Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils

Inoculation of Sinorhizobium saheli YH1 Leads to Reduced Metal Uptake for Leucaena leucocephala Grown in Mine Tailings and Metal-Polluted Soils

Metalliferous mine tailings have a negative impact on the soil environment near mining areas and render cultivable lands infertile. Phytoremediation involving the synergism of legume and rhizobia provides a useful technique in tackling this issue with cost-effective, environmentally friendly, and easy-to-use features under adverse soil conditions. Leucaena leucocephala has been found to build symbiotic relationships with native rhizobia in the iron-vanadium-titanium oxide (V-Ti magnetite) mine tailing soil. Rhizobia YH1, isolated from the root nodules of L. leucocephala, was classified as Sinorhizobium saheli according to similarity and phylogenetic analyses of 16S rRNA, housekeeping and nitrogen fixation genes. Besides nitrogen fixation, S. saheli YH1 also showed capabilities to produce indole-acetic acid (IAA) (166.77 ± 2.03 mg l⁻¹) and solubilize phosphate (104.41 ± 7.48 mg l⁻¹). Pot culture experiments showed that strain YH1 increased the biomass, plant height and root length of L. leucocephala by 67.2, 39.5 and 27.2% respectively. There was also an average increase in plant N (10.0%), P (112.2%) and K (25.0%) contents compared to inoculation-free control. The inoculation of YH1 not only reduced the uptake of all metals by L. leucocephala in the mine tailings, but also resulted in decreased uptake of Cd by up to 79.9% and Mn by up to 67.6% for plants grown in soils contaminated with Cd/Mn. It was concluded that S. saheli YH1 possessed multiple beneficial effects on L. leucocephala grown in metalliferous soils. Our findings highlight the role of S. saheli YH1 in improving plant health of L. leucocephala by reducing metal uptake by plants grown in heavy metal-polluted soils. We also suggest the idea of using L. leucocephala-S. saheli association for phytoremediation and revegetation of V-Ti mine tailings and soils polluted with Cd or Mn.

Dracaena sanderiana endophytic bacteria interactions: Effect of endophyte inoculation on bisphenol A removal

Bisphenol A (BPA) is one of the most abundant endocrine-disrupting compounds which is found in the aquatic environment. However, actual knowledge regarding the effect of plant-bacteria interactions on enhancing BPA removal is still lacking. In the present study, Dracaena sanderiana endophytic bacteria interactions were investigated to evaluate the effect of bacterial inoculation on BPA removal under hydroponic conditions. Two plant growth-promoting (PGP) bacterial strains, Bacillus thuringiensis and Pantoea dispersa, which have high BPA tolerance and can utilize BPA for growth, were used as plant inocula. P. dispersa-inoculated plants showed the highest BPA removal efficiency at 92.32 ± 1.23% compared to other inoculated and non-inoculated plants. This was due to a higher population of the endophytic inoculum within the plant tissues which resulted in maintained levels of indole-3-acetic acid (IAA) for the plant's physiological needs and lower levels of reactive oxygen species (ROS). In contrast, B. thuringiensis-inoculated plants had a lower BPA removal efficiency. However, individual B. thuringiensis possessed a significantly higher BPA removal efficiency compared to P. dispersa. This study provides convincing evidence that not all PGP endophytic bacteria-plant interactions could improve the BPA removal efficiency. Different inocula and inoculation times should be investigated before using plant inoculation to enhance phytoremediation.

Diversity and Applications of Endophytic Actinobacteria of Plants in Special and Other Ecological Niches

Actinobacteria are wide spread in nature and represent the largest taxonomic group within the domain Bacteria. They are abundant in soil and have been extensively explored for their therapeutic applications. This versatile group of bacteria has adapted to diverse ecological habitats, which has drawn considerable attention of the scientific community in recent times as it has opened up new possibilities for novel metabolites that may help in solving some of the most challenging problems of the day, for example, novel drugs for drug-resistant human pathogens, affordable means to maintain ecological balance in various habitats, and alternative practices for sustainable agriculture. Traditionally, free dwelling soil actinobacteria have been the subject of intensive research. Of late, symbiotic actinobacteria residing as endophytes within the plant tissues have generated immense interest as potential source of novel compounds, which may find applications in medicine, agriculture, and environment. In the light of these possibilities, this review focuses on the diversity of endophytic actinobacteria isolated from the plants of extreme habitats and specific ecological niches. Furthermore, an attempt has been made to assign chemical class to the compounds obtained from endophytic actinobacteria. Potential therapeutic applications of these compounds and the utility of endophytic actinobacteria in agriculture and environment are discussed.

Geochemical fractions and phytoavailability of Zinc in a contaminated calcareous soil affected by biotic and abiotic amendments

Many studies have conducted to determine the best management practice to reduce the mobility and phytoavailability of the trace metals in contaminated soils. In this study, geochemical speciation and phytoavailability of Zn for sunflower were studied after application of nanoparticles (SiO₂ and zeolite, with an application rate of 200 mg kg^-1) and bacteria [Bacillus safensis FO-036b(T) and Pseudomonas fluorescens p.f.169] to a calcareous heavily contaminated soil. Results showed that the biotic and abiotic treatments significantly reduced the Zn concentration in the aboveground to non-toxicity levels compared to the control treatment, and the nanoparticle treatments were more effective than the bacteria and control treatments. The concentration of CaCl₂-extractable Zn in the treated soils was significantly lower than those of the control treatment. The results of sequential extraction showed that the maximum portion of total Zn belonged to the fraction associated with iron and manganese oxides. On the contrary, the minimum percent belonged to the exchangeable and water-soluble Zn (F₁). From the environmental point of view, the fraction associated with iron and manganese oxides is less bioavailable than the F₁ and carbonated fractions. On the basis of plant growth promotion, simultaneous application of the biotic and abiotic treatments significantly increased the aboveground dry biomass yield and also significantly reduced the CaCl₂-extractable form, uptake by aboveground and translocation factor of Zn compared to the control treatment. Therefore, it might be suggested as an efficient strategy to promote the plant growth and reduce the mobile and available forms of toxic metals in calcareous heavily contaminated soils.

Increased biomass and reduced rapeseed Cd accumulation of oilseed rape in the presence of Cd-immobilizing and polyamine-producing bacteria

Two Cd-immobilizing and polyamine-producing bacteria Serratia liquefaciens CL-1 and Bacillus thuringiensis X30 were characterized for their effects on Cd immobilization, pH, and polyamine production in the solution and the rapeseed biomass and Cd uptake of Brassica napus Qinyou-10 in Cd-contaminated soil. These strains significantly increased pH and reduced water-soluble Cd concentration (25-76%) compared to the controls. Furthermore, strain CL-1 produced more polyamine (71-192%) in the solution than strain X30. Cell surface absorbed Cd content was increased by 23-56% in the presence of strain CL-1 compared to strain X30. The strains significantly increased the rapeseed biomass (12-32%), pH, polyamine content (70-244%), and relative abundance (21-49%) of arginine decarboxylase-producing bacteria (ADPB) of the rhizosphere soils but decreased DTPA-extractable Cd content and rapeseed Cd uptake compared to the controls. Notably, strain CL-1 had higher ability to reduce the rapeseed Cd and DTPA-extractable Cd contents and increase the abundance of ADPB than strain X30. Our results showed the distinct impact of these strains on the rapeseed Cd uptake and available Cd content and suggested that these strains reduced the available Cd and rapeseed Cd uptake by increasing the cell adsorption of Cd, abundance of ADPB, polyamine production, and pH in the rhizosphere soils.

Bacterial communities in mining soils and surrounding areas under regeneration process in a former ore mine

Human activities on the Earth's surface change the landscape of natural ecosystems. Mining practices are one of the most severe human activities, drastically altering the chemical, physical and biological properties of the soil environment. Bacterial communities in soil play an important role in the maintenance of ecological relationships. This work shows bacterial diversity, metabolic repertoire and physiological behavior in five ecosystems samples with different levels of impact. These ecosystems belong to a historical area in Iron Quadrangle, Minas Gerais, Brazil, which suffered mining activities until its total depletion without recovery since today. The results revealed Proteobacteria as the most predominant phylum followed by Acidobacteria, Verrucomicrobia, Planctomycetes, and Bacteroidetes. Soils that have not undergone anthropological actions exhibit an increase ability to degrade carbon sources. The richest soil with the high diversity was found in ecosystems that have suffered anthropogenic action. Our study shows profile of diversity inferring metabolic profile, which may elucidate the mechanisms underlying changes in community structure in situ mining sites in Brazil. Our data comes from contributing to know the bacterial diversity, relationship between these bacteria and can explore strategies for natural bioremediation in mining areas or adjacent areas under regeneration process in iron mining areas.

Microbe and plant assisted-remediation of organic xenobiotics and its enhancement by genetically modified organisms and recombinant technology: A review

Environmental problems such as the deterioration of groundwater quality, soil degradation and various threats to human, animal and ecosystem health are closely related to the presence of high concentrations of organic xenobiotics in the environment. Employing appropriate technologies to remediate contaminated soils is crucial due to the site-specificity of most remediation methods. The limitations of conventional remediation technologies include poor environmental compatibility, high cost of implementation and poor public acceptability. This raises the call to employ biological methods for remediation. Bioremediation and microbe-assisted bioremediation (phytoremediation) offer many ecological and cost-associated benefits. The overall efficiency and performance of bio- and phytoremediation approaches can be enhanced by genetically modified microbes and plants. Moreover, phytoremediation can also be stimulated by suitable plant-microbe partnerships, i.e. plant-endophytic or plant-rhizospheric associations. Synergistic interactions between recombinant bacteria and genetically modified plants can further enhance the restoration of environments impacted by organic pollutants. Nevertheless, releasing genetically modified microbes and plants into the environment does pose potential risks. These can be minimized by adopting environmental biotechnological techniques and guidelines provided by environmental protection agencies and other regulatory frameworks. The current contribution provides a comprehensive overview on enhanced bioremediation and phytoremediation approaches using transgenic plants and microbes. It also sheds light on the mitigation of associated environmental risks.

Effect of Phosphate-Solubilizing Bacteria on the Mobility of Insoluble Cadmium and Metabolic Analysis

Phosphate-solubilizing bacteria (PSB) can promote plant growth by dissolving insoluble phosphate. Therefore, PSB may have the potential to improve the mobility of heavy metals in soils and enhance phytoextraction. This study isolated a few PSB strains that could dissolve CdCO₃ and solid Cd in soil. Two typical PSB, namely, high- and low-Cd-mobilizing PSB (Pseudomonas fluorescens gim-3 and Bacillus cereus qh-35, respectively), were selected to analyze the metabolic profiles, metabolic pathways, and mechanisms of mobilization of insoluble Cd. A total of 34 metabolites secreted by the two PSB strains were identified. Gluconic acid was the main contributor to Cd dissolution (42.4%) in high-Cd-mobilizing PSB. By contrast, gluconic acid was not secreted in low-Cd-mobilizing PSB. Metabolic pathway analysis showed that gluconic acid was produced by the peripheral direct oxidation pathway. Hence, PSB with peripheral direct oxidation pathway were likely to have high-Cd-mobilizing capacity.

Enhancement of toxic Cr (VI), Fe, and other heavy metals phytoremediation by the synergistic combination of native Bacillus cereus strain and Vetiveria zizanioides L

Bioremediation of Cr (VI), Fe, and other heavy metals (HMs) through plant-microbes interaction is one of the efficient strategies due to its high efficiency, low cost, and ecofriendly nature. The aim of the study was to isolate, characterize, and assess the potential of rhizospheric bacteria to enhance growth and metal accumulation by the chromium hyperaccumulator Vetiveria zizanoides. The bacterial strain isolated from mine tailings was identified to be Bacillus cereus (T1B3) strain exhibited plant growth-promoting traits including, 1-aminocyclopropane-1-carboxylate deaminase, indole acetic acid, and siderophores production, nitrogen fixation, and P solubilization. Removal capacity (mg L^-1) of T1B3 strain was 82% for Cr⁺⁶ (100), 92% for Fe (100), 67% for Mn(50), 36% for Zn (50), 31% for Cd (30), 25% for Cu (30), and 43% for Ni (50) during the active growth cycle in HM-amended, extract medium. Results indicate that inoculating the native V. zizanioides with T1B3 strain improves its phytoremediation efficiency of HMs. The mineralogical characteristics of chromite ore tailings and soil were also confirmed by X-ray diffraction, Fourier Transform Infrared, scanning electron microscope-energy dispersive spectroscopy analysis.

Impacts of environmental factors on the whole microbial communities in the rhizosphere of a metal-tolerant plant: Elsholtzia haichowensis Sun

Rhizospheric microbes play important roles in plant growth and heavy metals (HMs) transformation, possessing great potential for the successful phytoremediation of environmental pollutants. In the present study, the rhizosphere of Elsholtzia haichowensis Sun was comprehensively studied to uncover the influence of environmental factors (EFs) on the whole microbial communities including bacteria, fungi and archaea, via quantitative polymerase chain reaction (qPCR) and high-throughput sequencing. By analyzing molecular ecological network and multivariate regression trees (MRT), we evaluated the distinct impacts of 37 EFs on soil microbial community. Of them, soil pH, HMs, soil texture and nitrogen were identified as the most influencing factors, and their roles varied across different domains. Soil pH was the main environmental variable on archaeal and bacterial community but not fungi, explaining 25.7%, 46.5% and 40.7% variation of bacterial taxonomic composition, archaeal taxonomic composition and a-diversity, respectively. HMs showed important roles in driving the whole microbial community and explained the major variation in different domains. Nitrogen (NH₄-N, NO₃-N, NO₂-N and TN) explained 47.3% variation of microbial population composition and 15.9% of archaeal taxonomic composition, demonstrating its influence in structuring the rhizospheric microbiome, particularly archaeal and bacterial community. Soil texture accounted for 10.2% variation of population composition, 28.9% of fungal taxonomic composition, 19.2% of fungal a-diversity and 7.8% of archaeal a-diversity. Rhizosphere only showed strong impacts on fungi and bacteria, accounting for 14.7% and 4.9% variation of fungal taxonomic composition and bacterial a-diversity. Spatial distance had stronger influence on bacteria and archaea than fungi, but not as significant as other EFs. For the first time, our study provides a complete insight into key influential EFs on rhizospheric microbes and how their roles vary across microbial domains, giving a hand for understanding the construction of microbial communities in rhizosphere.

Production of bioproducts by endophytic fungi: chemical ecology, biotechnological applications, bottlenecks, and solutions

Endophytes are microorganisms that colonize the interior of host plants without causing apparent disease. They have been widely studied for their ability to modulate relationships between plants and biotic/abiotic stresses, often producing valuable secondary metabolites that can affect host physiology. Owing to the advantages of microbial fermentation over plant/cell cultivation and chemical synthesis, endophytic fungi have received significant attention as a mean for secondary metabolite production. This article summarizes currently reported results on plant-endophyte interaction hypotheses and highlights the biotechnological applications of endophytic fungi and their metabolites in agriculture, environment, biomedicine, energy, and biocatalysts. Current bottlenecks in industrial development and commercial applications as well as possible solutions are also discussed.

Complete Genome Sequence of Cd(II)-Resistant Arthrobacter sp. PGP41, a Plant Growth-Promoting Bacterium with Potential in Microbe-Assisted Phytoremediation

Microbe-assisted phytoremediation has great potential for practical applications. Plant growth-promoting bacteria (PGPB) with heavy metal (HM) resistance are important for the implementation of PGPB-assisted phytoremediation of HM-contaminated environments. Arthrobacter sp. PGP41 is a Cd(II)-resistant bacterium isolated from the rhizosphere soils of a Cd(II) hyperaccumulator plant, Solanum nigrum. Strain PGP41 can significantly improve plant seedling and root growth under Cd(II) stress conditions. This bacterium exhibited the ability to produce high levels of indole-3-acetic acid (IAA), as well as the ability to fix nitrogen and solubilize phosphate, and it possessed 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. Here, we present the complete genome sequence of strain PGP41. The genome consists of a single chromosome with a G+C content of 65.38% and no plasmids. The genome encodes 3898 genes and contains 49 tRNA and 12 rRNA genes. Multiple genes associated with plant growth promotion were identified in the genome. The whole genome sequence of PGP41 provides information useful for further clarifying the molecular mechanisms behind plant growth promotion by PGPB and facilitates its potential use as an inoculum in the bioremediation of HM-contaminated environments.

Root bacterial endophytes confer drought resistance and enhance expression and activity of a vacuolar H+ -pumping pyrophosphatase in pepper plants

It has been previously shown that the transgenic overexpression of the plant root vacuolar proton pumps H+ -ATPase (V-ATPase) and H+ -PPase (V-PPase) confer tolerance to drought. Since plant-root endophytic bacteria can also promote drought tolerance, we hypothesize that such promotion can be associated to the enhancement of the host vacuolar proton pumps expression and activity. To test this hypothesis, we selected two endophytic bacteria endowed with an array of in vitro plant growth promoting traits. Their genome sequences confirmed the presence of traits previously shown to confer drought resistance to plants, such as the synthesis of nitric oxide and of organic volatile organic compounds. We used the two strains on pepper (Capsicuum annuum L.) because of its high sensitivity to drought. Under drought conditions, both strains stimulated a larger root system and enhanced the leaves' photosynthetic activity. By testing the expression and activity of the vacuolar proton pumps, H+ -ATPase (V-ATPase) and H+ -PPase (V-PPase), we found that bacterial colonization enhanced V-PPase only. We conclude that the enhanced expression and activity of V-PPase can be favoured by the colonization of drought-tolerance-inducing bacterial endophytes.

Remediation of arsenic in mung bean (Vigna radiata) with growth enhancement by unique arsenic-resistant bacterium Acinetobacter lwoffii

Arsenic, a carcinogenic and toxic contaminant of soil and water, affects human health adversely. During last few decades, it has been an important global environmental issue. Among several arsenic detoxification methods remediation using arsenic resistant microbes is proved to be environment-friendly and cost-effective. This study aimed to test the effects of arsenic utilizing bacterial strain Acinetobacter lwoffii (RJB-2) on arsenic uptake and growth of mung bean plants (Vigna radiata). RJB-2 exhibited tolerance up to 125mM of arsenic (V) and 50mM of arsenic (III). RJB-2 produced plant growth promoting substances e.g. indole acetic acid (IAA), siderophores, exopolysaccharide (EPS) and phosphate solubilization in the absence and in presence of arsenic. Pot experiments were used to scrutinize the role of RJB-2 on arsenic uptake and growth of mung bean plants grown in soil amended with 22.5mgkg^-1 of sodium arsenate (Na₂HAsO₄·7H₂O). RJB-2 could arrest arsenic uptake in just 7days and increase plant growth, number of plants per pot, chlorophyll and carotenoid content of the mung bean plants. RJB-2 formed biofilm and its root-association helped to abate arsenic uptake in mung bean. Confocal and light microscopic studies also revealed the abatement of arsenic uptake and increase in chlorophyll content in mung bean plants in presence of RJB-2. RJB-2 was also responsible for less production of reactive oxygen species (ROS) in mung bean plants reducing the oxidative damage caused by arsenic. The lower percentage of electrolytic leakage (EL) in RJB-2 inoculated mung bean plants proved arsenic abatement. The study also reported the distribution of arsenic in various parts of mung bean plant. RJB-2 owing to its intrinsic abilities of plant growth promotion even in presence of high concentrations of arsenic could inhibit arsenic uptake completely and therefore it could be used in large-scale cultivation for phytostabilization of plants.

Role of Burkholderia cepacia CS8 in Cd-stress alleviation and phytoremediation by Catharanthus roseus

The current study was performed to assess the effect of Burkholderia cepacia CS8 on the phytoremediation of cadmium (Cd) by Catharanthus roseus grown in Cd-contaminated soil. The plants cultivated in Cd amended soil showed reduced growth, dry mass, gas-exchange capacity, and chlorophyll contents. Furthermore, the plants exhibited elevated levels of malondialdehyde (MDA) and hydrogen peroxide (H2O2) under Cd stress. The bacterized plants showed higher shoot length, root length; fresh and dry weight. The improved stress tolerance in inoculated plants was attributed to the reduced quantity of MDA and H2O2, enhanced synthesis of protein, proline, phenols, flavonoids, and improved activity of antioxidant enzymes including peroxidase, superoxide dismutase, ascorbate peroxidase, and catalase. Similarly, the 1-aminocyclopropane-1-carboxylate deaminase activity, phosphate solubilization, auxin, and siderophore production capability of B. cepacia CS8 improved growth and stress alleviation in treated plants. The bacterial inoculation enhanced the amount of water extractable Cd from soil. Furthermore, the inoculated plants showed higher bioconcentration factor and translocation factor. The current study exhibits that B. cepacia CS8 improves stress alleviation and phytoextraction potential of C. roseus plants growing under Cd stress.

Degradation of polycyclic aromatic hydrocarbons in soil mesocosms by microbial/plant bioaugmentation: Performance and mechanism

In order to study the degradation of polycyclic aromatic hydrocarbons (PAHs) in an aged and highly contaminated soil, four bioremediation strategies (indigenous microorganisms, microbial bioaugmentation with a PAH-degrading and bioemulsifier-producing strain, Rhodococcus ruber Em1, plant bioaugmentation with Orychophragmus violaceus and their combination) were compared and the enhanced degradation mechanism was investigated in soil mesocosms. Degradation rates over a period of 175 days showed that Em1 combined with Orychophragmus violaceus promoted a significant enhancement of PAHs degradation. In inoculated microcosms with Rhodococcus ruberEm1, mineralization reached a lower level in the absence than in the presence of plants. Elimination of PAHs was significantly enhanced (increased by 54.45%) in the bioaugmented mesocosms. Quantitative PCR indicated that copy numbers of linA and RHD-like gene (encoding PAH-ring hydroxylating dioxygenase) in the mesocosm with plant were three and five times higher than those in the mesocosm without plant, respectively. Transcript copy numbers of RHD-like gene and 16S rRNA gene of strain Em1 in mesocosm with plant were two and four times higher than those in the mesocosm without plant, respectively. Taken together, the results of this study show that plants or Rhodococcus ruber Em1 enhance total PAHs removal, moreover their effects are necessarily cumulative by combined strains and plants.

Distinct Communities of Poplar Endophytes on an Unpolluted and a Risk Element-Polluted Site and Their Plant Growth-Promoting Potential In Vitro

Numerous studies demonstrated that endophytic microbes can promote plant growth and increase plant stress resistance. We aimed at isolating poplar endophytes able to increase their hosts' fitness both in nutrient-limited and polluted environments. To achieve this goal, endophytic bacteria and fungi were isolated from roots and leaves of hybrid poplars (Populus nigra × P. maximowiczii clone Max-4) on an unpolluted and a risk element-polluted site in the Czech Republic and subsequently screened by a number of in vitro tests. Bacterial communities at the unpolluted site were dominated by Gammaproteobacteria with Pseudomonas sp. as the prominent member of the class, followed by Bacilli with prevailing Bacillus sp., whereas Alphaproteobacteria, mostly Rhizobium sp., prevailed at the polluted site. The fungal endophytic community was dominated by Ascomycetes and highly distinct on both sites. Dothideomycetes, mostly Cladosporium, prevailed at the non-polluted site while unclassified Sordariomycetous fungi dominated at the polluted site. Species diversity of endophytes was higher at the unpolluted site. Many tested endophytic strains solubilized phosphate and produced siderophores, phytohormones, and antioxidants. Some strains also exhibited ACC-deaminase activity. Selected bacteria showed high tolerance and the ability to accumulate risk elements, making them promising candidates for use in inocula promoting biomass production and phytoremediation. Graphical Abstract ᅟ.

Effects of exogenously applied salicylic acid and putrescine alone and in combination with rhizobacteria on the phytoremediation of heavy metals and chickpea growth in sandy soil

The present attempt was made to study the role of exogenously applied salicylic acid (SA) and putrescine (Put) on the phytoremediation of heavy metals and on the growth parameters of chickpea grown in sandy soil. The SA and Put were applied alone as well as in combination with plant growth promoting rhizobacteria (PGPR). The PGPRs, isolated from the rhizosphere of chickpea, were characterized on the basis of colony morphology and biochemical traits through gram staining, catalase and oxidase tests, and identified by 16S-rRNA gene sequencing as Bacillus subtilis, Bacillus thuringiensis and Bacillus megaterium. The chickpea seeds were soaked in 24 h old fresh cultures of isolates for 2-3 h prior to sowing. The growth regulators (PGRs), SA and Put (150 mg/L), were applied to the seedlings as foliar spray at three-leaf stage. The result revealed that plants treated with SA and Put alone or in combination with PGPRs, significantly enhanced the accumulation of heavy metals in plant shoot. PGPR induces Ni accumulation in sensitive variety and Pb in both the varieties, the PGR in combination augment the bioremediation effects of PGPR and both sensitive and tolerant variety showed significant accumulation of Ni, Cd, and Pb. SA was more effective in accumulating Ni and Cd whereas, significant accumulation of Pb was recorded in Put. PGPRs further augmented the PGRs induced accumulation of heavy metals and macronutrients in chickpea shoot and in rhizosphere. SA increased the proline content of tolerant variety while decreasing the lipid peroxidation and proline content of the sensitive variety but decreased the stimulating effect of PGPR in proline production. Interactive effects of PGPR and PGRs are recommended for inducing phytoremediation in chickpea plants under drought stress.

Heavy Metal Stress, Signaling, and Tolerance Due to Plant-Associated Microbes: An Overview

Several anthropogenic activities including mining, modern agricultural practices, and industrialization have long-term detrimental effect on our environment. All these factors lead to increase in heavy metal concentration in soil, water, and air. Soil contamination with heavy metals cause several environmental problems and imparts toxic effect on plant as well as animals. In response to these adverse conditions, plants evolve complex molecular and physiological mechanisms for better adaptability, tolerance, and survival. Nowadays conventional breeding and transgenic technology are being used for development of metal stress resistant varieties which, however, are time consuming and labor intensive. Interestingly the use of microbes as an alternate technology for improving metal tolerance of plants is gaining momentum recently. The use of these beneficial microorganisms is considered as one of the most promising methods for safe crop-management practices. Interaction of plants with soil microorganisms can play a vital role in acclimatizing plants to metalliferous environments, and can thus be explored to improve microbe-assisted metal tolerance. Plant-associated microbes decrease metal accumulation in plant tissues and also help to reduce metal bioavailability in soil through various mechanisms. Nowadays, a novel phytobacterial strategy, i.e., genetically transformed bacteria has been used to increase remediation of heavy metals and stress tolerance in plants. This review takes into account our current state of knowledge of the harmful effects of heavy metal stress, the signaling responses to metal stress, and the role of plant-associated microbes in metal stress tolerance. The review also highlights the challenges and opportunities in this continued area of research on plant-microbe-metal interaction.

Co-inoculation of Lolium perenne with Funneliformis mosseae and the dark septate endophyte Cadophora sp. in a trace element-polluted soil

The presence of dark septate endophytes (DSEs) or arbuscular mycorrhizal fungi (AMF) in plant roots and their effects on plant fitness have been extensively described. However, little is known about their interactions when they are simultaneously colonizing a plant root, especially in trace element (TE)-polluted soils. We therefore investigated the effects of Cadophora sp. and Funneliformis mosseae on ryegrass (Lolium perenne) growth and element uptake in a Cd/Zn/Pb-polluted soil. The experiment included four treatments, i.e., inoculation with Cadophora sp., inoculation with F. mosseae, co-inoculation with Cadophora sp. and F. mosseae, and no inoculation. Ryegrass biomass and shoot Na, P, K, and Mg concentrations significantly increased following AMF inoculation as compared to non-inoculated controls. Similarly, DSE inoculation increased shoot Na concentration, whereas dual inoculation significantly decreased shoot Cd concentration. Moreover, oxidative stress determined by ryegrass leaf malondialdehyde concentration was alleviated both in the AMF and dual inoculation treatments. We used quantitative PCR and microscope observations to quantify colonization rates. They demonstrated that DSEs had no effect on AMF colonization, while AMF colonization slightly decreased DSE frequency. We also monitored fluorescein diacetate (FDA) hydrolysis and alkaline phosphatase (AP) activity in the rhizosphere soils. FDA hydrolysis remained unchanged in the three inoculated treatments, but AMF colonization increased AP activity and P mobility in the soil whereas DSE colonization did not alter AP activity. In this experiment, we unveiled the interactions between two ecologically important fungal groups likely to occur in roots which involved a decrease of oxidative stress and Cd accumulation in shoots. These results open promising perspectives on the fungal-based phytomanagement of TE-contaminated sites by the production of uncontaminated and marketable plant biomass.

Recent advances in conventional and contemporary methods for remediation of heavy metal-contaminated soils

Remediation of heavy metal-contaminated soils has been drawing our attention toward it for quite some time now and a need for developing new methods toward reclamation has come up as the need of the hour. Conventional methods of heavy metal-contaminated soil remediation have been in use for decades and have shown great results, but they have their own setbacks. The chemical and physical techniques when used singularly generally generate by-products (toxic sludge or pollutants) and are not cost-effective, while the biological process is very slow and time-consuming. Hence to overcome them, an amalgamation of two or more techniques is being used. In view of the facts, new methods of biosorption, nanoremediation as well as microbial fuel cell techniques have been developed, which utilize the metabolic activities of microorganisms for bioremediation purpose. These are cost-effective and efficient methods of remediation, which are now becoming an integral part of all environmental and bioresource technology. In this contribution, we have highlighted various augmentations in physical, chemical, and biological methods for the remediation of heavy metal-contaminated soils, weighing up their pros and cons. Further, we have discussed the amalgamation of the above techniques such as physiochemical and physiobiological methods with recent literature for the removal of heavy metals from the contaminated soils. These combinations have showed synergetic effects with a many fold increase in removal efficiency of heavy metals along with economic feasibility.

From phytoremediation of soil contaminants to phytomanagement of ecosystem services in metal contaminated sites

Since the emergence of phytoremediation, much research has focused on its development for (i) the removal of metals from soil and/or (ii) the reduction of metal bioavailability, mobility, and ecotoxicity in soil. Here, we review the lights and shades of the two main strategies (i.e., phytoextraction and phytostabilization) currently used for the phytoremediation of metal contaminated soils, irrespective of the level of such contamination. Both strategies face limitations to become successful at commercial scale and, then, often generate skepticism regarding their usefulness. Recent innovative approaches and paradigms are gradually establishing these phytoremediation strategies as suitable options for the management of metal contaminated soils. The combination of these phytotechnologies with a sustainable and profitable site use (a strategy called phytomanagement) grants value to the many benefits that can be obtained during the phytoremediation of metal contaminated sites, such as, for instance, the restoration of important ecosystem services, e.g. nutrient cycling, carbon storage, water flow regulation, erosion control, water purification, fertility maintenance, etc.

Potential applications of Pseudomonas sp. (strain CPSB21) to ameliorate Cr6+ stress and phytoremediation of tannery effluent contaminated agricultural soils

Contamination of agricultural soil with heavy metals has become a serious concern worldwide. In the present study, Cr⁶⁺ resistant plant growth promoting Pseudomonas sp. (strain CPSB21) was isolated from the tannery effluent contaminated agricultural soils and evaluated for the plant growth promoting activities, oxidative stress tolerance, and Cr⁶⁺ bioremediation. Assessment of different plant growth promotion traits, such as phosphate solubilization, indole-3-acetic acid production, siderophores, ammonia and hydrogen cyanide production, revealed that the strain CPSB21 served as an efficient plant growth promoter under laboratory conditions. A pot experiment was performed using sunflower (Helianthus annuus L.) and tomato (Solanum lycopersicum L.) as a test crop. Cr⁶⁺ toxicity reduced plant growth, pigment content, N and P uptake, and Fe accumulation. However, inoculation of strain CPSB21 alleviated the Cr⁶⁺ toxicity and enhanced the plant growth parameters and nutrient uptake. Moreover, Cr toxicity had varied response on oxidative stress tolerance at graded Cr⁶⁺ concentration on both plants. An increase in superoxide dismutase (SOD) and catalase (CAT) activity and reduction in malonialdehyde (MDA) was observed on inoculation of strain CPSB21. Additionally, inoculation of CPSB21 enhanced the uptake of Cr⁶⁺ in sunflower plant, while no substantial enhancement was observed on inoculation in tomato plant.

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.

Effect of bacterial inoculants on phytomining of metals from waste incineration bottom ash

Waste incineration bottom ash is considered a secondary resource for valuable trace elements (TE), which is currently neglected in most European countries. Phytomining could potentially recover valuable TE from such waste materials but is still at an exploratory stage with many challenges. The use of bioaugmentation to improve plant growth and TE accumulation of metal-tolerant high biomass plants growing on waste incineration bottom ash was evaluated. Bacterial strains that were previously isolated from rhizosphere, roots and contaminated soil were selected according to their plant growth promoting characteristics and tolerance to the bottom ash substrate. Those selected bacterial strains were tested for their beneficial effects on Nicotiana tabacum and Salix smithiana with regards to phytomining. The rhizobacterial strain Rhodococcus erythropolis P30 enhanced the shoot dry weight of N. tabacum by on average 57% compared to the control plants. Several bacterial inoculants enhanced biomass production and the nutritional status of S. smithiana. Moreover, those bacterial strains previously described to enhance biomass production of N. tabacum and members of the Salicaceae on TE-contaminated soils, also enhanced biomass production of these species on bottom ash. However, bacterial inoculants could not enhance trace element accumulation in plants.

Increased biomass and quality and reduced heavy metal accumulation of edible tissues of vegetables in the presence of Cd-tolerant and immobilizing Bacillus megaterium H3

A Cd-resistant and immobilizing Bacillus megaterium H3 was characterized for its impact on the biomass and quality and heavy metal uptake of edible tissues of two vegetables (Brassica campestris L. var. Aijiaohuang and Brassica rapa L. var. Shanghaiqing) grown in heavy metal-polluted soil. The impact of strain H3 on the soil quality was also evaluated. The increase in the edible tissue biomass and the contents of soluble proteins and vitamin C of the vegetables inoculated with strain H3 ranged from 18% to 33%, 17% to 31%, and 15% to 19%, respectively, compared with the controls. Strain H3 significantly decreased the edible tissue Cd and Pb contents of the two greens (41-80%), DTPA-extractable Cd content (35-47%) of the rhizosphere soils, and Cd and Pb translocation factors (25-56%) of the greens compared with the controls. Moreover, strain H3 significantly increased the organic matter content (17-21%) and invertase activity (13-14%) of the rhizosphere soils compared with the controls. Our results demonstrated the increased edible tissue biomass and quality, decreased Cd and Pb uptake of the edible tissues, and improved soil quality in the presence of strain H3. The results also suggested an effective bacterial-enhanced technique for decreased metal uptake of greens and improved vegetable and soil qualities in the metal-contaminated soils.

Modeling of Cd adsorption to goethite-bacteria composites

The accurate modeling of heavy metal adsorption in complex systems is fundamental for risk assessments in soils and associated environments. Bacteria-iron (hydr)oxide associations in soils and sediments play a critical role in heavy metal immobilization. The reduced adsorption of heavy metals on these composites have been widely reported using the component additivity (CA) method. However, there is a lack of a mechanism model to account for these deviations. In this study, we established models for Cd adsorption on goethite-Pseudomonas putida composites at 1:1 and 5:1 mass ratios. Cadmium adsorption on the 5:1 composite was consistent with the additivity method. However, the CA method over predicted Cd adsorption by approximately 8% on the 1:1 composite at high Cd concentration. The deviation was corrected by adding the site blockage reactions between P. putida and goethite. Both CA and "CA-site masking" models for Cd adsorption onto the composites were in line with the ITC data. These results indicate that CA method in simulating Cd adsorption on bacteria-iron oxides composites is limited to low bacterial and Cd concentrations. Therefore the interfacial complexation reactions that occur between iron (hydr)oxides and bacteria should be taken into account when high concentrations of bacteria and heavy metals are present.

Effect of rhizobacteria on arsenic uptake by macrophyte Eichhornia crassipes (Mart.) Solms

Wastewater flowing in streams and nallahs across India carries several trace metals, including metalloid arsenic (As), which are considered serious environmental contaminants due to their toxicity, and recalcitrant nature. In this study, we determined the phytoremediation of As by Eichhornia crassipes (Mart.) Solms either alone or in association with plant growth-promoting rhizobacteria. Pseudomonas and Azotobacter inoculation to E. crassipes resulted in enhanced As removal compared to uninoculated control. Co-inoculation with a consortium of Pseudomonas, Azotobacter, Azospirillum, Actinomyces, and Bacillus resulted in a higher As (p < 0.05) phytoaccumulation efficiency. P. aeruginosa strain jogii was found particularly effective in augmenting As removal by E. crassipes. Our findings indicate that the synergistic association of E. crassipes and various rhizobacteria is an effective strategy to enhance removal of As and thus may be utilized as an efficient biological alternative for the removal of this metalloid from wastewaters.

Titania (TiO2) nanoparticles enhance the performance of growth-promoting rhizobacteria

A novel use of nanotitania (TNs) as agents in the nanointerface interaction between plants and colonization of growth promoting rhizobacteria (PGPR) is presented. The effectiveness of PGPRs is related to the effectiveness of the technology used for their formulation. TNs produced by the Captigel patented SolGel approach, characterized by the transmission and scanning electron microscopy were used for formulation of the harsh environment PGPR strains. Changes in the biomass of wheat seedlings and in the density of single and double inoculants with and without TNs were monitored during two weeks of stress induced by drought salt and by the pathogen Fusarium culmorum. We show that double inoculants with TNs can attach stably to plant roots. Regression analysis indicates that there is a positive interaction between seedling biomass and TN-treated second inoculant colonization. We conclude that TN treatment provides an effectual platform for PGPR rational application via design of root microbial community. Our studies illustrate the importance of considering natural soil nanoparticles for PGPR application and thereby may explain the generally observed inconsistent behavior of PGPRs in the field. These new advancements importantly contribute towards solving food security issues in changing climates. The model systems established here provide a basis for new PGPR nanomaterials research.

Isolation of vanadium-resistance endophytic bacterium PRE01 from Pteris vittata in stone coal smelting district and characterization for potential use in phytoremediation

This study investigates the V-resistant endophytic bacteria isolated from V-accumulator Pteris vittata grown on stone coal smelting district. Among all the ten isolates, the strain PRE01 identified as Serratia marcescens ss marcescens by Biolog GEN III MicroPlate™ was screened out by ranking first in terms of heavy metal resistance and plant growth promoting traits. The S. marcescens PRE01 had strong V, Cr and Cd resistance especially for V up to 1500mg/L. In addition, it exhibited ACC deaminase activity, siderophore production and high indoleacetic acid production (60.14mg/L) and solubilizing P potential (336.41mg/L). For heavy metal detoxification tests, PRE01 could specifically assimilate 97.6%, 21.7% and 6.6% of Cd(II), Cr(VI) and V(V) within 72h incubation. Despite the poor absorption of the two anions, most V(V) and Cr(VI) were detoxified and reduced to lower valence states by the strain. Furthermore, the isolate had the potential to facilitate the metals uptake of their hosts by changing heavy metal speciation. Our research may open up further scope of utilizing the endophyte for enhancing phytoextraction of vanadium industry contaminated soils.

Root-induced changes of Zn and Pb dynamics in the rhizosphere of sunflower with different plant growth promoting treatments in a heavily contaminated soil

Root induced changes are deemed to have an important role in the success of remediation techniques in contaminated soils. Here, the effects of two nano-particles [SiO₂ and zeolite] with an application rate of 200mgkg^-1, and two bacteria [Bacillus safensis FO-036b(T) and Pseudomonas fluorescens p.f.169] in the rhizosphere of sunflower on Zn and Pb dynamics were studied in greenhouse conditions. The treatments reduced the exchangeable Zn (from 13.68% to 30.82%) and Pb (from 10.34% to 25.92%) in the rhizosphere compared to the control. The EC and microbial respiration/population of the rhizosphere and bulk soil had an opposite trend with the exchangeable fraction of Zn and Pb, but dissolved organic carbon followed a similar trend with the more bioavailable fractions. As a result, the accumulation of Pb and Zn in the plant tissues was significantly (p < 0.05) reduced by the application of amendments, which might be due to the shift of the metals to immobile forms induced by the nature of the treatments and changes in the rhizosphere process. The empirical conditions of this research produced the intensification of the rhizosphere process because the findings highlight those changes in the rhizosphere EC, pH and dissolved organic carbon can affect the efficiency of zeolite/SiO₂ NPs and bacteria to immobilize Pb and Zn in the soil, depending on the chemical character of the metals and the treatments. Generally, the affinity of the biotic treatment for Pb was more than the abiotic and conversely, the abiotic treatment showed a higher ability to immobilize Zn than the biotic treatment.

Bacterial mediated alleviation of heavy metal stress and decreased accumulation of metals in plant tissues: Mechanisms and future prospects

Heavy metal pollution of agricultural soils is one of main concerns causing some of the different ecological and environmental problems. Excess accumulation of these metals in soil has changed microbial community (e.g., structure, function, and diversity), deteriorated soil, decreased the growth and yield of plant, and entered into the food chain. Plants' tolerance to heavy metal stress needs to be improved in order to allow growth of crops with minimum or no accumulation of heavy metals in edible parts of plant that satisfy safe food demands for the world's rapidly increasing population. It is well known that PGPRs (plant growth-promoting rhizobacteria) enhance crop productivity and plant resistance to heavy metal stress. Many recent reports describe the application of heavy metal resistant-PGPRs to enhance agricultural yields without accumulation of metal in plant tissues. This review provides information about the mechanisms possessed by heavy metal resistant-PGPRs that ameliorate heavy metal stress to plants and decrease the accumulation of these metals in plant, and finally gives some perspectives for research on these bacteria in agriculture in the future.

Nickel phytoextraction through bacterial inoculation in Raphanus sativus

A pot experiment was conducted to evaluate the potential of two plant growth promoting rhizobacteria (PGPR) viz. Bacillus sp. CIK-516 and Stenotrophomonas sp. CIK-517Y for improving the growth and Ni uptake of radish (Raphanus sativus) in the presence of four different levels of Ni contamination (0, 50, 100, 150 mg Ni kg^-1 soil). Plant growth, dry biomass, chlorophyll and nitrogen contents were significantly reduced by the exogenous application of Ni, however, bacterial inoculation diluted the negative impacts of Ni stress on radish by improving these parameters. PGPR strain CIK-516 increased root length (9-27%), shoot length (8-27%), root dry biomass (2-32%), shoot dry biomass (9-51%), root girth (6-48%), total chlorophyll (4-38%) and shoot nitrogen contents (11-15%) in Ni contaminated and non-contaminated soils. Positive regulation of chlorophyll and nitrogen contents by the inoculated plants shows plant tolerance mechanism of Ni stress. Bacterial strain (CIK-516) exhibited indole acetic acid and 1-amino-cyclopropane-1-carboxylate deaminase potentials which would have helped radish plant to stabilize in Ni contaminated soil and thereby increased Ni uptake (24-257 in shoot and 58-609 in root mg kg^-1 dry biomass) and facilitated accumulation in radish (bioaccumulation factor = 0.6-1.7) depending upon soil Ni contamination. Based on the findings of this study, it might be suggested that inoculation with bacterial strain CIK-516 could be an efficient tool for enhanced Ni phytoextraction in radish.

Revitalization of plant growth promoting rhizobacteria for sustainable development in agriculture

The progression of life in all forms is not only dependent on agricultural and food security but also on the soil characteristics. The dynamic nature of soil is a direct manifestation of soil microbes, bio-mineralization, and synergistic co-evolution with plants. With the increase in world's population the demand for agriculture yield has increased tremendously and thereby leading to large scale production of chemical fertilizers. Since the use of fertilizers and pesticides in the agricultural fields have caused degradation of soil quality and fertility, thus the expansion of agricultural land with fertile soil is near impossible, hence researchers and scientists have sifted their attention for a safer and productive means of agricultural practices. Plant growth promoting rhizobacteria (PGPR) has been functioning as a co-evolution between plants and microbes showing antagonistic and synergistic interactions with microorganisms and the soil. Microbial revitalization using plant growth promoters had been achieved through direct and indirect approaches like bio-fertilization, invigorating root growth, rhizoremediation, disease resistance etc. Although, there are a wide variety of PGPR and its allies, their role and usages for sustainable agriculture remains controversial and restricted. There is also variability in the performance of PGPR that may be due to various environmental factors that might affect their growth and proliferation in the plants. These gaps and limitations can be addressed through use of modern approaches and techniques such as nano-encapsulation and micro-encapsulation along with exploring multidisciplinary research that combines applications in biotechnology, nanotechnology, agro biotechnology, chemical engineering and material science and bringing together different ecological and functional biological approaches to provide new formulations and opportunities with immense potential.

Microbial and Plant-Assisted Bioremediation of Heavy Metal Polluted Environments: A Review

Environmental pollution from hazardous waste materials, organic pollutants and heavy metals, has adversely affected the natural ecosystem to the detriment of man. These pollutants arise from anthropogenic sources as well as natural disasters such as hurricanes and volcanic eruptions. Toxic metals could accumulate in agricultural soils and get into the food chain, thereby becoming a major threat to food security. Conventional and physical methods are expensive and not effective in areas with low metal toxicity. Bioremediation is therefore an eco-friendly and efficient method of reclaiming environments contaminated with heavy metals by making use of the inherent biological mechanisms of microorganisms and plants to eradicate hazardous contaminants. This review discusses the toxic effects of heavy metal pollution and the mechanisms used by microbes and plants for environmental remediation. It also emphasized the importance of modern biotechnological techniques and approaches in improving the ability of microbial enzymes to effectively degrade heavy metals at a faster rate, highlighting recent advances in microbial bioremediation and phytoremediation for the removal of heavy metals from the environment as well as future prospects and limitations. However, strict adherence to biosafety regulations must be followed in the use of biotechnological methods to ensure safety of the environment.

Retrieved 16S rRNA and nifH sequences reveal co-dominance of Bradyrhizobium and Ensifer (Sinorhizobium) strains in field-collected root nodules of the promiscuous host Vigna radiata (L.) R. Wilczek

In the present study, the relative distribution of endophytic rhizobia in field-collected root nodules of the promiscuous host mung bean was investigated by sequencing of 16S ribosomal RNA (rRNA) and nifH genes, amplified directly from the nodule DNA. Co-dominance of the genera Bradyrhizobium and Ensifer was indicated by 32.05 and 35.84% of the total retrieved 16S rRNA sequences, respectively, and the sequences of genera Mesorhizobium and Rhizobium comprised only 0.06 and 2.06% of the recovered sequences, respectively. Sequences amplified from rhizosphere soil DNA indicated that only a minor fraction originated from Bradyrhizobium and Ensifer strains, comprising about 0.46 and 0.67% of the total retrieved sequences, respectively. 16S rRNA gene sequencing has also identified the presence of several non-rhizobial endophytes from phyla Proteobacteria, Actinobacteria, Bacteroides, and Firmicutes. The nifH sequences obtained from nodules also confirmed the co-dominance of Bradyrhizobium (39.21%) and Ensifer (59.23%) strains. The nifH sequences of the genus Rhizobium were absent, and those of genus Mesorhizobium comprised only a minor fraction of the sequences recovered from the nodules and rhizosphere soil samples. Two bacterial isolates, identified by 16S rRNA gene sequence analysis as Bradyrhizobium strain Vr51 and Ensifer strain Vr38, successfully nodulated the original host (mung bean) plants. Co-dominance of Bradyrhizobium and Ensifer strains in the nodules of mung bean indicates the potential role of the host plant in selecting specific endophytic rhizobial populations. Furthermore, successful nodulation of mung bean by the isolates showed that strains of both the genera Bradyrhizobium and Ensifer can be used for production of inoculum.

Leguminosae native nodulating bacteria from a gold mine As-contaminated soil: Multi-resistance to trace elements, and possible role in plant growth and mineral nutrition

Efficient N₂-fixing Leguminosae nodulating bacteria resistant to As may facilitate plant growth on As-contaminated sites. In order to identify bacteria holding these features, 24 strains were isolated from nodules of the trap species Crotalaria spectabilis (12) and Stizolobium aterrimum (12) growing on an As-contaminated gold mine site. 16S rRNA gene sequencing revealed that most of the strains belonged to the group of α-Proteobacteria, being representatives of the genera Bradyrhizobium, Rhizobium, Inquilinus, Labrys, Bosea, Starkeya, and Methylobacterium. Strains of the first four genera showed symbiotic efficiency with their original host, and demonstrated in vitro specific plant-growth-promoting (PGP) traits (production of organic acids, indole-3-acetic-acid and siderophores, 1-aminocyclopropane-1-carboxylate deaminase activity, and Ca₃(PO₄)₂ solubilization), and increased resistance to As, Zn, and Cd. In addition, these strains and some type and reference rhizobia strains exhibited a wide resistance spectrum to β-lactam antibiotics. Both intrinsic PGP abilities and multi-element resistance of rhizobia are promising for exploiting the symbiosis with different legume plants on trace-element-contaminated soils.

Role of Streptomyces pactum in phytoremediation of trace elements by Brassica juncea in mine polluted soils

The industrial expansion, smelting, mining and agricultural practices have increased the release of toxic trace elements (TEs) in the environment and threaten living organisms. The microbe-assisted phytoremediation is environmentally safe and provide an effective approach to remediate TEs contaminated soils. A pot experiment was conducted to test the potential of an Actinomycete, subspecies Streptomyces pactum (Act12) along with medical stone compost (MSC) by growing Brassica juncea in smelter and mines polluted soils of Feng County (FC) and Tongguan (TG, China), respectively. Results showed that Zn (7, 28%), Pb (54, 21%), Cd (16, 17%) and Cu (8, 10%) uptake in shoot and root of Brassica juncea was pronounced in FC soil. Meanwhile, the Zn (40, 14%) and Pb (82, 15%) uptake in the shoot and root were also increased in TG soil. Shoot Cd uptake remained below detection, while Cu decreased by 52% in TG soil. The Cd and Cu root uptake were increased by 17% and 33%, respectively. Results showed that TEs uptake in shoot increased with increasing Act12 dose. Shoot/root dry biomass, chlorophyll and carotenoid content in Brassica juncea were significantly influenced by the application of Act12 in FC and TG soil. The antioxidant enzymatic activities (POD, PAL, PPO and CAT) in Brassica juncea implicated enhancement in the plant defense mechanism against the TEs induced stress in contaminated soils. The extraction potential of Brasssica was further evaluated by TF (translocation factor) and MEA (metal extraction amount). Based on our findings, further investigation of Act12 assisted phytoremediation of TEs in the smelter and mines polluted soil and hyperaccumulator species are suggested for future studies.

Cultivar and Metal-Specific Effects of Endophytic Bacteria in Helianthus tuberosus Exposed to Cd and Zn

Plant growth promoting endophytic bacteria (PGPB) isolated from Brassica napus were inoculated in two cultivars of Helianthus tuberosus (VR and D19) growing on sand supplemented with 0.1 mM Cd or 1 mM Zn. Plant growth, concentrations of metals and thiobarbituric acid (TBA) reactive compounds were determined. Colonization of roots of H. tuberosus D19 by Pseudomonas sp. 262 was evaluated using confocal laser scanning microscopy. Pseudomonas sp. 228, Serratia sp. 246 and Pseudomonas sp. 262 significantly enhanced growth of H. tuberosus D19 exposed to Cd or Zn. Pseudomonas sp. 228 significantly increased Cd concentrations in roots. Serratia sp. 246, and Pseudomonas sp. 256 and 228 resulted in significantly decreased contents of TBA reactive compounds in roots of Zn exposed D19 plants. Growth improvement and decrease of metal-induced stress were more pronounced in D19 than in VR. Pseudomonas sp. 262-green fluorescent protein (GFP) colonized the root epidermis/exodermis and also inside root hairs, indicating that an endophytic interaction was established. H. tuberosus D19 inoculated with Pseudomonas sp. 228, Serratia sp. 246 and Pseudomonas sp. 262 holds promise for sustainable biomass production in combination with phytoremediation on Cd and Zn contaminated soils.

Remediation Performance and Mechanism of Heavy Metals by a Bottom Up Activation and Extraction System Using Multiple Biochemical Materials

Soil contamination with heavy metals has caused serious environmental problems and increased the risks to humans and biota. Herein, we developed an effective bottom up metals removal system based on the synergy between the activation of immobilization metal-resistant bacteria and the extraction of bioaccumulator material (Stropharia rugosoannulata). In this system, the advantages of biochar produced at 400 °C and sodium alginate were integrated to immobilize bacteria. Optimized by response surface methodology, the biochar and bacterial suspension were mixed at a ratio of 1:20 (w:v) for 12 h when 2.5% sodium alginate was added to the mixture. Results demonstrated that the system significantly increased the proportion of acid soluble Cd and Cu and improved the soil microecology (microbial counts, soil respiration, and enzyme activities). The maximum extractions of Cd and Cu were 8.79 and 77.92 mg kg^-1, respectively. Moreover, details of the possible mechanistic insight into the metal removal are discussed, which indicate positive correlation with the acetic acid extractable metals and soil microecology. Meanwhile, the "dilution effect" in S. rugosoannulata probably plays an important role in the metal removal process. Furthermore, the metal-resistant bacteria in this system were successfully colonized, and the soil bacteria community were evaluated to understand the microbial diversity in metal-contaminated soil after remediation.

Alleviation of Heavy Metal Stress in Plants and Remediation of Soil by Rhizosphere Microorganisms

Increasing concentration of heavy metals (HM) due to various anthropogenic activities is a serious problem. Plants are very much affected by HM pollution particularly in contaminated soils. Survival of plants becomes tough and its overall health under HM stress is impaired. Remediation of HM in contaminated soil is done by physical and chemical processes which are costly, time-consuming, and non-sustainable. Metal–microbe interaction is an emerging but under-utilized technology that can be exploited to reduce HM stress in plants. Several rhizosphere microorganisms are known to play essential role in the management of HM stresses in plants. They can accumulate, transform, or detoxify HM. In general, the benefit from these microbes can have a vast impact on plant’s health. Plant–microbe associations targeting HM stress may provide another dimension to existing phytoremediation and rhizoremediation uses. In this review, applied aspects and mechanisms of action of heavy metal tolerant-plant growth promoting (HMT-PGP) microbes in ensuring plant survival and growth in contaminated soils are discussed. The use of HMT-PGP microbes and their interaction with plants in remediation of contaminated soil can be the approach for the future. This low input and sustainable biotechnology can be of immense use/importance in reclaiming the HM contaminated soils, thus increasing the quality and yield of such soils.