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

Remediation mechanism of high concentrations of multiple heavy metals in contaminated soil by Sedum alfredii and native microorganisms

Pollution accident of nonferrous metallurgy industry often lead to serious heavy metal pollution of the surrounding soil. Phytoremediation of contaminated soil is an environmental and sustainable technology, and soil native microorganisms in the process of phytoremediation also participate in the remediation of heavy metals. However, the effects of high concentrations of multiple heavy metals (HCMHMs) on plants and native soil microorganisms remain uncertain. Thus, further clarification of the mechanism of phytoremediation of HCMHMs soil by plants and native soil microorganisms is required. Using the plant Sedum alfredii (S. alfredii) to restore HCMHM-contaminated soil, we further explored the mechanism of S. alfredii and native soil microorganisms in the remediation of HCMHM soils. The results showed that (i) S. alfredii can promote heavy metals from non-rhizosphere soil to rhizosphere soil, which is conducive to the effect of plants on heavy metals. In addition, it can also enrich the absorbed heavy metals in its roots and leaves; (ii) native soil bacteria can increase the abundance of signal molecule-synthesizing enzymes, such as trpE, trpG, bjaI, rpfF, ACSL, and yidC, and promote the expression of the pathway that converts serine to cysteine, then synthesize substances to chelate heavy metals. In addition, we speculated that genes such as K19703, K07891, K09711, K19703, K07891, and K09711 in native bacteria may be involved in the stabilization or absorption of heavy metals. The results provide scientific basis for S. alfredii to remediate heavy metals contaminated soils, and confirm the potential of phytoremediation of HCMHM contaminated soil.

PGPR-Enabled bioremediation of pesticide and heavy metal-contaminated soil: A review of recent advances and emerging challenges

The excessive usage of agrochemicals, including pesticides, along with various reckless human actions, has ensued discriminating prevalence of pesticides and heavy metals (HMs) in crop plants and the environment. The enhanced exposure to these chemicals is a menace to living organisms. The pesticides may get bioaccumulated in the food chain, thereby leading to several deteriorative changes in the ecosystem health and a rise in the cases of some serious human ailments including cancer. Further, both HMs and pesticides cause some major metabolic disturbances in plants, which include oxidative burst, osmotic alterations and reduced levels of photosynthesis, leading to a decline in plant productivity. Moreover, the synergistic interaction between pesticides and HMs has a more serious impact on human and ecosystem health. Various attempts have been made to explore eco-friendly and environmentally sustainable methods of improving plant health under HMs and/or pesticide stress. Among these methods, the employment of PGPR can be a suitable and effective strategy for managing these contaminants and providing a long-term remedy. Although, the application of PGPR alone can alleviate HM-induced phytotoxicities; however, several recent reports advocate using PGPR with other micro- and macro-organisms, biochar, chelating agents, organic acids, plant growth regulators, etc., to further improve their stress ameliorative potential. Further, some PGPR are also capable of assisting in the degradation of pesticides or their sequestration, reducing their harmful effects on plants and the environment. This present review attempts to present the current status of our understanding of PGPR's potential in the remediation of pesticides and HMs-contaminated soil for the researchers working in the area.

Effect of various phosphorus levels on the extraction of Cd, the transformation of P, and phosphorus-related gene during the phytoremediation of Cd contaminated soil

Phytoremediation has emerged as a common technique for remediating Cd pollution in farmland soil. Moreover, phosphorus, an essential element for plants, can alter the pectin content of plant cell walls and facilitate the accumulation of Cd in plant tissues, thereby enhancing phytoremediation efficiency. Therefore, pot experiments were conducted in order to investigate the effect of phosphorus levels on Cd extraction, phosphorus transformation and phosphorus-related genes during phytoremediation. The results revealed that an optimal application of suitable phosphate fertilizers elevated the soil's pH and electrical conductivity (EC), facilitated the conversion of soil from insoluble phosphorus into available forms, augmented the release of pertinent enzyme activity, and induced the expression of phosphorus cycling-related genes. These enhancements in soil conditions significantly promoted the growth of ryegrass. When applying phosphorus at a rate of 600 mg/kg, ryegrass exhibited plant height, dry weight, and chlorophyll relative content that were 1.27, 1.26, and 1.18 times higher than those in the control group (P0), while the Cd content was 1.12 times greater than that of P0. The potentially toxic elements decline ratio and bioconcentration factor were 42.86% and 1.17 times higher than those of P0, respectively. Consequently, ryegrass demonstrated the highest Cd removal efficiency under these conditions. Results from redundancy analysis (RDA) revealed a significant correlation among pH, total phosphorus, heavy metal content, phosphorus forms, soil enzyme activity, and phosphorus-related genes. In conclusion, this study suggests applying an optimal amount of suitable phosphate fertilizers can enhance restoration efficiency, leading to a reduction in soil Cd content and ultimately improving the safety of crop production in farmlands.

Phytobial remediation advances and application of omics and artificial intelligence: a review

Industrialization and urbanization increased the use of chemicals in agriculture, vehicular emissions, etc., and spoiled all environmental sectors. It causes various problems among living beings at multiple levels and concentrations. Phytoremediation and microbial association are emerging as a potential method for removing heavy metals and other contaminants from soil. The treatment uses plant physiology and metabolism to remove or clean up various soil contaminants efficiently. In recent years, omics and artificial intelligence have been seen as powerful techniques for phytobial remediation. Recently, AI and modeling are used to analyze large data generated by omics technologies. Machine learning algorithms can be used to develop predictive models that can help guide the selection of the most appropriate plant and plant growth-promoting rhizobacteria combination that is most effective at remediation. In this review, emphasis is given to the phytoremediation techniques being explored worldwide in soil contamination.

Understanding boron toxicity in aquatic plants (Salvinia natans and Lemna minor) in the presence and absence of EDTA

Even though boron is a widely used element in various industries and a contributor to water pollution worldwide, few studies have examined the toxicity of boron in aquatic plants. EDTA is used to maintain aquatic plants cultures, however it is possible to modify the toxicity of metals. The objective of this study is to assess the toxicity of boron in aquatic plants and explore the impact of EDTA presence on the resulting toxic responses. Floating watermoss Salvinia natans and duckweed Lemna minor were exposed to concentrations ranging from 5 to 100 mg/L for 7 days and 1 to 60 mg/L for 3 days, respectively. Growth and photosynthetic activity parameters were investigated in the presence and absence of EDTA. Growth inhibitions in both aquatic plants were observed in a concentration-dependent manner, irrespective of the presence or absence of EDTA. For instance, based on the specific growth rate (leaves coverage), EC10 values for S. natans were calculated as 12.7 (9.9-15.3) mg/L and 8.0 (5.8-10.3) mg/L with and without EDTA, respectively. In the case of L. minor, EC10 values were calculated as 1.3 (0.8-1.89) mg/L and 2.0 (0.4-4.3) mg/L with EDTA without EDTA, respectively. Significant effects were also observed on the photosynthetic capacity, however there was no change in the increase of boron concentration. Generally, negligible effects of EDTA to the toxicity of boron were observed in the present study. By comparing toxicity results based on the presence and absence of EDTA, which is an essential element in the test medium, the results of this study are expected to be utilized for the ecological risk assessment of boron in aquatic ecosystems.

Biodegradable chelating agents for enhancing phytoremediation: Mechanisms, market feasibility, and future studies

Heavy metals in soil significantly threaten human health, and their remediation is essential. Among the various techniques used, phytoremediation is one of the safest, most innovative, and effective. In recent years, the use of biodegradable chelators to assist plants in improving their remediation efficiency has gained popularity. These biodegradable chelators aid in the transformation of metal ions or metalloids, thereby facilitating their mobilization and uptake by plants. Developed countries are increasingly adopting biodegradable chelators for phytoremediation, with a growing emphasis on green manufacturing and technological innovation in the chelating agent market. Therefore, it is crucial to gain a comprehensive understanding of the mechanisms and market prospects of biodegradable chelators for phytoremediation. This review focuses on elucidating the uptake, translocation, and detoxification mechanisms of chelators in plants. In this study, we focused on the effects of biodegradable chelators on the growth and environmental development of plants treated with phytoremediation agents. Finally, the potential risks associated with biodegradable chelator-assisted phytoremediation are presented in terms of their availability and application prospects in the market. This study provides a valuable reference for future research in this field.

Exogenous chelating agents influence growth, physiological characteristics and cell ultrastructure of Robinia pseudoacacia seedlings under lead-cadmium stress

Heavy metal pollution of soil, especially by lead (Pb) and cadmium (Cd), is a serious problem worldwide. The application of safe chelating agents, combined with the growing of tolerant trees, constitutes an approach for phytoremediation of heavy-metal-contaminated soil. This study aimed to determine whether the two safe chelators, tetrasodium glutamate diacetate (GLDA) and citric acid (CA), could improve the phytoremediation capacity of black locust (Robinia pseudoacacia L.) in a Pb-Cd-contaminated soil and to find the key factors affecting the biomass accumulation of stressed black locust. In Pb- and Cd-stressed black locust plants, medium- and high-concentration GLDA treatment inhibited the growth, chlorophyll synthesis and maximum photochemical efficiency (Fv/Fm), promoted the absorption of Pb and Cd ions and resulted in the shrinkage of chloroplasts and starch grains when compared with those in Pb- and Cd-stressed plants that were not treated with GLDA. The effects of CA on plant growth, ion absorption, chlorophyll content, chlorophyll fluorescence and organelle size were significantly weaker than those of GLDA. The effect of both agents on Cd absorption was greater than that on Pb absorption in all treatments. The levels of chlorophyll a and plant tissue Cd and rates of starch metabolism were identified as the key factors affecting plant biomass accumulation in GLDA and CA treatments. In the future, GLDA can be combined with functional bacteria and/or growth promoters to promote the growth of Pb- and Cd-stressed plants and to further improve the soil restoration efficiency following pollution by heavy metals. Application of CA combined with the growing of black locust plants has great potential for restoring the Cd-polluted soil. These findings also provide insights into the practical use of GLDA and CA in phytoremediation by R. pseudoacacia and the tolerant mechanisms of R. pseudoacacia to Pb-Cd-contaminated soil.

The effect of exogenous plant growth regulators on elevated Cd phytoremediation by Solanum nigrum L. in contaminated soil

Maximizing amendment potential is an emphasis in the HM-contaminated field of phytoremediation by hyperaccumulators due to the low bioavailability of HMs in soils and small biomass yields of plants. This study investigated the influence of different types and concentrations of plant growth regulators on Cd phytoremediation by Solanum nigrum in contaminated soil. Our conclusions showed that the shoot Cd extractions (μg plant-1) and the root and shoot biomasses at all the treatments remarkedly increased compared with that of the CK (p < 0.05), while the Cd concentrations at root and aboveground parts by S. nigrum, the extractable Cd concentrations, and pH value of soils did not change significantly compared with the CK (p < 0.05). Furthermore, correlation analysis showed that the shoot Cd phytoaccumulation and the root and aboveground biomasses of S. nigrum were particularly dependent upon the application of CTK and GA3 concentration gradient (p < 0.05). Moreover, some related physicochemical indexes were determined for supervising the growth conditions of plants, and these results pointed out that after exogenous PGRs treatments, the chlorophyll content and antioxidative enzymes POD and SOD activities in vivo of plants clearly advanced, while the H2O2 and MDA contents and CAT apparently declined. These consequence demonstrated that the exogenous PGR addition prominently reinforced the Cd phytoextraction capacity of S. nigrum in contaminated soil by stimulating plant growth and increasing shoot yields.

Pseudomonas fluorescens and L-tryptophan application triggered the phytoremediation potential of sunflower (Heliantus annuus L.) in lead-contaminated soil

Lead, a toxic heavy metal present in soil, hampers biological activities and affects the metabolism of plants, animals, and human beings. Its higher concentration may disturb the various physio-chemical processes, which result in stunted and poor plant growth. An interactive approach of plant growth promoting rhizobacteria (PGPR) and L-tryptophan can be used to mitigate the lethal effects of lead. A pot experiment was conducted, and two weeks before sowing, the level of lead (300 mg kg-1) was maintained by spiking the PbCl2 salt. Pseudomonas fluorescens and L-tryptophan were applied individually as well as in combination to segregate the effect of both in contaminated soil under a completely Randomized Design (CRD). Statistical analysis revealed that plant growth was significantly reduced up to 22% due to lead contamination. However, the interactive approach of PGPR and L-tryptophan significantly improved the plant growth, physiology, and yield with relative productive index (RPI) under a lead-stressed environment. Moreover, integrated use of PGPR and L-tryptophan demonstrated a considerable increase (22%) in lead removal efficiency (LRE) by improving bioconcentration factor (BCF) and translocation factor (TF) for shoot without increasing the lead concentration in achenes. The reduced lead concentration in achene was due to its immobilization in shoot and root by negatively charged particles and improved the lead sequestration in vegetative parts which abridged the translocation of lead into achenes.

The Difference between Rhizosphere and Endophytic Bacteria on the Safe Cultivation of Lettuce in Cr-Contaminated Farmland

Chromium (Cr) is a major pollutant affecting the environment and human health and microbial remediation is considered to be the most promising technology for the restoration of the heavily metal-polluted soil. However, the difference between rhizosphere and endophytic bacteria on the potential of crop safety production in Cr-contaminated farmland is not clearly elucidated. Therefore, eight Cr-tolerant endophytic strains of three species: Serratia (SR-1~2), Lysinebacillus (LB-1~5) and Pseudomonas (PA-1) were isolated from rice and maize. Additionally, one Cr-tolerant strain of Alcaligenes faecalis (AF-1) was isolated from the rhizosphere of maize. A randomized group pot experiment with heavily Cr-contaminated (a total Cr concentration of 1020.18 mg kg^-1) paddy clay soil was conducted and the effects of different bacteria on plant growth, absorption and accumulation of Cr in lettuce (Lactuca sativa var. Hort) were compared. The results show that: (i) the addition of SR-2, PA-1 and LB-5 could promote the accumulation of plant fresh weight by 10.3%, 13.5% and 14.2%, respectively; (ii) most of the bacteria could significantly increase the activities of rhizosphere soil catalase and sucrase, among which LB-1 promotes catalase activity by 224.60% and PA-1 increases sucrase activity by 247%; (iii) AF-1, SR-1, LB-1, SR-2, LB-2, LB-3, LB-4 and LB-5 strains could significantly decrease shoot the Cr concentration by 19.2-83.6%. The results reveal that Cr-tolerant bacteria have good potential to reduce shoot Cr concentration at the heavily contaminated soil and endophytic bacteria have the same or even better effects than rhizosphere bacteria; this suggests that bacteria in plants are more ecological friendly than bacteria in soil, thus aiming to safely produce crops in Cr-polluted farmland and alleviate Cr contamination from the food chain.

Meeting the heavy-metal safety requirements for food crops by using biochar: An investigation using sunflower as a representative plant under different atmospheric CO2 concentrations

Global warming impacts on plant growth and food safety are emerging topics of concern, while biochar as a soil additive benefits plants. This study investigates (1) sunflower plant growth at various biochar concentrations in a soil-compost growing substrate under both ambient (420 ppm) and elevated (740 ppm) atmospheric CO₂ concentrations, and (2) concentrations of heavy metals in the growing substrates and organs of the plants. The elevated CO₂ concentration benefits the vegetative parts but harms the reproductive parts of the plants. Additionally, the elevated CO₂ concentration inhibits the beneficial effects that biochar confers on the plants at the ambient concentration. The optimum biochar concentration at both CO₂ levels was found to be 15%. At the time of harvest, most of the heavy-metal concentrations in the growing substrate increased. It was demonstrated that biochar can reduce the amount of heavy metals that accumulate in the roots and seeds whose heavy-metal concentrations complied with Singapore food safety regulations, while those for the biochar met the proposed Singapore biochar standard's thresholds. Our results show that the proposed Singapore biochar standard is practical and sound.

Challenges and opportunities for improving the environmental quality of cadmium-contaminated soil in China

Considering the soil cadmium pollution problem, the Chinese government proposed to estimate the costs and practicality "to completely improve the soil quality by the middle of this century". This study analyzed the challenges in achieving this goal using biophysical data from 10 typical demonstration soil phytoextraction projects. The current annual phytoextraction efficiency was determined as 14.8-490 g ha^-1 a^-1 at 319 RMB g^-1 cadmium. A total of 798 billion RMB and 5 years were required for remediation of cadmium contamination, which was 22 times the investment in soil remediation during 2016-2022. The break-even point of phytoextraction projects was 29 years. The heavy financial burden was considered the primary challenge in improving the environmental quality of such soil. The cost could be reduced by 5.5-35.3 % through optimization measures such as resourcefulness of hyperaccumulator harvests, large-scale breeding, and mechanized management. The break-even point could be shortened to 6-15 years by intercropping/rotating crops, contributing to the goal. Active exploration of phytoextraction efficiency-more efficient accumulators, optimized agronomic measures-is worth practicing.

Effects of microwave and plastic content on the sulfur migration during co-pyrolysis of biomass and plastic

In order to reduce the risks of sulfur-containing contaminants present in biofuels, the effects of microwave and content of hydrogen donor on the cracking of C-S bonds and the migration of sulfur were studied by co-pyrolysis of biomass and plastic. The synergistic mechanism of microwave and hydrogen donor was explored from the perspective of deducing the evolution of sulfur-containing compounds based on microwave thermogravimetric analysis. By combining temperature-weight curves, it was found that microwaves and hydrogen radicals promoted the cracking of sulfur-containing compounds and increased the mass loss of biomass during pyrolysis. The mixing ratio of hydrogen donor (plastic) was the key parameter resulting in the removal of sulfur from oil. By adjusting the mixing ratio, the yield of co-pyrolyzed oil was three times higher than that of cow dung pyrolysis alone and the relative removal rate of sulfur reached 73.67%. The relative content of sulfur in the oil was reduced by 73.77% due to the escape of sulfur-containing gases (H₂S, COS and C₂H₅SH) and the formation of sulfate crystals in the char. Microwave selectively heated sulfur-containing organics and hydrogen radicals stimulated the breaking of C-S bonds, which improved the cracking efficiency of the oil. This breaking will provide a theoretical and technological reference for the environmentally friendly treatment of biomass and biofuels.

Phytoremediation of Potentially Toxic Elements: Role, Status and Concerns

Environmental contamination with a myriad of potentially toxic elements (PTEs) is triggered by various natural and anthropogenic activities. However, the industrial revolution has increased the intensity of these hazardous elements and their concentration in the environment, which, in turn, could provoke potential ecological risks. Additionally, most PTEs pose a considerable nuisance to human beings and affect soil, aquatic organisms, and even nematodes and microbes. This comprehensive review aims to: (i) introduce potentially toxic elements; (ii) overview the major sources of PTEs in the major environmental compartments; (iii) briefly highlight the major impacts of PTEs on humans, plants, aquatic life, and the health of soil; (iv) appraise the major methods for tackling PTE-caused pollution; (v) discuss the concept and applications of the major eco-technological/green approaches (comprising phytoextraction, rhizofiltration, phytostabilization, phytovolatilization, and phytorestoration); (vi) highlight the role of microbes in phytoremediation under PTE stress; and (vii) enlighten the major role of genetic engineering in advancing the phytoremediation of varied PTEs. Overall, appropriate strategies must be developed in order to stop gene flow into wild species, and biosafety issues must be properly addressed. Additionally, consistent efforts should be undertaken to tackle the major issues (e.g., risk estimation, understanding, acceptance and feasibility) in order to guarantee the successful implementation of phytoremediation programs, raise awareness of this green technology among laymen, and to strengthen networking among scientists, stakeholders, industrialists, governments and non-government organizations.

Effect of different amounts of fruit peel-based activator combined with phosphate-solubilizing bacteria on enhancing phytoextraction of Cd from farmland soil by ryegrass

To improve the uptake of heavy metals by plants and increase the effectiveness of phytoextraction, chelating agents are employed to change the speciation of heavy metals in soil and increase their bioavailability. However, the effect of a single activator is limited. In recent years, compound activators have been applied widely to improve phytoextraction efficiency. In this study, a fruit peel-based activator (OG) was prepared, containing a mixture of orange peel extracts and tetrasodium glutamate diacetate (GLDA) (1.6% v/v) in a ratio of 1:1 (v/v). The pot experiment was used to investigate the effects of different amounts of OG combined with phosphate-solubilizing bacteria (Acinetobacter pitti, AP) on the extraction of Cd from farmland soil by ryegrass (Lolium perenne L). The results indicated that the addition of OG and AP increased the pH and EC of the soil and improved the content of nutrient elements in the soil. The optimal combination of the application rates of OG and AP improved the growth of ryegrass and enhanced the phytoextraction of Cd. Redundancy analysis (RDA) showed that total soil nitrogen had the greatest influence on phytoextraction, with a contribution rate of 85.3%, followed by pH, with a contribution rate of 7.7%. Total nitrogen, pH, available phosphorus, alkaline nitrogen, and total organic matter were correlated positively with plant Cd, soil Cd decrease ratio, and the bioaccumulation factor but negatively with total Cd and available Cd. Based on the findings of this study, it is feasible to apply the fruit peel-based activator (amended with GLDA) and phosphate-solubilizing bacteria to enhance phytoextraction of Cd, which will provide a valuable reference for the treatment of heavy metal-contaminated soils and the reutilization of fruit peel waste. When applying the compound activator, it is recommended to consider the influence of the additional amount of compound activator on the extraction efficiency.

A 24-epibrassinolide treatment and intercropping willow with alfalfa increase the efficiency of the phytoremediation of cadmium-contaminated soil

Cadmium contamination in agricultural soils threatens food security and human health, and that has caused widespread concern worldwide. Willow and alfalfa are widely used for the phytoremediation of cadmium (Cd)-contaminated soil, and willow NJU513 is the promising plant for remediating Cd-contaminated soil. In order to discuss the effect of intercropping willow NJU513 with alfalfa on the phytoremediation of Cd-contaminated soil, a pot-culture experiment was conducted in the greenhouse. The result showed that the phytoremediation of Cd-contaminated soil was enhanced by this intercropping because of the 25.90 % increase in the available Cd content. In order to increase the phytoremediation efficiency of Cd in the intercropping treatment, a 24-epibrassinolide (Brs) treatment was designed in the current study. The results showed that the phytoremediation of Cd-contaminated soil by willow and alfalfa improved following a Brs treatment because of the 16.32-74.15 % and 16.91-44.48 % increases in the plant biomass and available Cd content, respectively. Additionally, the extracted Cd by plants in the intercropping treatments with and without Brs was 0.56 and 0.31 mg pot-1, respectively. Transcriptome analyses of willow leaves revealed that Brs up-regulated the expression of genes related to calcium channel activity, calcium and zinc transmembrane transport, photosynthesis, catalase/antioxidant activity, glutathione metabolic processes and detoxification, phagosomes, and vacuoles, and that these upregulated genes promoted plant remediation efficiency and resistance to Cd stress. Brs promoted the phosphate ion transporter activity in willow leaves, which may have enhanced the solubilization of insoluble phosphate minerals by bacterial species (e.g., Vicinamibacterales, Bacillus, and Gaiella) to release Cd, ultimately leading to increased phytoremediation efficiency. In addition, plants with and without Brs treatments induced the bacteria-mediated transformation of available Cd to stable Cd. The study findings may be useful for improving the phytoremediation of Cd-contaminated paddy soil.

Titanium Oxide and Zinc Oxide Nanoparticles in Combination with Cadmium Tolerant Bacillus pumilus Ameliorates the Cadmium Toxicity in Maize

The efficiency of Cd-tolerant plant growth-promoting bacteria (PGPB), zinc oxide nanoparticles (ZnO NPs), and titanium dioxide nanoparticles (TiO₂) in maize growing in Cd-rich conditions was tested in the current study. The best Cd-tolerant strain, Bacillus pumilus, exhibited plant growth stimulation in vivo and in vitro experiments. We determined the toxic concentrations (30 (ppm)) of both NPs for plant growth. B. pumilus, ZnO NPs (20 (ppm)), and TiO₂ NPs (10 (ppm)) had a synergistic effect on plant growth promotion in Cd-contaminated soil (120 (ppm)) in a pot experiment. Both alone and in combination, these therapies reduced Cd toxicity, resulting in improved stress metabolism and defense responses. The combined treatments showed increased relative water content, photosynthetic pigments, proline, total sugars, and proteins and significantly reduced lipid peroxidation. Moreover, this combination increased the levels of minerals and antioxidants and reduced Cd bioaccumulation in shoots and roots by 40–60%. Our in silico pipeline presented a novel picture of the participation of ZnO–TiO₂ protein interaction in both B. pumilus and maize. These findings provide fresh insights on the use of B. pumilus, ZnO, and TiO₂ NPs, both separately and in combination, as a viable and environmentally benign strategy for reducing Cd stress in maize.

Chelator Iminodisuccinic Acid Regulates Reactive Oxygen Species Accumulation and Improves Maize (Zea mays L.) Seed Germination under Pb Stress

To explore the effects of iminodisuccinic acid (a chelating agent) on maize (Zea mays L.) seed germination under lead (Pb) stress, we comparatively analyzed the effects of applying different concentrations of iminodisuccinic acid (0, 5, 20, and 100 mmol·dm⁻³) and combined an addition of exogenous substances regulating reactive oxygen species production on maize seed germination, seedling growth, H₂O₂ content, NADPH oxidase activity, and antioxidant enzyme activities under Pb-stressed and Pb-free conditions. Iminodisuccinic acid (100 mmol·dm⁻³) significantly delayed seed germination under normal germination conditions and alleviated the inhibitory effects of Pb stress (20 mmol·dm⁻³) on seed germination. Under normal conditions (without Pb stress), the iminodisuccinic acid-induced inhibition of seed germination was enhanced by treatment with dimethylthiourea (a specific scavenger of reactive oxygen species) or diphenyleneiodonium chloride (a specific inhibitor of NADPH oxidase), but diminished by treatment with H₂O₂, CaCl₂, diethyldithiocarbamic acid (a specific inhibitor of superoxide dismutase), or aminotriazole (a specific inhibitor of catalase). Under Pb stress, iminodisuccinic acid partially eliminated the excessive H₂O₂ accumulation, improved superoxide dismutase and catalase activity, and weakened the high NADPH oxidase activity. In addition, Ca²⁺ chelation may be essential for maintaining the reactive oxygen species’ balance and improving seed germination and seedling growth by iminodisuccinic acid supplementation in maize under Pb stress. The proposed iminodisuccinic acid supplementation-based method improved maize seed germination in Pb-polluted soil.

Endophyte colonization enhanced cadmium phytoremediation by improving endosphere and rhizosphere microecology characteristics

This study investigated the phytoremediation efficiency of Cd-contaminated soils by hyperaccumulator P. acinosa and its endophyte B. cereus, and evaluated the variation of rhizosphere/endosphere microecology characteristics. The result showed that endophyte PE31, which could successfully colonize on P. acinosa root, increased plant Cd uptake by 42.90% and 28.85% in low and high Cd contaminated soils by promotion of plant biomass and Cd concentration in plant tissues. The improved phytoremediation may attribute to the endophyte inoculation, which significantly improved the bioavailable heavy metal (HM) percentage, nutrient cycling related enzyme activities and nutrient contents including available potassium, phosphorus and organic matter. Additionally, the relative abundance beneficial bacteria Bacillus (significantly increased by 81.23% and 34.03% in the endosphere, and by 4.86% and 8.54% in rhizosphere in low and high Cd contaminated soils) and Lysobacter, showed positive and close correlation with plant growth and HM accumulation. These results indicated that endophyte inoculation could reshape rhizosphere and endosphere microecology characteristics, which enhanced the potential for phytoremediation of Cd contaminated soils.

Integration of manganese accumulation, subcellular distribution, chemical forms, and physiological responses to understand manganese tolerance in Macleaya cordata

Macleaya cordata (Willd.) R. Br. are proposed for the application in phytoremediation of heavy metal-contaminated soil. In this paper, the physiological response, subcellular distribution, chemical form, ultrastructure, and manganese (Mn) absorption characteristics of M. cordata under the stress of 0, 3, 6, 9, 12, and 15 mmol/L manganese concentration were studied by sand culture experiment. The results showed that M. cordata seedlings show high tolerance to Mn stress with a concentration of less than 6 mmol/L, while higher Mn concentration showed a significant toxic effect. A low concentration of Mn (≤ 6 mmol/L) can promote the synthesis of chlorophyll and soluble protein; furthermore, superoxide dismutase and peroxidase activities responded positively. The accumulation of Mn in the inactive metabolic part (cell wall and vacuole) of M. cordata leaves might be one of the main Mn detoxification mechanism. According to the ultrastructure of M. cordata, high-concentration Mn²⁺ (≥ 12 mmol/L) stress can cause M. cordata cells to be distorted and deformed, black precipitates appeared in the intercellular space, mitochondria decrease, chloroplasts shrink, hungry particles increased, and starch granules decrease. The uptake ability of different tissues for Mn is leaf > root > stem, and transport coefficient decreases with the increase of Mn concentration. Clearly, M. cordata has a certain tolerance to manganese, which has the ecological application potential in Mn-polluted areas.

Hydrogen sulphide regulates the growth of tomato root cells by affecting cell wall biosynthesis under CuO NPs stress

Copper oxide nanoparticles (CuO NPs) show strong nano-toxic effects on organisms. Hydrogen sulphide (H₂ S) plays a pivotal role in plant response to abiotic stress. In this study, we examine the crucial role of the cell wall as regulated by H₂ S in response to CuO NPs stress. The digestion method was employed to determine Cu content using atomic absorption spectrometry. The TraKine pro-tubulin staining kit was used to investigate the microtubule cytoskeleton using confocal laser-scanning microscopy. Cell wall component analysis utilized the ICS-3000 HPLC system. Application of H₂ S reduced growth inhibition caused by CuO NPs. Furthermore, most of the CuO NPs accumulates in roots, indicating a low transfer rate, and H₂ S significantly decreased CuO NPs content in roots, leaves and stems. Subcellular distribution analysis implied most Cu accumulated in root cell walls, and that H₂ S reduced the content of Cu in root cell walls. Cortical microtubules in the plasma membrane, guide cell wall biosynthesis. H₂ S obviously alleviated microtubule cytoskeleton disorders caused by CuO NPs. In addition, the content of cellulose, hemicellulose, pectin and other monosaccharides in root cell walls was reduced by CuO NPs treatment. H₂ S enhanced the monosaccharide and polysaccharide contents compared with that after CuO NPs treatment. In conclusion, H₂ S regulates cell wall development in response to CuO NPs stress by stabilizing microtubules. H₂ S affected Cu distribution and alleviated growth inhibition of tomato seedlings. The research results provide a theoretical basis for further study of nano-toxicity regulation in plants.

Enhanced Cd Phytoextraction by Solanum nigrum L. from Contaminated Soils Combined with the Application of N Fertilizers and Double Harvests

It is very important to increase phytoremediation efficiency in practice in suitable climatic conditions for plant growth through multiple harvests. Solanum nigrum L. is a Cd hyperaccumulator. In the present experiment, after applying different types of N fertilizers (NH4HCO3, NH4Cl, (NH4)2SO4, CH4N2O), root and shoot biomasses and Cd phytoextraction efficiency of S. nigrum effectively improved (p < 0.05). Shoot biomasses of S. nigrum harvested at the first florescence stage plus the amounts at the second florescence stage were higher than those harvested at the maturation stage, which indicates that S. nigrum Cd phytoaccumulation efficiency was higher in the former compared to the latter as there was no clear change in Cd concentration (p < 0.05). The pH value and extractable Cd contents showed no changes, regardless of whether N fertilizer was added or not at different growth stages. In addition, after N fertilizer was applied, H2O2 and malondialdehyde (MDA) contents in S. nigrum in vivo were lower compared to those that had not received N addition (CK); similarly, the concentration of proline was decreased as well (p < 0.05). The activity of the antioxidant enzyme catalase (CAT), harvested at different growth periods after four types of N fertilizer applications, obviously decreased in S. nigrum shoots, while peroxidase (POD) and superoxide dismutase) (SOD) activities increased (p < 0.05). Our study demonstrated that (NH4)2SO4 treatment exerted the most positive effect and CH4N2O the second most positive effect on S. nigrum Cd phytoremediation efficiency in double harvests at florescence stages, and the growth conditions were better than others.

A review on bioremediation approach for heavy metal detoxification and accumulation in plants

Nowadays, the accumulation of toxic heavy metals in soil and water streams is considered a serious environmental problem that causes various harmful effects on plants and animals. Phytoremediation is an effective, green, and economical bioremediation approach by which the harmful heavy metals in the contaminated ecosystem can be detoxified and accumulated in the plant. Hyperaccumulators exude molecules called transporters that carry and translocate the heavy metals present in the soil to different plant parts. The hyperaccumulator plant genes can confine higher concentrations of toxic heavy metals in their tissues. The efficiency of phytoremediation relies on various parameters such as soil properties (pH and soil type), organic matters in soil, heavy metal type, nature of rhizosphere, characteristics of rhizosphere microflora, etc. The present review comprehensively discusses the toxicity effect of heavy metals on the environment and different phytoremediation mechanisms for the transport and accumulation of heavy metals from polluted soil. This review gave comprehensive insights into plants tolerance for the higher heavy metal concentration their responses for heavy metal accumulation and the different mechanisms involved for heavy metal tolerance. The current status and the characteristic features that need to be improved in the phytoremediation process are also reviewed in detail.

Phytoremediation of cadmium contaminated alkaline soil using the ornamental hyperaccumulator Mirabilis jalapa L. enhanced by double harvesting: a field study

Ornamental hyperaccumulators are considered ideal plants for phytoremediation of cadmium (Cd)-contaminated soils because of their high accumulation capacity and minimal potential to enter the food chain. Aiming to estimate the phytoremediation efficiency and explore the influence of double harvesting, a filed study was carried out in Cd-contaminated alkaline soil using the ornamental hyperaccumulator Mirabilis jalapa L. Seeds of M. jalapa were sawn with spacing in rows of 15 cm and in lines of 20 cm on April 20^th. Three treatments with different harvesting strategies were arranged: all plants were harvested once on September 10th (marked with H0), and the plants were harvested on July 20^th for the first time at a height above ground of 50 cm and 30 cm (marked with H1 and H2, respectively), and then, they were harvested on September 10th for the second time. The plant samples were digested with a mixture of HNO₃ and HClO₄ and determined using ICP-MS. The results showed that the total biomass of the shoots increased from 505.81 ± 8.29 g/m² in the H0 treatment to 849.72 ± 59.73 and 933.14 ± 96.12 g/m² in the H1 and H2 treatments, respectively, while the total Cd accumulation in the shoots was 42.32 ± 4.44, 52.99 ± 7.32, and 56.30 ± 6.95 g/ha in the H0, H1, and H2 treatments, respectively. Phytoremediation efficiencies increased by at least 20% after the application of double harvesting. However, double harvesting also decreased the translocation and bioconcentration factors with a reduction of 30-55% because the Cd concentration in the shoots was low at the first harvest. Our results suggested that double harvesting is a powerful method to improve phytoremediation efficiency with low costs and minimal risk, and other technologies should be applied together to address the accumulation and translocation of Cd.

Abscisic acid-catabolizing bacteria: A useful tool for enhancing phytoremediation

Bacteria-facilitated phytoextraction has been gaining recognition for the phytoremediation of heavy metal (HM)-contaminated soils. Nevertheless, it remains unclear whether catabolizing abscisic acid (ABA) in hyperaccumulating plants via rhizobacteria could facilitate HM phytoextraction. In this study, inoculation with the ABA-catabolizing bacterium, Rhodococcus qingshengii, increased HM (Cd, Zn, Pb, and Cu) concentrations in the shoots of hyperaccumulators Vetiveria zizanioides, Brassica juncea, Lolium perenne L., Solanum nigrum L., and Sedum alfredii Hance grown in mildly and severely contaminated soils by 28.8%-331.3%, 8.5%-393.4%, 21.2%-222.5%, 14.7%-115.5%, and 28.3%-174.2%, respectively, compared with non-inoculated plants. The fresh biomass of these hyperaccumulators was elevated by 16.5%-94.4%, compared to that of the bacteria-free control. Phytoremediation potential indices, including bioconcentration and translocation factors, also revealed that the bacteria markedly boosted the phytoextraction efficacy from soil. Furthermore, principal component analysis (PCA) suggested that the effects of bacteria on the concentrations of Cd and Zn in hyperaccumulators were significantly correlated with ABA metabolism, but not with Pb and Cu. Combined with the synergistic effects on plant biomass, the bacteria also improved the phytoextraction of Pb and Cu in hyperaccumulators. Overall, the application of microorganism-assisted remediation based on ABA-catabolizing bacteria might be an alternative strategy for enhancing phytoremediation efficiency in HM-contaminated soils.

The effects of different electrode materials on seed germination of Solanum nigrum L. and its Cd accumulation in soil

The effects of different electrode on Solanum nigrum L. seed germination were determined. The result showed that germination percentage (GP) of seeds in treatment T2 (titanium electrode) was 26.6% higher than in control (CK, without electric field). High potassium and calcium concentrations were beneficial for seed enzymatic activity in treatment T2, which could partly explain the increase in GP. Cd accumulation (μg/pot) in S. nigrum treated with any electric field was significantly higher (p<0.05) than in CK without electric field. Specifically, Cd accumulation under the treatment T3 (stainless steel electrode) was the highest both in roots and shoots; this accumulation in shoots and roots were 74.7 % and 67.4 % higher for stainless steel than in CK. This increase must have been associated with a higher Cd concentration in plants and did not exert a significant effect on the biomass. In particular, Cd concentrations in roots and shoots under stainless steel treatment were both significantly higher than in CK (p<0.05), which had to be related to the higher available Cd concentration in the soil in the middle region. Furthermore, it could be attributed to altered soil pH and other soil properties. Moreover, none of the biomasses were significantly affected (p<0.05) by different electrode materials compared to CK.

GLDA and EDTA assisted phytoremediation potential of Sedum hybridum 'Immergrunchen' for Cd and Pb contaminated soil

Exogenous application of chelants is a common way to enhance the phytoextraction of heavy metals. A pot experiment was conducted to investigate the influences of cadmium (Cd), lead (Pb), Cd and Pb, L-glutamic acid N, N-diacetic acid (GLDA) and ethylene diamine tetraacetate (EDTA) on the growth, Cd and Pb accumulation of Sedum hybridum 'Immergrunchen'. The results showed that Sedum hybridum 'Immergrunchen' had a high tolerance to Pb treatment, followed by Cd-Pb treatment. The plant was sensitive to Cd stress. EDTA treatment was more harmful to plant growth than that of GLDA treatment. The optimal Cd concentration of shoot and root reached 27.6 mg·kg^-1 and 32.6 mg·kg^-1, 757 mg·kg^-1 and 1,025 mg·kg^-1for Pb accumulation at 100-1,500 mg·kg^-1. The maximum Cd and Pb phytoextraction from 3 mmol·kg^-1 GLDA treatment were 1.40 and 1.73 times as much as that of the control, 1.21 and 1.02 times under 6 mmol·kg^-1 EDTA treatment. Therefore, the enhanced phytoremediation of GLDA to Cd and Pb co-contaminated soil was better than that of EDTA. GLDA-assisted phytoextraction of Cd and Pb by Sedum hybridum 'Immergrunchen' can be considered as a promising way to phytoremediate Cd and Pb co-contaminated soil.

Rhodococcus qingshengii facilitates the phytoextraction of Zn, Cd, Ni, and Pb from soils by Sedum alfredii Hance

The enhanced heavy metal (HM) phytoextraction efficiency of hyperaccumulating plants via plant-growth-promoting microbes has been proposed as an effective strategy to remove HMs from contaminated soil. Nevertheless, it remains unclear whether catabolizing the abscisic acid (ABA) in hyperaccumulating plants via rhizobacteria can facilitate HM phytoextraction. In the present study, a hyperaccumulator, Sedum alfredii Hance, inoculated with an ABA-catabolizing bacterium Rhodococcus qingshengii, showed higher concentrations of Zn, Cd, Ni, and Pb in the contaminated paddy-grown plant shoots by 35%, 63%, 49%, and 49%, and in plants grown in mine soils by 112%, 105%, 46%, and 49%, respectively, than in the control_{bacteria-free} plants. However, no significant changes were observed in Cu content between these plants. Furthermore, parameters indicating phytoremediation potential, including the translocation factor (TF) and bioconcentration factor (BCF), revealed that bacterial inoculation could markedly increase the efficacy of Zn, Cd, Ni, and Pb phytoextraction from the soil. Notably, the bioavailabilities of HMs in soils were not influenced by R. qingshengii; however, the expression of transporters related to the uptake of these HMs, including SaIRT1, SaZIP1, SaZIP2, SaZIP3, SaNramp1, SaNramp3, SaNramp6, SaHMA2, and SaHMA3, was upregulated. These findings indicate that R. qingshengii inoculation could increase the HM-uptake ability of plants by catabolizing ABA and may provide a promising strategy for enhancing the phytoremediation efficacy in HM-contaminated soils.

Enhanced effects of walnut green husk solution on the phytoextraction of soil Cd and Zn and corresponding microbial responses

Walnut green husk (WGH) is a common agricultural waste, but it may be conducive to phytoremediation of heavy metals owing to its abundant phenolic hydroxyl, carboxyl, and other functional groups. In this study, WGH solution was used as an enhancer in the phytoextraction process via Sedum plumbizincicola, a hyperaccumulator of Zn and Cd. Microbial responses in the soil and plants were seamlessly analyzed to determine the underlying mechanisms of heavy metal extraction in this process. The results showed that the addition of 0.05 g/mL WGH solution increased the accumulation of Cd (by 153%) and Zn (by 220%) in plants as well as the availabilities of Cd (by 29.46%) and Zn (by 9.71%) in the soil, which can be attributed to an increase in the relative abundance of plant growth-promoting rhizobacteria that benefit phytoextraction. Furthermore, co-occurrence network analysis indicated the keystone taxa in the microbial community. Particularly, one of the keystone taxa, Pseudomonas, was also identified as distinct taxon in soil, and it was dominant among the endophytic bacteria. These results indicated that Pseudomonas, a taxon responded to WGH solution, may play a key role in enhanced phytoextraction in both soil and plant root. These results help to a better understanding of the microbial mechanisms for heavy metal accumulation in hyperaccumulator.

Heterogeneity of humic/fulvic acids derived from composts explains the differences in accelerating soil Cd-hyperaccumulation by Sedum alfredii

The hyperaccumulating mechanism concerning heavy metal activation or passivation and plant response triggered by fulvic acid (FA) and humic acid (HA) recruitments are investigated herein. We carefully examine the Cd activation effect by various FA and HA, tracing from pig, goat, and duck manure composts to straw compost and commercial materials (i.e., PC, GC, DC, SC, and CM), as well as their roles in plant growth promotion and Cd uptake. Our results indicate that due to the decrease of soil pH and their multiple functional groups, the contents of available Cd (AE-Cd) increased by 4.3-4.8% and 3.6-6.3% when all FA and HA sources were applied for 30 days. A 13.1-19.9% increase in AE-Cd was observed when CFA, DFA, and PFA were applied for five days, and a 9.5% increment was found when PHA was applied for 10 days. In the pot experiment, the Cd accumulation in plants increased by 2.78 and 2.17 folds with PFA and PHA applications, respectively, compared to the blank control group. This result can be attributed to the stimulative effects of the simultaneous Sedum alfredii growth and Cd phytoavailability. Notably, the Cd accumulation increased by 2.26 times with the SFA amendment due to the predominant stimulation effect to the phytoavailable Cd rather than plant growth. However, slight inhibitory effects were observed upon plant growth or Cd uptake, which led to the reduction of the Cd accumulation with DHA, SHA, and CHA employments. Consistently, the corresponding soil Cd removal efficiencies were 43.5% and 34.6% with PFA and PHA, respectively, which hold abundant O- and N-containing groups. Our research aims to gain insights into the ternary interaction in the presence of heavy metal, humic substances, and S. alfredii to simultaneously accelerate Cd activation and hyperaccumulation.

Inoculation With Indigenous Rhizosphere Microbes Enhances Aboveground Accumulation of Lead in Salix integra Thunb. by Improving Transport Coefficients

The application of plant-microbial remediation of heavy metals is restricted by the difficulty of exogenous microbes to form large populations and maintain their long-term remediation efficiency. We therefore investigated the effects of inoculation with indigenous heavy-metal-tolerant rhizosphere microbes on phytoremediation of lead (Pb) by Salix integra. We measured plant physiological indexes and soil Pb bioavailability and conducted widespread targeted metabolome analysis of strains to better understand the mechanisms of enhance Pb accumulation. Growth of Salix integra was improved by both single and co-inoculation treatments with Bacillus sp. and Aspergillus niger, increasing by 14% in co-inoculated plants. Transfer coefficients for Pb, indicating mobility from soil via roots into branches or leaves, were higher following microbial inoculation, showing a more than 100% increase in the co-inoculation treatment over untreated plants. However, Pb accumulation was only enhanced by single inoculation treatments with either Bacillus sp. or Aspergillus niger, being 10% greater in plants inoculated with Bacillus sp. compared with uninoculated controls. Inoculation mainly promoted accumulation of Pb in aboveground plant parts. Superoxide dismutase and catalase enzyme activities as well as the proline content of inoculated plants were enhanced by most treatments. However, soil urease and catalase activities were lower in inoculated plants than controls. Proportions of acid-soluble Pb were 0.34 and 0.41% higher in rhizosphere and bulk soil, respectively, of plants inoculated with Bacillus sp. than in that of uninoculated plants. We identified 410 metabolites from the microbial inoculations, of which more than 50% contributed to heavy metal bioavailability; organic acids, amino acids, and carbohydrates formed the three major metabolite categories. These results suggest that both indigenous Bacillus sp. and Aspergillus niger could be used to assist phytoremediation by enhancing antioxidant defenses of Salix integra and altering Pb bioavailability. We speculate that microbial strains colonized the soil and plants at the same time, with variations in their metabolite profiles reflecting different living conditions. We also need to consider interactions between inocula and the whole microbial community when applying microbial inoculation to promote phytoremediation.

Application of inorganic selenium to reduce accumulation and toxicity of heavy metals (metalloids) in plants: The main mechanisms, concerns, and risks

Anthropogenic activities such as mining, industrialization and subsequent emission of industrial waste, and agricultural practices have led to an increase in the accumulation of metal(loid)s in agricultural soils and crops, which threatens the health of people; the risk is more pronounced for individuals whose survival depends on food sources from several contaminated regions. Selenium (Se) is an element essential for the normal functioning of the human body and is a beneficial element for plants. Se deficiency in the diet is a common issue in many countries around the world, such as China and Egypt. >40 diseases are associated with Se deficiency. In practice, Se compounds have been applied through foliar sprays or via base application of fertilizers to increase Se concentration in the edible parts of crops and to satisfy the daily Se intake. Moreover, Se at low concentrations has been used to mitigate the toxicity of many metal(loid)s. In this review, we present an overview of the latest knowledge and practices with regards to the utilization of Se to reduce the uptake/toxicity of metal(loid)s in plants. We have focused on the following issues: 1) the current status of understanding the mechanisms of detoxification and uptake restriction of metal(loid)s regulated by Se; 2) the optimal dose and speciation of Se, and stage of plant growth that is optimal for application; 3) the differences in the efficiency of different application methods of Se including seed priming, base application, and foliar spray of Se fertilizers; 4) the possibility of using Se along with other methods to reduce multiple metal(loid) accumulation in crops; and 5) potential risks when Se is used to reduce metal(loid) accumulation in crops.

Study of soil microorganisms modified wheat straw and biochar for reducing cadmium leaching potential and bioavailability

The application of crops straw and biochar in trace metals remediation from the contaminated environment attracted more and more attention during the past decade. Although there has been some review work on the mechanism of trace metals stabilization by crops straw, the effects and mechanisms of interaction among soil indigenous-microbes and crops-straw for trace metal adsorption and stabilization is still unclear. In this study, the dynamic effects along with potential mechanisms of wheat-straw (WS), wheat-straw biochar (WBC) and biologically modified wheat-straw (BMWS) were conducted to investigate the adsorption, leaching behaviour, chemical fractions and bioavailability of cadmium (Cd). The results showed that the biosorption capacity (q_e) was most elevated in the BMWS treatment (14.42 mg g^-1) as compared to WBC (6.28 mg g^-1) and WS (4.20 mg g^-1). The application of BMWS, WBC and WS at the rate of 3% significantly reduced Cd concentration in leachate to 53, 45 and 21% respectively, as compared to control. The addition of BMWS reduced the exchangeable Cd fraction resulted an increase in organic matter and carbonate bound Cd fraction in the soil. The DTPA extractable Cd was significantly decreased by 31.2 and 28.6% with the application of BMWS and WBC at 3% w/w respectively as compared to control. The research results may provide a novel perceptive for the development of functional materials and strategies for eco-friendly and sustainable trace metal remediation in contaminated soil and water by combination of straw and soil-indigenous microorganisms.

Effect of plant extracts and citric acid on phytoremediation of metal-contaminated soil

Application of activating agents can significantly improve efficiency of plants for remediation of soils contaminated by heavy metals, however, damage to soil and plants limits application of traditional activating agents. The aim of our experiments is to select an efficient,green and low-cost activating agent to improve efficiency of plant extraction technology. In this study, contaminated soil was remediated by Sedum alfredii. The effects of two plant extracts (i.e., Oxalis corniculata,OX and Medicago sativaextract, ME) in addition to citric acid (CA) were studied in oscillatory activation experiment and pot experiment. The oscillation activation experiment revealed that extraction quantity of heavy metals in the soil was enhanced significantly with concentration of plant extract. The extraction quantity of Zn from 100% OX extract and ME extract were significantly higher than 10 mmol/L CA (54.04% and 33.09%, respectively). The 10 mmol/L CA has best extraction efficiency for Cd, up to 41.36 µg/kg, which is significantly higher than CK (control) (p < 0.05). The pot experiment exhibited that application of CA has significantly reduced soil pH and organic matter content by 8.63% and 28.21%, respectively, however the two extracts have no significant effect on soil properties. The study indicated that application of CA has negative effects on root morphological parameters and chlorophyll fluorescence parameters of Sedum alfredii.The addition of extracts of two plants have not caused any harm to Sedum alfredii. The application of three activating agents was beneficial for purification of Cd and Zn in soils, and its repairing efficiency was improved by 3.92, 3.37, 3.33 times and 0.44, 0.20, 0.86 times, respectively. The combination of plant extracts and hyperaccumulators can effectively remove heavy metals from contaminated soils, which provided a theoretical basis for mitigation of pollution in soils.

Rhizosphere iron and manganese-oxidizing bacteria stimulate root iron plaque formation and regulate Cd uptake of rice plants (Oryza sativa L.)

Iron plaque is the amorphous and/or crystalline layer of Fe and Mn (hydr)oxides formed on the root surface of wetland plants. It could adsorb and co-precipitate metal(loid)s at the rhizosphere, thus modulating the uptake and accumulation of metal elements in plants. In this study, the Fe(II)/Mn(II)-oxidizing bacteria Burkholderia sp. D416 (D416) and Pseudomonas sp. YGL (YGL) were isolated from Cd-contaminated rice field, both hydroponic experiment and pot experiment were performed to assess the impact of bacterial inoculation on iron plaque formation, elemental content of the plaque, plant dry mass, antioxidant enzyme activity and Cd content in rice plants. The results revealed that inoculation with D416, YGL, and D416+YGL stimulated iron plaque formation on the root surface of the hydroponic rice. The content of C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Fe and Ca in the root plaque were affected by the bacterial inoculation and varied among different plant growth stages. The pot experiment indicated that inoculation with D416 increased the root dry biomass by 58.89%, and the combined inoculation of D416 and YGL increased the dry biomass of root, shoot and grain by 16.89%, 21.66% and 23.26%, respectively. Importantly, YGL inoculation decreased the Cd translocation from root to shoot and from glume to brown rice grain by 50.00% and 50.27%, respectively, and the Cd content in shoot and brown rice grain were decreased by 20.00% and 34.48%, respectively. Taken together, the elemental content of the iron plaque and Cd content in rice plants varied among different plant growth stages and when plants were inoculated with different bacterial strains. YGL dramatically reduced the Cd content in brown rice grain, thus it could potentially be used to reduce Cd content in rice crop grown in Cd-contaminated soils.

Lead phytoextraction by Pelargonium hortorum: Comparative assessment of EDTA and DIPA for Pb mobility and toxicity

Soil amendments like ethylene-diamine-tetraacetic-acid (EDTA) have extensively been used for enhancing lead (Pb) phytoextraction. But due to its toxic effects, environment friendly substitute is required. Therefore, the present study was conducted to investigate the effect of EDTA and Di-iso-propanol-amine (DIPA) to enhance Pb phytoavailability and uptake by Pelargonium hortorum along with comparative toxicities of both organic amendments. For this purpose, soil was spiked with Pb concentrations (0, 500, 750, 1000 and 1500 mg kg¹) and amended with EDTA and DIPA at dosage levels (0, 1.5, 3, 5, 7.5, 10 mmol kg^-1) for plantation of Pelargonium hortorum. Soil samples were extracted with MgCl₂, plant samples were acid digested and analyzed for metal content. Biomass and root/shoot length of Pelargonium hortorum was decreased with increase in concentration of Pb and chelating agents. Phytoavailability of Pb at 1500 mg kg^-1 with EDTA 10 mmol kg^-1 was 0.3-folds in comparison to DIPA at the same dosage. Pelargonium hortorum plants amended with EDTA and DIPA at 10 mmol kg^-1 with Pb 1000 mg kg^-1 were found to uptake Pb 5.3-fold and 2.5-folds, respectively in comparison to Pb 1000 mg kg^-1 alone. Pb uptake decreased at 1500 mg kg^-1 with both chelating agents. The EDTA alone and in combination with 1500 mg Pb kg^-1 showed maximum genotoxicity by reducing the mitotic index and increasing the micronuclei formation. EDTA+Pb showed maximum toxicity followed by Pb and DIPA. Overall, 10 mmol kg^-1 of EDTA and DIPA performed better among all dosages in enhancing phytoavailability and uptake of Pb. DIPA showed less toxicity than that caused by EDTA, with comparable ability to promote Pb phytoextraction.

Promotion of growth and phytoextraction of cadmium and lead in Solanum nigrum L. mediated by plant-growth-promoting rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) are a specific category of microbes that improve plant growth and promote greater tolerance to metal stress through their interactions with plant roots. We evaluated the effects of phytoremediation combining the cadmium accumulator Solanum nigrum L. and two Cd- and Pb-resistant bacteria isolates. To understand the interaction between PGPR and their host plant, we conducted greenhouse experiments with inoculation treatments at Nanjing Agricultural University (Jiangsu Province, China), in June 2018. Two Cd- and Pb-resistant PGPR with various growth-promoting properties were isolated from heavy metal-contaminated soil. 16S rRNA analyses indicated that the two isolates were Bacillus genus, and they were named QX8 and QX13. Pot experiments demonstrated that inoculation may improve the rhizosphere soil environment and promote absorption of Fe and P by plants. Inoculation with QX8 and QX13 also enhanced the dry weight of shoots (1.36- and 1.7-fold, respectively) and roots (1.42- and 1.96-fold) of plants growing in Cd- and Pb-contaminated soil, and significantly increased total Cd (1.28-1.81 fold) and Pb (1.08-1.55 fold) content in aerial organs, compared to non-inoculated controls. We also detected increases of 23% and 22% in the acid phosphatase activity of rhizosphere soils inoculated with QX8 and QX13, respectively. However, we did not detect significant differences between inoculated and non-inoculated treatments in Cd and Pb concentrations in plants and available Cd and Pb content in rhizosphere soils. We demonstrated that PGPR-assisted phytoremediation is a promising technique for remediating heavy metal-contaminated soils, with the potential to enhance phytoremediation efficiency and improve soil quality.

An overview on heavy metal resistant microorganisms for simultaneous treatment of multiple chemical pollutants at co-contaminated sites, and their multipurpose application

Anthropogenic imbalance of chemical pollutants in environment raises serious threat to all life forms. Contaminated sites often possess multiple heavy metals and other types of pollutants. Elimination of chemical pollutants at co-contaminated sites is imperative for the safe ecosystem functions, and simultaneous removal approach is an attractive scheme for their remediation. Different conventional techniques have been applied as concomitant treatment solution but fall short at various parameters. In parallel, use of microorganisms offers an innovative, cost effective and ecofriendly approach for simultaneous treatment of various chemical pollutants. However, microbiostasis due to harmful effects of heavy metals or other contaminants is a serious bottleneck facing remediation practices in co-contaminated sites. But certain microorganisms have unique mechanisms to resist heavy metals, and can act on different noxious wastes. Considering this significant, my review provides information on different heavy metal resistant microorganisms for bioremediation of different chemical pollutants, and other assistance. In this favour, the integrated approach of simultaneous treatment of multiple heavy metals and other environmental contaminants using different heavy metal resistant microorganisms is summarized. Further, the discussion also intends toward the use of heavy metal resistant microorganisms associated with industrial and environmental applications, and healthcare. PREFACE: Simultaneous treatment of multiple chemical pollutants using microorganisms is relatively a new approach. Therefore, this subject was not well received for review before. Also, multipurpose application of heavy metal microorganisms has certainly not considered for review. In this regard, this review attempts to gather information on recent progress on studies on different heavy metal resistant microorganisms for their potential of treatment of co-contaminated sites, and multipurpose application.

Pseudomonas fluorescens promote photosynthesis, carbon fixation and cadmium phytoremediation of hyperaccumulator Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote photosynthesis and biomass production of hyperaccumulators, achieving enhanced phytoremediation efficiency of cadmium (Cd). A better understanding of the mechanisms controlling photosynthesis of hyperaccumulating plants by PGPB is necessary for developing strategies that promote the practical phytoextraction of Cd-polluted soils. In this study, chlorophyll fluorescence, gas exchange, and transcriptome sequencing were conducted to evaluate the physiological and transcriptional changes on photosynthesis and carbon fixation in hyperaccumulator Sedum alfredii after inoculation with PGPB Pseudomonas fluorescens. The results showed that bacterial inoculation significantly enhanced maximum quantum yield of PS II (Fv/Fm), effective quantum yield of PS II (ΦPSII), photochemical quenching (qP) and chlorophyll concentration, while reduced non-photochemical quenching (NPQ) of S. alfredii. Further, inoculation resulted in an increased net photosynthetic rates (Pn), intercellular CO₂ concentration (Ci), transpiration rate (Tr) and stomatal conductance (Gs) of the studied plant. At the transcriptional level, 70 photosynthetic genes and 42 C4-pathway carbon fixation related genes were significantly up-regulated in response to inoculation, which could be the reason for enhanced photosynthesis and dry biomass. To sum up, this P. fluorescens strain can simultaneously promote growth and Cd uptake of S. alfredii, which can be a promising bacterial agent applied to Cd phytoremediation practices.

Prospects and applications of plant growth promoting rhizobacteria to mitigate soil metal contamination: A review

The rapid increase in world population has generated the issues of hunger, poverty, food insecurity and malnutrition. To meet the challenge of increased food production of better quality, the farmers were compelled to use more chemical fertilizers, especially in developing countries. The higher use of chemical fertilizers interrupts the food chain through eutrophication, the polluting air and soil by incorporating metals. Trace metals have a deleterious effect on soil microbial and plant growth. To minimize metal toxicity and maximize the production of food, there are different approaches that can lead to lessen the use of chemical fertilizers. Plant growth promoting rhizobacteria (PGPR) are capable to enhance the plant growth and can remediate metal contaminated soils. PGPR has the ability to improve food production with diverse attributes e.g. producing siderophores that promote rhizosphere trace metal sequestration and production of organic and inorganic acids thus affecting trace metal bioavailability and plant induced systemic tolerance (IST) to limit the crop metal accumulation. In this review paper, we have discussed the biological approach which is environmentally friendly and cost-effective mean for metal polluted soils and gives some new insights for safety use of PGPR in trace metal contaminated fields.