 |
University of Pennsylvania, Philadelphia, PA, USA   |
|
Home page
|
Transport and bioaccumulation of cadmium
in seeds of agronomic crops Global industrialization has been accelerating the release of heavy metals and other pollutants into the soil, water and atmosphere. Two heavy metals that frequently occur together are cadmium and zinc. These metals can enter the food chain, have toxic effects on plants and animals and disrupt natural ecosystems. Studies on different agronomic crops such as wheat, soybeans and Indian mustard have shown that cadmium accumulates in high concentration in the seeds. The main objectives of this study are to examine the accumulation of cadmium in vegetative tissue and seeds as a function of concentration, developmental stage, and interaction with zinc. Interaction between cadmium and zinc may be important because zinc treatments may help mitigate cadmium phytotoxicity and accumulation in edible tissues, as has been shown in some studies. This research utilizes a combination of greenhouse experiments and single or dual radiotracer studies for examination, and bioaccumulation. The results are expected to provide empirical data concerning whole plant cadmium transport and phytotoxicity as well as information pertinent to potential agricultural use of cadmium-contaminated soils. Responses to cadmium in different populations
of Thlaspi caerulescens In this study, we carried out experiments in hydroponic conditions
to better understand the differences and the similarities between
seven geographically isolated European populations of Thlaspi caerulescens
in terms of Cd tolerance and accumulation. Six of the populations
studied were isolated from heavy metal polluted sites showing high
concentrations of Zn, Pb and Cd. Some of the seeds of one population
were collected from a serpentine area containing high levels of Ni
but low levels of the other heavy metals. Cd exerted an adverse effect
on most populations. By contrast, Cd had a stimulatory effect on the
two populations from Southern France. The populations did not have
the same ability to hyperaccumulate these heavy metals (5000 to 10000
µg Zn /g DW and 3000 to 10000 µg Cd /g DW). The concentrations
of Zn and Cd in the shoot were, however, always greater than in the
root. Moreover, Cd and Zn concentrations in the shoot were always
positively correlated, but the two populations that exhibited Cd-stimulated
growth presented a higher Cd/Zn ratio. This is confirmed by a higher
Cd concentration in the xylem sap. Data also suggested that tolerance
and Cd hyperaccumulation are independent characters and that the root/shoot
ratio is a major determinant of the Cd and Zn hyperaccumulation. Populations
that differ greatly in their capacity to hyperaccumulate and tolerate
Cd have been selected for molecular analysis. Regulation and expression of metallothioneins
in response to heavy metal stress in the aquatic fern Azolla Our laboratory has been focused on the characterization and application
of active uptake and passive absorption of metals by the aquatic fern
Azolla filiculoides. The content of cadmium, copper, nickel,
zinc, chromium, iron and uranium in Azolla plants grown in
the presence of 10 mg/l metal attained Azolla dry weight up
to 5-6 mg metal/gr. In the present study we suggest that metallothioneins
(MTs) are involved in the accumulation of metals in the plant cell.
MTs are cysteine-rich, heavy metal-binding protein molecules and they
are gene-encoded polypeptides. We isolated type 2 MT- AzMT2 (accession
no. AF482470) from Azolla filiculoides; it contains two cysteine-rich
domains separated by spacer of 27 amino acid residues. The identity
and homology of this protein to that in other plants - Brassica
oleracea and Avicennia marina - is approximately 42% and
54%, respectively. Our results showed that AzMT2 protein has been
over-expressed in E. coli in the present of 200 µM Cd(NO3)2. Identifying the genes involved in cadmium
hyperaccumulation in Thlaspi caerulescens Thlaspi caerulescens is the only plant presently documented
as hyperaccumulating Cd in its natural environment. Understanding
mechanisms of Cd uptake and tolerance would lead to improvement of
remediation technologies and plant nutrition. Two T. caerulescens
populations (Prayon and Ganges) have been reported to show differences
in their capacity to take up Cd, with Ganges accumulating five times
more Cd than Prayon. Cd influx kinetics for Ganges show a saturable
component at low Cd concentrations. This is not seen in Prayon, suggesting
the existence of a transporter with high affinity for Cd, present
predominantly in the Ganges ecotype. Cadmium transport and tolerance in the Zn/Cd
hyperaccumulator Thlaspi caerulescens Thlaspi caerulescens, a hyperaccumulator of Zn and Cd, represents an interesting model system for the study of metal homeostasis in plants as well as a genetic resource of relevance to phytoremediation. Current research in our lab is focusing on Cd transport and tolerance in this species. We report here results from two ongoing studies. The first examines the effect of Cd-Zn interactions on the unidirectional influx of Cd into roots using radiotracer techniques to determine the time and concentration dependent kinetics of unidirectional influx. Previous studies have provided mixed results concerning the effect of Zn on Cd uptake. We will investigate this interaction using different ecotypes of Thlaspi caerulescens and the non-accumulator Thlaspi arvense. The second study will determine whether homologs of CAX1 and/or CAX2, Ca2+/H+ antiporters, potentially involved in heavy metal tolerance, are present in T. caerulescens and whether their expression increases during Cd exposure. Data from both studies will be compared between T. caerulescens ecotypes and between the hyperaccumulator and the non-accumulator. The results of these studies will contribute to our understanding of heavy metal homeostasis in plants and demonstrate the role of these specific physiological and biochemical processes to hyperaccumulation and tolerance. Metal hyperaccumulation does not increase
drought resistance in Thlaspi caerulescens and Alyssum
murale Metal hyperaccumulation mechanisms must bear substantial energetic 'costs' and might confer selective advantages besides defense against herbivores and pathogens. We tested the theory proposed by B.C. Severne (Nature 204, 807, 1974) that metal hyperaccumulation might increase drought resistance. The Zn-hyperaccumulator Thlaspi caerulescens was treated with 1 or 50 µM Zn, and the Ni-hyperaccumulator Alyssum murale with 0 or 20 µM Ni, and exposed to controlled water stress of -0.4 MPa in hydroponic culture using well-aerated aqueous polyethylene glycol (PEG6000). Water stress inhibited the growth of both species. Metal hyperaccumulation (>10 mg Zn or Ni g-1 d.wt.) did not ameliorate the growth inhibition by water stress in either species compared to low-metal plants (~1 mg g-1 d.wt.), nor did hyperaccumulation decrease water stress resistance as found for related non-accumulating species. Metal hyperaccumulation did not increase the relative water content of the plants under drought or the osmolality of cell-sap extracted from the leaves. This suggests that hyperaccumulation of Zn or Ni does not augment plant capacity for osmotic adjustment. We conclude that metal hyperaccumulation does not enhance drought resistance in T. caerulescens and A. murale. Further studies should focus on the osmotic effects of high concentrations of metals at the level of individual cells. Metal uptake, allocation and release from
Spartina alterniflora and Phragmites australis:
species' matter in natural attenuation of a metal-contaminated salt
marsh, Hackensack Meadowlands Due to differences in heavy metal uptake, allocation, release from leaf tissues, and litter dynamics, our research implies that metal bioavailability in contaminated sites may be reduced when Phragmites australis replaces Spartina alterniflora in salt marshes. At a contaminated site in the Hackensack Meadowlands, the proportional allocation of Hg, Cr and Pb to aboveground biomass was consistently greater in S. alterniflora, while P. australis occasionally had higher Cu and Zn above ground. Leaf tissue of S. alterniflora had consistently higher concentrations of Cr and Hg than P. australis throughout the growing season. In a greenhouse experiment, both plants acquired Pb, but S. alterniflora allocated more to leaf tissue than P. australis. Whereas both species were found to release metals onto leaf surfaces, rates of Hg release were 2-3 fold greater from S. alterniflora than from P. australis. For both species, Hg release declined from May to July, as did leaf concentrations of Hg. The correlation of Hg with Na release in S. alterniflora, but not with individual leaf transpiration, suggests that Hg follows a similar pathway out of salt glands. Although aboveground litter from S. alterniflora was initially greater in metal concentrations, rapid adsorption of Hg, Pb, Cu, Cr and Zn onto litter erased any specific differences. Stems of P. australis, however, decomposed more slowly than S. alterniflora. Due to differences in tolerance, allocation, excretion, and decomposition, the natural replacement of S. alterniflora with P. australis may lead to a decrease in Hg and Cr (and possibly Pb) export from aboveground biomass into the water column and the detrital pathway. Conversely, ongoing management to remove P. australis and replace it with S. alterniflora is likely to promote Hg and Cr export from salt marshes. A comparative study of root-cap cell
functions using a cytostatic cis-DDPt, cycloheximide, and cadmium
nitrate In 3-6 day-old maize seedlings, rhizodermis cell differentiation into trichoblasts and atrichoblasts occurred at the basal portion of the meristem. A DNA-related cytostatic cis-DDPt (1 mg/ml) inhibited cell division during the 1st and 2nd mitotic cycles (MC), reducing twice the number of meristematic files and the number of cells in each row. The cell shape changed: cells elongated and spread, their walls became thinner. The cells treated at the stage of the last but one MC, but not the last MC, did not differentiate into root hairs. It seems evident that this last but one MC is the stage competent for trichoblast differentiation. Cycloheximide (1 mg/ml) suppressed cell division and growth during the first MC and prevented growth of root hairs. Later, cell division was resumed, and half of cells abandoned the meristem. Cadmium nitrate (0.1 mM) induced waves of inhibition and stimulation of division along the cell files as dependent on the exposure duration and the number of the cell in the files. Cadmium nitrate enhanced cell differentiation into trichoblasts, which later grew into long root hairs. Dividing rhizodermis cells synthesize and secrete mucilage. cis-DDPt and cycloheximide both inhibited mucilage synthesis and secretion. Higher plant metallothionein-like gene responsiveness
to cytokinin While metallothionein-like genes have been isolated from numerous plant species, their biological significance remains uncertain. The metallothionein-like cDNA clones pECD7, isolated from Mesembryanthemum crystallinum leaves and pCkn16A1 from Nicotiana plumbaginifolia were used to characterize metallothionein-like mRNA prevalence during cytokinin treatment, copper and NaCl stress. DNA analysis suggested that both metallothionein-like cDNAs are members of small gene families. In Mesembryanthemum, copper and salt stress resulted in a 3-5 fold increase in mRNA hybridizing to pECD7. The growth regulator cytokinin was also found to greatly increase pECD7 mRNA hybridization compared to controls. Conversely, copper stress resulted in a significant loss of pCkn16A1 mRNA transcript in leaves of N. tabacum. When the Agrobacterium tumefaciens gene isopentenyl transferase was transformed and expressed in tobacco under the control of a light inducible promoter, transgenic tobacco demonstrated greater pCkn16A1 transcript accumulation compared to non-transformed controls. Tobacco pCkn16A1 mRNA levels in the cytokinin expressing and copper stressed leaves were similar to unstressed leaves. In cytokinin over-expressing tobacco plants, elevated pCkn16A1 mRNA concentration may contribute to the 8-fold accumulation of copper in older leaves compared to non-transformed control plants following copper stress. Metallothionein-like protein mRNA levels and stress responsiveness may depend on endogenous cytokinin supply.
|
|
Illustrations: Heavy Metal Plant cartoon by Sam
Day. Arabidopsis thaliana - the model plant (Philip Rea). Micrograph
of the leaf surface of the Ni-hyperaccumulator Alyssum lesbiacum (Ute
Kraemer). Arabidopsis halleri growing at the bottom of a heap of minewaste
(Ute Kraemer) Last updated: February 18, 2003 |
