Thlaspi caerulescens on nonmetalliferous soil in Luxembourg: ecological niche and genetic variation in mineral element composition

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).

Phytoremediation of urban soils contaminated with trace metals using Noccaea caerulescens: comparing non-metallicolous populations to the metallicolous 'Ganges' in field trials

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.

Extensive variation in cadmium tolerance and accumulation among populations of Chamaecrista fasciculata

Plant populations may vary substantially in their tolerance for and accumulation of heavy metals, and assessment of this variability is important when selecting species to use in restoration or phytoremediation projects. We examined the population variation in cadmium tolerance and accumulation in a leguminous pioneer species native to the eastern United States, the partridge pea (Chamaecrista fasciculata). We assayed growth, reproduction and patterns of cadmium accumulation in six populations of C. fasciculata grown on a range of cadmium-contaminated soils. In general, C. fasciculata exhibited tolerance in low to moderate soil cadmium concentrations. Both tolerance and accumulation patterns varied across populations. C. fasciculata exhibited many characteristics of a hyperaccumulator species, with high cadmium uptake in shoots and roots. However, cadmium was excluded from extrafloral nectar. As a legume with tolerance for moderate cadmium contamination, C. fasciculata has potential for phytoremediation. However, our findings also indicate the importance of considering the effects of genetic variation on plant performance when screening plant populations for utilization in remediation and restoration activities. Also, there is potential for cadmium contamination to affect other species through contamination of leaves, fruits, flowers, pollen and root nodules.

Contrasting patterns of genetic divergence in two sympatric pseudo-metallophytes: Rumex acetosa L. and Commelina communis L

BACKGROUND

Patterns of genetic divergence between populations of facultative metallophytes have been investigated extensively. However, most previous investigations have focused on a single plant species making it unclear if genetic divergence shows common patterns or, conversely, is species-specific. The herbs Rumex acetosa L. and Commelina communis L. are two pseudo-metallophytes thriving in both normal and cupriferous soils along the middle and lower reaches of the Yangtze River in China. Their non-metallicolous and metallicolous populations are often sympatric thus providing an ideal opportunity for comparative estimation of genetic structures and divergence under the selective pressure derived from copper toxicity.

RESULTS

In the present study, patterns of genetic divergence of R. acetosa and C. communis , including metal tolerance, genetic structure and genetic relationships between populations, were investigated and compared using hydroponic experiments, AFLP, ISSR and chloroplast genetic markers. Our results show a significant reduction in genetic diversity in metallicolous populations of C. communis but not in R. acetosa . Moreover, genetic differentiation is less in R. acetosa than in C. communis , the latter species also shows a clustering of its metallicolous populations.

CONCLUSIONS

We propose that the genetic divergences apparent in R. acetosa and C. communis , and the contrasting responses of the two species to copper contamination, might be attributed to the differences in their intrinsic physiological and ecological properties. No simple and generalised conclusions on genetic divergence in pseudo-metallophytes can thus be drawn.

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.

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.

Quantitative trait loci analysis of mineral element concentrations in an Arabidopsis halleri x Arabidopsis lyrata petraea F2 progeny grown on cadmium-contaminated soil

This study describes the quantitative trait locus (QTL) analysis of cadmium (Cd), zinc (Zn), iron (Fe), potassium (K), magnesium (Mg) and calcium (Ca) accumulation in the pseudometallophyte Arabidopsis halleri under conditions of Cd excess using an interspecific A. halleri x Arabidopsis lyrata F(2) population. *Our data provide evidence for the implication of one major QTL in Cd hyperaccumulation in A. halleri, and suggests that Cd tolerance and accumulation are not independent in A. halleri. Moreover, the major loci responsible for Zn hyperaccumulation in the absence of Cd appear to be the same when Cd is present at high concentrations. *More than twofold higher Fe concentrations were measured in A. halleri shoots than in A. lyrata, suggesting a different regulation of Fe accumulation in the hyperaccumulator. *With the exception of Ca, the accumulation of Cd was significantly correlated with the accumulation of all elements measured in the F(2) progeny, suggesting pleiotropic gene action. However, QTL analysis identified pleiotropic QTLs only for Cd, Zn and Fe. Mg accumulation was negatively correlated with Cd accumulation, as well as with dry shoot biomass, suggesting that it might indicate cellular damage.

Metal hyperaccumulation in plants

During the history of life on Earth, tectonic and climatic change repeatedly generated large territories that were virtually devoid of life and exhibited harsh environmental conditions. The ability of a few specialist pioneer plants to colonize such hostile environments was thus of paramount ecological importance for the continuous maintenance of primary production over time. Yet, we know very little about how extreme traits evolve and function in plants. Recent breakthroughs have given first insights into the molecular basis underlying the complex extreme model trait of metal hyperaccumulation and associated metal hypertolerance. This review gives an introduction into the hyperaccumulator research field and its history; provides an overview of hyperaccumulator germplasm; describes the state of the art of our understanding of the physiological, molecular, and genetic basis underlying metal hyperaccumulation and its evolution; and highlights future research needs and opportunities.

Cost of adaptation to a metalliferous environment for Thlaspi caerulescens: a field reciprocal transplantation approach

Field reciprocal transplantations of two metallicolous populations (Mpops) and two nonmetallicolous populations (NMpops) of Thlaspi caerulescens were performed here to determine the pattern of local adaptation and to assess the cost of adaptation of Mpops to a metalliferous environment (Menv). The role of herbivores as an important selective pressure in the nonmetalliferous environment (NMenv) was also examined. Growth, survival, fitness, life cycle and herbivore consumption were monitored for each transplant for 2 yr. Local adaptation of Mpops to their own environment was clearly demonstrated, as Mpops consistently outperformed NMpops in Menv. In NMenv, no advantage of NMpops over Mpops was detected. However, the fitness of Mpops was generally lower in NMenv than in Menv. Herbivore consumption appeared to be a significant selective pressure for Mpops in NMenv. An imbalance of selective forces between Menv and NMenv probably explains the greater local adaptation of Mpops. Therefore, colonization of NMenv by Mpops appears possible. Although Mpops were able to survive and reproduce in NMenv, they nevertheless expressed a cost attributable in part to their higher susceptibility to herbivores.

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.

Zinc in plants

Zinc (Zn) is an essential component of thousands of proteins in plants, although it is toxic in excess. In this review, the dominant fluxes of Zn in the soil-root-shoot continuum are described, including Zn inputs to soils, the plant availability of soluble Zn(2+) at the root surface, and plant uptake and accumulation of Zn. Knowledge of these fluxes can inform agronomic and genetic strategies to address the widespread problem of Zn-limited crop growth. Substantial within-species genetic variation in Zn composition is being used to alleviate human dietary Zn deficiencies through biofortification. Intriguingly, a meta-analysis of data from an extensive literature survey indicates that a small proportion of the genetic variation in shoot Zn concentration can be attributed to evolutionary processes whose effects manifest above the family level. Remarkable insights into the evolutionary potential of plants to respond to elevated soil Zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous Zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri.

Reaction norms of life history traits in response to zinc in Thlaspi caerulescens from metalliferous and nonmetalliferous sites

* We examined phenotypic plasticity of fitness components in response to zinc (Zn) in the Zn hyperaccumulator, Thlaspi caerulescens. * Two populations from Zn-enriched soils (M) and two populations from normal soils (NM) were grown in pots at three Zn concentrations (0, 1000 and 8000 mg kg(-1) Zn), for an entire life cycle. Growth, Zn accumulation and fitness components were assessed. * Based on vegetative growth, M and NM populations had similar Zn tolerance at 1000 mg kg(-1) Zn. However, reproductive output was markedly decreased in NM at 1000 and 8000 mg kg(-1) Zn. In M populations, Zn did not affect fitness. However, low Zn status enhanced reproductive output in year 1 compared with year 2 and decreased survival after the first flowering season. * M populations are able to achieve equal fitness across a broad range of Zn concentrations in soil by different combinations of fecundity and longevity. No cost of higher tolerance was demonstrated in M populations. Reproductive traits appeared to be a more sensitive indicator of tolerance than vegetative growth.

Cadmium tolerance and hyperaccumulation by Thlaspi caerulescens populations grown in hydroponics are related to plant uptake characteristics in the field

In order to fully understand the hyperaccumulation process and to increase the potential of plants for phytoextraction purposes, there is a need for more investigation of hyperaccumulating species or populations. Five Swiss populations of Thlaspi caerulescens J. & C. Presl originating from non-metalliferous but naturally Cd-rich soils (1.1-9.2 mg Cd kg^-1) were compared with Ganges and Prayon populations and a non-accumulating species, Thlaspi perfoliatum (L.) F.K. Meyer, for their tolerance (shoot and root dry weight and root length) and Cd hyperaccumulation in hydroponics (0, 1, 5, 10, 20 and 50 μm Cd). In the field, the Swiss populations accumulated Zn and clearly hyperaccumulated Cd (up to 505 mg Cd kg^-1 dry weight). The general response was significantly different between populations but in general an increasing Cd concentration in solution led to a decrease in dry weight production and an increase in Cd concentration in shoots. The shoot dry weight was a more discriminating parameter for tolerance than root dry weight and total root length. The Swiss populations behaved similarly to the Ganges population but differently from the Prayon population. Cadmium concentrations in shoots were above 100 mg kg^-1 when plants were grown in 1 μm Cd, except for the Prayon population and T. perfoliatum. In addition, as 1 μm Cd did not induce any visible toxicity symptoms, it was found to be adequate to test Cd hyperaccumulation. However, the most striking feature was the positive linear relationship observed between the transfer factor (TF) calculated in the field and the response of a population to increasing Cd concentrations in solution, indicating that plant uptake in the field had an influence on the plant response in solution.

Testing of outstanding individuals of Thlaspi caerulescens for cadmium phytoextraction

Thlaspi caerulescens is known to hyperaccumulate high quantities of Cd with Cd concentrations up to 3000 mg kg(-1) in some populations from south of France. However, within these populations, the Cd concentrations can vary widely from plant to plant in a way that appears to be not entirely due to variations in soil Cd. The aim of this work was to investigate the variability in the Cd uptake ability of individual plants within a population and among seedlings grown from seeds from a single plant. Ten populations of T. caerulescens plants were selected from four locations (V: Viviez; SF: Saint Félix-de-Pallières; LB: Le Bleymard; CMA: Col du Mas de l'Air) depending of the extent and soil homogeneity of the site. One population from CMA consisted of the progeny of a single maternal plant. Hundred plants of each population were grown for three months in the same homogeneous and lightly Cd-polluted soil (about 20 mg total Cd kg(-1) dry soil). Cadmium uptake behavior of the plants was monitored by labeling the soil with 109Cd. To allow partial plant destruction, radioanalysis was performed on the largest leaf of each plant as an indicator of the total Cd concentration in plant shoots. Results showed significant differences in biomass production and Cd uptake by T. caerulescens between sites and between populations within sites. We observed a wide intra-population variation in biomass, Cd concentration and total Cd uptake. For these properties, 1 to 5 percents of the plants in each population varied by more than a factor of two from the mean values. The mean Cd uptake by the single-plant population from CMA was more than 40% higher than for the population at large. T. caerulescens would respond to traditional selection methods, which would significantly improve the phytoextraction of Cd.

Cadmium hyperaccumulation and reproductive traits in natural Thlaspi caerulescens populations

During the last decade, the metal hyperaccumulating plants have attracted considerable attention because of their potential use in decontamination of heavy metal contaminated soils. However, in most species, little is known regarding the function, the ecological and the evolutionary significances of hyperaccumulation. In our study, we investigated the parameters influencing the Cd concentration in plants as well as the biological implications of Cd hyperaccumulation in nine natural populations of Thlaspi caerulescens. First, we showed that Cd concentration in the plant was positively correlated with plant Zn, Fe, and Cu concentrations. This suggested that the physiological and/or molecular mechanisms for uptake, transport and/or accumulation of these four heavy metals interact with each other. Second, we specified a measure of Cd hyperaccumulation capacity by populations and showed that T. caerulescens plants originating from populations with high Cd hyperaccumulation capacity had better growth, by developing more and bigger leaves, taller stems, and produced more fruits and heavier seeds. These results suggest a tolerance/disposal role of Cd hyperaccumulation in this species.

Construction of a genetic linkage map of Thlaspi caerulescens and quantitative trait loci analysis of zinc accumulation

Zinc (Zn) hyperaccumulation seems to be a constitutive species-level trait in Thlaspi caerulescens. When compared under conditions of equal Zn availability, considerable variation in the degree of hyperaccumulation is observed among accessions originating from different soil types. This variation offers an excellent opportunity for further dissection of the genetics of this trait. A T. caerulescens intraspecific cross was made between a plant from a nonmetallicolous accession [Lellingen (LE)], characterized by relatively high Zn accumulation, and a plant from a calamine accession [La Calamine (LC)], characterized by relatively low Zn accumulation. Zinc accumulation in roots and shoots segregated in the F3 population. This population was used to construct an LE/LC amplified fragment length polymorphism (AFLP)-based genetic linkage map and to map quantitative trait loci (QTL) for Zn accumulation. Two QTL were identified for root Zn accumulation, with the trait-enhancing alleles being derived from each of the parents, and explaining 21.7 and 16.6% of the phenotypic variation observed in the mapping population. Future development of more markers, based on Arabidopsis orthologous genes localized in the QTL regions, will allow fine-mapping and map-based cloning of the genes underlying the QTL.

Palatability of Thlaspi caerulescens for snails: influence of zinc and glucosinolates

* The hypothesis that zinc (Zn) hyperaccumulation defends Thlaspi caerulescens against herbivores is tested with the snail Helix aspersa. We investigated the effects of leaf zinc, cadmium, glucosinolate, nitrogen and dry matter concentrations on the feeding preferences of snails. * Four T. caerulescens populations from southern France (two from metalliferous and two from normal soils) were grown on low- and high-Zn soils to obtain contrasting leaf Zn concentrations. Plants were also collected in the field, and binary feeding choices involving low- and high-Zn leaves were conducted. * Foliar Zn, Cd, N and dry matter concentrations did not affect the feeding choices of snails, whereas glucosinolate had a significant negative effect on herbivore preferences. Compared with metallicolous plants, nonmetallicolous ones appeared to be better protected against snails, whatever their Zn concentration. * These results do not support the defence hypothesis, as glucosinolates appear to decrease the degree of herbivory when Zn does not.