Zinc, copper, or cerium accumulation from metal oxide nanoparticles or ions in sweet potato: Yield effects and projected dietary intake from consumption

The potential release of metal oxide engineered nanoparticles (ENP) into agricultural systems has created the need to evaluate the impact of these materials on crop yield and food safety. The study here grew sweet potato (Ipomoea batatas) to maturity in field microcosms using substrate amended with three concentrations (100, 500 or 1000 mg kg DW^-1) of either nZnO, nCuO, or nCeO₂ or equivalent amounts of Zn²⁺, Cu²⁺, or Ce⁴⁺. Adverse effects on tuber biomass were observed only for the highest concentration of Zn or Cu applied. Exposure to both forms of Ce had no adverse effect on yield and a slight positive benefit at higher concentrations on tuber diameter. The three metals accumulated in both the peel and flesh of the sweet potato tubers, with concentrations higher in the peel than the flesh for each element. For Zn, >70% of the metal was in the flesh and for Cu >50%. The peels retained 75-95% of Ce in the tubers. The projected dietary intake of each metal by seven age-mass classes from child to adult only exceeded the oral reference dose for chronic toxicity in a scenario where children consumed tubers grown at the highest metal concentration. The results throughout were generally not different between the ENP- and ionic-treatments, suggesting that the added ENPs underwent dissolution to release their component ions prior to accumulation. The results offer insight into the fate and impact of these ENPs in soils.

Bioavailability of cerium oxide nanoparticles to Raphanus sativus L. in two soils

Cerium oxide nanoparticles (CeO₂ NP) are a common component of many commercial products. Due to the general concerns over the potential toxicity of engineered nanoparticles (ENPs), the phytotoxicity and in planta accumulation of CeO₂ NPs have been broadly investigated. However, most previous studies were conducted in hydroponic systems and with grain crops. For a few studies performed with soil grown plants, the impact of soil properties on the fate and transport of CeO₂ NPs was generally ignored even though numerous previous studies indicate that soil properties play a critical role in the fate and transport of environmental pollutants. The objectives of this study were to evaluate the soil fractionation and bioavailability of CeO₂ NPs to Raphanus sativus L (radish) in two soil types. Our results showed that the silty loam contained slightly higher exchangeable fraction (F1) of cerium element than did loamy sand soil, but significantly lower reducible (F2) and oxidizable (F3) fractions as CeO₂ NPs concentration increased. CeO₂ NPs associated with silicate minerals or the residue fraction (F4) dominated in both soils. The cerium concentration in radish storage root showed linear correlation with the sum of the first three fractions (r² = 0.98 and 0.78 for loamy sand and silty loam respectively). However, the cerium content in radish shoots only exhibited strong correlations with F1 (r² = 0.97 and 0.89 for loamy sand and silty loam respectively). Overall, the results demonstrated that soil properties are important factors governing the distribution of CeO₂ NPs in soil and subsequent bioavailability to plants.

The β-cyanoalanine synthase pathway: beyond cyanide detoxification

Production of cyanide through biological and environmental processes requires the detoxification of this metabolic poison. In the 1960s, discovery of the β-cyanoalanine synthase (β-CAS) pathway in cyanogenic plants provided the first insight on cyanide detoxification in nature. Fifty years of investigations firmly established the protective role of the β-CAS pathway in cyanogenic plants and its role in the removal of cyanide produced from ethylene synthesis in plants, but also revealed the importance of this pathway for plant growth and development and the integration of nitrogen and sulfur metabolism. This review describes the β-CAS pathway, its distribution across and within higher plants, and the diverse biological functions of the pathway in cyanide assimilation, plant growth and development, stress tolerance, regulation of cyanide and sulfide signalling, and nitrogen and sulfur metabolism. The collective roles of the β-CAS pathway highlight its potential evolutionary and ecological importance in plants.

Projected Dietary Intake of Zinc, Copper, and Cerium from Consumption of Carrot (Daucus carota) Exposed to Metal Oxide Nanoparticles or Metal Ions

The expanding production and use of engineered nanomaterials (ENMs) have raised concerns about the potential risk of those materials to food safety and human health. In a prior study, the accumulation of Zn, Cu, and Ce from ZnO, CuO, or CeO2, respectively, was examined in carrot (Daucus carota L.) grown in sand culture in comparison to accumulation from exposure to equivalent concentrations of ionic Zn(2+), Cu(2+), or Ce(4+). The fresh weight concentration data for peeled and unpeeled carrots were used to project dietary intake of each metal by seven age-mass classes from child to adult based on consumption of a single serving of carrot. Dietary intake was compared to the oral reference dose (oral RfD) for chronic toxicity for Zn or Cu and estimated mean and median oral RfD values for Ce based on nine other rare earth elements. Reverse dietary intake calculations were also conducted to estimate the number of servings of carrot, the mass of carrot consumed, or the tissue concentration of Zn, Cu, or Ce that would cause the oral RfD to be exceeded upon consumption. The projections indicated for Zn and Cu, the oral RfD would be exceeded in only a few highly unrealistic scenarios of exceedingly high Zn or Cu concentrations in the substrate from ZnO or CuO or consumption of excessive amounts of unpeeled carrot. The implications associated with the presence of Ce in the carrot tissues depended upon whether the mean or median oral RfD value from the rare earth elements was used as a basis for comparison. The calculations further indicated that peeling carrots reduced the projected dietary intake by one to two orders of magnitude for both ENM- and ionic-treated carrots. Overall in terms of total metal concentration, the results suggested no specific impact of the ENM form on dietary intake. The effort here provided a conservative view of the potential dietary intake of these three metals that might result from consumption of carrots exposed to nanomaterials (NMs) and how peeling mitigated that dietary intake. The results also demonstrate the potential utility of dietary intake projections for examining potential risks of NM exposure from agricultural foods.

Projected Dietary Intake of Zinc, Copper, and Cerium from Consumption of Carrot (Daucus carota) Exposed to Metal Oxide Nanoparticles or Metal Ions

The expanding production and use of engineered nanomaterials (ENMs) have raised concerns about the potential risk of those materials to food safety and human health. In a prior study, the accumulation of Zn, Cu, and Ce from ZnO, CuO, or CeO2, respectively, was examined in carrot (Daucus carota L.) grown in sand culture in comparison to accumulation from exposure to equivalent concentrations of ionic Zn(2+), Cu(2+), or Ce(4+). The fresh weight concentration data for peeled and unpeeled carrots were used to project dietary intake of each metal by seven age-mass classes from child to adult based on consumption of a single serving of carrot. Dietary intake was compared to the oral reference dose (oral RfD) for chronic toxicity for Zn or Cu and estimated mean and median oral RfD values for Ce based on nine other rare earth elements. Reverse dietary intake calculations were also conducted to estimate the number of servings of carrot, the mass of carrot consumed, or the tissue concentration of Zn, Cu, or Ce that would cause the oral RfD to be exceeded upon consumption. The projections indicated for Zn and Cu, the oral RfD would be exceeded in only a few highly unrealistic scenarios of exceedingly high Zn or Cu concentrations in the substrate from ZnO or CuO or consumption of excessive amounts of unpeeled carrot. The implications associated with the presence of Ce in the carrot tissues depended upon whether the mean or median oral RfD value from the rare earth elements was used as a basis for comparison. The calculations further indicated that peeling carrots reduced the projected dietary intake by one to two orders of magnitude for both ENM- and ionic-treated carrots. Overall in terms of total metal concentration, the results suggested no specific impact of the ENM form on dietary intake. The effort here provided a conservative view of the potential dietary intake of these three metals that might result from consumption of carrots exposed to nanomaterials (NMs) and how peeling mitigated that dietary intake. The results also demonstrate the potential utility of dietary intake projections for examining potential risks of NM exposure from agricultural foods.

Uptake and accumulation of bulk and nanosized cerium oxide particles and ionic cerium by radish (Raphanus sativus L.)

The potential toxicity and accumulation of engineered nanomaterials (ENMs) in agricultural crops has become an area of great concern and intense investigation. Interestingly, although below-ground vegetables are most likely to accumulate the highest concentrations of ENMs, little work has been done investigating the potential uptake and accumulation of ENMs for this plant group. The overall objective of this study was to evaluate how different forms of cerium (bulk cerium oxide, cerium oxide nanoparticles, and the cerium ion) affected the growth of radish (Raphanus sativus L.) and accumulation of cerium in radish tissues. Ionic cerium (Ce(3+)) had a negative effect on radish growth at 10 mg CeCl3/L, whereas bulk cerium oxide (CeO2) enhanced plant biomass at the same concentration. Treatment with 10 mg/L cerium oxide nanoparticles (CeO2 NPs) had no significant effect on radish growth. Exposure to all forms of cerium resulted in the accumulation of this element in radish tissues, including the edible storage root. However, the accumulation patterns and their effect on plant growth and physiological processes varied with the characteristics of cerium. This study provides a critical frame of reference on the effects of CeO2 NPs versus their bulk and ionic counterparts on radish growth.

Alteration of root growth by lettuce, wheat, and soybean in response to wear debris from automotive brake pads

Brakes from motor vehicles release brake pad wear debris (BPWD) with increased concentrations of heavy metals. Germination and root-elongation assays with lettuce, wheat, and soybean were used to provide an initial evaluation of the phytotoxicity of either a water extract of BPWD or BPWD particulates. In terms of germination, the only effect observed was that lettuce germination decreased significantly in the BPWD particulate treatment. Lettuce and wheat showed decreased root length and root-elongation rate in the presence of the BPWD particulates, whereas lettuce produced a significantly greater number of lateral roots in response to BPWD extract. There was no significant effect of either BPWD treatment on soybean root elongation or lateral roots. Treatment with BPWD extracts or particulates caused significant alterations in the bending pattern of the plant roots. These initial results suggest that BPWD may have effects on the early growth and development of plants.

Dissolution of copper and iron from automotive brake pad wear debris enhances growth and accumulation by the invasive macrophyte Salvinia molesta Mitchell

Automotive vehicles release particulate matter into the environment when their brakes are applied. The environmental effects of this automotive brake pad wear debris (BPWD) on the environment is a matter of growing debate yet the effects on plants have been largely untested. In this study, the effect of BPWD on the growth of the aquatic invasive Salvinia molesta Mitchell was examined. Salvinia molesta, plants were grown hydroponically in distilled water or in a distilled water extract containing BPWD. Growth of floating leaves, submerged leaves, and leaf nodes were measured over 20 d at 4-d intervals. At the conclusion of the study the amount of BPWD present in solutions and plant tissues was quantified using atomic absorption spectrometry (AAS). Cultivation of S. molesta in the water containing BPWD resulted in greater dissolution of Cu and Fe than occurred in the absence of plants. The tissue Cu and Fe concentrations of plants cultivated in the BPWD were significantly higher than plants grown in the absence of BPWD. Growth of S. molesta significantly increased when cultivated in the BPWD solutions in comparison to the distilled water. The results suggest that S. molesta and similar aquatic plants may be capable of increasing the dissolution of metal micronutrients from BPWD and utilizing those micronutrients to increase growth. Such growth responses could indicate that BPWD may interact with invasive floating macrophytes to more rapidly degrade the quality and stability of aquatic communities.

The β-cyanoalanine pathway is involved in the response to water deficit in Arabidopsis thaliana

The β-cyanoalanine pathway is primarily responsible for detoxification of excess cyanide produced by plants. Recent evidence suggests that cyanide detoxification via this pathway may be involved in the response and tolerance to water deficit in plants. The aim of this study was to explore this role in Arabidopsis thaliana in greater detail. The first objective was to establish responsiveness of the pathway to the magnitude and duration of water deficit. The second objective was to examine how interruption of single genes (AtCysA1, AtCysC1 and AtNIT4) encoding enzymes of the pathway influenced the ability to metabolize cyanide and withstand water deficit. Arabidopsis plants were exposed to conditions which emulated acute and chronic water deficit, followed by measurement of tissue cyanide concentration, activity of enzymes, and physiological parameters. The results for wild-type Arabidopsis demonstrated a transient increase in cyanide concentration and β-cyanoalanine synthase activity, followed by a decrease in both. The increase in enzyme activity was localized to the tissue in direct proximity to the stress. The knockdown AtCysA1 mutant did not differ from wild-type while AtCysC1 mutants were slightly more sensitive to water deficit. The AtNIT4 mutant was the most sensitive showing decreased growth along with altered chlorophyll content under water deficit as compared to wild-type. Collectively, the results indicated that the pathway is responsive to water deficit although the severity of stress did not alter the nature of the response, implying that the capacity to remove cyanide generated during water deficit may contribute to tolerance to this stress in Arabidopsis.

Nitrogen supply and cyanide concentration influence the enrichment of nitrogen from cyanide in wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor L.)

Cyanide assimilation by the beta-cyanoalanine pathway produces asparagine, aspartate and ammonium, allowing cyanide to serve as alternate or supplemental source of nitrogen. Experiments with wheat and sorghum examined the enrichment of (15)N from cyanide as a function of external cyanide concentration in the presence or absence of nitrate and/or ammonium. Cyanogenic nitrogen became enriched in plant tissues following exposure to (15)N-cyanide concentrations from 5 to 200 microm, but when exposure occurred in the absence of nitrate and ammonium, (15)N enrichment increased significantly in sorghum shoots at solution cyanide concentrations of > or =50 microm and in wheat roots at 200 microm cyanide. In an experiment with sorghum using (13)C(15)N, there was also a significant difference in the tissue (13)C:(15)N ratio, suggestive of differential metabolism and transport of carbon and nitrogen under nitrogen-free conditions. A reciprocal (15)N labelling study using KC(15)N and (15)NH(4)(+) and wheat demonstrated an interaction between cyanide and ammonium in roots in which increasing solution ammonium concentrations decreased the enrichment from 100 microm cyanide. In contrast, with increasing solution cyanide concentrations there was an increase in the enrichment from ammonium. The results suggest increased transport and assimilation of cyanide in response to decreased nitrogen supply and perhaps to ammonium supply.

Cadmium sorption, influx, and efflux at the mesophyll layer of leaves from ecotypes of the Zn/Cd hyperaccumulator Thlaspi caerulescens

Differential sorption and transport characteristics of the leaf mesophyll layer of the Prayon and Ganges ecotypes of the hyperaccumulator Thlaspi caerulescens were examined. (109)Cd influx and efflux experiments were conducted with leaf sections, and X-ray absorption near edge structure (XANES) data were collected from leaves as a general comparison of in vivo cadmium (Cd) coordination. There were modest differences in cell wall sorption of Cd between ecotypes. There were obvious differences in time- and concentration-dependent Cd influx, including a greater V(MAX) for Prayon but a lower K(M) for Ganges for concentration-dependent Cd uptake and a notably greater Cd uptake by Ganges leaf sections at 1000 microm Cd. Leaf sections of Prayon had a greater Cd efflux than Ganges. The XANES spectra from the two ecotypes suggested differences in Cd coordination. The fundamental differences observed between the two ecotypes may reflect differential activity and/or expression of plasma membrane and tonoplast transporters. More detailed study of these transporters and the in vivo coordination of Cd are needed to determine the contribution of these processes to metal homeostasis and tolerance.

Transport of ferrocyanide by two eucalypt species and sorghum

The wastes from some industrial processes and the tailings from gold mining contain elevated concentrations of cyanide, which reacts with iron in the media to form iron cyanide complexes. This research examined the transport and possible metabolism of ferrocyanide by two native Australian trees, blue mallee and sugar gum, and by sorghum. Hydroponic studies using 15N-labeled ferrocyanide showed that both tree species transported ferrocyanide into roots and displayed significant increases in 15N enrichment and concentration with no evidence of phytotoxicity. A subsequent experiment with blue mallee and membrane-transport inhibitors showed that 15N enrichment was significantly inhibited in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone, suggesting that ferrocyanide uptake is mediated partly by H+ -symporters. A study of the time dependence of 15N translocation showed a rapid equilibration of 15N from ferrocyanide in the root of blue mallee, accompanied by a slow increase in shoot 15N, suggestive of the metabolism of ferrocyanide in plant roots. A similar experiment with sorghum showed a more rapid translocation of 15N, suggesting that the transport and/or metabolism of ferrocyanide by roots of this species may differ. The results offer additional incentive for the use of these species as vegetative cover over cyanidation wastes and for cyanide phytoremediation.

Transport of Cd and Zn to seeds of Indian mustard (Brassica juncea) during specific stages of plant growth and development

The accumulation of excess Cd in the seeds of cereal and other crops compromises their commercial value and presents a potential risk to human health. Indian mustard [Brassica juncea (L.) Czern.] is a moderate accumulator of heavy metals such as Cd and Zn, and the seeds are consumed throughout the world, particularly in the Indian subcontinent. The study here examined the transport of Cd into Indian mustard plants and to seeds as a function of external Cd and the stage of the life cycle (vegetative growth, flowering and seed set) to identify critical developmental windows where transport from roots to seeds was the greatest. Plants were also treated simultaneously with Zn to determine if Zn fertilization mitigated the transport of Cd to seeds. Plants treated with Cd during the seed set accumulated the highest concentrations of Cd, exceeding 8 mg kg(-1) dry weight in some instances. Cadmium accumulated during vegetative growth was not highly redistributed to seeds. No effects of Zn were observed with regard to Cd redistribution to seeds. This may be because of the relatively small Zn : Cd ratios tested. However, the results suggest that if Zn fertilization is to be used to reduce the Cd accumulation in seeds of this species, that plants should be treated during the seed set stage. As the seeds of Indian mustard consistently accumulated Cd to concentrations that exceed acceptable limits for food crops, additional study of Cd redistribution in this species is warranted.

Cadmium accumulation in deer tongue grass (Panicum clandestinum L.) and potential for trophic transfer to microtine rodents

Site 36 at the Crab Orchard National Wildlife Refuge includes a Cd-contaminated soil dominated by deer tongue grass (Panicum clandestinum L.). Analysis of deer tongue grass from this site indicated that biomass and leaf surface area were reduced and that there was a linear relationship between both plant bioavailable soil Cd and total soil Zn and tissue Cd concentration. The Cd concentrations in stems and leaves were also used to estimate the dietary Cd exposures that might be experienced by prairie voles (Microtus ochrogaster) and pine voles (M. pinetorum) consuming deer tongue grass. Renal and hepatic Cd burdens predicted from exclusive consumption of deer tongue grass would be comparable to those that have resulted in chronic toxicity in rodents. The results suggest that for the contaminated soil at Site 36, conditions could allow for the accumulation of Cd in deer tongue grass to concentrations that may pose an ecological risk.

Development of a plant uptake model for cyanide

A model for cyanide species uptake by willow (Salix eriocephala L. var. Michaux) was developed to interpret data from hydroponic experiments quantitatively. While the potential for cyanide phytoremediation has been demonstrated modeling will aid in determining plant processes that contribute to cyanide transport and metabolism in willow and will target specific physiological parameters for field-scale phytoremediation design and optimization. The objective of the model development was to gain insight into the relative role of different processes with respect to dissolved free and iron-complexed cyanide transport and assimilation in plants and to determine rates at which these processes occur within the willow plant under the experimental conditions. A physiologically-based model describing plant uptake, transport, and metabolism of cyanide species was developed to reflect the processes that influence the movement of cyanide into and throughout the plant. Plant compartmentalization (root, stem, and leaf) corresponded to the level of detail in the data collected via hydroponic experiments. Inclusion of more detailed intra- and intercellular processes would create a model inconsistent with the macroscale nature of the data. Mass balances around each compartment were developed via kinetic representations for the mass transfer processes and were combined to form a model describing the fate of cyanide species within plant-water systems.

Parameter estimation of a plant uptake model for cyanide: application to hydroponic data

A plant uptake model is applied to describe free cyanide and ferrocyanide transport and fate in willow (Salix eriocephala var. Michaux) grown in hydroponics. The model is applied to experimental data to determine best-fit parameter values, their associated uncertainty, and their relative importance to field-scale phytoremediation applications. The fitted model results, using least-squares optimization of the observed log concentrations, indicate that free cyanide volatilization from leaf tissue and free cyanide cell wall adsorption were negligible. The free cyanide maximum uptake rate and assimilate (noncyanide 15N) first-order leaf loss rate were the only coefficients that significantly affected the model goodness of fit and were concurrently sensitive to data uncertainty in the parameter optimization. Saturation kinetics may be applicable for free cyanide uptake into plants, but not for ferrocyanide uptake, which may occur via preferential protein-mediated or inefficient transpiration stream uptake. Within the free cyanide system, the relative magnitudes of the saturation uptake parameters and the demonstration of an active role for plants in uptake relative to transpiration suggest the potential importance of preferential diffusion through the cell membranes as reported in the literature, rather than protein-mediated uptake. The fitted 13-parameter model matched the observed data well except for the predicted stem and leaf tissue assimilate concentrations, which were significantly underestimated, particularly in the free cyanide system. These low predicted values, combined with the slightly underestimated solution free cyanide removal, suggest that noncyanide 15N redistribution in phloem should be considered.

Simplified extraction of ginsenosides from American ginseng (Panax quinquefolius L.) for high-performance liquid chromatography-ultraviolet analysis

Four methods were tested for extraction and recovery of six major ginsenosides (Rb1, Rb2, Rc, Rd, Re, and Rg1) found in roots of American ginseng (Panax quinquefolius): method A, sonication in 100% methanol (MeOH) at room temperature (rt); method B, sonication in 70% aqueous MeOH at rt; method C, water extraction (90 degrees C) with gentle agitation; and method D, refluxing (60 degrees C) in 100% MeOH. After 0.5-1 h, the samples were filtered and analyzed by high-performance liquid chromatography (HPLC)-UV. A second extraction by methods C and D was done, but 85-90% of ginsenosides were obtained during the first extraction. Lyophilization of extracts did not influence ginsenoside recovery. Method D resulted in the highest significant recoveries of all ginsenosides, except Rg1. Method C was the next most effective method, while method A resulted in the lowest ginsenoside recoveries. Method B led to similar recoveries as method C. All methods used one filtration step, omitted time-consuming cleanup, but maintained clear peak resolution by HPLC, and can be used for quantitative screening of ginsenosides from roots and commercial ginseng preparations.

Phytoextraction of Zn and Cu from sewage sludge and impact on agronomic characteristics

The presence of elevated concentrations of heavy metals limits the usage of sewage sludge as a fertilizer and soil amendment. Experiments were carried out to examine the extent to which seven plant species phytoextracted Zn and Cu from dewatered sludge. The hyperaccumulators Thlaspi caerulescens and Sedum alfredii showed the greatest removal of Zn, while shoots and tubers of two species of Alocasia showed the greatest Cu removal. Cultivation of plants in the sludge resulted in significant decreases in total Zn and changes in the partitioning of Zn between soil pools. However, Cu levels were largely unchanged and remained associated predominantly with the organic matter pool. Agronomic characteristics of the sludge material, such as pH, organic matter content, and nitrogen, phosphorus, and potassium concentrations, did not change significantly during the four-month growth period, indicating that subsequent crops could be sustained by this material. These results suggest that Zn can be phytoextracted from sludge material, provided the rate of metal uptake exceeds the rate of mobilization to the exchangeable fraction. Since there was no appreciable accumulation of Zn and Cu in seeds of Zea mays in this study, some tissues from sludge-grown plants could potentially be used as animal fodder.

Possible evidence for transport of an iron cyanide complex by plants

Barley (Hordeum vulgare L.), oat (Avena sativa L.), and wild cane (Sorghum bicolor L.), were exposed to 15N-labeled ferrocyanide to determine whether these plant species can transport this iron cyanide complex. Plants were treated with ferrocyanide in a nutrient solution that simulated iron cyanide contaminated groundwater and soil solutions. This nutrient solution has been shown to maintain ferrocyanide speciation with minimal dissociation to free cyanide. Following treatment, all three plants showed dramatic enrichments in roots (delta 15N per thousand =1000-1500) and shoots (delta 15N per thousand =500). Barley and oat showed enrichment primarily in roots while wild cane showed a near equal enrichment in root and shoot tissues. Nitrogen-deficient barley plants treated with ferrocyanide showed a significantly greater 15N enrichment as compared to nitrogen-sufficient plants. While the results are suggestive of ferrocyanide transport by these plant species, additional study will be required to verify these results.

Uptake of cesium-137 and strontium-90 from contaminated soil by three plant species; application to phytoremediation

A field test was conducted to determine the ability of three plant species to extract 137Cs and 90Sr from contaminated soil. Redroot pigweed (Amaranthus retroflexus L.), Indian mustard [Brassica juncea (L.) Czern.], and tepary bean (Phaseolus acutifolius A. Gray) were planted in a series of spatially randomized cells in soil that was contaminated in the 1950s and 1960s. We examined the potential for phytoextraction of 90Sr and 137Cs by these three species. Concentration ratios (CR) for 137Cs for redroot pigweed, Indian mustard, and tepary bean were 2.58, 0.46, and 0.17, respectively. For 90Sr they were substantially higher: 6.5, 8.2, and 15.2, respectively. The greatest accumulation of both radionuclides was obtained with redroot pigweed, even though its CR for 90Sr was the lowest, because of its relatively large biomass. There was a linear relationship between the 137Cs concentration in plants and its concentration in soil only for redroot pigweed. Uptake of 90Sr exhibits no relationship to 90Sr concentrations in the soil. Estimates of time required for removal of 50% of the two contaminants, assuming two crops of redroot pigweed per year, are 7 yr for 90Sr and 18 yr for 137Cs.

The molecular physiology of heavy metal transport in the Zn/Cd hyperaccumulator Thlaspi caerulescens

An integrated molecular and physiological investigation of the fundamental mechanisms of heavy metal accumulation was conducted in Thlaspi caerulescens, a Zn/Cd-hyperaccumulating plant species. A heavy metal transporter cDNA, ZNT1, was cloned from T. caerulescens through functional complementation in yeast and was shown to mediate high-affinity Zn(2+) uptake as well as low-affinity Cd(2+) uptake. It was found that this transporter is expressed at very high levels in roots and shoots of the hyperaccumulator. A study of ZNT1 expression and high-affinity Zn(2+) uptake in roots of T. caerulescens and in a related nonaccumulator, Thlaspi arvense, showed that alteration in the regulation of ZNT1 gene expression by plant Zn status results in the overexpression of this transporter and in increased Zn influx in roots of the hyperaccumulating Thlaspi species. These findings yield insights into the molecular regulation and control of plant heavy metal and micronutrient accumulation and homeostasis, as well as provide information that will contribute to the advancement of phytoremediation by the future engineering of plants with improved heavy metal uptake and tolerance.

Molecular physiology of zinc transport in the Zn hyperaccumulator Thlaspi caerulescens

In this manuscript, recent research from this laboratory into physiological and molecular aspects of heavy metal (Zn) transport in the hyperaccumulating plant species, Thlaspi caerulescens is reviewed. This research is aimed at elucidating the processes that underlie the accumulation of extraordinarily high levels of Zn in the T. caerulescens shoot (up to 3% Zn dry wt.) without any associated toxicity symptom. Physiological studies focused on the use of radiotracer flux techniques (65Zn2+) to characterize zinc transport and compartmentation in the root, and translocation and accumulation in the shoot of T. caerulescens in comparison with a related non-accumulator, T. arvense. These studies indicated that Zn transport was stimulated at a number of sites in T. caerulescens, contributing to the hyperaccumulation trait. The transport processes that were stimulated included Zn influx into both root and leaf cells, and Zn loading into the xylem. The 4- to 5-fold stimulation of Zn influx into the root was hypothesized to be due to an increased abundance of Zn transporters in T. caerulescens root cells. Additionally, compartmental analysis (radiotracer wash out or efflux techniques) was used to show that Zn was sequestered in the vacuoles of T. arvense root cells which retarded Zn translocation to the shoot in this non-accumulator species. Molecular studies have focused on the cloning and characterization of Zn transport genes in T. caerulescens. Complementation of a yeast Zn transport-defective mutant with a T. caerulescens cDNA library resulted in the recovery of a cDNA, ZNT1, that encodes a Zn transporter. Sequence analysis of ZNT1 indicated it is a member of a recently discovered micronutrient transport gene family which includes the Arabidopsis Fe transporter, IRT1, and the ZIP Zn transporters. Expression of ZNT1 in yeast allowed for a physiological characterization of this transporter. It was shown to encode a high affinity Zn transporter which can also mediate low affinity Cd transport. Northern analysis of ZNT1 and its homologue in the two Thlaspi species indicated that enhanced Zn transport in T. caerulescens results from a constitutively high expression of the ZNT1 gene in roots and shoots. In T. arvense, ZNT1 is expressed at far lower levels and this expression is stimulated by imposition of Zn deficiency.