ABSTRACT
Although Arabidopsis halleri (=Cardaminopsis halleri) is known as a Zn hyperaccumulator, there have been no detailed studies on Zn accumulation, tolerance and cellular distribution in this ...species. In a hydroponic experiment, A. halleri grew healthily with Zn concentrations varying from 1 to 1000 μM, without showing phytotoxicity or reduction in root or shoot dry weights. The concentration of Zn in the shoots increased from 300 μg g−1 dry weight in the 1 μM Zn treatment to 32 000 μg g−1 in the 1000 μM Zn treatment. Approximately 60% of the total Zn in the shoots were water‐soluble, and there was no evidence of Zn and P co‐precipitation. Both citric and malic acid concentrations in the shoots were not significantly affected by the Zn treatments, whereas in the roots there was a positive response in both organic acids to increasing Zn in solution. Cellular distribution of Zn, Ca and K in frozen hydrated leaf tissues was examined using energy‐dispersive X‐ray microanalysis. Zinc was sequestered in the base of trichomes, whereas the middle and upper parts of trichomes were highly enriched with Ca. Mesophyll cells appeared to have more Zn than the epidermis, probably because the latter were very small in size. Similarities and differences between A. halleri and the other well‐known Zn hyperaccumulator, Thlaspi caerulescens, are discussed.
Field trials were undertaken to investigate the effect of the application of metal mobilizing agents, different sowing strategies and length of growing season on the extraction of Cd and Zn from ...soils by
Thlaspi caerulescens and
Arabidopsis halleri. None of the mobilizing agents used enhanced metal accumulation by
T. caerulescens. Between 1998 and 2000, on average across plots where Cd or Zn exceeded allowable limits,
T. caerulescens removed 1.3 and 0.3% of the total soil Cd and Zn. In one season when
T. caerulescens was grown for 14
months, 21.7 and 4.4% of the total soil Cd and Zn was removed. This was larger than values found when
T. caerulescens was grown for 4
months.
A. halleri accumulated similar concentrations of Zn, but lower Cd concentrations than
T.
caerulescens. The results indicate that metal phytoextraction using
T. caerulescens can be used to clean up soils moderately contaminated by Cd.
The hyperaccumulator Thlaspi caerulescens (the Ganges ecotype) is more efficient at phytoextracting Cd than Zn from contaminated soil.
Mining activities generate spoils and effluents with extremely high metal concentrations of heavy metals that might have adverse effects on ecosystems and human health. Therefore, information on soil ...and plant metal concentrations is needed to assess the severity of the pollution and develop a strategy for soil reclamation such as phytoremediation. Here, we studied soils and vegetation in three heavily contaminated sites with potential toxic metals and metalloids (Zn, Pb, Cd, As, TI) in the mining district of Les Malines in the Languedoc region (southern France). Extremely high concentrations were found at different places such as the Les Aviniéres tailing basins (up to 160,000 mg kg
–1
Zn, 90,000 mg kg
–1
Pb, 9,700 mg kg
–1
of As and 245 mg kg
–1
of Tl) near a former furnace. Metal contamination extended several kilometres away from the mine sites probably because of the transport of toxic mining residues by wind and water. Spontaneous vegetation growing on the three mine sites was highly diversified and included 116 plant species. The vegetation cover consisted of species also found in non-contaminated soils, some of which have been shown to be metal-tolerant ecotypes (
Festuca arvernensis, Koeleria vallesiana
and
Armeria arenaria
) and several Zn, Cd and Tl hyperaccumulators such as
Anthyllis vulneraria, Thlaspi caerulescens, Iberis intermedia
and
Silene latifolia
. This latter species was highlighted as a new thallium hyperaccumulator, accumulating nearly 1,500 mg kg
–1
. These species represent a patrimonial interest for their potential use for the phytoremediation of toxic metal-polluted areas.
The effect of N form (NO₃ ⁻ versus NH₄ ⁺) on growth and uptake of Cd and Zn by the hyperaccumulator Thlaspi caerulescens (Ganges ecotype) was investigated in hydroponic and rhizobox experiments. In ...the hydroponic experiments, NO₃ ⁻ or NH₄ ⁺ was supplied to plants with the pH of the nutrient solution being unbuffered or buffered at around 6.0. A moderately contaminated soil was used in the rhizobox experiment with or without additions of NO₃ ⁻, NH₄ ⁺ or NH₄ ⁺ + DCD (dicyanodiamide, a nitrification inhibitor). A higher biomass was obtained when N was supplied as NO₃ ⁻ in both experiments, indicating that T. caerulescens prefers NO₃ ⁻ over NH₄ ⁺. In the hydroponic experiments, supplying NO₃ ⁻ resulted in a doubling of Cd concentration in the shoots compared with the NH₄ ⁺ treatment, regardless whether solution pH was buffered or not. The form of N also had a noticeable effect on root Zn concentrations. In the rhizosphere box experiment, rhizosphere pH was markedly influenced by the N treatment. The acidification in the NH₄ ⁺ and NH₄ ⁺ + DCD treatments increased the concentrations of extractable Cd and Zn, both of which showed a considerable depletion in the rhizosphere. However, total uptake of Cd and Zn were highest in the NO₃ ⁻ treatment, despite the fact that concentrations of extractable Cd and Zn in the rhizosphere were the lowest in this treatment. The results showed that supplying N as NO₃ ⁻ promoted growth and phytoextraction of Cd and Zn by T. caerulescens compared with NH₄ ⁺.
Noccaea caerulescens is an extremophile plant species belonging to the Brassicaceae family. It has adapted to grow on soils containing high, normally toxic, concentrations of metals such as nickel, ...zinc, and cadmium. Next to being extremely tolerant to these metals, it is one of the few species known to hyperaccumulate these metals to extremely high concentrations in their aboveground biomass. In order to provide additional molecular resources for this model metal hyperaccumulator species to study and understand the mechanism of adaptation to heavy metal exposure, we aimed to provide a comprehensive database of transcript sequences for N. caerulescens. In this study, 23,830 transcript sequences (isotigs) with an average length of 1025 bp were determined for roots, shoots and inflorescences of N. caerulescens accession "Ganges" by Roche GS-FLEX 454 pyrosequencing. These isotigs were grouped into 20,378 isogroups, representing potential genes. This is a large expansion of the existing N. caerulescens transcriptome set consisting of 3705 unigenes. When translated and compared to a Brassicaceae proteome set, 22,232 (93.2%) of the N. caerulescens isotigs (corresponding to 19,191 isogroups) had a significant match and could be annotated accordingly. Of the remaining sequences, 98 isotigs resembled non-plant sequences and 1386 had no significant similarity to any sequence in the GenBank database. Among the annotated set there were many isotigs with similarity to metal homeostasis genes or genes for glucosinolate biosynthesis. Only for transcripts similar to Metallothionein3 (MT3), clear evidence for an additional copy was found. This comprehensive set of transcripts is expected to further contribute to the discovery of mechanisms used by N. caerulescens to adapt to heavy metal exposure.
The two main features of plant hyper-accumulator species are the massive translocation of heavy metal ions to the aerial parts and their tolerance to such high metal concentrations. Recently, several ...lines of evidence have indicated a role for nicotianamine (NA) in metal homeostasis, through the chelation and transport of NA-metal complexes. The function of transport of NA-metal chelates, required for the loading and unloading of vessels, has been assigned to the Yellow Stripe 1 (YSL)-Like family of proteins. We have characterized three YSL genes in Thlaspi caerulescens in the context of hyper-accumulation. The three YSL genes are expressed at high rates compared with their Arabidopsis thaliana homologs but with distinct patterns. While TcYSL7 was highly expressed in the flowers, TcYSL5 was more highly expressed in the shoots, and the expression of TcYSL3 was equivalent in all the organs tested. In situ hybridizations have shown that TcYSL7 and TcYSL5 are expressed around the vasculature of the shoots and in the central cylinder in the roots. The exposure to heavy metals (Zn, Cd, Ni) does not affect the high and constitutive expression of the TcYSL genes. Finally, we have demonstrated by mutant yeast complementation and uptake measurements that TcYSL3 is an Fe/Ni-NA influx transporter. This work provides therefore molecular, histological and biochemical evidence supporting a role for YSL transporters in the overall scheme of NA and NA-metal, particularly NA-Ni, circulation in a metal hyper-accumulator plant.
Hyperaccumulators store accumulated metals in the vacuoles of large leaf epidermal cells (storage cells). For investigating cadmium uptake, we incubated protoplasts obtained from leaves of Thlaspi ...caerulescens (Ganges ecotype) with a Cd-specific fluorescent dye. A fluorescence kinetic microscope was used for selectively measuring Cd-uptake and photosynthesis in different cell types, so that physical separation of cell types was not necessary. Few minutes after its addition, cadmium accumulated in the cytoplasm before its transport into the vacuole. This demonstrated that vacuolar sequestration is the rate-limiting step in cadmium uptake into protoplasts of all leaf cell types. During accumulation in the cytoplasm, Cd-rich vesicle-like structures were observed. Cd uptake rates into epidermal storage cells were higher than into standard-sized epidermal cells and mesophyll cells. This shows that the preferential heavy metal accumulation in epidermal storage cells, previously observed for several metals in intact leaves of various hyperaccumulator species, is due to differences in active metal transport and not differences in passive mechanisms like transpiration stream transport or cell wall adhesion. Combining this with previous studies, it seems likely that the transport steps over the plasma and tonoplast membranes of leaf epidermal storage cells are driving forces behind the hyperaccumulation phenotype.
Urban soil contamination with trace metals is a major obstacle to the development of urban agriculture as crops grown in urban gardens are prone to accumulate trace metals up to toxic levels for ...human consumption. Phytoextraction is considered as a potentially cost-effective alternative to conventional methods such as excavation. Field trials of phytoextraction with
Noccaea caerulescens
were conducted on urban soils contaminated with Cd, Cu, Pb, and Zn (respectively around 2, 150–200, 400–500, and 400–700 μg g
−1
of dry soil). Metallicolous (Ganges population) and non-metallicolous (NMET) populations were compared for biomass production and trace metal uptake. Moreover, we tested the effect of compost and fertilizer addition. Maximal biomass of 5 t ha
−1
was obtained with NMET populations on some plots. Compared to Ganges— the high Cd-accumulating ecotype from South of France often used in phytoextraction trials— NMET populations have an advantage for biomass production and for Zn accumulation, with an average Zn uptake of 2.5 times higher. The addition of compost seems detrimental due to metal immobilization in the soil with little or no effect on plant growth. In addition to differences between populations, variations of growth and metal accumulation were mostly explained by soil Cd and Zn concentrations and texture. Our field trials confirm the potential of using
N. caerulescens
for both Cd and Zn remediation of moderately contaminated soils—with uptake values of up to 200 g Cd ha
−1
and 47 kg Zn ha
−1
—and show the interest of selecting the adequate population according to the targeted metal.
Efficient root-to-shoot translocation is a key trait of the zinc/cadmium hyperaccumulators Thlaspi caerulescens and Thlaspi praecox, but the extent of variation among different accessions and the ...underlying mechanisms remain unclear. Root-to-shoot translocation of Cd and Zn and apoplastic bypass flow were determined in 10 accessions of T. caerulescens and one of T. praecox, using radiolabels ¹⁰⁹Cd and ⁶⁵Zn. Two contrasting accessions (Pr and Ga) of T. caerulescens were further characterized for TcHMA4 expression and metal compartmentation in roots. Root-to-shoot translocation of ¹⁰⁹Cd and ⁶⁵Zn after 1 d exposure varied 4.4 to 5-fold among the 11 accessions, with a significant correlation between the two metals, but no significant correlation with uptake or the apoplastic bypass flow. The F₂ progeny from a cross between accessions from Prayon, Belgium (Pr) and Ganges, France (Ga) showed a continuous phenotype pattern and transgression. There was no significant difference in the TcHMA4 expression in roots between Pr and Ga. Compartmentation analysis showed a higher percentage of ¹⁰⁹Cd sequestered in the root vacuoles of Ga than Pr, the former being less efficient in translocation than the latter. Substantial natural variation exists in the root-to-shoot translocation of Cd and Zn, and root vacuolar sequestration may be an important factor related to this variation.
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