Electrochemical survey of the chain length influence in phytochelatins competitive binding by cadmium

Effect of pH and citric acid on the growth, arsenic accumulation, and phytochelatin synthesis in Eupatorium cannabinum L., a promising plant for phytostabilization

Heavy-metal contamination of soils has increased in the last decades due to anthropogenic and industrial activities. Arsenic is one of the pollutants that is commonly found in industrial soils and is toxic for both plants and humans. The pH of the soil or the culture medium is one of the most important factors that interferes with the bioavailability of this metalloid to the plant. The addition of chelating agents, such as citric acid (CA), can increase the absorption of As by plants. Therefore, the objective of this work is to study the effect of the pH and the exogenous addition of citric acid on the growth, As accumulation, and thiol compounds in Eupatorium cannabinum; this plant grows naturally in contaminated soils in Asturias, Spain, and has a potential use in phytoremediation. The results showed that E. cannabinum was able to tolerate As stress even at extreme pH values and accumulated a high amounts of As in its roots, which makes it a promising species for the phytostabilization of soils polluted with this metalloid. An addition of 20 mg CA L^-1 led to increased biomass and As accumulation at acidic pH. In order to determine if thiolic compounds, such as phytochelatins, are involved in As accumulation and detoxification in E. cannabinum, we analyzed the synthesis of these compounds in the presence and absence of As and/or citric acid. Our results suggest that these thiolic compounds play a major role in As detoxification, since the presence of CA as a chelating agent reduced the amount of thiols necessary to cope with the toxicity caused by As.

Strategies for Cd accumulation in Dittrichia viscosa (L.) Greuter: role of the cell wall, non-protein thiols and organic acids

Dittrichia viscosa (L.) Greuter is plant species commonly found in degraded zones of Asturias (Spain), where it accumulates high levels of Cd, but the mechanisms involved in this response in non-model plants have not been elucidated. In this way, we analysed the fraction of the total Cd bound to the cell walls, the ultrastructural localization of this metal, and non-protein thiol and organic acid concentrations of two clones of D. viscosa: DV-A (from a metal-polluted soil) and DV-W (from a non-polluted area). After 10 days of hydroponic culture with Cd, fractionation and ultrastructural localisation studies showed that most of the Cd accumulated by D. viscosa was kept in the cell wall. The non-protein thiol content rose in D. viscosa with Cd exposure, especially in the non-metallicolous DV-W clone, and in both clones we found with Cd exposure a synthesis de novo of phytochelatins PC2 and PC3 in shoots and roots and also of other phytochelatin-related compounds, particularly in roots. Regarding organic acids, their concentration in both clones decreased in shoots after Cd treatment, but increased in roots, mainly due to changes in the citric acid concentration. Thus, retention of Cd in the cell wall seems to be the first strategy in response to metal entry in D. viscosa and once inside cells non-protein thiols and organic acids might also participate in Cd tolerance.

A central role for thiols in plant tolerance to abiotic stress

Abiotic stress poses major problems to agriculture and increasing efforts are being made to understand plant stress response and tolerance mechanisms and to develop new tools that underpin successful agriculture. However, the molecular mechanisms of plant stress tolerance are not fully understood, and the data available is incomplete and sometimes contradictory. Here, we review the significance of protein and non-protein thiol compounds in relation to plant tolerance of abiotic stress. First, the roles of the amino acids cysteine and methionine, are discussed, followed by an extensive discussion of the low-molecular-weight tripeptide, thiol glutathione, which plays a central part in plant stress response and oxidative signalling and of glutathione-related enzymes, including those involved in the biosynthesis of non-protein thiol compounds. Special attention is given to the glutathione redox state, to phytochelatins and to the role of glutathione in the regulation of the cell cycle. The protein thiol section focuses on glutaredoxins and thioredoxins, proteins with oxidoreductase activity, which are involved in protein glutathionylation. The review concludes with a brief overview of and future perspectives for the involvement of plant thiols in abiotic stress tolerance.

SEC ICP MS and CZE ICP MS investigation of medium and high molecular weight complexes formed by cadmium ions with phytochelatins

Size-exclusion chromatography (SEC) and capillary zone electrophoresis (CZE) coupled with inductively coupled plasma mass spectrometry were applied to characterize low, medium, and high molecular weight cadmium complexes with glutathione and phytochelatins (PCs). The dominant stoichiometry of the complexes formed in vitro was established as 1:1 using electrospray ionization mass spectrometry. Calculated molecular masses of Cd₁L₁ complexes were used for calibration of the SEC and CZE methods. The results showed a lower (2 kDa) SEC column exclusion limit for cadmium complexes compared with free peptides (10 kDa), and most of the high molecular weight cadmium species were eluted in the void volume of the column. Moreover, the CZE method based on the semiempirical model of Offord to elucidate peptide migration allowed us to show a high propensity of Cd–PC complexes for polymorphism on complexation, which was also observed for extracts of Arabidopsis thaliana treated with cadmium. All the information presented is vital for understanding the mechanism of metal deactivation in plants.

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Cadmium tolerance and phytochelatin content of Arabidopsis seedlings over-expressing the phytochelatin synthase gene AtPCS1

Previous studies demonstrated that expression of the Arabidopsis phytochelatin (PC) biosynthetic gene AtPCS1 in Nicotiana tabacum plants increases the Cd tolerance in the presence of exogenous glutathione (GSH). In this paper, the Cd tolerance of Arabidopsis plants over-expressing AtPCS1 (AtPCSox lines) has been analysed and the differences between Arabidopsis and tobacco are shown. Based on the analysis of seedling fresh weight, primary root length, and alterations in root anatomy, evidence is provided that, at relatively low Cd concentrations, the Cd tolerance of AtPCSox lines is lower than the wild type, while AtPCS1 over-expressing tobacco is more tolerant to Cd than the wild type. At higher Cd concentrations, Arabidopsis AtPCSox seedlings are more tolerant to Cd than the wild type, while tobacco AtPCS1 seedlings are as sensitive as the wild type. Exogenous GSH, in contrast to what was observed in tobacco, did not increase the Cd tolerance of AtPCSox lines. The PC content in wild-type Arabidopsis at low Cd concentrations is more than three times higher than in tobacco and substantial differences were also found in the PC chain lengths. These data indicate that the differences in Cd tolerance and in its dependence on exogenous GSH between Arabidopsis and tobacco are due to species-specific differences in the endogenous content of PCs and GSH and may be in the relative abundance of PCs of different length.