Enhancement of Cd phytoextraction by two Amaranthus species with endophytic Rahnella sp. JN27

Transcriptome analysis shows that Glomus versiforme decrease the accumulation and toxicity of cadmium in Ipomoea aquatic Forsk

So far, the physiological and molecular mechanisms of the impact of arbuscular mycorrhizal fungus (AMF) on Cd absorption, transport and detoxification in Ipomoea aquatica (water spinach) are still unclear. In the present study, a pot experiment was performed to investigate the impact of AMF-Glomus versiforme (Gv) on the photosynthetic characteristics, Cd uptake, antioxidative system and transcriptome in water spinach in the soils supplemented with 5 mg Cd kg-1. Gv inoculation improved significantly the photosynthetic characteristics and growth of water spinach. Furthermore, Gv colonization significantly promoted the activities of catalase (CAT), peroxidase (POD) and glutathione reductase (GR), contents of glutathione (GSH) and ascorbic acid (AsA), and the total antioxidant capacity (TCA), but decreased malondialdehyde (MDA) content in water spinach. In addition, Gv inoculation significantly increased pH in rhizosphere soils and decreased the Cd concentrations and uptakes in water spinach. Importantly, 2670 differentially expressed genes (DEGs) were screened in water spinach root colonized with Gv in 5 mg Cd kg-1 soil, of which 2008 DEGs were upregulated and 662 DEGs were downregulated. Especially, the expression levels of POD, CAT, GR, dehydroascorbate reductase 2 (DHAR2), glutathione S-transferase U8 (GSTU8) and glutathione synthetase (GSHS) and cytochrome P450 (Cyt P450) genes were significantly up-regulated in water spinach inoculated with Gv. Meanwhile, the plant cadmium resistance protein 2 (PCR2), metal tolerance protein 4 (MTP4), ATP-binding cassette transporter C family member (ABCC), ABC-yeast cadmium factor 1 (ABC-YCF1) and metallothionein (MT) genes were also up-regulated in mycorrhizal water spinach. Our results firstly elucidated the mechanism by which AMF reduced the uptake and phytotoxicity of Cd in water spinach through a transcriptome analysis.

Rhizophagus intraradices and Azospirillum brasilense improve growth of herbaceous plants and soil biological activity in revegetation of a recovering coal-mining area

We assessed, in a field experiment, the effects of arbuscular mycorrhizal fungi (Rhizophagus intraradices) and plant growth-promoting bacteria (Azospirillum brasilense) on the soil biological activity and the growth of key pioneer species used in the revegetation of coal-mining areas undergoing recovery. We applied four inoculation treatments to the pioneer plant species (Lablab purpureus, Paspalum notatum, Crotalaria juncea, Neonotonia wightii, Stylosanthes guianensis, Andropogon gayanus and Trifolium repens) used in the recovery process: NI (Control - Non-inoculated), AZO (A. brasilense), AMF (R. intraradices), and co-inoculation of AZO and AMF. On the 75th and 180th days, we measured plant dry mass, mycorrhizal colonization, N and P concentration, and accumulation in plant tissue. We collected soil to quantify glomalin content and soil enzyme activity. After 180 days, we did a phytosociological characterization of the remaining spontaneous plants.The both microorganisms, singly or co-inoculated, promoted increases in different fractions of soil glomalin, acid phosphatase activity, and fluorescein diacetate activity at 75 and 180 days. The inoculation was linked to higher plant biomass production (62-89%) and increased plant P and N accumulation by 34-75% and 70-85% at 180 days, compared with the non-inoculated treatment. Among the pioneer species sown Crotalaria juncea produced the highest biomass at the 75th and 180th days (67% and 76% of all biomass), followed by Lablab purpureus (3% and 0.5%), while the other species failed to establish. At 180 days, we observed twenty spontaneous plant species growing in the area, primarily from the Poaceae family (74%). That suggests that the pioneer species present in the area do not hinder the ecological succession process. Inoculation of R. intraradices and A. brasilense, isolated or combined, increases soil biological activity, growth, and nutrient accumulation in key pioneer plant species, indicating the potential of that technique for the recovery of lands degraded by coal mining.

Glomus versiforme and intercropping with Sphagneticola calendulacea decrease Cd accumulation in maize

Little information is available on the influence of the compound use of intercropping (IN) and arbuscular mycorrhizal fungus (AMF) on Cd accumulation and the expression of Cd transporter genes in two intercropped plants. A pot experiment was conducted to study the influences of IN and AMF-Glomus versiforme on growth and Cd uptake of two intercropped plants-maize and Cd hyperaccumulator Sphagneticola calendulacea, and the expression of Cd transporter genes in maize in Cd-polluted soils. IN, AMF and combined treatments of IN and AMF (IN + AMF) obviously improved biomass, photosynthesis and total antioxidant capacities of two plants. Moreover, single and compound treatments of IN and AMF evidently reduced Cd contents in maize, and the greatest decreases appeared in the compound treatment. However, Cd contents of S. calendulacea in IN, AMF and IN + AMF groups were notably improved. Furthermore, the single and compound treatments of IN and AMF significantly downregulated the expression levels of Nramp1, HMA1, ABCC1 and ABCC10 in roots and leaves, and the largest decreases were observed in the combined treatment. Our work first revealed that the combined use of IN and AMF appeared to have a synergistic effect on decreasing Cd content by downregulating the expression of Cd transporter genes in maize.

Effects of Arbuscular Mycorrhizal Fungi on Robinia pseudoacacia L. Growing on Soils Contaminated with Heavy Metals

Arbuscular mycorrhizal fungi (AMF) have been shown to assist plants in increasing metal tolerance and accumulation in heavy metal (HM)-contaminated soils. Herein, a greenhouse pot experiment was conducted to assess the interactions of growth substrates (S1, S2, and S3, respectively) with various HM contamination and nutrient status sampling from a typical contaminated soil and tailings in Shuikoushan lead/zinc mining in Hunan province, China, and AMF inoculation obtained from plants in uncontaminated areas (Glomus mosseae, Glomus intraradices, and uninoculated, respectively) on the biomass and uptake of HMs and phosphorus (P) by the black locust plant (Robinia pseudoacacia L.). The results indicated that the inoculation with AMF significantly enhanced the mycorrhizal colonization of plant roots compared with the uninoculated treatments, and the colonization rates were found to be higher in S1 and S2 compared with S3, which were characterized with a higher nutrient availability and lead concentration. The biomass and heights of R. pseudoacacia were significantly increased by AMF inoculation in S1 and S2. Furthermore, AMF significantly increased the HM concentrations of the roots in S1 and S2 but decreased the HM concentrations in S3. Shoot HM concentrations varied in response to different AMF species and substrate types. Mycorrhizal colonization was found to be highly correlated with plant P concentrations and biomass in S1 and S2, but not in S3. Moreover, plant biomass was also significantly correlated with plant P concentrations in S1 and S2. Overall, these findings demonstrate the interactions of AMF inoculation and growth substrates on the phytoremediation potential of R. pseudoacacia and highlights the need to select optimal AMF isolates for their use in specific substrates for the remediation of HM-contaminated soil.

Identification of natural CTXM-15 inhibitors from aqueous extract of endophytic bacteria Cronobactersakazaki

Nyctanthes arbor-tristis is one of India's valuable and populous medicinal plants which belongs to the family Oleaceae, and widely recognize as night jasmine. Over the years till date, different parts of the plant are used to treat or cure different ailments via various means of traditional medicine. Endophytes are organisms that live in the cell or body of other organisms with no apparent negative impact on the host which they inhabit and are of great source of novel bioactive compounds possessing important economic value. Secondary metabolites were identified in the aqueous extract of Cronobactersakazakii through quantitative phytochemical and GC-MS analysis. Antibacterial activity of the extract against clinical and ATCC strains of E. coli was assessed. Biological activity spectra of these compounds were predicted and categorized either as probably active (Pa) or probably inactive (Pi). Drug-likeness of bioactive compounds was determined as well as their ability to target protein (CTXM-15) responsible for antibiotic resistance in Gram-negative bacteria. Results revealed the presence of active compounds with pharmacological activities and considerable pharmacokinetics parameters. In addition, ligand-protein interactions of compounds with CTXM-15 proteins were identified. These results suggest that bioactive compounds of endophytic Cronobactersakazakii could contain novel chemical entities for the development of antibiotics against pathogenic microbes and other drugs for the amelioration of several infections.

Regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi

Endophytic fungi exist widely in plants and play an important role in the growth and adaptation of plants. They could be used in phytoremediation techniques against heavy metal contaminated soil since beneficial microbial symbionts can endow plants with resistance to external heavy metal stresses. This review summarized the regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi. Potential endophytic fungi in enhancing plant's adaption to heavy metal stresses include arbuscular mycorrhizal fungi, dark septate endophytic fungi, plant growth promoting endophytic fungi. The mechanisms involve coevolution strategy, immune regulation and detoxification transport to improve the ability of plants to adapt to heavy metal stress. They can increase the synthesis of host hormones and maintaining the balance of endogenous hormones, strengthen osmotic regulation, regulate carbon and nitrogen metabolism, and increase immune activity, antioxidant enzyme and glutathione activity. They also help to improve the detoxification transport and heavy metal emission capacity of the host by significantly producing iron carrier, metallothionein and 1-aminocyclopropane-1-carboxylic acid deaminase. The combination of endophytic fungi and hyperaccumulation plants provides a promising technology for the ecological restoration of heavy metal contaminated soil. Endophytic fungi reserves further development on enhancing host plant's adaptability to heavy metal stresses.

Heavy metal and metalloid toxicity in horticultural plants: Tolerance mechanism and remediation strategies

Heavy metal/metalloids (HMs) are among the primary soil pollutants that limit crop production worldwide. Plants grown in HM contaminated soils exhibit reduced growth and development, resulting in a decrease in crop production. The exposure to HMs induces plant oxidative stress due to the formation of free radicals, which alter plant morphophysiological and biochemical mechanisms at cellular and tissue levels. When exposed to HM toxicity, plants evolve sophisticated physiological and cellular defense strategies, such as sequestration and transportation of metals, to ensure their survival. Plants also have developed efficient strategies by activating signaling pathways, which induce the expression of HM transporters. Plants either avoid the uptake of HMs from the soil or activate the detoxifying mechanism to tolerate HM stress, which involves the production of antioxidants (enzymatic and non-enzymatic) for the scavenging of reactive oxygen species. The metal-binding proteins including phytochelatins and metallothioneins also participate in metal detoxification. Furthermore, phytohormones and their signaling pathways also help to regulate cellular activities to counteract HM stress. The excessive levels of HMs in the soil can contribute to plant morpho-physiological, biochemical, and molecular alterations, which have a detrimental effect on the quality and productivity of crops. To maintain the commercial value of fruits and vegetables, various measures should be considered to remove HMs from the metal-polluted soils. Bioremediation is a promising approach that involves the use of tolerant microorganisms and plants to manage HMs pollution. The understanding of HM toxicity, signaling pathways, and tolerance mechanisms will facilitate the development of new crop varieties that help in improving phytoremediation.

Modeling the Carbon Sequestration Potential of Multifunctional Agroforestry-Based Phytoremediation (MAP) Systems in Chinandega, Nicaragua

Global sustainability challenges associated with increasing resource demands from a growing population call for resource-efficient land-use strategies that address multiple sustainability issues. Multifunctional agroforestry-based phytoremediation (MAP) is one such strategy that can simultaneously capture carbon, decontaminate soils, and provide diverse incomes for local farmers. Chinandega, Nicaragua, is a densely populated agricultural region with heavily polluted soils. Four different MAP systems scenarios relevant to Chinandega were created and carbon sequestration potentials were calculated using CO2FIX. All scenarios showed the potential to store significantly more carbon than conventional farming practices, ranging from 2.5 to 8.0 Mg CO2eq ha−1 yr−1. Overall, carbon sequestration in crops is relatively small, but results in increased soil organic carbon (SOC), especially in perennials, and the combination of crops and trees provide higher carbon sequestration rates than monoculture. Changes in SOC are crucial for long-term carbon sequestration, here ranging between 0.4 and 0.9 Mg C ha−1 yr−1, with the most given in scenario 4, an alley cropping system with pollarded trees with prunings used as green mulch. The adoption rate of multifunctional strategies providing both commodity and non-commodity outputs, such as carbon sequestration, would likely increase if phytoremediation is included. Well-designed MAP systems could help reduce land-use conflicts, provide healthier soil, act as climate change mitigation, and have positive impacts on local health and economies.

Organic Acid Excretion in Root Exudates as a Mechanism of Cadmium Uptake in a Sonchus Asper-Zea Mays Intercropping System

This study assessed how the Sonchus asper L.-Zea mays intercropping influenced the plant Cd level, as well as the organic acid (low molecular mass) types and concentrations in root secretions, which was accomplished by conducting pot experiments with soil that was collected from a farmland surrounding a mining area. The results showed that the oxalic and citric acids were the prevailing organic acids in the plant root secretions, and that the intercropping led to prominently elevated root exudate concentrations of the two acids for S. asper by 43.8 and 75.4%, respectively, while decreased such concentrations by 18.5 and 18.7% for Z. mays, compared to monoculture. According to observations, the root exudate concentrations of citric and oxalic acids were significantly positively related to the available rhizosphere soil Cd and plant Cd. The results indicate that changes in root exudation of oxalic acid and citric acid result in different available Cd contents of the rhizosphere soil, thereby affecting the bioavailability of soil Cd, which increases Cd uptake and accumulation in S. asper but inhibits Cd accumulation in Z. mays.

The intercropping and arbuscular mycorrhizal fungus decrease Cd accumulation in upland rice and improve phytoremediation of Cd-contaminated soil by Sphagneticola calendulacea (L.) Pruski

Little is known about the impact of the combined application of intercropping and arbuscular mycorrhizal fungus (AMF) on the plant growth and Cd accumulation in the two intercropped plants. A greenhouse pot experiment was performed to investigate the effects of intercropping (IC) and AMF-Glomus versiforme (GV) on the growth, photosynthesis, Cd accumulation and antioxidant activities in the two intercropped plants-upland rice and Cd hyperaccumulator Sphagneticola calendulacea (L.) Pruski in the soils added with 5 mg Cd kg^-1. It was found that the GV inoculation and the combined treatment of IC and GV (IC + GV) significantly (p < 0.05) increased the biomasses and the P contents of upland rice and S. calendulacea. In addition, the Cd concentrations and uptakes of plants in IC, GV and IC + GV treatments were significantly (p < 0.05) dropped in upland rice but increased in S. calendulacea compared with the monocropping control, and the compound treatment showed better effect on decreasing Cd accumulation in upland rice (especially grains) and increasing Cd uptake by S. calendulacea compared with the single intercropping or AMF treatment. Moreover, IC, GV and IC + GV treatments significantly (p < 0.05) improved the net photosynthetic rate, stomatal conductance and transpiration rate of the two intercropped plants. Finally, IC, GV and IC + GV treatments all significantly increased the catalase activities and total antioxidant capacities, while decreased the malondialdehyde contents in upland rice and S. calendulacea. The present work could provide a feasible strategy for safe production of upland rice and phytoremediation of Cd contaminated soils.

Effects of plant growth regulator and chelating agent on the phytoextraction of heavy metals by Pfaffia glomerata and on the soil microbial community

Pfaffia glomerata is a candidate for the remediation of heavy metal-contaminated soil, but phytoremediation efficiency requires enhancement. In this study, we evaluated how application of DA-6, EDTA, or CA affected the growth and heavy metal accumulation of P. glomerata and soil microorganisms. We found that P. glomerata removed more Cd and Zn than Pb or Cu from contaminated soil. When compared to the control, application of DA-6, CA, or CA + DA-6 increased plant biomass and increased stem Cd concentration by 1.28-, 1.20-, and 1.31-fold respectively; increased leaf Cd concentration by 1.25-, 1.28-, and 1.20-fold, respectively; and increased the total quantity of Cd extracted by 1.37-, 1.37-, and 1.38-fold, respectively. When compared to the control, application EDTA or EDTA + DA-6 significantly increased the soil available metal and Na concentrations, which harmed plant growth. Application of EDTA or EDTA + DA-6 also significantly decreased the Cd concentration in roots and stems. 16S rRNA high-throughput sequencing analysis revealed that application of EDTA or CA alone to soil significantly reduced the richness and diversity of soil bacteria, while foliar spraying of DA-6 combined with EDTA or CA slightly alleviated this reduction. EDTA or CA addition significantly changed the proportion of Actinobacteria and Proteobacteria. In addition, EDTA or CA addition caused changes in soil properties (e.g. heavy metal availability, K concentration, Na concentration, soil pH, soil CEC, and soil DOC concentration) that were associated with changes in the bacterial community. EDTA addition mainly affected the soil bacterial community by changing soil DOC concentration, the soil available Pb and Na concentration, and CA addition mainly affected the soil bacterial community by changing the soil available Ca concentration.

Enhanced reduction of lead bioavailability in phosphate mining wasteland soil by a phosphate-solubilizing strain of Pseudomonas sp., LA, coupled with ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.)

Due to ecologically unsustainable mining strategies, there remain large areas of phosphate mining wasteland contaminated with accumulated lead (Pb). In this study, a Pb-resistant phosphate-solubilizing strain of Pseudomonas sp., LA, isolated from phosphate mining wasteland, was coupled with two species of native plants, ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.), for use in enhancing the reduction of bioavailable Pb in soil from a phosphate mining wasteland. The effect of PbCO₃ solubilization by Pseudomonas sp. strain LA was evaluated in solution culture. It was found that strain LA could attain the best solubilization effect on insoluble Pb when the PbCO₃ concentration was 1% (w/v). Pot experiments were carried out to investigate the potential of remediation by ryegrass and sonchus in phosphate mining wastelands with phosphate rock application and phosphate-solubilizing bacteria inoculation. Compared to the control group without strain LA inoculation, the biomass and length of ryegrass and sonchus were markedly increased, available P and Pb in roots increased by 22.2%-325% and 23.3%-368%, respectively, and available P and Pb in above-ground parts increased by 4.44%-388% and 1.67%-303%, respectively, whereas available Pb in soil decreased by 14.1%-27.3%. These results suggest that the combination of strain LA and plants is a bioremediation strategy with considerable potential and could help solve the Pb-contamination problem in phosphate mining wastelands.

Effects of Soil Amendment With Wood Ash on Transpiration, Growth, and Metal Uptake in Two Contrasting Maize (Zea mays L.) Hybrids to Drought Tolerance

Wood ash as a soil amendment has gained wide spread acceptance in the recent years as a sustainable alternative to chemical fertilizers, although information regarding the effects of its application on maize growth and yield in the context of climate change and increasing drought severity is lacking till date. In the present study, field and pot trials were carried out at the experimental farm of the University of Padova at Legnaro (NE Italy) in a silty-loam soil in order to investigate the effects of soil amendment with wood ash (0.1% w/w, incorporated into the 0.2-m top soil) on the bioavailability of mineral elements and their uptake by maize. Characteristics analyzed included plant growth, leaf transpiration dynamics, and productivity in two contrasting hybrids, P1921 (drought sensitive) and D24 (drought tolerant). Wood ash contained relevant amounts of Ca, K, Mg, P, and S, and hazardous levels of Zn (732 mg kg^-1), Pb (527 mg kg^-1), and Cu (129 mg kg^-1), although no significant changes in total soil element concentration, pH, and electrical conductivity were detected in open field. Ash application led to a general increasing trend of diethylene triamine penta-acetic acid (DTPA)-extractable of various elements, bringing to higher grain P in D24 hybrid, and Zn and Ni reductions in P1921 hybrid. Here, the results demonstrated that ash amendment enhanced shoot growth and the number of leaves, causing a reduction of harvest index, without affecting grain yield in both hybrids. The most relevant result was a retarded inhibition of leaf transpiration under artificial progressive water stress, particularly in the drought-tolerant D24 hybrid that could be sustained by root growth improvements in the field across the whole 0-1.5 m soil profile in D24, and in the amended top soil in P1921. It is concluded that woody ash can be profitably exploited in maize fertilization for enhancing shoot and root growth and drought tolerance, thanks to morphological and physiological improvements, although major benefits are expected to be achieved in drought tolerant hybrids. Attention should be payed when using ash derived by metal contaminated wood stocks to avoid any health risk in food uses.

The effect of Funneliformis mosseae on the plant growth, Cd translocation and accumulation in the new Cd-hyperaccumulator Sphagneticola calendulacea

The screening and identification of hyperaccumulators is the key to the phytoremediation of soils contaminated by heavy metal (HM). Arbuscular mycorrhizal fungus (AMF) can improve plant growth and tolerance to HM; therefore, AMF-assisted phytoextraction has been regarded as a potential technique for the remediation of HM-polluted soils. A greenhouse pot experiment was conducted to determine whether Sphagneticola calendulacea is a Cd-hyperaccumulator and to investigate the effect of the AMF-Funneliformis mosseae (FM) on plant growth and on the accumulation, subcellular distribution and chemical form of Cd in S. calendulacea grown in soils supplemented with different Cd levels. At 25, 50 and 100 mg Cd kg^-1 level, S. calendulacea showed high Cd tolerance, the translocation factor and the bioconcentration factor exceeded 1, and accumulation of more than 100 mg Cd kg^-1 was observed in the aboveground parts of the plant, meeting the requirements for a Cd-hyperaccumulator. Moreover, FM colonization significantly increased both biomasses and Cd concentration in S. calendulacea. After FM inoculation, the Cd concentrations and proportions increased in the cell walls, but exhibited no significant change in the organelles of the shoots. Meanwhile, FM symbiosis contributed to the conversion of Cd from highly toxic chemical forms (extracted by 80% ethanol and deionized water) to less toxic chemical forms (extracted by 1 M NaCl, 2% acetic acid, 0.6 M HCl) of Cd in the shoots. Overall, S. calendulacea is a typical Cd-hyperaccumulator, and FM symbiosis relieved the phytotoxicity of Cd and promoted plant growth and Cd accumulation, and thus greatly increasing the efficiency of phytoextraction for Cd-polluted soil. Our study provides a theoretical basis and application guidance for the remediation of Cd-contaminated soil by the symbiont of S. calendulacea with FM.

Assessment of physiological and biochemical responses of Amaranthus retroflexus seedlings to the accumulation of heavy metals with regards to phytoremediation potential

The aim of this research was to assess, under laboratory conditions, how the accumulation of four heavy metals (HMs) (lead (Pb), copper (Cu), nickel (Ni), and zinc (Zn)), prepared as aqueous solutions from 1 μM to 1 mM, affected biochemical and physiological parameters of Amaranthus retroflexus seedlings. Seedlings showed considerably high resistance to all investigated HMs and no significant oxidative stress in leaves. After chronic exposure to high doses of any of the HMs, seedlings remained viable, but with slightly slower axial growth. We propose the use of biochemical indices (lipid peroxidation (LPO) intensity; level of total peroxides) as criteria to assess the adaptive potential of amaranth plants to HMs. These indices had very high correlation coefficients (r) with the accumulation of HMs in A. retroflexus roots, stems and leaves: 0.86-0.89 for malone dialdehyde (MDA) content for Ni and Zn, and 0.79-0.94 for total peroxides (for Cu, Pb, and maximum in Ni). At 1 mM of any HM, seedlings accumulated Pb and Ni at levels of HM-hyperaccumulating species. If soil is contaminated (in terms of maximum permissible concentration, MPC) by Pb (8.2 ± 2.2 MPC) or Ni (3.5 ± 1.0 MPC) (equivalent to 1 mM of the HM in solution), A. retroflexus is a strong candidate for the phytoremediation of Pb- and Ni-contaminated soils.

Evaluation of phytoremediation potential of five Cd (hyper)accumulators in two Cd contaminated soils

A phytoextraction experiment with five Cd hyperaccumulators (Amaranthus hypochondriacus, Celosia argentea, Solanum nigrum, Phytolacca acinosa and Sedum plumbizincicola) was conducted in two soils with different soil pH (5.93 and 7.43, respectively). Most accumulator plants grew better in the acidic soil, with 19.59-39.63% higher biomass than in the alkaline soil, except for S. plumbizincicola. The potential for a metal-contaminated soil to be cleaned up using phytoremediation is determined by the metal uptake capacity of hyperaccumulator, soil properties, and mutual fitness of plant-soil relationships. In the acidic soil, C. argentea and A. hypochondriacus extracted the highest amount of Cd (1.03 mg pot^-1 and 0.92 mg pot^-1, respectively). In the alkaline soil, S. plumbizincicola performed best, mainly as a result of high Cd accumulation in plant tissue (541.36 mg kg^-1). Most plants achieved leaf Cd bioconcentration factor (BCF) of >10 in the acidic soil, compared to <4 in the alkaline soil. Soil Cd availability was chiefly responsible for such contrasting metal extraction capacity, with 5.02% fraction and 48.50% fraction of total Cd being available in the alkaline and acidic soil, respectively. In the alkaline soil, plants tended to increase rhizosphere soil available Cd mainly through excreting more low molecular weight organic acids, not through changing the soil pH. In the acidic soil, plants slightly decreased soil available Cd. Those species which have high Ca, Zn, Fe uptake capacity extract more Cd from soil, and a positive correlation was found between the concentrations of Cd and Ca, Zn, Fe in leaves. Soil available Ca²⁺, Mg²⁺, SO₄^2-, Cl^- did not play a key role in Cd uptake by plants. In summary, acidic soil was of higher potential to recover from Cd contamination by phytoextraction, while in the alkaline soil, S. plumbizincicola showed potential for Cd phytoextraction.

Morphological Responses and Gene Expression of Grain Amaranth (Amaranthus spp.) Growing under Cd

Phytoremediation efficiency depends on the ability of plants to accumulate, translocate and resist high levels of metals without symptoms of toxicity. This study was conducted to evaluate the potential of grain amaranth for remediation of soils contaminated with Cd. Three grain amaranth varieties, “Pribina” (A. cruentus), “Zobor” (A. hypochondriacus x A. hybridus) and Plainsman (A. hypochondriacus x A. hybridus) were tested under different level of Cd (0, 5, 10 and 15 mg/L) in a hydroponic experimental treatment. All could be classified as Cd excluders or Cd-hypertolerant varieties able to grow and accumulate significant amounts of Cd from the hydroponic solution, preferentially in the roots. Under the highest level of Cd exposure, qRT-PCR expression analysis of five stress-related genes was examined in above- and below-ground biomass. The results show that the Cd concentration significantly increased the mRNA level of chitinase 5 (Chit 5) in amaranth roots as the primary site of metal stress. The involvement of phytochelatin synthase (PCS1) in Cd detoxification is suggested. Based on our findings, we can conclude that variety “Pribina” is the most Cd-tolerant among three tested and can be expected to be used in the phytomanagement of Cd loaded soils as an effective phytostabiliser.

Improving cadmium mobilization by phosphate-solubilizing bacteria via regulating organic acids metabolism with potassium

Organic acids secreted by phosphorus-solubilizing bacteria (PSB) is one of the main biological metabolites with cadmium (Cd) mobilization capacity in the conversion of insoluble precipitate forms to bioavailable forms in contaminated soil. However, the fluctuating concentrations of nutrient elements caused by agricultural activities may result in the substantial variances of carbohydrate metabolism of microorganisms involved in Cd remediation, it is therefore essential to study how metabolic strategies, especially for organic acids, affected by the environmentally friendly fertilizers, such as potassium (K). In this study, adding K⁺ (KCl) concentrations from 0.0 to 100.0 mg/L in medium clearly accelerated Cd mobilization from 15.9 to 35.9 mg/L via inducing the secretion of tartaric acid, 3-hydroxybutyrate, fumaric and succinic acids, increased by 10.0-, 7.5-, 4.3- and 4.1-fold changes, respectively. Current data revealed that the significant differences of metabolic pathways and genes expressions with the varied K⁺ concentrations included: ⅰ) K⁺ induces a substantial up-regulation in metabolic pathway of pyruvic acid to oxaloacetate and tartaric acids; ⅱ) the varied expression of genes involved in encoding enzymes of tricarboxylic acid cycle result in the up-regulated fumaric acid, succinic acid and 3-hydroxybutyrate; ⅲ) the expression of genes related enzyme cysteine and glutamate metabolism processes promoted with the increasing bioavailable Cd concentrations. Besides, P-type ATPase activity increased with K⁺ levels, indicating that H⁺ efflux and medium acidification were strengthened. In general, an appropriate enhancement of K based fertilizer is an effective manner for soil Cd remediation via the regulation of organic acids metabolism and H⁺ secretion of PSB.

Significance of manganese resistant bacillus cereus strain WSE01 as a bioinoculant for promotion of plant growth and manganese accumulation in Myriophyllum verticillatum

Endophytic bacteria are generally helpful for plant growth and protection. Strain WSE01, which was identified as bacillus cereus, was isolated from the stem of Myriophyllum verticillatum and it displayed a high tolerance to Mn (1500 mg/L). The strain was found to be able to produce indole-3-acetic acid (IAA) and siderophores, fix the atmospheric nitrogen and dissolve potassium from insoluble K-bearing minerals. In hydroponic culture experiments, the inoculation of strain WSE01 significantly promoted the growth and increased the leaf enzyme activity in the inoculated plant M. verticillatum. Furthermore, the manganese content was increased by 36.4% in stems and by 54.7% in leaves of the inoculated plant under Mn stress at 400 mg/L, compared to the non-inoculated group. This study suggests that the strain WSE01 has the potential to be used as biocontrol and/or biofertilizing agents for application in macrophyte M. verticillatum and conduces to achieving more effective phytoremediation of metal-contaminated waters.

Diversity of endophytic plant-growth microorganisms from Gentianella weberbaueri and Valeriana pycnantha, highland Peruvian medicinal plants

Microbial diversity in Peruvian mountain areas is poorly know, specially endophytic microorganisms of medicinal native plants from the Cordillera Blanca. So, nine bacterial and six fungal species were isolated from Gentianella weberbaueri and Valeriana pycnantha. According to 16S rDNA analysis, bacterial strains belong to genera Rahnella, Pseudomonas, Serratia, Rouxiella, and Bacillus; while ITS analysis showed that fungi belong to Pyrenochaeta, Scleroconidioma, Cryptococcus, and Plenodomus genera. Rahnella sp. GT24B and P. trivialis VT20B solubilized tricalcium phosphate and produced siderophores at 10 and 24 °C. Five bacteria strains produced indol-3-acetic acid (IAA) at 10 and 24 °C, where Rahnella sp. VT19B showed more production at 10 °C than 24 °C. Rahnella sp. GT24B, Serratia sp. VT28B, and Rahnella sp. GT25B inhibited Fusarium oxysporum growth up to 100, 78 and 74 %, respectively. R. inusitata VT25B and B. licheniformis GT10B showed high cellulolytic and proteolytic activities. On the other hand, only a few fungi moderately inhibited growth of F. oxysporum, and produced siderophores and cellulases. Most of bacteria inoculated on Medicago sativa "alfalfa" and Triticum aestivum "wheat" seeds got better root development, especially Rahnella sp. GT24B, Rouxiella sp.VT24B, Serratia sp. VT28B, and Rahnella sp. VT34B. Finally, this study is the first report of endophytic microorganisms associated to wild medicinal high-mountain Peruvian plants and it show a valuable microbial diversity and its possible role in promoting growth of crops and wild medicinal plants.

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

Phytoremediation potential of Miscanthus sinensis for mercury-polluted sites and its impacts on soil microbial community

Phytoremediation potential of Miscanthus sinensis and its impacts on soil microbial community and nutrients were evaluated by pot experiment at soil mercury concentration from 1.48 to 706 mg kg^-1. The changes in biomass yield in dry mass, chlorophyll content, and SOD activity indicated Miscanthus sinensis was tolerant to higher levels of soil mercury exposure, and could grow even if at soil mercury up to 706 mg kg^-1. Mercury bioconcentration and translocation factors were close to or greater than 1 when exposed to soil mercury up to 183 mg kg^-1, demonstrating Miscanthus sinensis a potential phytoremediator for mercury-polluted soils. Miscanthus sinensis planting could significantly improve the diversity and abundance of soil microbial community, but might cause potential loss of soil nitrogen and phosphorus in the early and middle of its growth. In a word, the study indicated Miscanthus sinensis was a promising energy crop linking biofuel production and phytoremediation of mercury-contaminated sites.

Metabolic responses and their correlations with phytochelatins in Amaranthus hypochondriacus under cadmium stress

Phytochelatins (PCs) play a vital role in the tolerance and enrichment of cadmium (Cd) in higher plants by chelating with Cd²⁺. The aim of this study was to perform a full-scale metabolomics analysis of metabolic responses highly correlated with PCs generation. These metabolites and metabolic pathways were expected to promote PCs generation and further optimize Cd absorption in plants. In the current study, Amaranthus hypochondriacus, a potential species for phytoremediation, was first adopted to investigate physiological responses to Cd stress via LCMS/MS-based metabolomics and the HPLC based determination of thiol compounds. The results showed that the leaves of A. hypochondriacus under high Cd stress accumulated 40 times the amount of Cd compared to the leaves of the plants not under Cd stress and had an increased content of three types of PCs. Metabolomics qualitatively identified 12084 substances in total, among which 41 were significantly different metabolites (SDMs) between the two groups and involved in 7 metabolic pathways. Among the SDMs, 12 metabolites were highly linearly correlated with PCs involved in three pathways (Val, Leu and Ile biosynthesis; Ala, Asp and Glu metabolism; and Arg and Pro metabolism). These results provide an innovative method to promote PCs synthesis for the restoration of Cd-contaminated-soil.

Mechanisms of Cr(VI) resistance by endophytic Sphingomonas sp. LK11 and its Cr(VI) phytotoxic mitigating effects in soybean (Glycine max L.)

Chromium Cr(VI) is highly toxic and leads to impaired phenotypic plasticity of economically important crops. The current study assessed an endophytic-bacteria assisted metal bio-remediation strategy to understand stress-alleviating mechanisms in Glycine max L (soybean) plants inoculated with Sphingomonas sp. LK11 under severe Cr(VI) toxicity. The screening analysis showed that high Cr concentrations (5.0 mM) slightly suppressed LK11 growth and metal uptake by LK11 cells, while significantly enhancing indole-3-acetic acid (IAA) production. Endophytic LK11 significantly upregulated its antioxidant system compared to control by enhancing reduced glutathione (GSH), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) activities to counteract Cr-induced oxidative stress. Cr toxicity induced cell morphological alteration, as shown by SEM-EDX analysis and triggered significant lipid peroxidation. The interaction between LK11 and soybean in Cr-contaminated soil significantly increased plant growth attributes and down-regulated the synthesis of endogenous defense-related phytohormones, salicylic acid and abscisic acid, by 20% and 37%, respectively, and reduced Cr translocation to the roots, shoot, and leaves. Additionally, Cr-induced oxidative stress was significantly reduced in LK11-inoculated soybean, regulating metal responsive reduced GSH and enzymatic antioxidant CAT. Current findings indicate that LK11 may be a suitable candidate for the bioremediation of Cr-contaminated soil and stimulation of host physiological homeostasis.

Augmentation with potential endophytes enhances phytostabilization of Cr in contaminated soil

The contamination of soil with heavy metals is a major environmental problem worldwide. The combined use of plants and their associated microbes has gained popularity in recent years for their potential to remediate heavy metal-contaminated soil. In the current study, the effect that augmentation of soil with plant growth-promoting endophytes has on the phytostabilization of chromium (Cr)-contaminated soil was investigated. Three potential endophytic bacterial strains (Enterobacter sp. HU38, Microbacterium arborescens HU33, and Pantoea stewartii ASI11) were inoculated individually as well as in combination to Leptochloa fusca and Brachiaria mutica vegetated in Cr-contaminated soil. The accumulation of Cr in the root and shoot of the plants was determined. Moreover, bacterial persistence in the rhizosphere and endosphere was determined. Augmentation with potential endophytes significantly increased root length (24-45%), shoot height (39-64%), chlorophyll content (20-55%), and the overall biomass (32-61%) of the plants. Although L. fusca and B. mutica showed potential to accumulate Cr in their root and shoot, endophytic augmentation increased uptake, translocation, and accumulation of Cr in the roots and shoots of both plant species. However, L. fusca showed more potential to phytostabilize Cr as compared to B. mutica. Furthermore, the potential endophytes showed more survival and persistence within the roots than in the rhizosphere and shoot interior. This study provides useful evidence of endophyte-assisted phytoremediation to be the most sustainable and affordable approach for in situ remediation of Cr-contaminated soil.

Phytoextraction of cadmium-contaminated soil and potential of regenerated tobacco biomass for recovery of cadmium

The aim of this study was to estimate the influence of regenerated tobacco on the extraction of Cd from two acidic soils as well as to address the problem of how to deal with contaminated leaves following phytoextraction. Results showed that a coppicing tobacco led to a decline in Cd concentration in regenerated leaves and stalks when plants were grown in pots, but increased concentrations in regenerated lower and middle leaves when plants were grown under field conditions. The highest recorded bioconcentration factors in Chaling and Guanxi soil were 37.53 and 19.21 in lower leaves in the field, respectively. Total Cd extraction efficiency in practice (9.43% for Chaling soil and 6.24% for Guanxi soil) under field conditions confirmed our theoretical calculations (10.0% for Chaling soil and 6.73% for Guanxi soil). Use of a 0.5% hydrochloric acid(HCl) solution was sufficient to reduce Cd (98.4%) in tobacco leaves to permissible levels as required by the Hygienic Standard for Feeds in China (≤0.5 mg kg-1). Regenerated tobacco has the potential to allow cultivation of Cd contaminated farmland to produce animal feed, assist in lowering total Cd content of soil, and allow income generation for farmers.

Role of Ni-tolerant Bacillus spp. and Althea rosea L. in the phytoremediation of Ni-contaminated soils

In our current study, four nickel-tolerant (Ni-tolerant) bacterial species viz, Bacillus thuringiensis 002, Bacillus fortis 162, Bacillus subtilis 174, and Bacillus farraginis 354, were screened using Ni-contaminated media. The screened microbes exhibited positive results for synthesis of indole acetic acid (IAA), siderophore production, and phosphate solubilization. The effects of these screened microbes on Ni mobility in the soil, root elongation, plant biomass, and Ni uptake in Althea rosea plants grown in Ni-contaminated soil (200 mg Ni kg^-1) were evaluated. Significantly higher value for water-extractable Ni (38 mg kg^-1) was observed in case of Ni-amended soils inoculated with B. subtilis 174. Similarly, B. thuringiensis 002, B. fortis 162, and B. subtilis 174 significantly enhanced growth and Ni uptake in A. rosea. The Ni uptake in the shoots and roots of B. subtilis 174-inoculated plants enhanced up to 1.7 and 1.6-fold, respectively, as compared to that in the un-inoculated control. Bacterial inoculation also significantly improved the root and shoot biomass of treated plants. The current study presents a novel approach for bacteria-assisted phytoremediation of Ni-contaminated areas.

Value added phytoremediation of metal stressed soils using phosphate solubilizing microbial consortium

The presence of heavy metals in the soil is a matter of growing concern due to their toxic and non-biodegradable nature. Lack of effectiveness of various conventional methods due to economic and technical constraints resulted in the search for an eco-friendly and cost-effective biological techniques for heavy metal removal from the environment. Until now, phytoremediation has emerged as an innovative technique to address the problem. However, the efficiency of phytoremediation process is hindered under the high metal concentration conditions. Hence, phosphate solubilizing microbes (PSM) assisted phytoremediation technique is gaining more insight as it can reduce the contamination load even under elevated metal stressed conditions. These microbes convert heavy metals into soluble and bioavailable forms, which consequently facilitate phytoremediation. Several studies have reported that the use of microbial consortium for remediation is considered more effective as compared to single strain pure culture. Therefore, this review paper focuses on the current trends in research related to PSM mediated uptake of heavy metal by plants. The efficiency of PSM consortia in enhancing the phytoremediation process has also been reviewed. Moreover, the role of phosphatase enzymes in the mineralization of organic forms of phosphate in soil is further discussed. Biosurfactant mediated bioremediation of metal polluted soils is a matter of extensive research nowadays. Hence, the recent advancement of using biosurfactants in enhanced phytoremediation of metal stressed soils is also described.

Accumulation of Cr, Cd, Pb, Cu, and Zn by plants in tanning sludge storage sites: opportunities for contamination bioindication and phytoremediation

Tanning sludge enriched with high concentrations of Cr and other metals has adverse effects on the environment. Plants growing in the metalliferous soils may have the ability to cope with high metal concentrations. This study focuses on potentials of using native plants for bioindication and/or phytoremediation of Cr-contaminated sites. In the study, we characterized plants and soils from six tanning sludge storage sites. Soil in these sites exhibited toxic levels of Cr (averaged 16,492 mg kg^-1) and other metals (e.g., 48.3 mg Cu kg^-1, 2370 mg Zn kg^-1, 44.9 mg Pb kg^-1, and 0.59 mg Cd kg^-1). Different metal tolerance and accumulation patterns were observed among the sampled plant species. Phragmites australis, Zephyranthes candida, Cynodon dactylon, and Alternanthera philoxeroides accumulated moderate-high concentrations of Cr and other metals, which could make them good bioindicators of heavy metal pollution. High Cr and other metal concentrations (e.g., Cd and Pb) were found in Chenopodium rubrum (372 mg Cr kg^-1), Aster subulatus (310 mg Cr kg^-1), and Brassica chinensis (300 mg Cr kg^-1), being considered as metal accumulators. In addition, Nerium indicum and Z. candida were able to tolerate high concentrations of Cr and other metals, and they may be used as preferable pioneer species to grow or use for restoration in Cr-contaminated sites. This study can be useful for establishing guidelines to select the most suitable plant species to revegetate and remediate metals in tanning sludge-contaminated fields.

Drought-tolerant Streptomyces pactum Act12 assist phytoremediation of cadmium-contaminated soil by Amaranthus hypochondriacus: great potential application in arid/semi-arid areas

Microbe-assisted phytoremediation provides an effective approach to clean up heavy metal-contaminated soils. However, severe drought may affect the function of microbes in arid/semi-arid areas. Streptomyces pactum Act12 is a drought-tolerant soil actinomycete strain isolated from an extreme environment on the Qinghai-Tibet Plateau, China. In this study, pot experiments were conducted to assess the effect of Act12 on Cd tolerance, uptake, and accumulation in amaranth (Amaranthus hypochondriacus) under water deficit. Inoculated plants had higher Cd concentrations (root 8.7-33.9 %; shoot 53.2-102.1 %) and uptake (root 19.9-95.3 %; shoot 110.6-170.1 %) than non-inoculated controls in Cd-treated soil. The translocation factor of Cd from roots to shoots was increased by 14.2-75 % in inoculated plants, while the bioconcentration factor of Cd in roots and shoots was increased by 10.2-64.4 and 53.9-114.8 %, respectively. Moreover, inoculation with Act12 increased plant height, root length, and shoot biomass of amaranth in Cd-treated soil compared to non-inoculated controls. Physiochemical analysis revealed that Act12 enhanced Cd tolerance in the plants by increasing glutathione, elevating superoxide dismutase and catalase activities, as well as reducing malondialdehyde content in the leaves. The drought-tolerant actinomycete strain Act12 can enhance the phytoremediation efficiency of amaranth for Cd-contaminated soils under water deficit, exhibiting potential for application in arid and semi-arid areas.

Biochemical and Molecular Mechanisms of Plant-Microbe-Metal Interactions: Relevance for Phytoremediation

Plants and microbes coexist or compete for survival and their cohesive interactions play a vital role in adapting to metalliferous environments, and can thus be explored to improve microbe-assisted phytoremediation. Plant root exudates are useful nutrient and energy sources for soil microorganisms, with whom they establish intricate communication systems. Some beneficial bacteria and fungi, acting as plant growth promoting microorganisms (PGPMs), may alleviate metal phytotoxicity and stimulate plant growth indirectly via the induction of defense mechanisms against phytopathogens, and/or directly through the solubilization of mineral nutrients (nitrogen, phosphate, potassium, iron, etc.), production of plant growth promoting substances (e.g., phytohormones), and secretion of specific enzymes (e.g., 1-aminocyclopropane-1-carboxylate deaminase). PGPM can also change metal bioavailability in soil through various mechanisms such as acidification, precipitation, chelation, complexation, and redox reactions. This review presents the recent advances and applications made hitherto in understanding the biochemical and molecular mechanisms of plant-microbe interactions and their role in the major processes involved in phytoremediation, such as heavy metal detoxification, mobilization, immobilization, transformation, transport, and distribution.

Beneficial role of bacterial endophytes in heavy metal phytoremediation

Phytoremediation is an emerging technology that uses plants and their associated microbes to clean up pollutants from the soil, water and air. In recent years, phytoremediation assisted by bacterial endophytes has been highly recommended for cleaning up of metal polluted soils since endophytic bacteria can alleviate metal toxicity in plant through their own metal resistance system and facilitate plant growth under metal stress. Endophytic bacteria improve plant growth in metal polluted soils in two different ways: 1) directly by producing plant growth beneficial substances including solubilization/transformation of mineral nutrients (phosphate, nitrogen and potassium), production of phytohormones, siderophores and specific enzymes; and 2) indirectly through controlling plant pathogens or by inducing a systemic resistance of plants against pathogens. Besides, they also alter metal accumulation capacity in plants by excreting metal immobilizing extracellular polymeric substances, as well as metal mobilizing organic acids and biosurfactants. The present work aims to review the progress of recent research on the isolation, identification and diversity of metal resistant endophytic bacteria and illustrate various mechanisms responsible for plant growth promotion and heavy metal detoxification/phytoaccumulation/translocation in plants.

Can arbuscular mycorrhizal fungi reduce Cd uptake and alleviate Cd toxicity of Lonicera japonica grown in Cd-added soils?

A greenhouse pot experiment was conducted to study the impact of arbuscular mycorrhizal fungi--Glomus versiforme (Gv) and Rhizophagus intraradices (Ri) on the growth, Cd uptake, antioxidant indices [glutathione reductase (GR), ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), ascorbate (ASA), glutathione (GSH) and malonaldehyde (MDA)] and phytochelatins (PCs) production of Lonicera japonica in Cd-amended soils. Gv and Ri significantly increased P acquisition, biomass of shoots and roots at all Cd treatments. Gv significantly decreased Cd concentrations in shoots and roots, and Ri also obviously reduced Cd concentrations in shoots but increased Cd concentrations in roots. Meanwhile, activities of CAT, APX and GR, and contents of ASA and PCs were remarkably higher in Gv/Ri-inoculated plants than those of uninoculated plants, but lower MDA and GSH contents in Gv/Ri-inoculated plants were found. In conclusion, Gv and Ri symbiosis alleviated Cd toxicity of L. japonica through the decline of shoot Cd concentrations and the improvement of P nutrition, PCs content and activities of GR, CAT, APX in inoculated plants, and then improved plant growth. The decrease of shoot Cd concentrations in L. japonica inoculated with Gv/Ri would provide a clue for safe production of this plant from Cd-contaminated soils.

Increased plant growth and copper uptake of host and non-host plants by metal-resistant and plant growth-promoting endophytic bacteria

The effects of inoculation with two metal-resistant and plant growth-promoting endophytic bacteria (Burkholderia sp. GL12 and Bacillus megaterium JL35) were evaluated on the plant growth and Cu uptake in their host Elsholtzia splendens and non-host Brassica napus plants grown in natural Cu-contaminated soil. The two strains showed a high level of ACC deaminase activities. In pot experiments, inoculation with strain GL12 significantly increased root and above-ground tissue dry weights of both plants, consequently increasing the total Cu uptake of E. splendens and Brassica napus by 132% and 48.2% respectively. Inoculation with strain JL35 was found to significantly increase not only the biomass of B. napus, consequently increasing the total Cu uptake of B. napus by 31.3%, but Cu concentration of E. splendens for above-ground tissues by 318% and roots by 69.7%, consequently increasing the total Cu uptake of E. splendens by 223%. The two strains could colonize the rhizosphere soils and root interiors of both plants. Notably, strain JL35 could colonize the shoot tissues and significantly increase the translocation factors and bioaccumulation factors of E. splendens. These results suggested that Burkholderia sp. GL12 and B. megaterium JL35 were valuable bacterial resource which had the potential in improving the efficiency of Cu phytoextraction by E. splendens and B. napus in a natural Cu-contaminated soil.

Effect of Inoculation with Glomus versiforme on Cadmium Accumulation, Antioxidant Activities and Phytochelatins of Solanum photeinocarpum

The plant growth, phosphate acquisition, Cd translocation, phytochelatins (PCs) production and antioxidant parameters [superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), glutathione (GSH), ascorbate (ASA) and malonaldehyde (MDA)] were investigated in Cd-hyperaccumulator Solanum photeinocarpum inoculated with Glomus versiforme BGC GD01C (Gv) in Cd-added soils (0, 5, 10, 20, 40 mg Cd kg-1 soil). Mycorrhizal colonization rates were generally high (from 77% to 94%), and hardly affected by Cd. Gv colonization significantly enhanced P acquisition, growth and total Cd uptakes in both shoots and roots of S. photeinocarpum at all Cd levels. Meanwhile, Gv symbiosis significantly increased Cd concentration in the roots, and decreased Cd concentration in the shoots at all Cd levels, which indicates that Gv could promote phytostabilization by enhancing Cd accumulation in the roots to inhibit its translocation to shoots and the "dilution effects" linked to an increase in plant dry matter yield and a reduced Cd partitioning to shoots. Moreover, the improvement of CAT, POD and APX activities in the leaves of mycorrhizal plants infers that Gv symbiosis helped S. photeinocarpum to relieve oxidative damage to biomolecules in Cd-contaminated soil. The evident decline of MDA content in the leaves of mycorrhizal plants indicates that Gv symbiosis evidently improved antioxidant activities, and the enhancement of PCs production in the leaves of mycorrhizal plants suggests that Gv-inoculated plant may be more efficient to relieve Cd phytotoxicity. Therefore, the possible mechanisms of Cd phytotoxicity alleviation by Gv can be concluded as the decline of Cd concentration in the shoots and the improvement of P acquisition, PCs production and activities of CAT, POD, APX in mycorrhizal plants.

Effect of short-term cadmium stress on Populus nigra L. detached leaves

Pollution by toxic metals, accumulating into soils as result of human activities, is a worldwide major concern in industrial countries. Plants exhibit different degrees of tolerance to heavy metals, as a consequence of their ability to exclude or accumulate them in particular tissues, organs or sub-cellular compartments. Molecular information about cellular processes affected by heavy metals is still largely incomplete. As a fast-growing, highly tolerant perennial plant species, poplar has become a model for environmental stress response investigations. To study the short-term effects of cadmium accumulation in leaves, we analyzed photosystem II (PSII) quantum yield, hydrogen peroxide (H2O2) generation, hormone levels variation, as well as proteome profile alteration of 50μM CdSO4 vacuum-infiltrated poplar (Populus nigra L.) detached leaves. Cadmium management brought about an early and sustained production of hydrogen peroxide, an increase of abscisic acid, ethylene and gibberellins content, as well as a decrease in cytokinins and auxin levels, whereas photosynthetic electron transport was unaffected. Proteomic analysis revealed that twenty-one proteins were differentially induced in cadmium-treated leaves. Identification of fifteen polypeptides allowed to ascertain that most of them were involved in stress response while the remaining ones were involved in photosynthetic carbon metabolism and energy production.

Combined endophytic inoculants enhance nickel phytoextraction from serpentine soil in the hyperaccumulator Noccaea caerulescens

This study assesses the effects of specific bacterial endophytes on the phytoextraction capacity of the Ni-hyperaccumulator Noccaea caerulescens, spontaneously growing in a serpentine soil environment. Five metal-tolerant endophytes had already been selected for their high Ni tolerance (6 mM) and plant growth promoting ability. Here we demonstrate that individual bacterial inoculation is ineffective in enhancing Ni translocation and growth of N. caerulescens in serpentine soil, except for specific strains Ncr-1 and Ncr-8, belonging to the Arthrobacter and Microbacterium genera, which showed the highest indole acetic acid production and 1-aminocyclopropane-1-carboxylic acid-deaminase activity. Ncr-1 and Ncr-8 co-inoculation was even more efficient in promoting plant growth, soil Ni removal, and translocation of Ni, together with that of Fe, Co, and Cu. Bacteria of both strains densely colonized the root surfaces and intercellular spaces of leaf epidermal tissue. These two bacterial strains also turned out to stimulate root length, shoot biomass, and Ni uptake in Arabidopsis thaliana grown in MS agar medium supplemented with Ni. It is concluded that adaptation of N. caerulescens in highly Ni-contaminated serpentine soil can be enhanced by an integrated community of bacterial endophytes rather than by single strains; of the former, Arthrobacter and Microbacterium may be useful candidates for future phytoremediation trials in multiple metal-contaminated sites, with possible extension to non-hyperaccumulator plants.