves and senescent leaves than in fresh material, and Zn was more water-soluble than Cd. In conclusion, the distribution of Cd and Zn in the hyperaccumulator T. caerulescensvaried according to the organ and plant age, and Cd and Zn were shown to have a different distribution within the plant.     

Investigating heavy-metal hyperaccumulation using Thlaspi caerulescens as a model system

Background

Metal-hyperaccumulating plant species are plants that are endemic to metalliferous soils and are able to tolerate and accumulate metals in their above-ground tissues to very high concentrations. One such hyperaccumulator, Thlaspi caerulescens, has been widely studied for its remarkable properties to tolerate toxic levels of zinc (Zn), cadmium (Cd) and sometimes nickel (Ni) in the soil, and accumulate these metals to very high levels in the shoot. The increased awareness regarding metal-hyperaccumulating plants by the plant biology community has helped spur interest in the possible use of plants to remove heavy metals from contaminated soils, a process known as phytoremediation. Hence, there has been a focus on understanding the mechanisms that metal-hyperaccumulator plant species such as Thlaspi caerulescens employ to absorb, detoxify and store metals in order to use this information to develop plants better suited for the phytoremediation of metal-contaminated soils.

Scope

In this review, an overview of the findings from recent research aimed at better understanding the physiological mechanisms of Thlaspi caerulescens heavy-metal hyperaccumulation as well as the underlying molecular and genetic determinants for this trait will be discussed. Progress has been made in understanding some of the fundamental Zn and Cd transport physiology in T. caerulescens. Furthermore, some interesting metal-related genes have been identified and characterized in this plant species, and regulation of the expression of some of these genes may be important for hyperaccumulation.

Conclusions

Thlaspi caerulescens is a fascinating and useful model system not only for studying metal hyperaccumulation, but also for better understanding micronutrient homeostasis and nutrition. Considerable future research is still needed to elucidate the molecular, genetic and physiological bases for the extreme metal tolerance and hyperaccumulation exhibited by plant species such as T. caerulescens.Key words: Zn, Cd, Ni, Thlaspi caerulescens, hyperacumulator, phytoremediation, heavy metal     

Metal concentration and metal mass of metallicolous, non metallicolous and serpentine Noccaea caerulescens populations, cultivated in different growth media

Aims

Evaluate the genetic and environmental variability of metal concentration and metal mass of Noccaea caerulescens, from metalliferous (MET), non metalliferous (NMET) and serpentine (SERP) soils.

Methods

18 populations were cultivated in 18 different growth conditions, such as a soil mine tailing, soils amended with zinc (Zn), cadmium (Cd) and nickel (Ni) salts (in mixtures or in monometallic salts) and a hydroponic solution with two Zn concentrations.

Results

MET populations had Zn concentrations lower than NMET and SERP in the different soils but higher Cd mass (the product of aerial biomass and foliar metal concentration). SERP had the highest Ni concentration and Ni mass values. The addition of Cd or Ni to a Zn-contaminated soil significantly decreases Zn concentration. In hydroponics, MET and NMET had equivalent Zn concentrations but these were three times higher than those obtained in soil experiments. Zn mass of NMET was significantly lower than MET with the latter having Zn mass values largely above those obtained in mine soil.

Conclusions

Results showed a large heterogeneity of responses among populations depending on the substrate used, and it was not possible to correctly assign a single population to its accurate origin with only one experiment. Finally, data on metal concentration obtained in culture soils are closer to those in field soils than those from hydroponics so that they could give a more accurate information on the accumulating capacity of Noccaea caerulescens and its use in phytoextraction of metals in field conditions.     

Do metal-rich plants deter herbivores? A field test of the defence hypothesis

Some plant species growing on metalliferous soils are able to accumulate heavy metals in their shoots up to very high concentrations, but the selective advantage of this behaviour is still unknown. The most popular hypothesis, that metals protect plants against herbivores, has been tested several times in laboratory conditions, with contradictory results. We carried out the first large-scale test of the defence hypothesis in eight natural populations of the model Zn hyperaccumulator Thlaspi caerulescens J. and C. Presl (Brassicaceae). In two climatic regions (temperate, Belgium–Luxembourg, and Mediterranean, southern France), we worked in metalliferous and in normal, uncontaminated environments, with plants spanning a wide range of Zn concentrations. We also examined the importance of glucosinolates (main secondary metabolites of Brassicaceae) as antiherbivore defences. When exposed to natural herbivore populations, T. caerulescens suffered lower herbivory pressures in metal-enriched soils than in normal soils, both in Belgium–Luxembourg and in southern France. The trapping of gastropods shows an overall lower population density in metalliferous compared to normal environments, which suggests that herbivory pressure from gastropods is lower on metalliferous soils. In addition, foliar concentration of glucosinolates was constitutively lower in all populations from metal-enriched soils, suggesting that these have evolved towards lower investment in organic defences in response to lower herbivory pressure. The Zn concentration of plants had a protective role only for Belgian metallicolous plants when transplanted in normal soils of Luxembourg. These results do not support the hypothesis that Zn plays a key role in the protection of T. caerulescens against enemies. In contrast, glucosinolates appear to be directly involved in the defence of this hyperaccumulator against herbivores.     

Nickel bioavailability in an ultramafic toposequence in the Vosges Mountains (France)

A serpentinised harzburgite outcrop located in the Vosges Mountains hosts a population of the Ni-hyperaccumulator Thlaspi caerulescens J. & C. Presl. A complete study was undertaken to relate the variability of Ni availability along the ultramafic toposequence to pedogenesis, soil mineralogy and functioning with X-Ray Diffraction, Transmission Electron Microscope observations coupled with Isotopic Exchange Kinetics and diethylenetriamine pentaacetic acid extraction of Ni. The soil profiles ranging from Dystric Cambisol to Hypermagnesic Hypereutric Cambisol were distributed unevenly along the toposequence probably due to geochemical variability of the bedrock and also complex quaternary erosion features. The richest soils were characterised by slight mineral weathering leading to Ni, Cr and Fe accumulation in the B horizons whereas the lowest saturated soils had very low-metal contents. Most soil minerals were inherited from the parent materials and there were only few traces of formation of secondary minerals. Primary minerals (e.g. serpentine, chlorite) contained low Ni concentrations (0.2%) whereas neoformed goethite, mainly in the B horizons of the richest soils, contained up to 4.3% Ni. Ni was probably sorbed onto amorphous Fe oxy−hydroxide particles (oxalate extraction) rather than incorporated within the crystal lattice of goethite. Ni availability in the B horizon of Hypereutric Cambisols was extremely high and so was the oxalate extractable Fe. At the toposequence level, there was a high level of Ni availability in the least weathered soils and a very low-availability level in the more intensively weathered soils (strongly acidic pH). Ni availability was unexpectedly positively correlated to pH and was controlled by soil mineralogy and Ni-bearing mineral phases. Ni hyperaccumulation (above 1,000 mg kg⁻¹) by native T. caerulescens was only reached in the Ni-rich soils as a consequence of the local edaphic factors. Ni uptake by T. caerulescens is strongly regulated by Ni availability in soils and therefore related to pedogenesis.     

Availability of cadmium and zinc accumulated in the leaves of Thlaspi caerulescens incorporated into soil

When grown on contaminated soil, hyperaccumulator plants contain high concentrations of metals which may return to the soil after senescence. This work was undertaken to assess the availability of Cd and Zn associated to the leaves of the hyperaccumulator Thlaspi caerulescens after incorporation into an uncontaminated soil. A Zn- and Cd- accumulator population of T. caerulescens was grown on a Cd- and Zn- contaminated soil previously labelled with ¹⁰⁹Cd. Leaves (TCL) were harvested, dried, ground and incorporated into the soil at a rate of 2.07 mg Cd kg⁻¹ and 51.9 mg Zn kg⁻¹. Then a pot experiment was conducted for 3 months with rye grass (Lolium perenne) and T. caerulescens. Rye grass was harvested monthly and T. caerulescens at the end of the experiment. Plant biomass was measured, along with the concentration of Cd, Zn and ¹⁰⁹Cd. Results showed that water-extractable metals in TCL were 69% for Zn and 33% for Cd. Addition of TCL to soil, depleted growth of rye grass, and improved that of T. caerulescens. At harvest, concentrations of both metals were increased in plants by TCL. Concentrations of Cd in rye grass increased with the cut number, while that of Zn decreased slightly. Rye grass extracted 1.6% of the total Cd and 0.9% of the total Zn, and T. caerulescens extracted up to 22.4% of the Cd and 7% of the Zn. About 94% of the Cd in rye grass and 86% in T. caerulescens was derived from TCL. In conclusion, metals associated with leaves of the hyperaccumulator T. caerulescens were very mobile after incorporation into the soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Organic Acids Accumulation and Antioxidant Enzyme Activities in Thlaspi caerulescens under Zn and Cd Stress

Growth, organic acid and phytochelatin accumulation, as well as the activity of several antioxidative enzymes, i.e. superoxide dismutase (SOD), ascorbate peroxidase (APX) guaiacol peroxidase (POX) and catalase (CAT) were investigated under Zn and Cd stress in hydroponically growing plants of Thlaspi caerulescens population from Plombières, Belgium. Tissue Zn and Cd concentration increased (the highest concentration of both was in roots) as the concentration of these metals increased in the nutrient solution. Increasing Zn concentration enhanced plant growth, while with Cd it declined compared to the control. Both metals stimulated malate accumulation in shoots, Zn also caused citrate to increase. Zn did not induce phytochelatin (PC) accumulation. In plants exposed to Cd, PC concentration increased with increasing Cd concentration, but decreased with time of exposure. Under Zn stress SOD activity increased, but APX activity was higher at 500 and 1000 μM Zn and CAT activity only at 500 μM Zn in comparison with the control. CAT activity decreased in Cd- and Zn-stressed plants. The results suggest that relative to other populations, a T. caerulescens population from Plombières, when grown in hydroponics, was characterized by low Zn and Cd uptake and their translocation to shoots and tolerance to both metals. The accumulation of malate and citrate, but not PC accumulation was responsible for Zn tolerance. Cd tolerance seems to be due to neither PC production nor accumulation of organic acids.     

Within and between population variation for zinc and nickel accumulation in two species of Thlaspi (Brassicaceae)

In this study, the differences in zinc (Zn) and nickel (Ni) hyperaccumulation were investigated between three populations of Thlaspi pindicum together with genetic variation within populations of T. pindicum and Thlaspi alpinum var. sylvium, both serpentine endemics. Three experiments were conducted under standard conditions in hydroponic assay. Each experiment contained three treatments of metal: 100 microm Zn, 100 microm Ni, and combined 100/100 microm Zn/Ni. Genetic variation within populations was determined using maternal families. No genetic variation within populations was found for either Zn or Ni hyperaccumulation for both T. pindicum and T. alpinum var. sylvium, but differences were observed for both Zn and Ni hyperaccumulation between populations of T. pindicum. In combined Zn/Ni treatments, Zn inhibited Ni translocation in both species, which is unexpected considering that these species are serpentine endemics and well known Ni hyperaccumulators. The lack of genetic variation for metal hyperaccumulation is possibly due to inbreeding. Since Zn hyperaccumulation is not manifested in the field, inadvertent uptake of Zn is a plausible hypothesis for its preferential uptake.     

Physiological responses to Cd and Zn in two Cd/Zn hyperaccumulating Thlaspi species

In a model hyperaccumulation study a Cd/Zn hyperaccumulator Thlaspi caerulescens accession Ganges and a recently reported Cd/Zn hyperaccumulator Thlaspi praecox grown in increasing Cd and Zn concentrations in the substrate and in field collected polluted soil were compared. Plant biomass, concentrations of Cd and Zn, total chlorophylls and anthocyanins, antioxidative stress parameters and activities of selected antioxidative enzymes were compared. Increasing Cd, but not Zn in the substrate resulted in the increase of biomass of roots and shoots of T. praecox and T. caerulescens. The two species hyperaccumulated Cd in the shoots to a similar extent, whereas T. caerulescens accumulated more Zn in the shoots than T. praecox. Cadmium amendment decreased total chlorophyll concentration and glutathione reductase activity, and increased non-protein thiols concentration only in T. praecox, suggesting that it is less tolerant to Cd than T. caerulescens. In the field-contaminated soil, T. caerulescens accumulated higher Cd concentrations; but as T. praecox produced higher biomass, both species have similar ability to extract Cd.     

Subcellular localisation of Cd and Zn in the leaves of a Cd-hyperaccumulating ecotype of<Emphasis Type="Italic"> Thlaspi caerulescens</Emphasis>

Thlaspi caerulescens (Ganges ecotype) is able to accumulate large concentrations of cadmium (Cd) and zinc (Zn) in the leaves without showing any toxicity, suggesting a strong internal detoxification. The distribution of Cd and Zn in the leaves was investigated in the present study. Although the Cd and Zn concentrations in the epidermal tissues were 2-fold higher than those of mesophyll tissues, 65–70% of total leaf Cd and Zn were distributed in the mesophyll tissues, suggesting that mesophyll is a major storage site of the two metals in the leaves. To examine the subcellular localisation of Cd and Zn in mesophyll tissues, protoplasts and vacuoles were isolated from plants exposed to 50 M Cd and Zn hydroponically. Pure protoplasts and vacuoles were obtained based on light-microscopic observation and the activities of marker enzymes of cytosol and vacuoles. Of the total Cd and Zn in the mesophyll tissues, 91% and 77%, respectively, were present in the protoplast, and all Cd and 91% Zn in the protoplast were localised in the vacuoles. Furthermore, about 70% and 86% of total Cd and Zn, respectively, in the leaves were extracted in the cell sap, suggesting that most Cd and Zn in the leaves is present in soluble form. These results indicate that internal detoxification of Cd and Zn in Thlaspi caerulescens leaves is achieved by vacuolar compartmentalisation.     

Nickel and zinc hyperaccumulation by Alyssum murale and Thlaspi caerulescens (Brassicaceae) do not enhance survival and whole-plant growth under drought stress

Nickel and Zn hyperaccumulation by Alyssum murale and Thlaspi caerulescens bear substantial energetic costs and should confer benefits to the plant. This research determined whether metal hyperaccumulation can increase osmotic adjustment and resistance to water stress (drought). Alyssum murale and Thlaspi caerulescens treated with low or high concentrations of Ni or Zn were exposed to moderate (?0·4 MPa) and severe (?1·0 MPa) water stresses using aqueous polyethylene glycol. In the absence of metals both water deficits inhibited shoot growth. Nickel and Zn hyperaccumulation did not ameliorate growth inhibition by either level of water stress. The water stress did not induce major changes in shoot metal concentrations of these constitutive hyperaccumulators. Moreover, metal hyperaccumulation had minimal effects on the osmolality of leaf‐sap extracts, relative water content of the shoots, or rate of evapotranspiration. It is concluded that Ni or Zn hyperaccumulation does not augment whole‐plant capacity for drought resistance in A. murale and T. caerulescens.     

Cadmium accumulation in populations of Thlaspi caerulescens and Thlaspi goesingense

The capacity to accumulate cadmium (Cd) and zinc (Zn) was compared in Thlaspi goesingense and four populations of Thlaspi caerulescens . Two populations of T. caerulescens were grown in hydroponics at five concentrations of Cd. In addition, plants were grown in pots containing compost in which three different concentrations of Cd and two concentrations of Zn were added. A field trial was conducted to compare Zn and Cd uptake by three populations of T. caerulescens on nine selected plots of the Woburn Market Garden Experiment (UK) which had been contaminated to different degrees with heavy metals owing to past applications of sewage sludge. Results show that the four populations of T. caerulescens had the same ability to hyperaccumulate Zn but were significantly different in terms of Cd accumulation. Two populations of T. caerulescens from Southern France accumulated much more Cd than the populations from Prayon (Belgium) and Whitesike (UK). Generally, uptake of Cd was not decreased by increased concentrations of Zn in the substrate. These results indicate that the mechanisms of Cd and Zn hyperaccumulation are not identical in this species. This is the first report of hyperaccumulation of Cd by T. goesingense , but the growth of this species was markedly reduced by the large concentrations of Zn in the substrate. Future work should focus on the differences between Cd and Zn uptake in hyperaccumulator plants at the species and population level.     

Phytoextraction of Zinc: Physiological and Molecular Mechanism

Zinc is an essential trace element, necessary for plants, animals, and microorganisms. Zn is required for many enzymes as a catalytic cofactor, for photosynthetic CO₂ fixation, and in maintaining the integrity of bio-membranes. However, Zn is potentially toxic when accumulated beyond cellular needs. Phytoextraction technique, which is a part of phytoremediation, has opened new avenues for remediation of Zn-contaminated places. Hyperaccumulators like Thlaspi caerulescens and Arabidopsis halleri have been identified, which can accumulate up to 40,000 mg kg^?1 Zn in the aerial parts of the plant body. Carboxylic acids, primarily malate, citrate, and oxalate, and amino acids are found to play an important role in Zn hyperaccumulation. Transmembrane metal transporters are assumed to play a key role in Zn metal uptake, xylem loading, and vacuolar sequestration. Members of CDF (cation diffusion facilitator) and ZIP (zinc-regulated transporter, iron-regulated transporter like protein) family have been implicated in Zn-metal-tolerance mechanisms. A potential metal-binding motif, containing multiple histidine residues, is found in the variable regions of almost all of the ZIP family, including ZIP1, ZIP2, ZIP4, ZRT1, and ZRT2. Overexpression of some Zn metal transporter genes like TcZNT1 (Thlaspi caerulescens Zn transporter1), TcHMA4 (Thlaspi caerulescens heavy metal ATPase) in Thlaspi caerulescens, AhMTP1;3 (Arabidopsis halleri metal transporter1;3) in Arabidopsis halleri, and PtdMTP1(Poplar metal transporter1) from a hybrid poplar confer Zn hypertolerance in Thlaspi, Arabidopsis, and Poplar plant species.