Overexpression of ZNT1 and NRAMP4 from the Ni Hyperaccumulator Noccaea caerulescens Population Monte Prinzera in Arabidopsis thaliana Perturbs Fe, Mn, and Ni Accumulation.

Metalliferous soils are characterized by a high content of metal compounds that can hamper plant growth. The pseudometallophyte Noccaea caerulescens is able to grow on metalliferous substrates by implementing both tolerance and accumulation of usually toxic metal ions. Expression of particular transmembrane transporter proteins (e.g., members of the ZIP and NRAMP families) leads to metal tolerance and accumulation, and its comparison between hyperaccumulator N. caerulescens with non-accumulator relatives Arabidopsis thaliana and Thlaspi arvense has deepened our knowledge on mechanisms adopted by plants to survive in metalliferous soils. In this work, two transporters, ZNT1 and NRAMP4, expressed in a serpentinic population of N. caerulescens identified on the Monte Prinzera (Italy) are considered, and their expression has been induced in yeast and in A. thaliana. In the latter, single transgenic lines were crossed to test the effect of the combined over-expression of the two transporters. An enhanced iron and manganese translocation towards the shoot was induced by overexpression of NcZNT1. The combined overexpression of NcZNT1 and NcNRAMP4 did perturb the metal accumulation in plants.

Influence of edaphic conditions and nitrogen fertilizers on cadmium and zinc phytoextraction efficiency of Noccaea caerulescens.

The success of cadmium phytoextraction operations with Noccaea caerulescens varies by a factor of 70 between sites of trials. However, soil factors driving the efficiency of cadmium (Cd) and zinc (Zn) phytoextraction are still poorly understood, as are the effects of nitrogen fertilizers. We studied biomass production and Cd and Zn uptake by two contrasting populations of N. caerulescens, Ganges (metallicolous) and Wilwerwiltz (non-metallicolous) grown in pots on a range of 24 field contaminated soils for 20 weeks. The addition of KNO3 and NH4NO3 fertilizers was also tested. Using model averaging of multiple regression models, we show that the major drivers of N. caerulescens growth are physical soil factors such as organic matter and soil bulk density while trace metal accumulation mainly relies on soil Cd and Zn exchangeable concentrations. We confirm the negative effect of soil copper (Cu) on growth, even at exchangeable concentrations below 30 mg kg-1, and therefore on uptake efficiency, while increasing soil lead (Pb) content was related to increased biomass probably due to a protective effect against soil pathogens. Finally, there is a small positive effect of nitrogen fertilization on biomass production only in soils with low initial nitrogen content (under 25 µg g-1 NO3-), while above this value, the positive impact of initial nitrogen content is offset by lower shoot Cd and Zn concentrations. Our data bring substantial information regarding the physico-chemical properties to ensure N. caerulescens growth: a soil bulk density under 1.05 kg/dm3, organic matter above 7% and pH under 7.5. We show that phytoextraction efficiency is maximal for moderate soil contamination in Cd (2-10 mg kg-1) and Zn (300-1000 mg kg-1).

Identification of target genes and processes involved in erucic acid accumulation during seed development in the biodiesel feedstock Pennycress (Thlaspi arvense L.).

We studied erucic acid accumulation in the biodiesel feedstock Pennycress (Thlaspi arvense L.) as a first step towards the development of a sustainable strategy for biofuel production in the EU territory. To that end, two inbred Pennycress lines of European origin, "NASC" and "French," were cultivated in a controlled chamber and in experimental field plots, and their growth, seed production and seed oil characteristics analyzed. Differences in some agronomical traits like vernalization (winter-French versus spring-NASC), flowering time (delayed in the French line) and seed production (higher in the French line) were detected. Both lines showed a high amount (35-39%) of erucic acid (22:1Δ13) in their seed oil. Biochemical characterization of the Pennycress seed oil indicated that TAG was the major reservoir of 22:1Δ13. Incorporation of 22:1Δ13 to TAG occurred very early during seed maturation, concomitant with a decrease of desaturase activity. This change in the acyl fluxes towards elongation was controlled by different genes at different levels. TaFAE1 gene, encoding the fatty acid elongase, seemed to be controlled at the transcriptional level with high expression at the early stages of seed development. On the contrary, the TaFAD2 gene that encodes the Δ12 fatty acid desaturase or TaDGAT1 that catalyzes TAG biosynthesis were controlled post-transcriptionally. TaWRI1, the master regulator of seed-oil biosynthesis, showed also high expression at the early stages of seed development. Our data identified genes and processes that might improve the biotechnological manipulation of Pennycress seeds for high-quality biodiesel production.

Aerobic methane emissions from stinkweed (Thlaspi arvense) capsules.

Aerobic methane (CH4) emission from plant vegetative parts has been confirmed by many studies. However, the origin of aerobic CH4 from plants and its emission from reproductive parts have not been well documented. We determined the effects of developmental stages (early, mid, late) and incubation conditions (darkness, dim light, bright light) on CH4 emissions from stinkweed (Thlaspi arvense) capsules. We found that CH4 emissions from capsules varied with developmental stage and incubation light. Methane emission was highest for the late harvested capsules and for those incubated under lower (dim) light condition. Our results also showed a significant negative correlation between CH4 emission and capsule moisture content. We conclude that CH4 emissions vary with capsule age and diurnal light environment.

New approaches to facilitate rapid domestication of a wild plant to an oilseed crop: example pennycress (Thlaspi arvense L.).

Oilseed crops are sources of oils and seed meal having a multitude of uses. While the domestication of soybean and rapeseed took extended periods of time, new genome-based techniques have ushered in an era where crop domestication can occur rapidly. One attractive target for rapid domestication is the winter annual plant Field Pennycress (Thlaspi arvense L.; pennycress; Brassicaceae). Pennycress grows widespread throughout temperate regions of the world and could serve as a winter oilseed-producing cover crop. If grown throughout the USA Midwest Corn Belt, for example, pennycress could produce as much as 840L/ha oils and 1470kg/ha press-cake annually on 16 million hectares of farmland currently left fallow during the fall through spring months. However, wild pennycress strains have inconsistent germination and stand establishment, un-optimized maturity for a given growth zone, suboptimal oils and meal quality for biofuels and food production, and significant harvest loss due to pod shatter. In this review, we describe the virtues and current shortcomings of pennycress and discuss how knowledge from studying Arabidopsis thaliana and other Brassicas, in combination with the advent of affordable next generation sequencing, can bring about the rapid domestication and improvement of pennycress and other crops.

Hyperaccumulation of zinc by Noccaea caerulescens results in a cascade of stress responses and changes in the elemental profile.

Noccaea caerulescens (J. & C. Presl) F. K. Meyer is a metal hyperaccumulating plant which can accumulate more than 2% zinc (Zn) dry tissue mass in its aerial tissues. At this concentration Zn is toxic to most plants due to inhibition of enzyme function, oxidative damage and mineral deficiencies. In this study the elemental and metabolite profiles of N. caerulescens plants grown in four different Zn concentrations were measured. This revealed broad changes in the metabolite and elemental profiles with the hyperaccumulation of Zn. The Zn treated plants exhibited no typical signs of stress such as chlorosis or reduced biomass, however, a range of metabolic stress responses, such as the modification of galactolipids and the major membrane lipids of plastids, and increases in oxylipins, which are precursors to the signalling molecules jasmonic and abscisic acids, as well as the increased synthesis of glucosinolates, was observed. Increases in particular organic acids and the ubiquitous metal cation chelator nicotianamine were also observed. The small molecule metabolite changes observed, however, did not account for the extreme Zn concentrations in the leaf tissue showing that the increase in nicotianamine production most likely negates Fe deficiency. The elemental analyses also revealed significant changes in other essential micronutrients, in particular, significantly lower Mn concentrations in the high Zn accumulating plants, yet higher Fe concentrations. This comprehensive elemental and metabolite analysis revealed novel metabolite responses to Zn and offers evidence against organic acids as metal-storage ligands in N. caerulescens.

TcOPT3, a member of oligopeptide transporters from the hyperaccumulator Thlaspi caerulescens, is a novel Fe/Zn/Cd/Cu transporter.

BACKGROUND: Thlaspi caerulescens is a natural selected heavy metal hyperaccumulator that can not only tolerate but also accumulate extremely high levels of heavy metals in the shoots. Thus, to identify the transportors involved in metal long-distance transportation is very important for understanding the mechanism of heavy metal accumulation in this hyperaccumulator. METHODOLOGY/PRINCIPAL FINDINGS: We cloned and characterized a novel gene TcOPT3 of OPT family from T. caerulescens. TcOPT3 was pronouncedly expressed in aerial parts, including stem and leaf. Moreover, in situ hybridization analyses showed that TcOPT3 expressed in the plant vascular systems, especially in the pericycle cells that may be involved in the long-distance transportation. The expression of TcOPT3 was highly induced by iron (Fe) and zinc (Zn) deficiency, especially in the stem and leaf. Sub-cellular localization showed that TcOPT3 was a plasma membrane-localized protein. Furthermore, heterogonous expression of TcOPT3 by mutant yeast (Saccharomyces cerevisiae) complementation experiments demonstrated that TcOPT3 could transport Fe(2+) and Zn(2+). Moreover, expression of TcOPT3 in yeast increased metal (Fe, Zn, Cu and Cd) accumulation and resulted in an increased sensitivity to cadmium (Cd) and copper (Cu). CONCLUSIONS: Our data demonstrated that TcOPT3 might encode an Fe/Zn/Cd/Cu influx transporter with broad-substrate. This is the first report showing that TcOPT3 may be involved in metal long-distance transportation and contribute to the heavy metal hyperaccumulation.

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.

Life history traits of the pseudometallophyte Thlaspi caerulescens in natural populations from Northern Europe.

We examined recruitment, survival, life cycle and fecundity of two metallicolous (M, on metalliferous calamine soils) and two non-metallicolous (NM, on normal soils) populations of Thlaspi caerulescens in Belgium and Luxemburg. In each population, permanent plots were monitored over two reproductive seasons. In M populations, plots were located in two contrasting environments (grass versus grove) in order to test the influence of vegetation cover on life strategy. Our results show that the monocarpic life cycle is dominant in all populations of T. caerulescens. However the length of the pre-reproductive period varies from several months (winter annuals) to 1 year or more (perennials), and is partly related to plant origin (M versus NM). Most plants growing in metalliferous environments were annuals, whereas NM plants were mostly perennials. These differences in life cycle were related to differences in survival during summer, which was better in NM than in M populations. Within each M population, different survival conditions and life cycles were observed according to vegetation cover. Plants growing in grass areas were mostly annuals and had a low survival rate in summer whereas grove plants were mostly perennials and survived better in summer. Our results suggest the selection of stress avoiders (shortening of life cycle) in M populations of T. caerulescens but only for individuals growing in grass areas. Summer survival seems to play a key role in selection of life strategy in T. caerulescens.

Generation of nonvernal-obligate, faster-cycling Noccaea caerulescens lines through fast neutron mutagenesis.

Noccaea caerulescens (formerly Thlaspi caerulescens) is a widely studied metal hyperaccumulator. However, molecular genetic studies are challenging in this species because of its vernal-obligate biennial life cycle of 7-9months. Here, we describe the development of genetically stable, faster cycling lines of N. caerulescens which are nonvernal-obligate. A total of 5500 M(0) seeds from Saint Laurent Le Minier (France) were subjected to fast neutron mutagenesis. Following vernalization of young plants, 79% of plants survived to maturity. In all, 80,000 M(2) lines were screened for flowering in the absence of vernalization. Floral initials were observed in 35 lines, with nine flowering in <12wk. Two lines (A2 and A7) were selfed to the M(4) generation. Floral initials were observed 66 and 87d after sowing (DAS) in A2 and A7, respectively. Silicle development occurred for all A2 and for most A7 at 92 and 123 DAS, respectively. Floral or silicle development was not observed in wild-type (WT) plants. Leaf zinc (Zn) concentration was similar in WT, A2 and A7 lines. These lines should facilitate future genetic studies of this remarkable species. Seed is publicly available through the European Arabidopsis Stock Centre (NASC).

Response of antioxidative enzymes and apoplastic bypass transport in Thlaspi caerulescens and Raphanus sativus to cadmium stress.

A hydroponics experiment using hyperaccumulator Thlaspi caerulescens (alpine pennycress) and non-specific accumulator Raphanus sativus (common radish) was conducted to investigate the short-term effect of increasing Cd concentrations (0, 25, 50, 75, 100 microM) on metal uptake, chlorophyll content, antioxidative enzymes, and apoplastic bypass flow. As expected, T. caerulescens generally showed better resistance to metal stress, which was reflected by higher Cd accumulation within plant tissues with no signs of chlorosis, or wilt. Glutathione reductase (GR) and superoxide dismutase (SOD) activities in fresh leaves were monitored as the plant metal-detoxifying response. In general, both plant species exhibited an increase trend of GR activity before declining at 100 microM likely due to excessive levels of phytotoxic Cd. SOD activity exhibited almost a similar variation pattern to GR and decreased also at 100 microM Cd. For both plant species, fluorescent PTS uptake (8-hydroxy-1,3,6-pyrenetrisulphonic acid) increased significantly with metal level in exposure solutions indicating that Cd has a comparable effect to drought or salinity in terms of the gain of relative importance in apoplastic bypass transport under such stress conditions.

Comparison of protein variations in Thlaspi caerulescens populations from metalliferous and non-metalliferous soils.

In this work we analysed the protein variations which occurred in two Thlaspi caerulescens populations when subjected to 0 and 10 microM nickel (Ni) treatments: the Ni hyperaccumulator T. caerulescensfrom a metalliferous soil in Italy and T. caerulescens from Czech Republic, adapted to grow on a non-metalliferous soil. Ni accumulation in roots and shoots and the effect on growth and morphology were examined. Leaves proteins profiles of Ni treated and untreated samples were analysed by two dimensional liquid chromatography technique. From the comparison of more than 500 proteins, few differences were observed between treated and untreated plants of the same population. Differences were found between the two Thlaspi populations, instead. Proteins involved in transport, metal chelation, and signal transduction increased in abundance in the 10 microM Ni treated samples while, in condition of absence of Ni, proteins involved in sulphur metabolism, protection against reactive oxygen species and stress response showed to increase in abundance in the two populations. These proteins can be used as biomarkers both for monitoring biodiversity in indigenous plants and for selection of Ni phytoremediation plants.

Impact of metal pollution and Thlaspi caerulescens growth on soil microbial communities.

Soil microorganisms drive critical functions in plant-soil systems. As such, various microbial properties have been proposed as indicators of soil functioning, making them potentially useful in evaluating the recovery of polluted soils via phytoremediation strategies. To evaluate microbial responses to metal phytoextraction using hyperaccumulators, a microcosm experiment was carried out to study the impacts of Zn and/or Cd pollution and Thlaspi caerulescens growth on key soil microbial properties: basal respiration; substrate-induced respiration (SIR); bacterial community structure as assessed by PCR-denaturing gradient gel electrophoresis (DGGE); community sizes of total bacteria, ammonia-oxidizing bacteria, and chitin-degrading bacteria as assessed by quantitative PCR (Q-PCR); and functional gene distributions as determined by functional gene arrays (GeoChip). T. caerulescens proved to be suitable for Zn and Cd phytoextraction: shoots accumulated up to 8,211 and 1,763 mg kg(-1) (dry weight [DW]) of Zn and Cd, respectively. In general, Zn pollution led to decreased levels of basal respiration and ammonia-oxidizing bacteria, while T. caerulescens growth increased the values of substrate-induced respiration (SIR) and total bacteria. In soils polluted with 1,000 mg Zn kg(-1) and 250 mg Cd kg(-1) (DW), soil bacterial community profiles and the distribution of microbial functional genes were most affected by the presence of metals. Metal-polluted and planted soils had the highest percentage of unique genes detected via the GeoChip (35%). It was possible to track microbial responses to planting with T. caerulescens and to gain insight into the effects of metal pollution on soilborne microbial communities.

Flow cytometric assessment of Cd genotoxicity in three plants with different metal accumulation and detoxification capacities.

Cadmium (Cd) is a widespread environmental contaminant, strongly mutagenic and known to cause DNA damage in plants. In this work, flow cytometry (FCM) was applied to determine if in vivo exposure to Cd would induce genotoxic effects at the genome level. The hyper-accumulator Thlaspi caerulescens (J. & C. Presl), the related non-accumulator Thlaspi arvense L. and the accumulator crop species Lactuca sativa L. were germinated in distilled water and grown in modified Hoagland's medium with increasing concentrations of Cd(NO3)2 (0, 1, 10 and 100 microM). After 28 days of exposure, shoot and root growth was recorded and the tissues were harvested for Cd and FCM analysis. In general, roots from treated plants contained higher content of Cd than leaves and growth inhibition was observed in the treated plants. Nuclear DNA content was estimated and the G0/G1 full peak coefficient of variation (FPCV), as an indicator of clastogenic damage, was recorded. In T. arvense and T. caerulescens no significant differences were detected between control and exposed plants. Leaves of L. sativa exposed to 10 microM Cd presented a statistically significant increase in FPCV values in comparison with the control group. Furthermore, roots exposed to 100 microM Cd presented a reduction in nuclear DNA content and an increase in FPCV when compared to the control. FCM data indicates that no major DNA damage was induced on both Cd-exposed Thlaspi species and L. sativa leaves. On the contrary, results obtained with L. sativa roots suggests clastogenic damage in these organs exposed to 100 microM of Cd.

Remediation of cadmium contaminated irrigation and drinking water: a large scale approach.

Cadmium is one of the most troublesome toxic heavy metals. It accumulates in the water reservoirs and agricultural soil as a result of intensive use of Cd contaminated phosphate fertilizers, e.g. in agriculture in the North Central Province (NCP) of Sri Lanka. The hyper-accumulator Thlaspi caerulescens, accumulates up to 1000 ppm Cd in shoots without exhibiting toxicity symptoms. The storage rhizomes of year old Nelumbo nucifera (lotus) natural vegetation in water reservoirs in NCP accumulated 253+/-12 mg Cd/kg. Seedlings of lotus grown in 5% Hoagland's solution at 0.75, 1.0 and 1.25 ppm cadmium sulphate showed a significant increase in Cd removal of 0.0334-0.121 ppm/week. However the removal rate of Cd from water failed to increase any further at higher concentrations of Cd in water. The slow growth rate and low rate of phytoextraction demands a more effective but an affordable method of remediation in order to combat the prevailing elevated cadmium levels in NCP that causes chronic renal failure (CRF). We have developed a large scale filtering device using rice husk. We have achieved successful results in sequestering Cd using raw rice husk as well as amorphous silica derived from rice husk.

[Effect of the soil bulk density on the root morphology and cadmium uptake by Thlaspi caerulescens grown on Cd-contaminated soil].

A pot experiment was conducted using a soil contaminated with 2.12 mg x kg(-1) Cd to study the effect of the variety of the soil bulk density on the Zn/Cd uptake by the hyperaccumulator Thlaspi caerulescens and the removal of Cd and Zn from the soil. The contaminated soil received 0, 0.1%, 2% of soil conditioner and the plants were harvested after 100 days. The results showed that soil amendment with the soil conditioner (EB. a) significantly decreased the soil bulk density. Compared to the control, the bulk density value decreased from 1.27 g x cm(-3) to 1.09 g x cm(-3) at the level of 2% soil conditioner. The increased biomass of shoot and root was observed at the treatment of EB. a amendment. The total root length, root hair length and root/shoot ratio were all significantly enhanced (p < 0.05) by the addition of EB. a. The significant positive relationships between the total root length and the removed Cd/Zn from soil were determined (p < 0.05). Compared with the control,the total root length was increased by 2.6 folds at the addition of 2% soil conditioner; the Cd concentration and removal of Cd from soil were significantly elevated by 20% and 30% respectively. The phytoextraction efficiency of Cd was improved from 15% to 19%. However, the Zn concentration and removal of Zn were not significantly elevated by the addition of soil conditioner. The present results demonstrate that the decreased soil bulk density may improve the root system of T. caerulescens and enhance the phytoextraction efficiency of Cd.

Complexation and toxicity of copper in higher plants. II. Different mechanisms for copper versus cadmium detoxification in the copper-sensitive cadmium/zinc hyperaccumulator Thlaspi caerulescens (Ganges Ecotype).

The cadmium/zinc hyperaccumulator Thlaspi caerulescens is sensitive toward copper (Cu) toxicity, which is a problem for phytoremediation of soils with mixed contamination. Cu levels in T. caerulescens grown with 10 microm Cu(2+) remained in the nonaccumulator range (<50 ppm), and most individuals were as sensitive toward Cu as the related nonaccumulator Thlaspi fendleri. Obviously, hyperaccumulation and metal resistance are highly metal specific. Cu-induced inhibition of photosynthesis followed the "sun reaction" type of damage, with inhibition of the photosystem II reaction center charge separation and the water-splitting complex. A few individuals of T. caerulescens were more Cu resistant. Compared with Cu-sensitive individuals, they recovered faster from inhibition, at least partially by enhanced repair of chlorophyll-protein complexes but not by exclusion, since the content of Cu in their shoots was increased by about 25%. Extended x-ray absorption fine structure (EXAFS) measurements on frozen-hydrated leaf samples revealed that a large proportion of Cu in T. caerulescens is bound by sulfur ligands. This is in contrast to the known binding environment of cadmium and zinc in the same species, which is dominated by oxygen ligands. Clearly, hyperaccumulators detoxify hyperaccumulated metals differently compared with nonaccumulated metals. Furthermore, strong features in the Cu-EXAFS spectra ascribed to metal-metal contributions were found, in particular in the Cu-resistant specimens. Some of these features may be due to Cu binding to metallothioneins, but a larger proportion seems to result from biomineralization, most likely Cu(II) oxalate and Cu(II) oxides. Additional contributions in the EXAFS spectra indicate complexation of Cu(II) by the nonproteogenic amino acid nicotianamine, which has a very high affinity for Cu(II) as further characterized here.

Heavy metal contamination of a mixed waste compost: metal speciation and fate.

The aims of this study were to assess changes in heavy metal availability in two contrasting feedstocks during aerobic composting, and the availability of said metals in the finished composts. A high C-to-N ratio mixed biodegradable municipal solid waste (MSW) feedstock was successfully composted on its own and in combination with green waste. Changes in heavy metal speciation throughout the composting process were studied using the modified BCR sequential extraction protocol. It was found that total Cu, Pb and Zn concentrations increased over time due to the progressive mineralization of the compost feedstock. Metals were fractionated differently within the two feedstocks, although only Cu showed significant redistribution (mostly to the oxidisable fraction) over the 5 month composting period. The MSW-derived composts performed comparably with other commercially-available composts in a series of plant growth trials. Plant metal accumulation was not influenced by the heavy metals present in the MSW-derived compost implying that they are not plant available. It is recommended that these relatively low value/quality composts may be used for remediation of acidic heavy metal contaminated sites.

Feasibility of phytoextraction to remediate cadmium and zinc contaminated soils.

A Cd and Zn contaminated soil was mixed and equilibrated with an uncontaminated, but otherwise similar soil to establish a gradient in soil contamination levels. Growth of Thlaspi caerulescens (Ganges ecotype) significantly decreased the metal concentrations in soil solution. Plant uptake of Cd and Zn exceeded the decrease of the soluble metal concentrations by several orders of magnitude. Hence, desorption of metals must have occurred to maintain the soil solution concentrations. A coupled regression model was developed to describe the transfer of metals from soil to solution and plant shoots. This model was applied to estimate the phytoextraction duration required to decrease the soil Cd concentration from 10 to 0.5 mg kg(-1). A biomass production of 1 and 5 t dm ha(-1) yr(-1) yields a duration of 42 and 11 yr, respectively. Successful phytoextraction operations based on T. caerulescens require an increased biomass production.