Phytochelatin synthesis in response to elevated CO2 under cadmium stress in Lolium perenne L

Phytochelatins formation kinetics and Cd-induced growth inhibition in Lolium perenne L. at elevated CO2 level under Cd stress

Elevated CO₂ levels may alleviate toxicities induced by environmental stresses in plants, such as heavy metals. To assess this possibility, seedlings of Lolium perenne L. were exposed to different Cd stress and CO₂ levels during hydroponic culture. The kinetics of growth, Cd accumulation, and thiol formation were investigated. Elevated CO₂ levels increased the growth rate from 30 to 75%, and decreased the Cd accumulation rate from 36 to 42%, leading to a decrease of Cd content in root and shoot. However, an increase in Cd transport from root to shoot was observed at elevated CO₂ under Cd stress. The production of phytochelatins (PCs) occurred earlier at elevated CO₂ level than at ambient CO₂ level after exposure to Cd stress. The mean SH/Cd ratio was relatively higher at elevated CO₂ level, but elevated CO₂ level significantly decreased thiol contents. The reduction of Cd contents, earlier production of PCs, and relatively higher SH/Cd ratio at the elevated CO₂ level alleviated Cd toxicity in root and shoot to some extent, causing significant yield increase of L. perenne after exposure to Cd stress. This study could provide an important data support and theoretical basis in understanding the effects of elevated CO₂ on plant growth, heavy metal accumulation, and thiol formation.

Effects of EDTA and plant growth-promoting rhizobacteria on plant growth and heavy metal uptake of hyperaccumulator Sedum alfredii Hance

Phytoremediation is a cost-effective and environment-friendly strategy for decontaminating heavy-metal-contaminated soil. However, the practical use of phytoremediation is constrained by the low biomass of plants and low bioavailability of heavy metals in soil. A pot experiment was conducted to investigate the effects of the metal chelator ethylenediaminetetraacetic acid (EDTA) and EDTA in combination with plant growth-promoting rhizobacteria (Burkholderia sp. D54 or Burkholderia sp. D416) on the growth and metal uptake of the hyperaccumulator Sedum alfredii Hance. According to the results, EDTA application decreased shoot and root biomass by 50% and 43%, respectively. The soil respiration and Cd, Pb, Zn uptake were depressed, while the photosynthetic rate, glutathione and phytochelatin (PC) contents were increased by EDTA application. Interestingly, Burkholderia sp. D54 and Burkholderia sp. D416 inoculation significantly relieved the inhibitory effects of EDTA on plant growth and soil respiration. Compared with the control, EDTA + D416 treatment increased the Cd concentration in shoots and decreased the Pb concentration in shoots and roots, but did not change the Zn concentration in S. alfredii plants. Furthermore, EDTA, EDTA + D54 and EDTA + D416 application increased the cysteine and PC contents in S. alfredii (p < 0.05); among all tested PCs, the most abundant species was PC2, and compared with the control, the PC2 content was increased by 371.0%, 1158.6% and 815.6%, respectively. These results will provide some insights into the practical use of EDTA and PGPR in the phytoremediation of heavy-metal-contaminated soil by S. alfredii.

Amending the seedling bed of eggplant with biochar can further immobilize Cd in contaminated soils

Untreated municipal sewage is a potential source of Cd but has been used for irrigating vegetables in many countries in recent years. In growing vegetables and fruits in greenhouses, seedling breeding method is generally used in which the seedlings are transplanted into soils together with their seedling culture. Biochar has been increasingly used to amend soils contaminated by heavy metals, but there are few studies on the effectiveness of different ways of applying the biochar. In this paper, we investigated the efficacy of immobilizing Cd by amending eggplant seedling bed with biochar before transplanting them to biochar-amended soil contaminated by Cd. The results showed that, in comparison with traditional seedling method (without adding biochar), amending the seedling bed by biochar not only had a positive effect on plant growth and production, but further reduced the Cd concentration in the roots, shoots and the fruits by 12.2%, 12.5% and 18.5%, respectively. Furthermore, it increased the pH in rhizosphere to 8.83, reduced the exchangeable Cd concentration in soil by 28.6%, and decreased the Cd bio-accumulation factor from 0.36 to 0.32. Phytochelatin synthesis could be induced when plants are exposed to Cd and it has been used in the literature as a biomarker for evaluating metal toxicity. Our results showed that the seedling culture amended with biochar reduced phytochelatin synthesis in both roots and shoots. It can therefore be concluded that amending the eggplant seedlings bed with biochar can further enhance the effectiveness of remediating Cd contamination in soil after transplanting the plants into soil also amended with biochar.

CAPSULE ABSTRACT

We found a new method to further immobilize Cd in contaminated soils by amending the seedling bed with biochar.

Increasing CO2 differentially affects essential and non-essential amino acid concentration of rice grains grown in cadmium-contaminated soils

Environmental pollution by both ambient CO2 and heavy metals has been steadily increasing, but we do not know how fluctuating CO2 concentrations influence plant nutrients under high Cd pollution, especially in crops. Here, we studied the effects of elevated CO2 and Cd accumulation on proteins and amino acids in rice under Cd stress. In this pot experiment, we analyzed the amino-acid profile of 20 rice cultivars that accumulate Cd differently; the plants were grown in Cd-containing soils under ambient conditions and elevated CO2 levels. We found that although Cd concentrations appeared to be higher in most cultivars under elevated CO2 than under ambient CO2, the effect was significant only in seven cultivars. Combined exposure to Cd and elevated CO2 strongly decreased rice protein and amino acid profiles, including essential and non-essential amino acids. Under elevated CO2, the ratios of specific amino acids were either higher or lower than the optimal ratios provided by FAO/WHO, suggesting that CO2 may flatten the overall amino-acid profile, leading to an excess in some amino acids and deficiencies in others when the rice is consumed. Thus, Cd-tainted rice limits the concentration of essential amino acids in rice-based diets, and the combination with elevated CO2 further exacerbates the problem.

Heavy-metal-induced reactive oxygen species: phytotoxicity and physicochemical changes in plants

As a result of the industrial revolution, anthropogenic activities have enhanced there distribution of many toxic heavy metals from the earth's crust to different environmental compartments. Environmental pollution by toxic heavy metals is increasing worldwide, and poses a rising threat to both the environment and to human health.Plants are exposed to heavy metals from various sources: mining and refining of ores, fertilizer and pesticide applications, battery chemicals, disposal of solid wastes(including sewage sludge), irrigation with wastewater, vehicular exhaust emissions and adjacent industrial activity.Heavy metals induce various morphological, physiological, and biochemical dysfunctions in plants, either directly or indirectly, and cause various damaging effects. The most frequently documented and earliest consequence of heavy metal toxicity in plants cells is the overproduction of ROS. Unlike redox-active metals such as iron and copper, heavy metals (e.g, Pb, Cd, Ni, AI, Mn and Zn) cannot generate ROS directly by participating in biological redox reactions such as Haber Weiss/Fenton reactions. However, these metals induce ROS generation via different indirect mechanisms, such as stimulating the activity of NADPH oxidases, displacing essential cations from specific binding sites of enzymes and inhibiting enzymatic activities from their affinity for -SH groups on the enzyme.Under normal conditions, ROS play several essential roles in regulating the expression of different genes. Reactive oxygen species control numerous processes like the cell cycle, plant growth, abiotic stress responses, systemic signalling, programmed cell death, pathogen defence and development. Enhanced generation of these species from heavy metal toxicity deteriorates the intrinsic antioxidant defense system of cells, and causes oxidative stress. Cells with oxidative stress display various chemical,biological and physiological toxic symptoms as a result of the interaction between ROS and biomolecules. Heavy-metal-induced ROS cause lipid peroxidation, membrane dismantling and damage to DNA, protein and carbohydrates. Plants have very well-organized defense systems, consisting of enzymatic and non-enzymatic antioxidation processes. The primary defense mechanism for heavy metal detoxification is the reduced absorption of these metals into plants or their sequestration in root cells.Secondary heavy metal tolerance mechanisms include activation of antioxidant enzymes and the binding of heavy metals by phytochelatins, glutathione and amino acids. These defense systems work in combination to manage the cascades of oxidative stress and to defend plant cells from the toxic effects of ROS.In this review, we summarized the biochemiCal processes involved in the over production of ROS as an aftermath to heavy metal exposure. We also described the ROS scavenging process that is associated with the antioxidant defense machinery.Despite considerable progress in understanding the biochemistry of ROS overproduction and scavenging, we still lack in-depth studies on the parameters associated with heavy metal exclusion and tolerance capacity of plants. For example, data about the role of glutathione-glutaredoxin-thioredoxin system in ROS detoxification in plant cells are scarce. Moreover, how ROS mediate glutathionylation (redox signalling)is still not completely understood. Similarly, induction of glutathione and phytochelatins under oxidative stress is very well reported, but it is still unexplained that some studied compounds are not involved in the detoxification mechanisms. Moreover,although the role of metal transporters and gene expression is well established for a few metals and plants, much more research is needed. Eventually, when results for more metals and plants are available, the mechanism of the biochemical and genetic basis of heavy metal detoxification in plants will be better understood. Moreover, by using recently developed genetic and biotechnological tools it may be possible to produce plants that have traits desirable for imparting heavy metal tolerance.