The effects of endophytic bacterium SaMR12 on Sedum alfredii Hance metal ion uptake and the expression of three transporter family genes after cadmium exposure

Enhancing the phytoextraction efficiency of heavy metals in acidic and alkaline soils by Sedum alfredii Hance: A study on the synergistic effect of plant growth regulator and plant growth-promoting bacteria

Plant growth regulators (PGR) and plant growth-promoting bacteria (PGPB) have the potential in phytoremediation of heavy metals (HMs) contaminated soils. However, their sole application may not yield the optimal results, thus necessitating the combined application. The present study aimed to enhance the phytoremediation efficiency of Sedum alfredii Hance (S. alfredii) in acidic and alkaline soils through the combination of PGR (Brassinolide, BR) and PGPB (Pseudomonas fluorescens, P. fluorescens). The combination of BR and P. fluorescens (BRB treatment) effectively increased the removal efficiency of S. alfredii for Cd, Pb, and Zn by 355.2 and 155.3 %, 470.1 and 128.9 %, and 408.4 and 209.6 %, in acidic and alkaline soils, respectively. Moreover, BRB treatment led to a substantial increase in photosynthetic pigments contents and antioxidant enzymes activities, resulting in a remarkable increase in biomass (86.71 and 47.22 %) and dry mass (101.49 and 42.29 %) of plants grown in acidic and alkaline soils, respectively. Similarly, BRB treatment significantly elevated the Cd (109.4 and 71.36 %), Pb (174.9 and 48.03 %), and Zn levels (142.8 and 104.3 %) in S. alfredii shoots, along with cumulative accumulation of Cd (122.7 and 79.47 %), Pb (183.8 and 60.49 %), and Zn (150.7 and 117.9 %), respectively. In addition, the BRB treatment lowered the soil pH and DTPA-HMs contents, while augmenting soil enzymatic activities, thereby contributing soil microecology and facilitating the HMs absorption and translocation by S. alfredii to over-ground tissues. Furthermore, the evaluation of microbial community structure in phyllosphere and rhizosphere after remediation revealed the shift in microbial abundance. The combined treatment altered the principal effects on S. alfredii HMs accumulation from bacterial diversity to the soil HMs availability. In summary, our findings demonstrated that synergistic application of BR and P. fluorescens represents a viable approach to strengthen the phytoextraction efficacy of S. alfredii in varying soils.

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.

NPs-Ca promotes Cd accumulation and enhances Cd tolerance of rapeseed shoots by affecting Cd transfer and Cd fixation in pectin

The use of rapeseed (Brassica napus) as a hyperaccumulator plant has shown great promise for the remediation of cadmium (Cd) contaminated soils. Nanosized materials (NPs) have been shown to mitigate heavy metal toxicity in plants, but it is unknown how l-aspartate nano-calcium (NPs-Ca) affects Cd uptake, transport, and tolerance in rapeseed. A soil pot experiment was conducted with two treatments: a control treatment (CK) with 2.16 g CaCl2 and NPs-Ca treatment with 6.00 g NPs-Ca, to evaluate the effects and mechanisms of NPs-Ca on Cd tolerance in rapeseed. Compared to CaCl2, NPs-Ca promoted Cd transportation from roots to shoots by up-regulating the expression of Cd transport genes (ABCC12, HMA8, NRAM6, ZIP6, CAX4, PCR2, and HIP6). Therefore, NPs-Ca increased Cd accumulation in rapeseed shoots by 39.4%. Interestingly, NPs-Ca also enhanced Cd tolerance in the shoots, resulting in lower hydrogen peroxide (H2O2) accumulation and proline content, as well as higher antioxidant enzyme activities (POD, CAT). Moreover, NPs-Ca reduced the activity of pectin-degrading enzymes (polygalacturonase: PG, β-galactosidase: β-GAL), promoted the activity of pectin methyl esterase (PME), and changed transcription levels of related genes (PME, PMEI, PG, PGIP, and β-GAL). NPs-Ca treatment also significantly increased the Cd content in cell walls by 59.8%, that is, more Cd was immobilized in cell walls, and less Cd entered organelles in shoots of NPs-Ca treatment due to increased pectin content and degree of pectin demethylation. Overall, NPs-Ca increased Cd accumulation in rapeseed shoots by promoting Cd transport from roots to shoots. And meantime, NPs-Ca enhanced Cd tolerance of shoots by inhibiting pectin degradation, promoting pectin demethylation and increasing Cd fixation in pectin. These findings suggest that NPs-Ca can improve the potential of rapeseed as a hyperaccumulator for the remediation of Cd-contaminated soil and the protection of the environment. Furthermore, the study provides a theoretical basis for the application of NPs-Ca in the phytoremediation of Cd-contaminated soils with hyperaccumulating plants.

Epigenetic Control of Plant Response to Heavy Metals

Plants are sessile organisms that must adapt to environmental conditions, such as soil characteristics, by adjusting their development during their entire life cycle. In case of low-distance seed dispersal, the new generations are challenged with the same abiotic stress encountered by the parents. Epigenetic modification is an effective option that allows plants to face an environmental constraint and to share the same adaptative strategy with their progeny through transgenerational inheritance. This is the topic of the presented review that reports the scientific progress, up to date, gained in unravelling the epigenetic response of plants to soil contamination by heavy metals and metalloids, collectively known as potentially toxic elements. The effect of the microbial community inhabiting the rhizosphere is also considered, as the evidence of a transgenerational transfer of the epigenetic status that contributes to the activation in plants of response mechanisms to soil pollution.

Bacterial Communities Associated with the Roots of Typha spp. and Its Relationship in Phytoremediation Processes

Heavy metal pollution is a severe concern worldwide, owing to its harmful effects on ecosystems. Phytoremediation has been applied to remove heavy metals from water, soils, and sediments by using plants and associated microorganisms to restore contaminated sites. The Typha genus is one of the most important genera used in phytoremediation strategies because of its rapid growth rate, high biomass production, and the accumulation of heavy metals in its roots. Plant growth-promoting rhizobacteria have attracted much attention because they exert biochemical activities that improve plant growth, tolerance, and the accumulation of heavy metals in plant tissues. Because of their beneficial effects on plants, some studies have identified bacterial communities associated with the roots of Typha species growing in the presence of heavy metals. This review describes in detail the phytoremediation process and highlights the application of Typha species. Then, it describes bacterial communities associated with roots of Typha growing in natural ecosystems and wetlands contaminated with heavy metals. Data indicated that bacteria from the phylum Proteobacteria are the primary colonizers of the rhizosphere and root-endosphere of Typha species growing in contaminated and non-contaminated environments. Proteobacteria include bacteria that can grow in different environments due to their ability to use various carbon sources. Some bacterial species exert biochemical activities that contribute to plant growth and tolerance to heavy metals and enhance phytoremediation.

Recent advances in bioremediation of heavy metals and persistent organic pollutants: A review

Heavy metals and persistent organic pollutants are causing detrimental effects on the environment. The seepage of heavy metals through untreated industrial waste destroys the crops and lands. Moreover, incineration and combustion of several products are responsible for primary and secondary emissions of pollutants. This review has gathered the remediation strategies, current bioremediation technologies, and their primary use in both in situ and ex situ methods, followed by a detailed explanation for bioremediation over other techniques. However, an amalgam of bioremediation techniques and nanotechnology could be a breakthrough in cleaning the environment by degrading heavy metals and persistant organic pollutants.

Highly promoted phytoremediation with endophyte inoculation in multi-contaminated soil: plant biochemical and rhizosphere soil ecological functioning behavior

Rhizosphere soil ecological functioning behavior is of critical importance for regulating phytoremediation efficiency during microbial-assisted phytoremediation for multi-heavy metal-polluted soils. In this study, Trifolium repens L. and its endophyte Pseudomonas putida were used to investigate the ecological responses of the microbe-plant-soil system in Cd, Cr, and Pb co-contaminated soil. The results showed that endophyte Pseudomonas putida significantly increased plant biomass by 22.26-22.78% and phytoremediation efficiency by 29.73-64.01%. The increased phytoremediation efficiency may be related to the improvement of photosynthetic pigment content and antioxidant enzyme activities in leaves and the enhancement of rhizosphere soil ecological functioning. With endophyte application, soil nutrient content was significantly increased and heavy metal bioavailability was enhanced that residual fraction was reduced by 3.79-12.87%. Besides, the relative abundance of ecologically beneficial rhizobacteria such as Bacteriovorax and Arthrobacter was increased by 3.04-8.53% and 0.80-1.64%, respectively. Endophyte inoculation also significantly increased all the functional genes involved in cellular processes, genetic information processing, environmental information processing, and metabolism. This study indicated that the application of endophytes has a positive effect on the biochemical responses of Trifolium repens L. and could significantly improve rhizosphere ecological functioning in multi-heavy metal contamination, which provided clear strategies for regulating phytoremediation.

The role of auxins and auxin-producing bacteria in the tolerance and accumulation of cadmium by plants

Cadmium (Cd) is one of the most toxic heavy metals for plant physiology and development. This review discusses Cd effects on auxin biosynthesis and homeostasis, and the strategies for restoring plant growth based on exogenous auxin application. First, the two well-characterized auxin biosynthesis pathways in plants are described, as well as the effect of exogenous auxin application on plant growth. Then, review describes the impacts of Cd on the content, biosynthesis, conjugation, and oxidation of endogenous auxins, which are related to a decrease in root development, photosynthesis, and biomass production. Finally, compelling evidence of the beneficial effects of auxin-producing rhizobacteria in plants exposed to Cd is showed, focusing on photosynthesis, oxidative stress, and production of antioxidant compounds and osmolytes that counteract Cd toxicity, favoring plant growth and improve phytoremediation efficiency. Expanding our understanding of the positive effects of exogenous auxins application and the interactions between bacteria and plants growing in Cd-polluted environments will allow us to propose phytoremediation strategies for restoring environments contaminated with this metal.

Enhanced Cd-Accumulation in Typha latifolia by Interaction with Pseudomonas rhodesiae GRC140 under Axenic Hydroponic Conditions

The Typha genus comprises plant species extensively studied for phytoremediation processes. Recently, Pseudomonas rhodesiae GRC140, an IAA-producing bacterium, was isolated from Typha latifolia roots. This bacterium stimulates the emergence of lateral roots of Arabidopsis thaliana in the presence and absence of cadmium. However, the bacterial influence on cadmium accumulation by the plant has not been determined. Moreover, the P. rhodesiae GRC140 effect in Cd phytoextraction by T. latifolia remains poorly understood. In this work, an axenic hydroponic culture of T. latifolia was established. The plants were used to evaluate the effects of cadmium stress in axenic plants and determine the effects of P. rhodesiae GRC140 and exogenous indole acetic acid (IAA) on Cd tolerance and Cd uptake by T. latifolia. Biomass production, total chlorophyll content, root electrolyte leakage, catalase activity, total glutathione, and Cd content were determined. The results showed that Cd reduces shoot biomass and increases total glutathione and Cd content in a dose-dependent manner in root tissues. Furthermore, P. rhodesiae GRC140 increased Cd translocation to the shoots, while IAA increased the Cd accumulation in plant roots, indicating that both treatments increase Cd removal by T. latifolia plants. These results indicate that axenic plants in hydroponic systems are adequate to evaluate the Cd effects in plants and suggest that T. latifolia phytoextraction abilities could be improved by P. rhodesiae GRC140 and exogenous IAA application.

Genome-Wide Characterization, Evolutionary Analysis of ARF Gene Family, and the Role of SaARF4 in Cd Accumulation of Sedum alfredii Hance

Auxin response factors (ARFs) play important roles in plant development and environmental adaption. However, the function of ARFs in cadmium (Cd) accumulation are still unknown. Here, 23 SaARFs were detected in the genome of hyperaccumulating ecotype of Sedum alfredii Hance (HE), and they were not evenly distributed on the chromosomes. Their protein domains remained highly conservative. SaARFs in the phylogenetic tree can be divided into three groups. Genes in the group Ⅰ contained three introns at most. However, over ten introns were found in other two groups. Collinearity relationships were exhibited among ten SaARFs. The reasons for generating SaARFs may be segmental duplication and rearrangements. Collinearity analysis among different species revealed that more collinear genes of SaARFs can be found in the species with close relationships of HE. A total of eight elements in SaARFs promoters were related with abiotic stress. The qRT-PCR results indicated that four SaARFs can respond to Cd stress. Moreover, that there may be functional redundancy among six SaARFs. The adaptive selection and functional divergence analysis indicated that SaARF4 may undergo positive selection pressure and an adaptive-evolution process. Overexpressing SaARF4 effectively declined Cd accumulation. Eleven single nucleotide polymorphism (SNP) sites relevant to Cd accumulation can be detected in SaARF4. Among them, only one SNP site can alter the sequence of the SaARF4 protein, but the SaARF4 mutant of this site did not cause a significant difference in cadmium content, compared with wild-type plants. SaARFs may be involved in Cd-stress responses, and SaARF4 may be applied for decreasing Cd accumulation of plants.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

Drought tolerance induction and growth promotion by indole acetic acid producing Pseudomonas aeruginosa in Vigna radiata

Drought accompanied with reduced precipitation is one of the key manacles to global agricultural throughput and is expected to escalate further hence posing major challenges to future food safety. For a sustainable agricultural environment, drought resistant plant growth promoting rhizobacteria (PGPR) are new encouraging prospect, which are inexpensive and have no side effects, as those of synthetic fertilizers. In the present study, five strains of Pseudomonas aeruginosa, the strain MK513745, strain MK513746, strain MK513747, strain MK513748, and strain MK513749 were used as drought tolerant PGPR with multiple traits of IAA production, N fixation, P solubilization, siderophore producing capabilities. The strain MK513745 and strain MK513749 produced higher quantities of indole acetic acid (116±0.13 and 108±0.26 μg ml^-1). MK513749 yielded 12 different indole compounds in GCMS analysis. The strain MK513748 yielded maximum S.I. (3.33mm) for phosphate solubilizing test. Maximum nitrogen concentration was produced (0.18 μg ml^-1) by strain MK513746. Percent siderophore units ranged from 2.65% to 2.83% as all five pseudomonas strains were siderophore positive. In all growth experiments of plant microbe interaction two varieties of Vigna radiata (AZRI-06, NM-11) plants inoculated with P. aeruginosa strains under drought stress responded significantly (P<0.05) better than control stressed plants. Maximum shoot length was enhanced up-to 125%, pod/plant 172%, number of grains 65%, 100 seed weight 95%, 100 seed straw weight 124% and total yield 293% were recorded in plants inoculated with drought stress tolerant PGPR in both varieties as compared to respective stressed control plants. Photosynthetic activity, membrane stability (45%), water content (68%) and antioxidant efficacy (19%) were improved with PGPR inoculations. The variety NM-11 (V2) was more tolerant to drought stress with inoculations of Pseudomonas strains than AZRI-06 (V1). Inoculations with these indole acetic acid producing strains would be suitable for plant growth promotion in areas facing water deficiency.

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.

Lowered Cd toxicity, uptake and expression of metal transporter genes in maize plant by ACC deaminase-producing bacteria Achromobacter sp

In this study, an ACC deaminase-producing bacterial strain Achromobacter sp. A1 was isolated from maize rhizosphere soil, characterized and evaluated for the effects on cadmium (Cd) immobilization in solution/rhizosphere, physiological characteristics and the tissue Cd contents in maize and the molecular mechanisms involved by hydroponic and pot experiments. ACC deaminase activity of strain A1 was significantly enhanced by Cd addition and Cd concentration decreased (55.54-63.62%) in solution supplemented with various Cd concentrations. Strain A1 significantly increased the maize dry weights (30.77-105%) and chlorophyll content (7.46-14.46%), decreased MDA content (25.16-36.87%) and ethylene production (20.93-35.86%) in hydroponic experiment. Strain A1 significantly reduced the above-ground tissue Cd uptake by 12.64-33.68% and 42-48% in hydroponic and pot experiments, reduced the DTPA-extractable Cd content and elevated invertase, urease and catalase activity in rhizosphere soils. In addition, the expression levels of Cd transporter genes HMA3 and Nramp5 were significantly reduced in root and shoot after strain A1 inoculation. These results indicate that strain A1 has great potential for application as a novel and environmentally friendly inoculant to immobilize Cd and reduce maize Cd uptake in Cd-contaminated environments, and will improve the understanding of the relative molecular mechanisms underlying the response to strain A1 in maize plant.

Plant Growth-Promoting Rhizobacteria as a Green Alternative for Sustainable Agriculture

Environmental stress is a major challenge for sustainable food production as it reduces yield by generating reactive oxygen species (ROS) which pose a threat to cell organelles and biomolecules such as proteins, DNA, enzymes, and others, leading to apoptosis. Plant growth-promoting rhizobacteria (PGPR) offers an eco-friendly and green alternative to synthetic agrochemicals and conventional agricultural practices in accomplishing sustainable agriculture by boosting growth and stress tolerance in plants. PGPR inhabit the rhizosphere of soil and exhibit positive interaction with plant roots. These organisms render multifaceted benefits to plants by several mechanisms such as the release of phytohormones, nitrogen fixation, solubilization of mineral phosphates, siderophore production for iron sequestration, protection against various pathogens, and stress. PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. Genetically engineered PGPR strains play significant roles to alleviate the abiotic stress to improve crop productivity. Thus, the present review will focus on the impact of PGPR on stress resistance, plant growth promotion, and induction of antioxidant systems in plants.

Exposure of cerium oxide nanoparticles to the hyperaccumulator Sedum alfredii decreases the uptake of cadmium via the apoplastic pathway

Cadmium (Cd) is harmful to the environment and threatens human health. With the increasing use of cerium oxide nanoparticles (CeO₂NPs) in extensive industries, investigating the combination of CeO₂NPs and plants has attracted research interests for phytoremediation. Here, we explored the effects of CeO₂NPs on Cd uptake, transport and the consequent Cd accumulation in Sedum alfredii. Exposure of 50 or 500 mg L^-1 CeO₂NPs alone had no apparent damaging effects on plant growth. However, upon Cd condition, the consistent CeO₂NPs decreased Cd concentrations in the roots and shoots by up to 37%. Furthermore, the application of a metabolic inhibitor revealed that CeO₂NPs mainly decreased the Cd uptake in roots by the apoplastic pathway. Simultaneously, CeO₂NPs accelerated the development of Casparian strips (CSs) and suberin, which was further proven by the elevated expression levels of genes associated with their formation, SaCASP, SaGPAT5, SaKCS20 and SaCYP86A1. Compared to CeO₂NPs added alone, the concurrent Cd decreased the Ce contents in the roots and altered its translocation from root to shoot. Taken together, both CeO₂NPs and Cd influence the interactional uptake of both chemicals in roots of S. alfredii mainly via the apoplastic pathway which is primarily regulated by the development of CSs and suberin.

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.

Strong accumulation capacity of hyperaccumulator Solanum nigrum L. for low or insoluble Cd compounds in soil and its implication for phytoremediation

This experiment is to explore whether one hyperaccumulator shows the strongly accumulative capacities for low or insoluble Cd compounds in soil. Soil potting experiment was conducted to analyze the accumulation capacity of Solanum nigrum L. for 10 different Cd compounds under two levels. The results clearly indicated: The Cd concentrations of shoots and roots were very high for different Cd compounds in soils even with low or insoluble Cd compounds compared with easily soluble Cd in the treatments of soil contaminated with Cd at different concentrations. Furthermore, the EFs and TFs were all larger than 1 either. Based on the results, although the bioavailabilities of some Cd compounds in soil were lower, S. nigrum's ability to accumulate them was still very strong. Phytoremediation may be widely used to treat with soil contaminated by different cadmium compounds. In addition, the total Cd content is also very important in evaluating the risk of Cd contamination in soil. Thus, phytoextraction is promising.

Pseudomonas fluorescens accelerates a reverse and long-distance transport of cadmium and sucrose in the hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote root uptake and shoot accumulation of cadmium (Cd) in hyperaccumulator plants, but the mechanisms by which PGPB accelerate root-to-shoot transport of Cd is still unknown. A better understanding of these mechanisms is necessary to develop the strategies that can promote the practical phytoextraction of Cd-polluted soils. In this study, we found that Pseudomonas fluorescens accelerates a reversed and a long-distance transport of Cd and sucrose in Sedum alfredii, by examining the xylem and phloem sap and by quantifying the concentrations of Cd and sucrose in shoot and root. The transcriptome sequencing has revealed the up-regulated expressions of starch metabolism and sucrose biosynthesis related genes in the shoots of Cd hyperaccumulator plant S. alfredii that was inoculated with PGPB P. fluorescens. In addition, the genes of sugar, cation and anion transporters were also up-regulated by bacterial treatment, showing a complicated co-expression network with sucrose biosynthesis related genes. The expression levels of Cd transporter genes, such as ZIP1, ZIP2, HMA2, HMA3 and CAX2, were elevated after PGPB inoculation. As a result, the PGPB successfully colonized the root, and promoted the sucrose shoot-to-root transport and Cd root-to-shoot transport in S. alfredii. Since non-photosynthetic root-associated bacteria usually obtain sugars from photosynthetic plants, our results highlight the importance of PGPB-induced changes in hyperaccumlator plants for both the host and the PGPB.

The plant-growth promoting bacteria promote cadmium uptake by inducing a hormonal crosstalk and lateral root formation in a hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) that inhabit hyperaccumulating plants assist cadmium (Cd) absorption, but the underlying mechanism has not been comprehensively studied. For this reason, we combined the fluorescence imaging, and transcriptomic and metabolomic methods in a Cd hyperaccumulator, Sedum alfredii, inoculated or not with PGPB Pseudomonas fluorescens. The results showed that the newly emerged lateral roots, that were heavily colonized by P. fluorescens, are the main entry for Cd influx in S. alfredii. Inoculation with P. fluorescens promoted a lateral root formation of its host plant, leading to a higher Cd phytoremediation efficiency. Furthermore, the plant transcriptome revealed that 146 plant hormone related genes were significantly up-regulated by the bacterial inoculation, with 119 of them showing a complex interaction, which suggests that a hormonal crosstalk participated root development. The targeted metabolomics analysis showed that P. fluorescens inoculation significantly increased indole acetic acid concentration and significantly decreased concentrations of abscisic acid, brassinolide, trans-zeatin, ethylene and jasmonic acid in S. alfredii roots, thereby inducing lateral root emergence. Altogether, our results highlight the importance of PGPB-induced lateral root formation for the increased Cd uptake in a hyperaccumulating plant.

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.

Low calcium-induced delay in development of root apoplastic barriers enhances Cd uptake and accumulation in Sedum alfredii

Mineral nutrients play an important role in heavy metal uptake and accumulation in plant. However, the effects of calcium (Ca) supply level on apoplastic transport in roots and consequences for uptake of cadmium (Cd) in hyperaccumulators are poorly understood. Here, we investigated how Ca regulated the development of apoplastic barriers in the roots of two ecotypes of Sedum alfredii and assessed its effects on Cd uptake. Results of correlation analysis indicated that Ca content was positively correlated with the development of Casparian strips (CSs) and suberin lamellae (SL) in the absence or presence of Cd. Simultaneously, low Ca supply was proven to delay the formation of endodermis CSs and suberin accumulation by decreasing the relative expressions of genes associated with CSs localization and lignin/suberin synthesis. Moreover, Cd in apoplastic fluid and cell walls (regarding the apoplastic transport) and symplastic fractions were elevated by low Ca supply. Contrary to high Ca supply, the expression levels of genes related to Cd influx and xylem loading were increased upon low Ca addition in roots of both ecotypes. All the results above suggested that low Ca supply promotes root Cd uptake via apoplastic pathway by delaying apoplastic barriers development and also regulating Cd transport to the xylem in S. alfredii.

Plant growth promotion and enhanced uptake of Cd by combinatorial application of Bacillus pumilus and EDTA on Zea mays L

In developing countries, Cd contamination is ubiquitous which limits agriculture productivity. The current study was designed to investigate the efficacy of plant-Bacillus pumilus-ethylene diamine tetraacetic acid (EDTA) and plant-microbe-chelator (PMC) synergy for enhanced plant growth and Cd-uptake potential of Zea mays in industrially contaminated and cadmium (Cd) spiked soil. A pot experiment was conducted by growing Z. mays seedlings either inoculated with B. pumilus or un-inoculated along with the application of 5 mM EDTA. Plants were exposed to two levels of Cd contamination for 45 days. An increase in Cd uptake was observed in Z. mays inoculated with B. pumilus followed by EDTA treatment as compared to non-inoculated and un-treated ones. Zea mays showed improved values with PMC approach for different growth parameters including root length (41%), shoot length (40%), fresh weight (59%), dry weight (49%), chlorophyll contents (49%), and relative water contents (30%). Higher tolerance index (117%) was observed for plants grown in soil spiked with 300 mg kg^-1 Cd (S2). PMC application markedly enhanced Cd uptake potential of Z. mays up to 12% and 68.8%, respectively, in S1 and S2 soil. While the PMC application increased Cd accumulation capacity of Z. mays by 71.2% and 52.5% in S1 and S2 soil. The calculated bioaccumulation and translocation factor revealed that Z. mays possess Cd uptake potential, and this ability can be significantly enhanced with PMC application.

Exogenous abscisic acid (ABA) promotes cadmium (Cd) accumulation in Sedum alfredii Hance by regulating the expression of Cd stress response genes

Sedum alfredii Hance is a zinc (Zn) and cadmium (Cd) hyperaccumulator plant. However, the regulatory role of plant hormones in the Zn or Cd uptake and accumulation of S. alfredii remains unclear. In this work, the growth, Cd accumulation, abscisic acid (ABA) synthesis and catabolism, malonaldehyde (MDA) content, and transcriptional level of some Cd stress response genes under ABA and Cd co-treatment were investigated to reveal the impact of ABA on Cd resistance and Cd accumulation of S. alfredii. The results show that 0.2 mg/L ABA and 100 μmol/L Cd co-treatment enhanced Cd accumulation and growth in S. alfredii, whereas lower or higher ABA concentrations weaken or even reverse this effect, which was positively correlated with endogenous ABA content. The increase in endogenous ABA content might be the results of the increasing ABA synthetase activities and decreasing ABA lytic enzyme, which was induced by the application of 0.2 mg/L ABA under 100 μmol/L Cd treatment. Principal component analysis (PCA) indicated that ABA impacted the expression pattern of Cd stress response genes, which coincided with the Cd accumulation pattern in the shoots of S. alfredii. Cross-over analysis of partial least squares-discriminant analysis (PLS-DA) and correlation analysis indicated that HsfA4c, HMA4 expression in roots, and HMA2, HMA3, CAD, NAS expression in shoots were correlated with endogenous ABA, which suggests that endogenous ABA improves Cd resistance of seedlings, switches the root-to-shoot transporter from HMA2 to HMA4, and transports more Cd into apoplasts to promote Cd accumulation in the shoots of S. alfredii. Taken together, ABA plays an essential role not only in Cd resistance but also in Cd transport from root to shoot in S. alfredii under Cd stress.

Effects of different soil pH and nitrogen fertilizers on Bidens pilosa L. Cd accumulation

Bidens pilosa L. was a Cd hyperaccumulator. This experiment determined the effects of different soil pH (adjusted by weak acid and alkali at 4.83, 6.81, and 7.84, respectively) and nitrogen ((NH₄)₂SO₄, Ca(NO₃)₂) on B. pilosa phytoextracting Cd in soil collected from a smelter (Cd concentration was 19.63 mg kg^-1). The results showed that the Cd concentrations in B. pilosa were significantly higher (p < 0.05) with soil pH 4.83 treatments than those of pH 6.81 and 7.84 ones. The Cd concentration of B. pilosa grown in pH 7.84 soil was significantly lower (p < 0.05) than that in pH 6.81 soil. The extractable Cd concentration in soil was decreased (p < 0.05) with the increase of pH. Under three different pH conditions, the rhizosphere pH of B. pilosa was basically 0.2 lower than that of pH in bulk soil respectively, indicating that the hyperaccumulator had a certain acidification effect on soil. Two kinds of nitrogen fertilizers (NH₄)₂SO₄ and Ca(NO₃)₂ had no significant difference (p < 0.05) on Cd concentrations of B. pilosa, which was probably caused by the acidification effect of its rhizosphere. The biomasses of B. pilosa were not affected (p < 0.05) by different pH of soil. The photosynthetic production, antioxidative enzymes, and lipid peroxidation change trends of B. pilosa were basically consistent with its biomasses. Generally speaking, B. pilosa showed high Cd accumulation potential and strong adaptability for different soil situations.

The endophytic bacterium Sphingomonas SaMR12 alleviates Cd stress in oilseed rape through regulation of the GSH-AsA cycle and antioxidative enzymes

BACKGROUND

Microbes isolated from hyperaccumulating plants have been reported to be effective in achieving higher phytoextraction efficiency. The plant growth-promoting bacteria (PGPB) SaMR12 from the cadmium (Cd)/zinc hyperaccumulator Sedum alfredii Hance could promote the growth of a non-host plant, oilseed rape, under Cd stress. However, the effect of SaMR12 on Brasscia juncea antioxidative response under Cd exposure was still unclear.

RESULTS

A hydroponic experiment was conducted to study the effects of Sphingomonas SaMR12 on its non-host plant Brassica juncea (L.) Czern. under four different Cd treatments. The results showed that SaMR12 could colonize and aggregate in the roots and then move to the shoots. SaMR12 inoculation promoted plant growth by up to 71% in aboveground biomass and 81% in root biomass over that of the non-inoculated plants. SaMR12-inoculated plants significantly enhanced root Cd accumulation in the 10 and 20 μM Cd treatments, with 1.72- and 0.86-fold increases, respectively, over that of the non-inoculated plants. SaMR12 inoculation not only decreased shoot hydrogen peroxide (H₂O₂) content by up to 38% and malondialdehyde (MDA) content by up to 60% but also reduced proline content by 7-30% in shoots and 17-32% in roots compared to the levels in non-inoculated plants. Additionally, SaMR12 inoculation promoted the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) and facilitated the relative gene expression levels of dehydroascorbate reductase (DHAR) and glutathione reductase (GR) involved in the glutathione (GSH)-ascorbic acid (AsA) cycle.

CONCLUSIONS

The results demonstrated that, under Cd stress, SaMR12 inoculation could activate the antioxidative response of B. juncea by decreasing the concentrations of H₂O₂, MDA and proline, increasing the activities of antioxidative enzymes, and regulating the GSH-AsA cycle. These results provide a theoretical foundation for the potential application of hyperaccumulator endophytic bacteria as remediating agents to improve heavy metal tolerance within non-host plant species, which could further improve phytoextraction efficiency.

Inoculation with abscisic acid (ABA)-catabolizing bacteria can improve phytoextraction of heavy metal in contaminated soil

Promotion of plant capacity for accumulation of heavy metals (HMs) is one of the key strategies in enhancing phytoremediation in contaminated soils. Here we report that, Rhodococcus qingshengii, an abscisic acid (ABA)-catabolizing bacteria, clearly boosts levels of Cd, Zn, and Ni in wild-type Arabidopsis by 47, 24, and 30%, respectively, but no increase in Cu was noted, when compared with non-inoculated Arabidopsis plants in contaminated growth substrate. Furthermore, when compared with wild-type plants, R.qingshengii-induced increases in Cd, Zn, and Ni concentrations were more pronounced in abi1/hab1/abi2 (ABA-sensitive mutant) strains of Arabidopsis, whereas little effect was observed in snrk2.2/2.3 (ABA insensitive mutant). This demonstrates that metabolizing ABA might be indispensable for R. qingshengii to improve metal accumulation in plants. Bacterial inoculation significantly elevated the expression of Cd, Zn, and Ni-related transporters; whereas the transcript levels of Cu transporters remained unchanged. This result may be a reasonable explanation for why the uptake of Cd, Zn, and Ni in plants was stimulated by bacterial inoculation, while no effect was observed on Cu levels. From our results, we clearly demonstrate that R. qingshengii can increase the accumulation of Cd, Zn, and Ni in plants via an ABA-mediated HM transporters-associated mechanism. Metabolizing ABA in the plants by ABA-catabolizing bacterial inoculation might be an alternative strategy to improve phytoremediation efficiency in HMs contaminated soil.

Effects of endophytes inoculation on rhizosphere and endosphere microecology of Indian mustard (Brassica juncea) grown in vanadium-contaminated soil and its enhancement on phytoremediation

We investigated the effects of endophytes inoculation on ecological factors such as root morphology, rhizosphere soil properties, heavy metal speciation, and rhizosphere and endophytic bacterial communities and their role on phytoremediation. Indian mustards were grown for two months in V-contaminated soil with three treatments (control, inoculation with Serratia PRE01 or Arthrobacter PRE05). Inoculation with PRE01 and PRE05 increased organic matter content by 6.94% and 4.6% respectively and significantly increased bioavailability of heavy metals in rhizosphere soils. Despite the endophyte inocula failed to flourish as stable endophytes, they significantly affected the specific composition and diversity of endophytic bacterial communities in roots, with no significant effect on rhizosphere bacterial communities. The test strains could greatly increase plant growth promotion-related biomarkers in the endosphere, especially those associated with Pseudomonas and Microbacterium genera. PICRUSt analysis predicted high relative abundances of functional genes related to environmental information processing especially in the endophytic microbiota. More biomass production (12.0%-17.4%) and total metals uptake (24.2%-32.0%) were acquired in inoculated treatments. We conclude that endophyte PRE01 or PRE05 inoculation could effectively enhance phytoremediation of V-contaminated soil by improving the rhizosphere and endosphere microecology without causing any ecological damage.

Understanding the molecular mechanisms for the enhanced phytoremediation of heavy metals through plant growth promoting rhizobacteria: A review

Rapid industrialization, modern agricultural practices and other anthropogenic activities add a significant quantity of toxic heavy metals into the environment, which induces severe toxic effects on all form of living organisms, alter the soil properties and its biological activity. Remediation of heavy metal contaminated sites has become an urgent necessity. Among the existing strategies, phytoremediation is an eco-friendly and much convincing tool for the remediation of heavy metals. However, the applicability of phytoremediation in contaminated sites is restricted by two prime factors such as i) slow growth rate at higher metal contaminated sites and ii) metal bioavailability. This circumstance could be minimized and accelerate the phytoremediation efficiency by incorporating the potential plant growth promoting rhizobacterial (PGPR) as a combined approach. PGPR inoculation might improve the plant growth through the production of plant growth promoting substances and improve the heavy metal remediation efficiency by the secretion of chelating agents, acidification and redox changes. Moreover, rhizobacterial inoculation consolidates the metal tolerance and uptake by regulating the expression of various metal transporters, tolerant and metal chelator genes. However, the exact underlying molecular mechanism of PGPR mediated plant growth promotion and phytoremediation of heavy metals is poorly understood. Thus, the present review provides clear information about the molecular mechanisms excreted by PGPR strains in plant growth promotion and phytoremediation of heavy metals.

Inoculation of plant growth promoting bacteria from hyperaccumulator facilitated non-host root development and provided promising agents for elevated phytoremediation efficiency

Plant growth promoting bacteria (PGPB) have been reported to have the ability to promote plant growth, development and increase heavy metals (HMs) uptake. Therefore, PGPB inoculation as soil remediation agents into plants with larger biomass and potential of phytoextraction is of great importance to increase bioremediation efficiency. In this study, 12 PGPB strains isolated from a cadmium (Cd)/zinc hyperaccumulator Sedum alfredii Hance were inoculated into non-host plant Brassica juncea and their effects on plant growth and Cd uptake were determined. The results showed that inoculation of most PGPB strains promoted plant growth, boosted root development and improved chlorophyll content in the absence of Cd. Inoculation of PGPB strains promoted plant growth up to 111% in shoot and 358% in root when treated with 2 μM Cd. In addition, PGPB inoculation not only ameliorated plant root morphology including the total root length (RL), total surface area (SA), total root volume (RV) and number of root tips (RT), but also facilitated Cd uptake up to 126%. Furthermore, inoculation of PGPB strains promoted plant Cd accumulation up to 261% in shoot and up to 8.93-fold increase in root. Among all the 12 PGPB strains, Burkholdria SaMR10 and Sphingomonas SaMR12 were identified as the promising microbes for improving phytoremediation efficiency of Cd contaminated soils. These results not only provided useful findings for further investigation of interacting mechanisms between different bacterial strains and plants, but also facilitated the development of microbe-assisted phytoremediation application for HM contaminated soil.

Bidens pilosa L. hyperaccumulating Cd with different species in soil and the role of EDTA on the hyperaccumulation

Investigating whether the same hyperaccumulator shows a high accumulation potential for different species of the same heavy metal in the soil has rarely been considered until now. In this experiment, Cd accumulation by a hyperaccumulator Bidens pilosa L. from soils spiked with 3 and 9 mg Cd kg^-1 in the form of Cd(NO₃)₂, CdCl₂, CdBr₂, CdI₂, CdSO₄, CdF₂, Cd(OH)₂, CdCO₃, Cd₃(PO₄)₂, and CdS and effect of soil amendment with EDTA were determined. The results showed that the Cd concentrations in B. pilosa for high-solubility species were basically higher. But the enrichment factors (EFs) (shoot to soil Cd concentration ratio) and translocation factors (TFs) (shoot to root Cd concentration ratio) of low-solubility Cd species were all greater than 1, either indicating that there was a high Cd hyperaccumulative potentials of B. pilosa without considering on Cd species in soil. EDTA significantly improved B. pilosa Cd hyperaccumulation, especially for low-solubility Cd forms in soils. These results can perfectly explain the accumulation properties of one hyperaccumulator to different species of the same heavy metal. Phytoremediation may be applied for a wide scope for different Cd species-contaminated soil. Moreover, the total amount of Cd in soil was important when assessing the risk of Cd-contaminated soils.

Assessing interactions between environmental factors and aquatic toxicity: Influences of dissolved CO2 and light on Cd toxicity in the aquatic macrophyte Potamogeton crispus

The objective of this study was to investigate the combined effects of varying dissolved CO₂ concentration (ambient CO₂, 3˜17 μmol L^-1, elevated CO₂, 48˜81 μmol L^-1) and light intensity (high light, c. 150 μmol photon m^-2 s^-1, low light, c. 25 μmol photon m^-2 s^-1) on the bioaccumulation and phytotoxicity of cadmium (Cd) in a macrophyte Potamogeton crispus, under constant Cd exposure. The data confirmed that 100 μM Cd led to adverse changes in morphology, ultrastructure and biochemistry in P. crispus. The toxic effects depended strongly on CO₂ concentration and light intensity: elevated CO₂ and high light both increased Cd concentrations in P. crispus, and there was a significant interaction between the two factors. Compared to high light grown plants, the photochemical efficiency and chlorophyll content of low light grown P. crispus were much less affected and the MDA content was lower, when exposed to 100 μM Cd. In addition, an antioxidative response was observed with a significant increase in SOD, POD and GST activities, indicating that low light grown P. crispus are more protected against Cd toxicity. When compared with ambient CO₂ concentrations, chlorophyll content, chlorophyll fluorescence, photosynthetic rate and starch content, as well as the activity of SOD and GST, were significantly enhanced in Cd treated P. crispus under elevated CO₂. This suggests that elevated CO₂ reduced Cd toxicity in P. crispus by increasing photosynthesis and enhancing the antioxidant system. Moreover, the statistical results showed that dissolved CO₂ and light had additive effects on Cd toxicity in P. crispus, reflected by the physiological parameters of total chlorophyll content, SOD activity and MDA content, indicating that the combination of high CO₂ and low light produced more protection against Cd toxicity than did the factors alone. Based on the results of this study, it appears clear that referring to a specific site in aquatic ecosystem, dissolved CO₂ concentration and light availability should be considered when assessing and managing Cd impacts on aquatic plants.

Promotion of the root development and Zn uptake of Sedum alfredii was achieved by an endophytic bacterium Sasm05

Endophyte-assisted phytoremediation has gained increasing attention. However, the interacting mechanisms of endophytes and metal hyperaccumulators are still not clear. An endophytic bacterium Pseudomonas fluorescens Sasm05 inoculation promoted Sedum alfredii Hance rooting and root development, in which the specific root length (SRL) and average number of root tips (ART) increased to 2.09- and 3.35-fold, respectively. Sasm05 inoculation promoted plant growth, increased the chlorophyll content, and elevated Zn uptake of plant at excess Zn supply. At 200 μM Zn treatment level, Sasm05 inoculation increased plant biomass and the chlorophyll content by more than 40%, and root Zn content by 40%. Furthermore, Sasm05 inoculation upregulated the expression of the Zn transporter SaIRT1 to 3.43-fold in the roots, while another transporter SaNramp1 expression was increased to 38.66-fold in the roots and 7.53-fold in the shoots. Time course study showed the best effects of Sasm05 on plant biomass and the chlorophyll content were detected at 30 d, while for Zn content at 3 d. These results firstly provided molecular evidences of endophytic bacteria in facilitating host plant Zn uptake, which will absolutely benefit the understanding of interacting mechanisms between hyperaccumulators and their endophytes.

Effects of elevated CO2 and endophytic bacterium on photosynthetic characteristics and cadmium accumulation in Sedum alfredii

Elevated CO₂ and use of endophytic microorganisms have been considered as efficient and novel ways to improve phytoextraction efficiency. However, the interactive effects of elevated CO₂ and endophytes on hyperaccumulator is poorly understood. In this study, a hydroponics experiment was conducted to investigate the combined effect of elevated CO₂ (eCO₂) and inoculation with endophyte SaMR12 (ES) on the photosynthetic characteristics and cadmium (Cd) accumulation in hyperaccumulator Sedum alfredii. The results showed that eCO₂ × ES interaction promoted the growth of S. alfredii, shoot and root biomass net increment were increased by 264.7 and 392.3%, respectively, as compared with plants grown in ambient CO₂ (aCO₂). The interaction of eCO₂ and ES significantly (P < 0.05) increased chlorophyll content (53.2%), Pn (111.6%), Pn_max (59.8%), AQY (65.1%), and Lsp (28.8%), but reduced Gs, Tr, Rd, and Lcp. Increased photosynthetic efficiency was associated with higher activities of rubisco, Ca²⁺-ATPase, and Mg²⁺-ATPase, and linked with over-expression of two photosystem related genes (SaPsbS and SaLhcb2). PS II activities were significantly (P < 0.05) enhanced with Fv/Fm and Φ(II) increased by 12.3 and 13.0%, respectively, compared with plants grown in aCO₂. In addition, the net uptake of Cd in the shoot and root tissue of S. alfredii grown in eCO₂ × ES treatment was increased by 260.7 and 434.9%, respectively, due to increased expression of SaHMA2 and SaCAX2 Cd transporter genes. Our results suggest that eCO₂ × ES can promote the growth of S. alfredii due to increased photosynthetic efficiency, and improve Cd accumulation and showed considerable potential of improving the phytoextraction ability of Cd by S. alfredii.

Ornamental hyperaccumulator Mirabilis jalapa L. phytoremediating combine contaminated soil enhanced by some chelators and surfactants

Mirabilis jalapa L. is an ornamental plant of the composite family, which was found hyperaccumulating Cd. Due to its larger biomass, developed root system, root exudation, and microbial interactions, certain organic pollutants in its rhizosphere can be effectively degraded. Thus, M. jalapacan be used to co-remediate heavy metal and organic pollutant co-contaminated soil. The aim of this paper is to explore the remediation capacity of M. jalapa for Cd-PAHs co-contaminated soil in the presence of five chelators or surfactants. The concentrations of Cd and PAHs in collected soil samples were 0.85 mg kg^-1 Cd and 1.138 mg kg^-1 PAHs (16 kinds of priority control polycyclic aromatic hydrocarbons by USEPA). The chelators or surfactants of EDTA, EGTA, CA, TW80, and SA were respectively spiked to the pots according to the experiment design at 1 month before the plant harvested. The results showed that the capacity of Cd in shoot of M. jalapa was 7.99 μg pot^-1 without any addition (CK4, M. jalapa in original soil without amendment). However, Cd capacity in shoot of M. jalapa was increased (p < 0.05) by 31.7%, 181.7%, and 107.4% in treatment of R_EGTA, R_CA and R_{EGTA + SA}, respectively. As for the degradation of PAHs in soil, there was no significant decrease (p < 0.05) in the treatment of CK2 (original soil spiked with 0.9 SA without M. jalapa), CK3 (original soil spiked with 0.3 TW80 without M. jalapa), and CK4 compared to the control CK1 (original soil without M. jalapa and amendment). When amendments were added to soils with M. jalapa,the PAHs concentrations in soils significantly decreased (p < 0.05) by 21.7%, 23.8%, 27.0%, 19.8%, 21.8%, 31.2%, and 25.5% for the treatment of R_{EDTA + SA}, R_{EDTA + T80}, R_{EGTA + SA}, R_{EGTA + T80}, R_{CA + T80}, R_{SA + T80 + EDTA}, and R_{SA + T80 + CA}, respectively. Basically, Cd capacity in shoot of M. jalapa was improved by chelators. PAHs degradation was caused by the existence of surfactants in rhizosphere of M. jalapa. But the roles of different chelators or surfactants were quite distinct. In short, the Cd capacity in the shoot and PAHs degradation in the rhizosphere of M. jalapa in the treatment of R_{EGTA + SA} were all significantly increased (p < 0.05), which was more practical for M. jalapa phytoremediating Cd-PAHs co-contaminated soil.

The Effects of the Endophytic Bacterium Pseudomonas fluorescens Sasm05 and IAA on the Plant Growth and Cadmium Uptake of Sedum alfredii Hance

Endophytic bacteria have received attention for their ability to promote plant growth and enhance phytoremediation, which may be attributed to their ability to produce indole-3-acetic acid (IAA). As a signal molecular, IAA plays a key role on the interaction of plant and its endomicrobes. However, the different effects that endophytic bacteria and IAA may have on plant growth and heavy metal uptake is not clear. In this study, the endophytic bacterium Pseudomonas fluorescens Sasm05 was isolated from the stem of the zinc (Zn)/cadmium (Cd) hyperaccumulator Sedum alfredii Hance. The effects of Sasm05 and exogenous IAA on plant growth, leaf chlorophyll concentration, leaf Mg²⁺-ATPase and Ca²⁺-ATPase activity, cadmium (Cd) uptake and accumulation as well as the expression of metal transporter genes were compared in a hydroponic experiment with 10 μM Cd. The results showed that after treatment with 1 μM IAA, the shoot biomass and chlorophyll concentration increased significantly, but the Cd uptake and accumulation by the plant was not obviously affected. Sasm05 inoculation dramatically increased plant biomass, Cd concentration, shoot chlorophyll concentration and enzyme activities, largely improved the relative expression of the three metal transporter families ZRT/IRT-like protein (ZIP), natural resistance associated macrophage protein (NRAMP) and heavy metal ATPase (HMA). Sasm05 stimulated the expression of the SaHMAs (SaHMA2, SaHMA3, and SaHMA4), which enhanced Cd root to shoot translocation, and upregulated SaZIP, especially SaIRT1, expression to increase Cd uptake. These results showed that although both exogenous IAA and Sasm05 inoculation can improve plant growth and photosynthesis, Sasm05 inoculation has a greater effect on Cd uptake and translocation, indicating that this endophytic bacterium might not only produce IAA to promote plant growth under Cd stress but also directly regulate the expression of putative key Cd uptake and transport genes to enhance Cd accumulation of plant.