Physiological Responses and Tolerance Mechanisms to Cadmium in Conyza canadensis

EcAGL enhances cadmium tolerance in transgenic Arabidopsis thaliana through inhibits cadmium transport and ethylene synthesis pathway

Cadmium (Cd) is a highly toxic heavy metal with severe impacts on plant growth and development. Although a multitude of plants have acquired strong tolerance to Cd stress, the underlying molecular mechanism has not been fully elucidated. Here, we identified a Agamous-like MADS-box gene (EcAGL) from Erigeron canadensis. The expression of EcAGL was obviously raised under Cd stress and subcellular localization indicated EcAGL was localized in the nucleus. Overexpression of EcAGL in Arabidopsis thaliana showed marked alleviation of the Cd-induced reduction; Compared to wild-type lines, the antioxidant enzymes activities were increased in EcAGL overexpressing lines under Cd stress. The roots Cd content of transgenic lines was not different with the control plants, whereas significant reduction in shoots Cd content was detected in the transgenic lines, indicating that this gene can enhance Cd tolerance by reducing Cd accumulation in Arabidopsis. Moreover, the expression levels of heavy metal ATPase (AtHMA2 and AtHMA3) and natural resistance-associated macrophage protein (AtNRAMP5) genes in the root of transgenic lines decreased under Cd stress, indicating that EcAGL likely hampered the Cd transport pathway. Gene expression profiles in shoot showed that EcAGL likely modulates the expression of 1-aminocyclopropane-1-carboxylic acid synthase gene (AtACS2), which is involved in the ethylene synthesis pathway, to strengthen the tolerance to Cd. Collectively, these results indicate that EcAGL plays a significant role in regulating Cd tolerance in E. canadensis by alleviating oxidative stress, Cd transport and affecting the ethylene biosynthesis pathway, providing new insight into the molecular mechanism underlying plant tolerance to Cd stress.

Integrative physiological and transcriptome analyses provide insights into the Cadmium (Cd) tolerance of a Cd accumulator: Erigeron canadensis

Cadmium (Cd) is a highly toxic pollutant in soil and water that severely hampers the growth and reproduction of plants. Phytoremediation has been presented as a cost-effective and eco-friendly method for addressing heavy metal pollution. However, phytoremediation is restricted by the limited number of accumulators and the unknown mechanisms underlying heavy metal tolerance. In this study, we demonstrated that Erigeron canadensis (Asteraceae), with its strong adaptability, is tolerant to intense Cd stress (2 mmol/L CdCl₂ solution). Moreover, E. canadensis exhibited a strong ability to accumulate Cd²⁺ when treated with CdCl₂ solution. The activity of some antioxidant enzymes, as well as the malondialdehyde (MDA) level, was significantly increased when E. canadensis was treated with different CdCl₂ solutions (0.5, 1, 2 mmol/L CdCl₂). We found high levels of superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities under 1 mmol/L CdCl₂ treatment. Comparative transcriptomic analysis identified 5,284 differentially expressed genes (DEGs) in the roots and 3,815 DEGs in the shoots after E. canadensis plants were exposed to 0.5 mM Cd. Functional annotation of key DEGs indicated that signal transduction, hormone response, and reactive oxygen species (ROS) metabolism responded significantly to Cd. In particular, the DEGs involved in auxin (IAA) and ethylene (ETH) signal transduction were overrepresented in shoots, indicating that these genes are mainly involved in regulating plant growth and thus likely responsible for the Cd tolerance. Overall, these results not only determined that E. canadensis can be used as a potential accumulator of Cd but also provided some clues regarding the mechanisms underlying heavy metal tolerance.

Difference in Cd2+ flux around the root tips of different soybean (Glycine max L.) cultivars and physiological response under mild cadmium stress

The purpose of the study was to compare differences in Cd2+ flux in the vicinity of root tips of 20 soybean cultivars under mild Cd stress conditions using non-invasive micro-test technology (NMT). The results indicated that Cd2+ influx to the root tips under mild Cd treatment was higher compared to controls. Cd2+ influx showed an obvious spatial distribution, with the highest Cd2+ influx measured 300 μm from the root tips, and a gradually decrease above and below this site. The cultivar Liaodou32 had a lower Cd uptake (3.40 pmol cm-2 s-1), while Liaodou23 had a relatively higher Cd uptake (66.37 pmol cm-2 s-1). Cluster analysis showed that the order of the average Cd2+ influx of the cultivars at a distance of 300 μm from the root tips was as follows: high-uptake cultivars (61.80 pmol cm-2 s-1)>medium-high-uptake cultivars (33.92 pmol cm-2 s-1)>medium-low-uptake cultivars (19.78 pmol cm-2 s-1)>low-uptake cultivars (4.84 pmol cm-2 s-1). We also analyzed physiological responses of different soybean cultivars to mild Cd stress. The results indicated that mild Cd stress could inhibit soluble protein production and root vigor among individual soybean cultivars. Moreover, stress increased SOD, CAT and POD activities and MDA content in root tissues. It should be noted that the physio-biochemical indicators of low-uptake cultivars did not change significantly after exposure to mild Cd stress compared to controls. Pearson's correlation analyses showed that all physio-biochemical indicators were significantly positively associated with influx, except of root SP and biomass. PCA analysis demonstrated that root vigor was a dominant factor causing the differences in Cd tolerance among different soybean seedling cultivars. NMT is of great significance for safe utilization of contaminated soil to distinguish the cultivars with different enrichment capacity for heavy metals from different crop cultivars.

Stabilization of heavy metals in biochar derived from plants in antimony mining area and its environmental implications

Heavy metals pollution in mining soils seriously threatens the ecological environment and human health worldwide. Phytoremediation is considered to be an ideal method to reduce the toxicity, mobility, and bioavailability of heavy metals in the soils. However, the disposal of plant-enriched heavy metals has become a thorny problem. To estimate the effect of pyrolysis on the stabilization of heavy metals in post-phytoremediation plant residues, different biochars were prepared from Conyza canadensis (CC), Gahnia tristis (GT), and Betula luminifera (BL) at different pyrolysis temperatures (300, 450, and 600 °C). Results indicated that pyrolysis was effective in the stabilization of heavy metals (Cr, Ni, As, Sb, Hg, and Pb) in plants and significantly (P < 0.05) decreased the bioavailability of most heavy metals. Among them, GT₆₀₀ prepared by pyrolysis of GT at 600 °C has the best stabilization effect on Sb, which increases the residual fraction by 7.32 times, up to 82.05%. The results of environmental risk assessment show that pyrolysis of biomass at high temperature (600 °C) can effectively mitigate the environmental impact of As, Sb, and Hg. Additionally, the reutilization potential of biochar produced by post-phytoremediation plant residues as adsorbents was investigated. The results of adsorption experiments revealed that all biochars have an excellent performance to adsorb Pb(II), and the maximum adsorption capacity is 139.16 mg g^-1 for CC₄₅₀. The adsorption mechanism could be attributed to complexation, electrostatic attraction, and cation exchange. This study demonstrates that pyrolysis is an effective and environment-friendly alternative method to stabilize heavy metals in plants, and their pyrolysis products can be reused for heavy metal adsorption.

Tolerance and detoxification mechanisms to cadmium stress by hyperaccumulator Erigeron annuus include molecule synthesis in root exudate

Cadmium (Cd) is one of the most toxic environmental pollutants affecting the growth and reproduction of various plants. Analysis of the biological adaptation and tolerance mechanisms of the hyperaccumulator Erigeron annuus to Cd stress may help identify new plant species for phytoremediation and in optimizing the process. This study is to the first to analyze the molecular composition and diversity of dissolved organic matter (DOM) secreted by roots using FT-ICR MS, and multiple physiological and biochemical indexes of E. annuus seedlings grown in solutions containing 0-200 Cd μmol L-1. The results showed that E. annuus had strong photosynthetic adaptation and protection ability under Cd stress. Cd was immobilized or compartmentalized by cell walls and vacuoles in the plant, thus alleviating Cd stress. Activation of anti-oxidation defense mechanisms also played an important role in alleviating or eliminating Cd toxicity in E. annuus. High Cd stress promoted production of a higher proportion of new molecules in DOM secreted by E. annuus roots compared to low Cd stress. DOM secreted by roots contributed to plant resistance to Cd-induced stress via producing more carbohydrates, aromatic structures and tannins. Results indicate the mechanisms underpinning the potential use of E. annuus as a phytoremediator in environments with moderate Cd pollution.

Growth and physiological responses of Pennisetum sp. to cadmium stress under three different soils

Pennisetum sp. was employed as a model species to detect the growth and physiological response to cadmium (Cd) stress at different Cd concentrations (0, 20, 50, and 100 mg kg^-1) in three types of soils (yellow brown soil, yellow soil, and red soil). Results showed that the growth of Pennisetum sp. was not significantly influenced by Cd in 20 mg kg^-1, but significantly inhibited at higher Cd concentrations in three types of soils. Besides, the higher Cd concentrations, the lower root, stem, and leaf biomass. With Cd concentration of soil increasing, Cd content of root, stem, and leaf increased. Compared with no Cd, high Cd concentrations (50 and 100 mg kg^-1) induced the physiological indices (photosynthetic rate, stomatal conductance, transpiration rate) and biochemical indices (nitrate reductase, glutamine synthetase, and glutamate synthase activities) decreasing, but the concentration of NO₃^- and NH₄⁺ increasing. The activity of antioxidative enzymes (SOD, POD, and CAT) was disrupted and the content of malondialdehyde (MDA) increasing. Pennisetum sp. could protect cells from damage and maintain normal physiological metabolism via increasing the production of soluble sugar and soluble protein, but soluble proteins and soluble sugars were limited in high concentrations of Cd (50 and 100 mg kg^-1). Moreover, the growth and physiological response to Cd are different in the three types of soils. The growth of Pennisetum sp. in yellow brown soil was better than that in other two soils, and the gas exchange rate, antioxidant enzyme activity, and nitrogen metabolism in yellow soil and red soil were more affected by Cd stress than that in yellow brown soil. Overall, Pennisetum sp. had certain tolerance and biosorption ability to Cd in different Cd concentrations and different types of soil. Hence, Pennisetum sp. was a suitable choice for Cd remediation, especially in yellow brown soil.

Response of sulfhydryl compounds in subcells of Cladophora rupestris under Pb stress

This study aimed to determine the role of sulfhydryl compounds in the subcells of C. rupestris under Pb stress. Different concentrations (0, 0.5, 1.0, 2.5, 5.0, 7.5, and 10 mg/L) and different exposure days (1, 3, 5, and 7 days) were designed to analyze the subcellular distribution of non-protein thiols (NPT), glutathione (GSH), and phytochelatins (PCs) in C. rupestris. NPT, GSH, and PCs increased significantly with increasing Pb stress in the cell wall and soluble fraction, especially NPT. NPT and GSH slowly increased, and PCs showed no significant difference in the organelle of C. rupestris at low concentrations (< 5.0 mg/L). PCs slightly increased under 5.0 mg/L of Pb stress. PCs/NPT gradually increased with Pb stress at a low Pb concentration. GSH detoxification response lagged behind those of NPT and PCs in response to time. PCs/NPT initially increased and then decreased with Pb stress duration. This study suggested that NPT, GSH, and PCs played an important role in the detoxification of the cell wall and the soluble fraction of C. rupestris under Pb stress. PCs were important in the organelle.

Uptake and accumulation of potentially toxic elements in colonized plant species around the world's largest antimony mine area, China

To provide information on reclamation of multi-heavy metal polluted soils with conception of phytostabilization, a field survey on the uptake and accumulation of potentially toxic elements such as antimony (Sb), arsenic (As), lead (Pb), cadmium (Cd), copper (Cu), and zinc (Zn) in colonized plant species around the world's largest antimony mine area, China, was conducted. Samples including leaves and shoots (including roots and stems) of colonized plants as well as rhizospheric soils were collected from eight sampling zones in the studied area. The results showed that the contents of Cu, Zn, and Pb in rhizospheric soils below plants were comparable to the corresponding background values of Hunan province, otherwise Sb, Cd, and As contents were extremely high (17-106, 17-87, and 3-7 times of the corresponding background values). The highest concentration of Sb was found in Aster subulatus (410 mg kg^-1); Cd, As, and Zn were in Herba bidentis bipinnatae (10.9, 264, and 265 mg kg^-1, respectively); and Cu was in Artemisia lavandulaefolia (27.1 mg kg^-1). It also exhibited that all the contents of As in leaves were several times of those in shoots of plants, Cd and other heavy metals showed in a similar pattern in several studied species, implying that the uptake route of these heavy metals via foliar might contribute to the accumulation. With high bioconcentration factors of heavy metals (more than 1, except for Zn), together with the growth abundance, Herba bidentis bipinnatae was considered as the most suitable colonized species for phytostabilization of the multi-heavy metal pollution in soils on this antimony mine area.

Improvement of the phytoremediation efficiency of Neyraudia reynaudiana for lead-zinc mine-contaminated soil under the interactive effect of earthworms and EDTA

Slow plant growth, low biomass, and low bioavailability of heavy metals in soil are important factors that limit remediation efficiencies. This study adopted a pot cultivation method to evaluate the phytoremediation efficiency of Neyraudia reynaudiana, planted in contaminated soil from a lead-zinc mining area. The soil was inoculated with earthworms (Eisenia fetida), and mixed with the chelating agent ethylenediaminetetraacetic acid (EDTA) one month after planting. The addition of earthworms significantly increased the aboveground biomass of N. reynaudiana and activated heavy metals in the soil, thus facilitating heavy metal uptake by N. reynaudiana. The addition of EDTA significantly increased the incorporation and transport of heavy metals, reduced the uptake of heavy metals by the plant cell wall, and increased the proportions of cellular soluble constituents. Especially with regard to lead, inoculation with earthworms and EDTA application significantly promoted the accumulation efficiency of N. reynaudiana, increasing it 7.1-16.9-fold compared to the control treatment without earthworms and EDTA, and 1.5-2.3-fold compared to a treatment that only used EDTA.

Bioaccumulation and detoxification mechanisms for lead uptake identified in Rhus chinensis Mill. seedlings

A greenhouse experiment was conducted to assay the bioaccumulation and tolerance characteristics of Rhus chinensis Mill. to lead (Pb). The effects of exposing R. chinensis Mill seedlings to increasing Pb concentrations (0, 250, 500, 100mgkg-1) in the soil were assessed by measuring Pb accumulation, subcellular distribution, ultrastructure, photosynthetic characteristics, antioxidative enzyme activity, malondialdehyde content, and phytochelatin content. The majority of Pb taken up by R. chinensis Mill was associated with the cell wall fraction in the roots, where the absorption of Ca increased to maintain cell wall stability, and Pb deposits were found in the intercellular space or in the cell wall structures. In leaves, Pb was primarily stored in the cell wall, while it was compartmentalized into the vacuolar structures in the stem. Pb concentrations adversely affected the morphology of Rhus chinensis Mill cellular substructures. Furthermore, increased Peroxidase (POD) and catalase (CAT) activity was observed in plants grown in Pb-amended soil, and this may have led to reduced ROS to maintain the function of the membrane. Changes in phytochelatin levels (PCs) that were observed in Pb treated plants suggest that PCs formed complexes with Pb in the cytoplasm to reduce Pb²⁺ toxicity in the metabolically active cellular compartment. This mechanism may allow for the plant to accumulate higher concentrations of toxic Pb and survive for a longer period of time. Our study provides a better understanding of how Rhus chinensis Mill detoxifies Pb.

Differential Cadmium Distribution and Translocation in Roots and Shoots Related to Hyper-Tolerance between Tall Fescue and Kentucky Bluegrass

Phytoremediation efficiency mainly depends upon mechanisms in the uptake and translocation of soil contaminants. Cadmium (Cd) distribution and translocation in roots and shoots of tall fescue (Festuca arundinacea) and Kentucky bluegrass (Poa pratensis) were observed using fluorescence spectroscopy with a laser confocal scanning microscope. No difference in root Cd accumulations was detected between these two turfgrass species. Kentucky bluegrass transported more Cd into the stele for root-to-shoot translocation and resulted in significantly higher Cd concentration in leaves. In tall fescue, less Cd was transported into the stele in roots and more Cd was excreted to the cuticle layer in leaves. These results suggested that both turfgrass species were hypertolerant to Cd through distinct distribution patterns in leaves and roots.

Changes in subcellular distribution and antioxidant compounds involved in Pb accumulation and detoxification in Neyraudia reynaudiana

The effects of increasing concentrations of lead (Pb) on Pb accumulation, subcellular distribution, ultrastructure, photosynthetic characteristics, antioxidative enzyme activity, malondialdehyde content, and phytochelatin contents were investigated in Neyraudia reynaudiana seedlings after a 21-day exposure. A Pb analysis at the subcellular level showed that the majority of Pb in the roots was associated with the cell wall fraction, followed by the soluble fraction. In contrast, the majority of the Pb in the leaves was located in the soluble fraction based on transmission electron microscopy and energy dispersive X-ray analyses. Furthermore, high Pb concentrations adversely affected N. reynaudiana cellular structure. The changes in enzyme activity suggested that the antioxidant system plays an important role in eliminating or alleviating Pb toxicity, both in the roots and leaves of N. reynaudiana. Additionally, the phytochelatin contents in the roots and leaves differed significantly between Pb-spiked treatments and control plants. Our results provide strong evidence that cell walls restrict Pb uptake into the protoplasm and establish an important protective barrier. Subsequent vacuolar compartmentalization in leaves could isolate Pb from other substances in the cell and minimize Pb toxicity in other organelles over time. These results also demonstrated that the levels of antioxidant enzymes and phytochelatin in leaves and roots are correlated with Pb toxicity. These detoxification mechanisms promote Pb tolerance in N. reynaudiana.

Lead tolerance mechanism in Conyza canadensis: subcellular distribution, ultrastructure, antioxidative defense system, and phytochelatins

We used hydroponic experiments to examine the effects of different concentrations of lead (Pb) on the performance of the Pb-tolerable plant Conyza canadensis. In these experiments, most of the Pb was accumulated in the roots; there was very little Pb accumulated in stems and leaves. C. canadensis is able to take up significant amounts of Pb whilst greatly restricting its transportation to specific parts of the aboveground biomass. High Pb concentrations inhibited plant growth, increased membrane permeability, elevated antioxidant enzyme activity in roots, and caused a significant increase in root H2O2 and malondialdehyde content. Analysis of Pb content at the subcellular level showed that most Pb was associated with the cell wall fraction, followed by the nucleus-rich fraction, and with a minority present in the mitochondrial and soluble fractions. Furthermore, transmission electron microscopy and energy dispersive X-ray analysis of root cells revealed that the cell wall and intercellular space in C. canadensis roots are the main locations of Pb accumulation. Additionally, high Pb concentrations adversely affected the cellular structure of C. canadensis roots. The increased enzyme activity suggests that the antioxidant system may play an important role in eliminating or alleviating Pb toxicity in C. canadensis roots. However, the levels of non-protein sulfhydryl compounds, glutathione, and phytochelatin did not significantly change in either the roots or leaves under Pb-contaminated treatments. Our results provide strong evidence that cell walls restrict Pb uptake into the root and act as an important barrier protecting root cells, while demonstrating that antioxidant enzyme levels are correlated with Pb exposure. These findings demonstrate the roles played by these detoxification mechanisms in supporting Pb tolerance in C. canadensis.