Selection, Isolation, and Characterization of Cadmium-Resistant Datura innoxia Suspension Cultures

Ornamental plants adapted to urban ecosystem pollution: lawn grasses and painted daisy tolerating copper

Lawns and flower are major aesthetical and environmental elements of the urban ecosystem. However, harsh urban conditions such as pollution by heavy metals are unfavorable for plants. For example, copper is toxic for ornamental plants, limiting the spread of lawn grass and flowering plants. Therefore, here we hypothesized that plants could be adapted to urban conditions by breeding. We first showed the possibility of using environmental biotechnology in urban greening to obtain, tolerating copper flowering plants and lawn grasses. We tested the adaptation of creeping bentgrass (Agrostis stolonifera L.) and painted daisy (Chrysanthemum carinatum Schousb.) to сopper. We measured Cu resistance in the next generations of those plant species. Results show that some next generations of plant regenerants have increased resistance up to 100 mg/kg Cu for Agrostis stolonifera, and up to 30 mg/kg for Chrysanthemum carinatum. Our findings thus imply that city plants may be adapted and improved by сell selection. Our approach thus represents a novel biotechnology consisting of adapting plants to pollution by сell selection.

Phytochelatins, a class of heavy-metal-binding peptides from plants, are functionally analogous to metallothioneins

Phytochelatins are a class of heavy-metal-binding peptides previously isolated from cell suspension cultures of several dicotyledonous and monocotyledonous plants. These peptides consist of repetitive gamma-glutamylcysteine units with a carboxyl-terminal glycine and range from 5 to 17 amino acids in length. In the present paper we show that all plants tested synthesized phytochelatins upon exposure to heavy metal ions. No evidence for the occurrence of metallothionein-like proteins was found. All data so far obtained indicate that phytochelatins are involved in detoxification and homeostasis of heavy metals in plants and thus serve functions analogous to those of metallothioneins in animals and some fungi. Phytochelatins are induced by a wide range of metal anions and cations. Phytochelatin synthesis in suspension cultures was inhibited by buthionine sulfoximine, a specific inhibitor of gamma-glutamylcysteine synthetase (EC 6.3.2.2). This finding and kinetic studies of phytochelatin induction point to a synthesis from glutathione or its precursor, gamma-glutamylcysteine, in a sequential manner, thereby generating the set of homologous peptides.

In vitro breeding of Brassica juncea L. to enhance metal accumulation and extraction properties

In vitro breeding and somaclonal variation were used as tools to improve the potential of Indian mustard (Brassica juncea L.) to extract and accumulate toxic metals. Calli from B. juncea were cultivated on a modified MS medium supplemented with 10-200 microM Cd or Pb. Afterwards, new B. juncea somaclones were regenerated from metal-tolerant callus cells. Three different phenotypes with improved tolerance of Cd, Zn and Pb were observed under hydroponic conditions: enhanced metal accumulation in both shoots and roots; limited metal translocation from roots to shoots; reduced accumulation in shoots and roots. Seven out of thirty individual variants showed a significantly higher metal extraction than the control plants. The improvement of metal shoot accumulation of the best regenerant (3 x Cd, 1.6 x Zn, 1.8 x Pb) and metal extraction (6.2 x Cd, 3.2 x Zn, 3.8 x Pb) indicated a successful breeding and selection of B. juncea, which could be used for phytoremediation purpose.

Phytochelatins

Phytochelatins (PCs) were first discovered as Cd-binding "Cadystins A and B" in a fission yeast and then in many plants as the major components of Cd-binding complexes. PCs have the general structure of (gamma-glutamyl-cysteinyl)n-glycine (n=2-11) and the variants with the repeated gamma-glutamyl-cysteinyl units are formed in some plants and yeast. They are capable of binding to various metals including Cd, Cu, Zn or As via the sulfhydryl and carboxyl residues, but their biosyntheses are controlled preferentially by the metal Cd or metalloid As. PCs are synthesized from glutathione (gamma-glutamyl-cysteinyl-glycine) in steps mediated by PC synthase. Genes (CAD1, PCS1) of the enzyme have been isolated from plants, fission yeast and some animals. Inhibition studies of PC biosynthesis via glutathione have demonstrated their fundamental roles in the metal detoxification in yeast and fungi, green algae and some aquatic plants, and also in the suspension-cultured cells and intact tissues in higher plants. Over-expression of PC synthase genes increases the Cd-tolerance in yeast and bacteria efficiently but not always in higher plant tissues especially in metal-accumulating species. "Hyperaccumulators" of Cd, Zn, Ni or As in terrestrial plants have a common feature where massive metal transport to shoots prevails, besides the ability of their roots to form PCs. This suggests that PC-based metal detoxification might be an ancient type of defense mechanism established in micro-algae or micro-fungi, and the additional PC-independent mechanism via vascular transport system became established later in higher plants. Readjustment of the PC-dependent and independent mechanisms at the metal-binding sites in the symplast and apoplast of shoots can be effective for further improvement of the metal detoxification activities and the tolerance characteristics of higher plants under various conditions.

Gene expression in cadmium-tolerant Datura innoxia: detection and characterization of cDNAs induced in response to Cd2+

The response of a metal tolerant plant to heavy metal stress involves a number of biochemical pathways. To investigate the overall molecular response of a metal-tolerant plant to heavy-metal exposure, suppressive subtractive hybridization was used to create a library enriched in cadmium-induced cDNAs from cadmium-tolerant Datura innoxia. Two differential screening steps were used to screen the cadmium-induced library resulting in 8 putative cadmium-specific cDNAs out of a pool of 94 clones. Reverse transcriptase polymerase chain reaction was used to confirm that 4 of these 8 clones were cadmium-specific, while the other 4 were induced under heat shock or in the no treatment cells in addition to cadmium exposure. All 8 cDNAs were sequenced and used to search for identification against GenBank. One of the 4 cadmium-specific cDNAs had homology to a sulfur transferase-family protein in Arabidopsis thaliana. The possible link between this result and the heavy-metal response of plants is discussed.

Azuki bean cells are hypersensitive to cadmium and do not synthesize phytochelatins

Suspension-cultured cells of azuki bean (Vigna angularis) as well as the original root tissues were hypersensitive to Cd (<10 microM). Repeated subculturings with a sublethal level of Cd (1-10 microM) did not affect the subsequent response of cells to inhibitory levels of Cd (10-100 microM). The azuki bean cells challenged to Cd did not contain phytochelatin (PC) peptides, unlike tomato (Lycopersicon esculentum) cells that have a substantial tolerance to Cd (>100 microM). Both of the cell suspensions contained a similar level of reduced glutathione (GSH) when grown in the absence of Cd. Externally applied GSH to azuki bean cells recovered neither Cd tolerance nor PC synthesis of the cells. Furthermore, enzyme assays in vitro revealed that the protein extracts of azuki bean cells had no activity converting GSH to PCs, unlike tomato. These results suggest that azuki bean cells are lacking in the PC synthase activity per se, hence being Cd hypersensitive. We concluded that the PC synthase has an important role in Cd tolerance of suspension-cultured cells.

Involvement of a metallothionein-like copper complex in the mechanism of copper tolerance in Mimulus guttatus

The possible involvement of a metallothionein-like copper complex in the mechanism of copper tolerance in Mimulus guttatus has been examined. In the roots of copper-tolerant plants, after exposure to copper, a constant proportion of the total copper content is bound to this complex. The complex is present in the roots of both copper-tolerant and non-tolerant plants, and is also induced in the roots of both by increased exposure to copper. Differences in the amount of the complex in the roots of copper-tolerant and non-tolerant plants suggest that it may be involved in the mechanism of copper tolerance. However, only a small proportion (6%) of the copper in the roots of the copper-tolerant clone is bound to the complex after growth in a high concentration (16 μm) of copper; this result suggests that the mechanism of tolerance does not simply involve the sequestration of all excess copper by the complex. The complex may have a transient function in the mechanism of copper tolerance.

Subcellular Location of O-Acetylserine Sulfhydrylase Isoenzymes in Cell Cultures and Plant Tissues of Datura innoxia Mill

O-Acetylserine sulfhydrylase (OASS; EC 4.2.99.8) catalyzes the formation of L-cysteine from O-acetylserine and inorganic sulfide. Three OASS isoenzymes that differ in molecular mass and subunit structure are present in shoot and root tissues and in cadmium-resistant and cadmium-susceptible cell cultures of Datura innoxia Mill. Different OASS forms predominate in leaves, roots, and suspension-cell cultures. To determine the subcellular location of the OASS isoenzymes, purified mitochondria, chloroplasts, and cytosolic fractions from protoplasts were obtained. The isoenzymes are compartmentalized in D. innoxia cells, with a different isoenzyme predominant in the chloroplast, cytosol, and mitochondria, suggesting that they serve different functions in the plant cell. The chloroplast form is most abundant in green leaves and leaf protoplasts. The cytosolic form is most abundant in roots and cell cultures. A mitochondrial form is abundant in cell cultures, but is a minor form in leaves or roots. Cadmium-tolerant cell cultures contain 1.8 times as much constitutive OASS activity as the wild-type cell line, and 2.9 times more than the cadmium-hypersensitive cell line. This may facilitate rapid production of glutathione and metal-binding phytochelatins when these cultures are exposed to cadmium.

[Characterization of low molecular weight zinc species in normal commercial vegetable foodstuffs]

In six different ordinary vegetables, namely kohlrabi, Chinese cabbage, chard, leak, spinach and Jerusalem artichoke, zinc was mainly found as low molecular weight species. In the present study, these important zinc compounds are further investigated. The determinations of the metal are performed by ET-AAS. The zinc complexes of all vegetables are anionic at pH 8.0 and show similar elution behaviour in gel permeation and anion exchange chromatography. Consequently, a great resemblance in structure between the low molecular weight zinc species from the different vegetables can be supposed. Exemplary, the zinc complexes of kohlrabi and Chinese cabbage are further examined. In more purified samples of these vegetables compared to zinc neither stoichiometric amounts of free protein amino acids nor nicotianamine, free malic acid, citric acid or phytic acid have been detected. Mainly glutamic acid is found in molar excess to zinc after acid hydrolysis in both cases. The cysteine contents of both zinc-binding fractions are very low. Conclusively, the well-known gamma-glutamylcysteinyl-glycines (phytochelatines) can not be responsible for the bonding of zinc in both ordinary vegetables. We suppose that zinc in kohlrabi and Chinese cabbage is bound to a glutamic acid derivative unknown as yet, possibly a malic acid ester.

Retention of cadmium in roots of maize seedlings. Role of complexation by phytochelatins and related thiol peptides

Cd from roots of maize was partitioned in seedlings exposed to 3 microM CdSO4 for 1 to 7 d. Most of the root Cd (92-94%) was buffer soluble and provided the classical metal-induced cysteine-rich, high-molecular-weight Cd-binding complex. This complex, however, bound only part of the Cd within the roots, from 19% after 1 d of exposure to 59% by d 7. Three families of peptides formed the Cd-binding complex: (gamma-glutamic acid-cysteine)n-glycine [(gamma-Glu-Cys)n-Gly], or phytochelatins, (gamma-Glu-Cys)n, and (gamma-Glu-Cys)n-Glu. The monothiols gamma-Glu-Cys-Gly (glutathione), gamma-Glu-Cys, and gamma-Glu-Cys-Glu were absent from the complex. The n2 oligomers of any peptide were the least concentrated, whereas the n3 and n4 oligomers increased in the complex with exposure to Cd. By d 7, 75% of (gamma-Glu-Cys)4-Gly, 80% of (gamma-Glu-Cys)4, and 73% of (gamma-Glu-Cys)3-Glu were complexed with Cd. The peptide thiol:Cd molar ratio for the complexes was 1.01 +/- 0.07, as if the minimal amount of thiol was used to bind Cd. Acid-labile sulfide occurred in the complexes from d 1 onward at the low S2-;Cd molar ratio of 0.18 +/- 0.02.

Phytochelatin accumulation and cadmium tolerance in selected tomato cell lines

Four cell lines of tomato, Lycopersicon esculentum Mill. cv VFNT-Cherry, were selected for their ability to grow in the presence of up to 6 millimolar CdCl(2). The intracellular Cd concentration in these cells was at least 2.3 times higher than in the medium. Growth in media containing higher concentrations of Cd was accompanied by increased production of Cd-binding phytochelatins and a trend toward accumulation of higher molecular weight phytochelatins. At least 90% of the Cd in the most tolerant cells was associated with Cd-phytochelatin complexes. Cell lines maintained an increased tolerance of Cd in the absence of continuous selection pressure.

Reversible accumulation of plant suspension cell cultures in g(1) phase and subsequent synchronous traverse of the cell cycle

The induction of DNA synthesis in Datura innoxia Mill. cell cultures was determined by flow cytometry. A large fraction of the total population of cells traversed the cell cycle in synchrony when exposed to fresh medium. One hour after transfer to fresh medium, 37% of the cells were found in the process of DNA synthesis. After 24 hours of culture, 66% of the cells had accumulated in G(2) phase, and underwent cell division simultaneously. Only 10% of the cells remained in G(0) or G(1). Transfer of cells into a medium, 80% (v/v) of which was conditioned by a sister culture for 2 days, was adequate to inhibit this simultaneous traverse of the cell cycle. A large proportion of dividing cells could be arrested at the G(0) + G(1)/S boundary by exposure to 10 millimolar hydroxyurea (HU) for 12 to 24 hours. Inhibition of DNA synthesis by HU was reversible, and when resuspended into fresh culture medium synchronized cells resumed the cell cycle. Consequently, a large fraction of the cell population could be obtained in the G(2) phase. However, reversal of G(1) arrested cells was not complete and a fraction of cells did not initiate DNA synthesis. Seventy-four percent of the cells simultaneously reached 4C DNA content whereas the frequency of cells which remained in G(0) + G(1) phase was approximately 17%. Incorporation of radioactive precursors into DNA and proteins identified a population of nondividing cells which represents the fraction of cells in G(0). The frequency of cells entering G(0) was 11% at each generation. Our results indicate that almost 100% of the population of dividing cells synchronously traversed the cell cycle following suspension in fresh medium.

Relationships between Cadmium, Zinc, Cd-Peptide, and Organic Acid in Tobacco Suspension Cells

Responses of tobacco (Nicotiana tabacum) suspension cells to Cd and Zn were studied in the presence and absence of ligand of Cd-peptide in order to understand the role of this peptide versus other mechanisms in Cd and Zn accumulation and accommodation in plants. With 45 micromolar Cd and 300 micromolar Zn (non-growth-inhibiting levels), metals appeared rapidly within cells, and intracellular Cd and Zn reached medium concentrations after 6 to 10 hours. Cd-peptide was observed in response to Cd after 2 hours, but this form only accounted for approximately 30% of soluble Cd after 24 hours. Peptide was not observed in cells exposed to 300 micromolar Zn for up to 7 days. Organic acid-to-metal stoichiometry indicated that endogenous organic acid content of cells was more than sufficient to complex absorbed metals and no evidence was found for stimulation of organic acid biosynthesis by Cd or Zn. Metal-complexing potential of organic acids for Cd and Zn versus endogenous cations is discussed as is vacuolar-extravacuolar distribution of metals. The absence of Cd-peptide does not limit Cd-accumulation in the system studied. Results suggest that tobacco suspension cells accommodate the presence of non-growth-inhibiting and growth-inhibiting levels of Cd and Zn by sequestration in the vacuole as complexes with endogenous organic acids and that this may be a principal means for accommodation of Cd as well as Zn in the presence and absence of Cd-peptide.

Effects of cadmium on gene expression in cadmium-tolerant and cadmium-sensitiveDatura innoxia cells

The effect of Cd on gene expression in suspension cultures of twoDatura innoxia cell lines with differing Cd tolerance was studied.In vivo labeling experiments using [(3)H] leucine showed that Cd induced the synthesis of a similar range of proteins in both cell lines at a concentration which will kill the sensitive but not the tolerant cells. Corresponding changes in levels of translatable mRNA were also observed. The induction of the synthesis of proteins by Cd was transient since Cd-tolerant cells growing continuously in 250 μM CdCl2 contained a similar set ofin vitro translation products to cells growing in the absence of Cd. Although Cd had a similar effect on gene expression in both cell lines, Cd-tolerant cells possess two abundant mRNAs which are constitutively produced. These mRNAs encode proteins of low molecular weight (about 11 kDa) and are either absent or present at a low level in Cd-sensitive cells. The functions of these proteins are not known but they may be involved in the tolerance mechanism. Two-dimensional gel electrophoresis ofin vitro translation products showed that many of the Cd-induced proteins are also induced by heat shock. A 42°C heat shock resulted in agreater range and more intense induction of translatable mRNAs than 4 h exposure to 250 μM CdCl2. However a subset of mRNAs were induced specifically by Cd while other mRNAs were heat shock-specific. There was no difference in the ability of the two cell lines to tolerate heat shock. This was also reflected by the same pattern of major proteins induced by heat shock in the two cell lines.

Precursor-product relationships of Poly(γ-glutamylcysteinyl)glycine biosynthesis in Datura innoxia

Suspension cultures of Datura innoxia cells were pulse-labeled with [(35)S]cysteine, then exposed to Cd to determine whether there is a direct precursor-product relationship amongst the different forms of the Cd-induced polypeptides, poly(γ-glutamylcysteinyl)glycines [(γEC)nG, n=2 to 5]. Degradation of the polypeptides and possible regeneration of the [(35)S]-labeled glutathione and cysteine pools were also examined. After 2 h of exposure to [(35)S]cysteine, about 70% of the [(35)S]cysteine in the soluble fraction of the cell was incorporated into [(35)S]glutathione before exposure of the cells to Cd. One h after Cd exposure, most of the cellular [(35)S]glutathione was depleted and label was incorporated into (γEC)nG. Analysis of [(35)S](γEC)nG by reverse phase HPLC showed no direct precursor-product relationship between the synthesis of the shorter and longer chain forms. However, the rate of synthesis of the different polypeptides was linear for 32 h after Cd exposure. There was no evidence of degradation of [(35)S](γEC)nG nor was it excreted into the medium within this period. From these results it is suggested that in the presence of Cd, a large pool of (γEC)nG is unavailable for elongation to (γEC)n+1G.

Poly(gamma-glutamylcysteinyl)glycine Synthesis in Datura innoxia and Binding with Cadmium : Role in Cadmium Tolerance

The effects of Cd on poly(gamma-glutamylcysteinyl)glycine [(gammaEC)(n)G] biosynthesis and formation of (gammaEC)(n)G:Cd complexes were measured in two cell lines of Datura innoxia with differing Cd tolerance. In addition, RNA synthesis, protein synthesis, and GSH concentrations were measured during a 48 hour exposure to Cd. Exposure to 250 micromolar CdCl(2) was toxic to the sensitive line, whereas the tolerant line survived and grew in its presence. Cd-sensitive cells synthesized the same amount of (gammaEC)(n)G as tolerant cells during an initial 24 hour exposure to 250 micromolar CdCl(2). However, rates of (gammaEC)(n)G:Cd complex formation differed between the two cell lines with the sensitive cells forming complexes later than tolerant cells. In addition, the complexes formed by sensitive cells were of lower molecular weight than those of tolerant cells and did not bind all of the cellular Cd. Pulse-labeling of cells with l-[(35)S]cysteine resulted in equivalent rates of incorporation into the (gammaEC)(n)G of both cell lines during the initial 24 hours after Cd. Rates of protein and RNA synthesis were similar for both cell lines during the initial 8 hours after Cd but thereafter declined rapidly in sensitive cells. This was reflected by a decline in viability of sensitive cells. The GSH content of both cell lines declined rapidly upon exposure to Cd but was higher in sensitive cells throughout the experiment. These results show that the biosynthetic pathway for (gammaEC)(n)G synthesis in sensitive cells is operational and that relative overproduction of (gammaEC)(n)G is not the mechanism of Cd-tolerance in a Cd-tolerant cell line of D. innoxia. Rapid formation of (gammaEC)(n)G:Cd complexes that bind all of the cellular Cd within 24 hours appears to correlate with tolerance in these cells.

Cadmium tolerance in tobacco cell culture and its relevance to temperature stress

Growth of unselected tobacco (Nicotiana tabacum W38) cell suspension cultures was reduced by 50-200 μM cadmium (Cd) in the culture medium and cells were killed by 400 μM Cd. Tolerance to Cd was increased either by using rapidly growing cells or by culturing cells at higher densities. Cell lines tolerant to 2 mM Cd were established by progressively elevating levels of Cd in the culture medium. The Cd tolerance was not due to differences in uptake between unselected and Cd-tolerant cell lines, and the tolerance to Cd was not lost during long term culture in the absence of Cd. Cd-tolerant cells also showed higher tolerance to heat shock (37.5°C, 2-8 hours) and cold treatments (4°C, 1-7 days) than the unselected cells.

Phytochelatin synthesis and glutathione levels in response to heavy metals in tomato cells

Cell suspension cultures of tomato, Lycopersicon esculentum Mill. cv VFNT-Cherry, produce phytochelatins (poly[gamma-glutamylcysteinyl]glycines) when exposed to cadmium. The synthesis of these peptides is accompanied by a decline in cellular levels of glutathione. Buthionine sulfoximine, an inhibitor of glutathione synthesis, inhibits the sustained production of phytochelatins. However, phytochelatin synthesis can occur in the presence of buthionine sulfoximine provided that sufficient glutathione is available. These results indicate that glutathione is a substrate for phytochelatin synthesis. The protein synthesis inhibitor cycloheximide does not affect the initial production of phytochelatin.

Poly(gamma-glutamylcysteinyl)glycine: its role in cadmium resistance in plant cells

Angiosperms can be selected for the ability to grow in the presence of normally toxic concentrations of certain trace metal ions. Addition of Cd and Cu to Cd-resistant Datura innoxia cell cultures results in the rapid synthesis and accumulation of sulfur-rich, metal-binding polypeptides. The structure of these compounds was determined using amino acid analysis, 13C NMR, and site-specific enzymic digestion. These compounds are poly(gamma-glutamylcysteinyl)glycines. Greater than 80% of the cellular Cd is bound to the bis and tris forms in Cd-resistant cells. There is a direct correlation between the maximum accumulation of the metal-binding polypeptides and the concentration of toxic ions to which the cells are resistant. In the presence of metal ions, the polypeptides form multimeric aggregates that can be resolved by gel chromatography. Cd binds to both the high and low molecular weight aggregates, whereas Cu preferentially binds to the higher molecular weight forms. The presence of gamma-carboxamide linkages between glutamyl and adjacent cysteinyl residues indicates that these polypeptides are products of biosynthetic pathways. Poly(gamma-glutamylcysteinyl)glycines bind metals and, in this respect, appear to be functional analogs of the protein metallothionein. However, in the absence of supraoptimal concentrations of trace metal ions, the functions of metallothionein in animals and microorganisms and poly(gamma-glutamylcysteinyl)glycines in plants may differ.

An avidin-biotin solid phase ELISA for femtomole isopentenyladenine and isopentenyladenosine measurements in HPLC purified plant extracts

A solid phase enzyme immunoassay was developed for isopentenyladenine (iP) and isopentenyladenosine (iPA) quantitation in HPLC purified plant extracts. It was performed on antigen-coated microtitration plates on which bound antibodies were indirectly labeled by the means of a biotinylated goat anti-rabbit antibody and an avidin-alkaline phosphatase conjugate. Less than 3 femtomoles of iP or iPA were easily detected and the measuring range extended from 3 femtomole to 1 picomole. The reproducibility has been tested and intra- and interassay variations did not exceed 5.0%. The specificity of iPA antibodies was good, as determined by cross-reactivity measurements with other adenylic compounds. The specificity of the measurements for iP and iPA was demonstrated by analysis of the immunoreactivity of fractions obtained by HPLC separation of a methanolic tobacco leaf extract.

Effects of buthionine sulfoximine on cd-binding Peptide levels in suspension-cultured tobacco cells treated with cd, zn, or cu

Inhibition of cell growth and accumulation of Cd-binding peptide were measured in cultured tobacco cells exposed to buthionine sulfoximine. This inhibitor of glutathione metabolism caused little or no reduction of growth (at 0.1 millimolar) in the absence of Cd, but growth was greatly reduced in cultures exposed to buthionine sulfoximine and >/=22 micromolar Cd. Decreased cell growth was directly correlated with decreased levels of Cd-binding peptide and increased levels of what is thought to be free Cd. Zinc inhibited growth of tobacco cells only at the highest levels examined (900-1800 micromolar Zn), but buthionine sulfoximine had no additional significant effect. Similar results were observed for Cu (45-90 micromolar). Results suggest that synthesis of plant Cd-peptide involves gamma-glutamylcysteine synthetase or a related enzyme and that Zn accumulation in tobacco cells does not cause formation of significant Cd-peptide ligand.

Properties of tobacco (Nicotiana tabacum) cadmium-binding peptide(s). Unique non-metallothionein cadmium ligands

The chemical and physical characteristics of Cd-binding peptides isolated from tobacco (Nicotiana tabacum) leaves and suspension-cultured tobacco cells were determined and compared with properties of rat liver Cd,Zn-thionein. Some emphasis was placed on metal-binding and specificity properties. Cd-peptides of apparent Mr 6000 and 2000 were induced in tobacco leaves by growth of plants with 90 microM-Cd. Only the apparent-Mr-2000 Cd-peptide was induced in the leaves of tobacco plants grown in the presence of 3 microM-Cd. In cultured tobacco cells exposed to a wide range of Cd levels (3-180 microM), a peptide of apparent Mr 2000 was observed. Under denaturing conditions [6 M-guanidinium chloride (GdmCl) with or without 100 mM-2-mercaptoethanol], all of the above forms were shown to have an Mr of approx. 1300, compared with an Mr of 6000 for Cd,Zn-thionein. The apparent disaggregation of the Mr-6000 form by GdmCl to what appears to be the unit Cd-binding peptide was reversible. Tobacco-derived Cd-peptide contained approx. 40, 35 and 15 residues of glutamate/glutamine, cysteine and glycine respectively, with serine, lysine, and aromatic residues being absent. Tobacco Cd-peptide had an isoelectric point (pI) of 3.15, which is lower than the pI greater than or equal to 4 reported for metallothionein. A 50% dissociation of Cd occurred at pH 5 and 3.5 for the tobacco Cd-peptide and Cd,Zn-thionein respectively, and GdmCl was shown to cause Cd dissociation from tobacco peptide, but not from metallothionein. No evidence was obtained for Zn induction in vivo of, or Zn binding in vitro to, tobacco Cd-peptide. Copper induced a low-Mr metal-binding component in cultured tobacco cells which did not appear to be identical with the peptide induced by Cd. Properties of tobacco Cd-peptide and Cd,Zn-thionein, including metal affinity and selectivity, are greatly different, except for the common presence of 30 residues of cysteine/100 residues.

Characterization of metal binding peptides from cadmium resistant plant cells

The majority of the cellular cadmium (less than 80%) in cadmium resistant Datura innoxia cells is bound to a small, metal induced peptide which is not metallothionein. This peptide consists of glutamate, cysteine and glycine in a ratio between 2:2:1 and 3:3:1 and has an apparent molecular weight of 776, under denaturing conditions. It is heat stable and complexes with cadmium to produce multimeric forms which are separable by gel filtration. Chemical analyses suggest that some amino acids are not joined by classical peptide linkages. This indicates that the synthesis of the peptide may not be directed by mRNA and that induction of its synthesis may not involve increased transcription from a putative gene corresponding directly to this peptide. A smaller proportion (greater than 15%) of the cellular cadmium is bound to a larger compound which is also heat stable and binds copper more readily than cadmium in vivo. This larger compound has an amino acid composition similar, in some respects, to metallothioneins.

The Cd-binding protein from tomato compared to those of other vascular plants

Cd-binding protein from tomato roots was partially purified and characterized. The Cd-protein complex was eluted as a single Cd peak from QAE-Sephadex A-25 and purified further on Sephadex G-75 in 1 M KC1 buffer. Circular dichroism measurements showed positive Cotton bands at 232 and 273 nm and a negative band at 253 nm, indicative of Cd-thiolate coordination. The major amino acids were Cys (25.6%), Glx (53.3%), Asx (5.4%) and Gly (12.8%) with no aromatic residues detected. The Cd:Cys ratio was 1:2.4. The material had an apparent molecular mass of 3,000 daltons on gel filtration through Sephadex G-50 fine in 1 M KC1 buffer. The tomato protein resembled the Cd-binding proteins isolated from black bentgrass, maize and cabbage. The presence of Cd-thiolate coordination and the high content of Cys are critical features typical of metallothioneins. The abundance of Glx creates a highly anionic protein which is unlike animal metallothioneins. It is proposed that the Cd-binding proteins from vascular plants be designated phytometallothioneins.

Purification and characterization of atypical cadmium-binding polypeptides from Zea mays

Exposure of corn seedlings to high concentrations of Cd ions induces formation of large quantities of polypeptidic compounds which are composed almost exclusively of Glu, Cys and Gly and which bind Cd through thiolate coordination. These polymers occur in classes with molecular weights of 4000 and 8000, respectively, and are made up of chains of about 2000 crosslinked by disulfide bridges. By derivatization and HPLC separation four different molecular species of similar amino acid composition were resolved. In the "native" state all Cys are either coordinated to Cd or linked in intra- and intermolecular disulfide bridges. The overall thiol-to-Cd ratio is close to 2. Judged from the refractoriness to endopeptidase digestion it is inferred that as in glutathione Glu is linked to the adjoining amino acid through its gamma-carboxyl group.

Variation in cadmium accumulation potential and tissue distribution of cadmium in tobacco

Variation in Cd accumulation between Nicotiana species but not varieties has been observed in seedlings grown in solution culture with moderate-to-low levels of Cd. Nicotiana tabacum has been characterized as a leaf and root accumulator while Nicotiana rustica is shown to be primarily a root accumulator, having about half the leaf Cd per gram dry weight of N. tabacum. This phenotype is retained in the mature N. rustica plant. To characterize these two species which differ in their modes of Cd accumulation, tissue Cd distribution, partitioning of metal in soluble and insoluble fractions and the contribution of soluble Cd-binding proteins (peptides) to total plant Cd was assessed using mature solution cultured plants. Metal accumulation was highest in the most mature leaves and in young roots. The preponderance of young roots in N. rustica may, in part, account for low leaf/high root Cd accumulation in this species. While Cd-binding peptides appear to be a principal form of Cd in leaves and roots of seedlings and these also occur in mature leaves, Cd is equally distributed between soluble (about 80% as Cd-binding peptide) and uncharacterized insoluble forms in mature plant roots.

Partial characterization of cadmium-binding protein from roots of tomato

Cd-binding protein was extracted from tomato roots and purified on QAE-Sephadex A-25 and on Sephadex G-75 in 1 molar KCl buffer. The protein preparation was light brown and contained predominantly Cd and small amounts of Zn and Cu. Polyacrylamide gel electrophoresis at pH 6.9 removed the brown material from protein which now bound mostly Cd and some Cu. The apparent molecular weight was 3,100 daltons in high ionic strength medium (1 molar KCl buffer) and 21,500 daltons at low ionic strength. Ionic strength also affected the apparent molecular weight of the Cd-binding protein in crude root extracts. The protein contained 26% cysteine, 53% glutamic acid/glutamine, and 2.8 gram atoms (Cd+Zn+Cu)/mole. The (Cd+Zn+Cu):cysteine ratio was 1:2.3. Circular dichroism measurements indicated Cd-thiolate coordination. The tomato Cd-binding protein was more similar to phytochelatins than to animal metallothioneins.

Selection and characterization of cadmium-resistant suspension cultures of the wild tomato Lycopersicon peruvianum

Suspension cultures of Lycopersicon peruvianum were selected for resistance to cadmium by stepwise exposure to increasing concentrations of cadmium sulfate. Resistant cells grow in 1500 micromolar Cd(++). This resistance was retained for thirty generations without selection. Both resistant and parental sensitive cultures take up Cd(++) at similar rates and to the same final levels. Exposure of sensitive or resistant cultures to Cd(++), Cu(++), or Zn(++) leads to the intracellular accumulation of a low molecular weight, cysteine-rich, cadmium-binding protein. This metallothionein is induced over fifteen fold by 100 μM cadmium and builds up to about five fold higher levels in the resistant cultures.