Characterization of metal-resistant plant-growth promotingBacillus weihenstephanensisisolated from serpentine soil in Portugal

Plant growth-promoting rhizobacterial secondary metabolites in augmenting heavy metal(loid) phytoremediation: An integrated green in situ ecorestorative technology

In recent times, increased geogenic and human-centric activities have caused significant heavy metal(loid) (HM) contamination of soil, adversely impacting environmental, plant, and human health. Phytoremediation is an evolving, cost-effective, environment-friendly, in situ technology that employs indigenous/exotic plant species as natural purifiers to remove toxic HM(s) from deteriorated ambient soil. Interestingly, the plant's rhizomicrobiome is pivotal in promoting overall plant nutrition, health, and phytoremediation. Certain secondary metabolites produced by plant growth-promoting rhizobacteria (PGPR) directly participate in HM bioremediation through chelation/mobilization/sequestration/bioadsorption/bioaccumulation, thus altering metal(loid) bioavailability for their uptake, accumulation, and translocation by plants. Moreover, the metallotolerance of the PGPR and the host plant is another critical factor for the successful phytoremediation of metal(loid)-polluted soil. Among the phytotechniques available for HM remediation, phytoextraction/phytoaccumulation (HM mobilization, uptake, and accumulation within the different plant tissues) and phytosequestration/phytostabilization (HM immobilization within the soil) have gained momentum in recent years. Natural metal(loid)-hyperaccumulating plants have the potential to assimilate increased levels of metal(loid)s, and several such species have already been identified as potential candidates for HM phytoremediation. Furthermore, the development of transgenic rhizobacterial and/or plant strains with enhanced environmental adaptability and metal(loid) uptake ability using genetic engineering might open new avenues in PGPR-assisted phytoremediation technologies. With the use of the Geographic Information System (GIS) for identifying metal(loid)-impacted lands and an appropriate combination of normal/transgenic (hyper)accumulator plant(s) and rhizobacterial inoculant(s), it is possible to develop efficient integrated phytobial remediation strategies in boosting the clean-up process over vast regions of HM-contaminated sites and eventually restore ecosystem health.

Sphingomonas panaciterrae PB20 increases growth, photosynthetic pigments, antioxidants, and mineral nutrient contents in spinach (Spinacia oleracea L.)

Plant growth promoting rhizobacteria (PGPR) have been intensively investigated in agricultural crops for decades. Nevertheless, little information is available on the application of Sphingomonas spp. as a PGPR particularly in vegetables, despite of potential plant growth promoting traits of this group. This study investigated the role of Sphingomonas panaciterrae (PB20) on growth and nutritional profile of spinach applied through seed priming (SP), soil drenching (SD), foliar application (FA), and bacterial culture filtrate foliar (BCF) applications. The results showed that, depending on different methods of application, PB20 significantly increased plant height (19.57-65.65 %), fresh weight (7.26-37.41 %), total chlorophyll (71.14-192.54 %), carotenoid (67.10-211.67 %) antioxidant (55.99-207.04), vitamin C (8.1-94.6 %) and protein content (6.7-21.5 %) compared to control in the edible part of spinach. Among the mineral nutrients, root nitrogen (N) showed greater response to bacterial application (18.65%-46.15 % increase over control) than shoot nitrogen (6.70%-21.52 % increased over control). Likewise, in all methods of application, phosphorus (P) content showed significant increase over control both in root (42.79-78.48 %) and in shoot (3.57-27.0 %). Seed priming and foliar application of PB20 increased the shoot calcium (Ca) content compared to control. BCF foliar application yielded maximum magnesium (Mg), iron (Fe) and zinc (Zn) in shoot. However, seed priming resulted in maximum Fe in root. Overall, seed priming outperformed in growth, vitamin C, antioxidants, N and P uptake, while BCF foliar application resulted in better uptake of several nutrients. Multivariate analysis validated the positive association of most of the growth parameters with SP while several nutrients with FA and BCF. Based on the findings it is evident that this rhizobacteria PB20 has the potentiality to be applied as a biofertilizer to produce nutrient-enriched spinach with an improved yield. Farmers can conveniently incorporate PR20 through seed priming before planting of spinach, with additional benefits through foliar spray.

Plant growth-promoting and heavy metal-resistant Priestia and Bacillus strains associated with pioneer plants from mine tailings

Open mine tailings dams are extreme artificial environments containing sizeable potentially toxic elements (PTEs), including heavy metals (HMs), transition metals, and metalloids. Furthermore, these tailings have nutritional deficiencies, including assimilable phosphorus sources, organic carbon, and combined nitrogen, preventing plant colonization. Bacteria, that colonize these environments, have mechanisms to tolerate the selective pressures of PTEs. In this work, several Priestia megaterium (formerly Bacillus megaterium), Bacillus mojavensis, and Bacillus subtilis strains were isolated from bulk tailings, anthills, rhizosphere, and endosphere of pioneer plants from abandoned mine tailings in Zacatecas, Mexico. Bacillus spp. tolerated moderate HMs concentrations, produced siderophores and indole-3-acetic acid (IAA), solubilized phosphates, and reduced acetylene in the presence of HMs. The strains harbored different PIB-type ATPase genes encoding for efflux pumps and Cation Diffusion Facilitator (CDF) genes. Moreover, nifH and nifD nitrogenase genes were detected in P. megaterium and B. mojavensis genomic DNA. They showed similarity with sequences of the beta-Proteobacteria species, which may represent likely horizontal transfer events. These Bacillus species precede the colonization of mine tailings by plants. Their phenotypic and genotypic features could be essential in the natural recovery of the sites by reducing the oxidative stress of HMs, fixing nitrogen, solubilizing phosphate, and accumulating organic carbon. These traits of the strains reflect the adaptations of Bacillus species to the mine tailings environment and could contribute to the success of phytoremediation efforts.

Endophytic Plant Growth-Promoting Bacterium Bacillus subtilis Reduces the Toxic Effect of Cadmium on Wheat Plants

Heavy metal ions, in particular cadmium (Cd), have a negative impact on the growth and productivity of major crops, including wheat. The use of environmentally friendly approaches, in particular, bacteria that have a growth-stimulating and protective effect, can increase the resistance of plants. The effects of the pre-sowing seed treatment with the plant growth-promoting endophyte Bacillus subtilis 10-4 (BS) on cadmium acetate (Cd)-stressed Triticum aestivum L. (wheat) growth, photosynthetic pigments, oxidative stress parameters, roots' lignin content, and Cd ions accumulation in plants were analyzed. The results showed that the tested Cd-tolerant BS improved the ability of wheat seeds to germinate in the presence of different Cd concentrations (0, 0.1, 0.5, and 1 mM). In addition, the bacterial treatment significantly decreased the damaging effects of Cd stress (1 mM) on seedlings' linear dimensions (lengths of roots and shoots), biomass, as well as on the integrity and permeability of the cell walls (i.e., lipid peroxidation and electrolyte leakage) and resulted in reduced H₂O₂ generation. The pretreatment with BS prevented the Cd-induced degradation of the leaf photosynthetic pigments chlorophyll (Chl) a, Chl b, and carotenoids. Moreover, the bacterial treatment intensified the lignin deposition in the roots under normal and, especially, Cd stress conditions, thereby enhancing the barrier properties of the cell wall. This manifested in a reduced Cd ions accumulation in the roots and in the restriction of its translocation to the aboveground parts (shoots) of the bacterized plants under Cd stress in comparison with non-bacterized controls. Thus, the pre-sowing seed treatment with the endophyte BS may serve as an eco-friendly approach to improve wheat production in Cd-contaminated areas.

Zinc and cadmium change the metabolic activities and vegetable cellulose degradation of Bacillus cellulasensis in vegetable soils

Bacillus cellulasensis Zn-B isolated from vegetable soil was highly adaptable to Zinc (Zn) and Cadmium (Cd). Cd, but not Zn, adversely affected the total protein spectrum and functional groups of Bacillus cellulasensis Zn-B. Up to 31 metabolic pathways and 216 metabolites of Bacillus cellulasensis Zn-B were significantly changed by Zn and Cd (Zn&Cd). Some metabolic pathways and metabolites related to functional groups of sulfhydryl (-SH) and amine (-NH-) metabolism were enhanced by Zn&Cd addition. The cellulase activity of Bacillus cellulasensis Zn-B was up to 8.58 U mL^-1, increased to 10.77 U mL^-1 in Bacillus cellulasensis Zn-B + 300 mg L^-1 Zn, and maintained at 6.13 U mL^-1 in Bacillus cellulasensis Zn-B + 50 mg L^-1 Cd. The vegetables' cellulose content was decreased by 25.05-52.37% and 40.28-70.70% under the action of Bacillus cellulasensis Zn-B and Bacillus cellulasensis Zn-B + 300 mg L^-1 Zn. Those results demonstrated that Zn could significantly enhance cellulase activity and biodegradability of Bacillus cellulasensis Zn-B to vegetable cellulose. Bacillus cellulasensis Zn-B can survive in vegetable soil accumulated with Zn&Cd. The tolerance concentration and adsorption capacity of Bacillus cellulasensis Zn-B to Zn were up to 300 mg L^-1 and 56.85%, indicating that Bacillus cellulasensis Zn-B acting as a thermostability biological agent had an essential advantage in accelerating the degradation of discarded vegetables by Zn and were beneficial to maintain organic matter content of vegetable soil.

Detection of Bacillus Species with Arsenic Resistance and Plant Growth Promoting Efficacy from Agricultural Soils of Nepal

Arsenic contamination in soil and water is one of the major environmental problems in multiple countries including Nepal imposing a serious threat to the ecosystem and public health. Many soil bacteria can detoxify arsenic, including genus Bacillus. With an objective to gauge the plant growth-promoting activities of arsenic-resistant Bacillus species, 36 samples (soil, rice, cauliflower, and beans) were collected from the Terai region of Nepal. For selective isolation of Bacillus species, each sample was heated at 80°C for 15 min before the inoculation into nutrient agar (NA). Following the standard protocol, arsenic-resistant Bacillus species were screened using NA supplemented with 100 ppm sodium arsenate and sodium arsenite. Among 158 randomly selected isolates, only five isolates were able to tolerate sodium arsenite concentration up to 600 ppm. Notably, all five isolates were able to produce indole acetic acid (IAA), a plant hormone, and solubilize phosphate. Based on biochemical analysis and 16S rRNA gene sequencing, isolates N4-1, RW, KR7-12, Bhw1-4, and BW2-2 were identified as B. subtilis subsp. stercosis, B. flexus, B. licheniformis, B. cereus, and B. flexus, respectively. To the best of our knowledge, this is the first study showing the presence of arsenic-resistant B. flexus in Nepalese soil with plant growth-promoting traits. Possible utilization of these Bacillus strains could facilitate the novel bioremediation pathway to reduce the toxic effect of arsenic from the soil and water in the Terai region of Nepal.

Enhanced phytoextraction of multi-metal contaminated soils under increased atmospheric temperature by bioaugmentation with plant growth promoting Bacillus cereus

The co-occurrence of environmental stresses such as heavy metals (HM) and increased atmospheric temperature (IAT) pose serious implications on plant growth and productivity. In this work, we evaluated the role of plant growth-promoting bacteria (PGPB) and its effectiveness on Zea mays growth, stress tolerance and phytoremediation potential in multi-metal (MM) contaminated soils under IAT stress conditions. The PGPB strain TCU11 was isolated from metal contaminated soils and identified as Bacillus cereus. TCU11 was able to resist abiotic stresses such as IAT (45 °C), MM (Pb, Zn, Ni, Cu, and Cd), antibiotics and induced in vitro plant growth promotion (PGP) by producing siderophores (catechol and hydroxymate) and indole 3-acetic acid even in the presence of MM under IAT. Inoculation of TCU11 significantly increased the biomass, chlorophyll, carotenoids, and protein content of Z. mays compared to the respective control under MM, IAT, and MM + IAT stress. A decrease of malondialdehyde and over-accumulation of total phenolics, proline along with the increased activity of superoxide dismutase, catalase and ascorbic peroxidase were observed in TCU11 inoculated plants under stress conditions. These results suggested MM and/or IAT significantly reduced the maize growth, whereas TCU11 inoculation mitigated the combined stress effects on maize performance. Moreover, the inoculation of TCU11 under IAT stress increased the MM (Pb, Zn, Ni, Cu, and Cd) accumulation in plant tissues and also increased the translocation of HM from root to shoot except for Ni. The results of soil HM mobilization further indicates that IAT increased the HM mobilizing activity of TCU11, thus increasing the concentrations of bio-available HM in soil. These results suggested that TCU11 not only alleviates MM and IAT stresses but also enhances the biomass production and HM accumulation in plants. Therefore, TCU11 can be exploited as inoculums for improving the phytoremediation efficiency in MM polluted soils under IAT conditions.

Evaluation of direct and biochar carrier-based inoculation of Bacillus sp. on As- and Pb-contaminated technosol: effect on metal(loid) availability, Salix viminalis growth, and soil microbial diversity/activity

Phytomanagement manipulates the soil-plant system to lower the risk posed by contaminated soils. In this process, the addition of amendments, such as biochar, and bacteria can improve the fertility of poor contaminated soils and consequently ameliorate plant growth. A number of studies based on the inoculation of soil with microorganisms of the genus Bacillus, previously isolated from contaminated sites, revealed positive effects on soil properties and plant growth. Furthermore, when the Bacillus isolates were used in association with biochar, better results were obtained, as biochar can ameliorate soil properties and serve as habitat for microorganisms. Accordingly, a mesocosm study was set-up using a mining technosol amended with biochar and inoculated with an endogenous Bacillus isolate, to evaluate the effect of inoculation on soil properties, metal(loid) immobilization, and Salix viminalis growth. Two inoculation methods were compared: (1) direct inoculation of bacteria (Bacillus sp.) and (2) inoculation using biochar as a carrier. Results showed that the Bacillus isolate modified soil properties and ameliorated plant growth, while having a reduced effect on metal(loid) accumulation. The microbial activity was also stimulated, and the community composition was shifted, more importantly when biochar was used as a carrier. In conclusion, this research revealed an improvement of the plant growth and microbial activity after the addition of the endogenous bacterium to the analyzed former mining soil, with better results recorded when a carrier was used.

Mechanistic elucidation of germination potential and growth of Sesbania sesban seedlings with Bacillus anthracis PM21 under heavy metals stress: An in vitro study

Soils contaminated with heavy metals such as Chromium (Cr) and Cadmium (Cd) severely impede plant growth. Several rhizospheric microorganisms support plant growth under heavy metal stress. In this study, Cr and Cd stress was applied to in vitro germinating seedlings of a Legume plant species, Sesbania sesban, and investigated the plant growth potential in presence and absence of Bacillus anthracis PM21 bacterial strain under heavy metal stress. The seedlings were exposed to different concentrations of Cr (25-75 mg/L) and Cd (100-200 mg/L) in Petri plates. Growth curve analysis of B. anthracis PM21 revealed its potential to adapt Cr and Cd stress. The bacteria supported plant growth by exhibiting ACC-deaminase activity (1.57-1.75 μM of α-ketobutyrate/h/mg protein), producing Indole-3-acetic acid (99-119 μM/mL) and exopolysaccharides (2.74-2.98 mg/mL), under heavy metal stress condition. Analysis of variance revealed significant differences in growth parameters between the seedlings with and without bacterial inoculation in metal stress condition. The combined Cr+Cd stress (75 + 200 mg/L) significantly reduced root length (70%), shoot length (24%), dry weight (54%) and fresh weight (57%) as compared to control. Conversely, B. anthracis PM21 inoculation to seedlings significantly increased (p ≤ 0.05) seed germination percentage (5%), root length (31%), shoot length (23%) and photosynthetic pigments (Chlorophyll a: 20%; Chlorophyll b: 16% and total chlorophyll: 18%), as compared to control seedlings without B. anthracis PM21 inoculation. The B. anthracis PM21 inoculation also enhanced activities of antioxidant enzymes, including superoxide dismutase (52%), peroxidase (66%), and catalase (21%), and decreased proline content (56%), electrolyte leakage (50%), and malondialdehyde concentration (46%) in seedlings. The B. anthracis PM21 inoculated seedlings of S. sesban exhibited significantly high (p ≤ 0.05) tissue deposition of Cr (17%) and Cd (16%) as compared to their control counterparts. Findings of the study suggested that B. anthracis PM21 endured metal stress through homeostasis of antioxidant activities, and positively impacted S. sesban growth and biomass. Further experiments in controlled conditions are necessary for investigating phytoremediation potential of S. sesban in metal-contaminated soils in presence of B. anthracis PM21 bacterial strain.

Enhanced uptake of Cd, Cr, and Cu in Catharanthus roseus (L.) G.Don by Bacillus cereus: application of moss and compost to reduce metal availability

Heavy metals (HMs) being the notorious and toxic are being introduced into the environment credited to natural and anthropogenic activities. The use of ornamental plants is being ignored as potential candidates for HMs phytoremediation. In this study, pot experiments were conducted on Catharanthus roseus (L.) G.Don to evaluate selected heavy metals tolerance and accumulation potential with reference to the bacterial endophyte (Bacillus cereus) and organic amendments (moss and compost at 5% v/v). Results indicated improvement in uptake of Cd (230 mg kg^-1), Cu (229 mg kg^-1), and Cr (458 mg kg^-1) by C. roseus with B. cereus. The concentration of Ni and Pb was found highest in controls (without strain) that were 420 and 904 mg kg^-1, respectively. Conversely, the addition of organic amendments enhanced biomass production, as compared to controls, 441, 471, and 763% by peat moss (T3), compost (T4), and peat moss + compost + inoculum treatments (T6), respectively, while reduction of plant HMs content was observed. Microbial-aided phytoremediation/phytoextraction could be a potential method for removal of Cd, Cr, and Cu, while organic amendments can significantly improve plant growth in the presence of heavy metals.

A comparative evaluation towards the potential of Klebsiella sp. and Enterobacter sp. in plant growth promotion, oxidative stress tolerance and chromium uptake in Helianthus annuus (L.)

Prevalence of metal pollutants exerts negative effects on human health and environment, thus propounding an urgent need for a safer substitute. This study was conducted to compare the chromium bioremediation and plant growth promotion ability of two bacterial strains, Klebsiella sp. strain CPSB4 (MH266218) and Enterobacter sp. strain CPSB49 (MH532567), isolated from the rhizospheric soils. A pot scale experiment was setup with Helianthus annuus (L.) as a test plant to compare the efficiency of both isolates in enhancement of plant growth, nutrients uptake, anti-oxidative enzymes production, lipid peroxidation, and chromium bioremediation. Inoculation of strains, CPSB4 and CPSB49 enhanced plant biomass, plant growth, nutrient uptake, anti-oxidative enzymes, and chromium bioremediation, while reduction in lipid peroxidation was observed compared to uninoculated control under chromium stress. The maximum increase in plant growth and nutrient uptake was found in treatments inoculated with CPSB49, while maximum chromium uptake by sunflower was observed in treatments inoculated with CPSB4. Moreover, an increase in anti-oxidative enzyme production and decrease in lipid peroxidation was observed on inoculation of the selected strains. Thus, the strains Klebsiella sp. and Enterobacter sp. can be effectively used in chromium bioremediation and plant growth promotion under chromium stress conditions.

Priming with ACC-utilizing bacterium attenuated copper toxicity, improved oxidative stress tolerance, and increased phytoextraction capacity in wheat

The major challenges for the plants growing in metal-contaminated soils are deficiency of nutrients, biomass reduction, and severe oxidative damages in the presence of heavy metals. In this regard, our aim was to overcome these challenges through the use of efficient microbial strains in metal-polluted soils and to assess its/their physiological and biochemical effects. In the current study, a copper (Cu)-resistant bacterium was isolated from the rhizospheric soil of 'Ziziphus nummularia' and evaluated for its ability to promote the wheat growth under the gradient stress of copper. Based on 16S rRNA gene sequencing, the isolate was identified as Pantoea sp. Among the plant growth promoting tests, the isolate showed the production of indole acetic acid, solubilization of inorganic phosphate, and ACC deaminase activity. Also, the isolate showed resistance to many heavy metals and antibiotics and increased the water-soluble copper in solution. The results of pot studies showed that bacterial application promoted various growth parameters of wheat plants and also enhanced the Cu uptake of wheat from the Cu-amended soil. The results showed that enhancement of Cu stress (100 to 300 mg kg^-1) resulted in a decrease in various compatible solutes such as proline, total soluble sugars, and total protein content, and increase in the level of malondialdehyde (MDA), latter of which is the indicator of oxidative stress. Bacterial treatment markedly increased the proline, soluble sugar, total protein content, and decreased the MDA content under Cu stress. In addition, bacterial inoculation significantly alleviated the harmful effect of metal toxicity by decreasing the activation of ROS molecules including superoxide (O₂^-) and hydrogen peroxide (H₂O₂). The activation of various antioxidative enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) was noted following bacterial inoculation under Cu stress. Therefore, the present study demonstrates the potential of the isolate Pantoea sp. ZNP-5 to improve the growth and phytoextraction of metal from the metal-polluted soil through the polyphasic mechanism of action.

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.

Draft Genome Sequence of the Endophyte Bacillus mycoides Strain GM6LP Isolated from Lolium perenne

Bacillus mycoides GM6LP is an endophyte isolated from plant tissues of Lolium perenne L. Here, we report its draft genome sequence (6.2 Mb), which contains 96 contigs and 6,129 protein-coding genes. Knowledge about its genome will enable us to evaluate the potential use of GM6LP as a plant growth-promoting bacterium.

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.

Biotoxic impact of heavy metals on growth, oxidative stress and morphological changes in root structure of wheat (Triticum aestivum L.) and stress alleviation by Pseudomonas aeruginosa strain CPSB1

Rapid industrialization and uncontrolled metal discharge into environment is a global concern for crop production. Metal tolerant bacterium isolated from chilli rhizosphere was identified as Pseudomonas aeruginosa by 16S rDNA sequence analysis. Pseudomonas aeruginosa tolerated high concentrations of Cu (1400 μg ml^-1), Cd (1000 μg ml^-1) and Cr (1000 μg ml^-1). Pseudomonas aeruginosa CPSB1 produced multiple plant growth promoting biomolecules in the presence and absence of metals. Strain CPSB1 solubilized P at 400 μg ml^-1 of Cd, Cr and Cu. The strain was positive for indole-3-acetic acid (IAA), siderophores, hydrogen cyanide (HCN), ammonia (NH₃) and 1-aminocyclopropane-1-carboxylate (ACC) deaminase when grown with/without metals. The phytotoxic effects on wheat increased with increasing Cd, Cr and Cu rates. The P. aeruginosa CPSB1 inoculated wheat in contrast had better growth and yields under Cu, Cd and Cr stress. The root dry biomass of inoculated plants was enhanced by 44, 28 and 48% at 2007 mg Cu kg^-1, 36 mg Cd kg^-1 and 204 mg Cr kg^-1, respectively. The bioinoculant enhanced number of spikes, grain and straw yields by 25, 17 and 12%, respectively. Pseudomonas aeruginosa CPSB1 significantly declined the levels of catalase (CAT), glutathione reductase (GR) and superoxide dismutase SOD), proline and malondialdehyde (MDA), and reduced metal uptake by wheat. The study demonstrated that P. aeruginosa CPSB1 possessed plant growth promoting potentials, showed metal tolerance capability and had ability to counteract deleterious metal impacts. Due to these, P. aeruginosa CPSB1 could be used as bioinoculant for enhancing wheat production even in metal contaminated soils.

The effect of plant growth-promoting rhizobacteria on the growth, physiology, and Cd uptake of Arundo donax L

In this study, plant growth-promoting potential isolates from rhizosphere of 10 weed species grown in heavy metal-contaminated areas were identified and their effect on growth, antioxidant enzymes, and cadmium (Cd) uptake in Arundo donax L. was explored. Plant growth-promoting traits of isolates were also analyzed. These isolates were found to produce siderophores and enzymes such as 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and aid in solubilization of mineral nutrients and modulate plant growth and development. Based on the presence of multiple plant growth-promoting traits, isolates were selected for molecular characterization and inoculation studies. Altogether, 58 isolates were obtained and 20% of them were able to tolerate Cd up to 400 ppm. The sequence analysis of the 16S rRNA genes indicates that the isolates belong to the phylum Firmicutes. Bacillus sp. along with mycorrhizae inoculation significantly improves the growth, the activity of antioxidants enzymes, and the Cd uptake in A. donax than Bacillus alone. Highly significant correlations were observed between Cd uptake, enzymatic activities, and plant growth characteristics at 1% level of significance. The synergistic interaction effect between these organisms helps to alleviate Cd effects on soil. Heavy metal-tolerant isolate along with arbuscular mycorrhizae (AM) could be used to improve the phytoremedial potential of plants.

Inoculation of Brassica oxyrrhina with plant growth promoting bacteria for the improvement of heavy metal phytoremediation under drought conditions

The aim of this study was to investigate the effects of drought resistant serpentine rhizobacteria on plant growth and metal uptake by Brassica oxyrrhina under drought stress (DS) condition. Two drought resistant serpentine rhizobacterial strains namely Pseudomonas libanensis TR1 and Pseudomonas reactans Ph3R3 were selected based on their ability to stimulate seedling growth in roll towel assay. Further assessment on plant growth promoting (PGP) parameters revealed their ability to produce indole-3-acetic acid, siderophore and 1-aminocyclopropane-1-carboxylate deaminase. Moreover, both strains exhibited high resistance to various heavy metals, antibiotics, salinity and extreme temperature. Inoculation of TR1 and Ph3R3 significantly increased plant growth, leaf relative water and pigment content of B. oxyrrhina, whereas decreased concentrations of proline and malondialdehyde in leaves under metal stress in the absence and presence of DS. Regardless of soil water conditions, TR1 and Ph3R3 greatly improved organ metal concentrations, translocation and bioconcentration factors of Cu and Zn. The successful colonization and metabolic activities of P. libanensis TR1 and P. reactans Ph3R3 represented positive effects on plant development and metal phytoremediation under DS. These results indicate that these strains could be used as bio-inoculants for the improvement of phytoremediation of metal polluted soils under semiarid conditions.

Characterization of cadmium-resistant bacteria for its potential in promoting plant growth and cadmium accumulation in Sesbania bispinosa root

The cadmium (Cd) resistant bacteria were isolated from soils of Damanganga river, Vapi, and identified 11 potential Cd resistant bacteria based on 16S rDNA sequences. The Cd resistant bacteria belonged to four different genera: Providencia spp., Morganella sp., Stenotrophomonas sp., and Bacillus spp. The assessment of plant growth-promoting (PGP) parameters revealed that the Cd tolerant bacteria showed one or more PGP properties. Further, a pot experiment was conducted to elucidate the effects of Cd resistant bacteria on the plant growth and the uptake of Cd by Sesbania bispinosa. The bacterized seedlings recorded 36.0-74.8% and 21.2-32.9% higher root and shoot lengths, respectively, in Cd amended soil compared with control. The Cd mobilization in the root of S. bispinosa by microbial inoculants ranged from 0.02 ± 0.01 to 1.11 ± 0.06 ppm. The enhanced concentrations of Cd accumulation in S. bispinosa roots correspond to the effect of the bacterial strains on metal mobilization in soil. The present observations showed that the Cd resistant strains protect the plants against the inhibitory effects of Cd, probably due to the production of PGP properties. The present results provided a new insight into the phytoremediation of Cd contaminated soil.

Bioaugmentation with Endophytic Bacterium E6S Homologous to Achromobacter piechaudii Enhances Metal Rhizoaccumulation in Host Sedum plumbizincicola

Application of hyperaccumulator-endophyte symbiotic systems is a potential approach to improve phytoremediation efficiency, since some beneficial endophytic bacteria are able to detoxify heavy metals, alter metal solubility in soil, and facilitate plant growth. The objective of this study was to isolate multi-metal resistant and plant beneficial endophytic bacteria and to evaluate their role in enhancing plant growth and metal accumulation/translocation. The metal resistant endophytic bacterial strain E6S was isolated from stems of the Zn/Cd hyperaccumulator plant Sedum plumbizincicola growing in metalliferous mine soils using Dworkin and Foster salts minimal agar medium with 1-aminocyclopropane-1-carboxylate (ACC) as the sole nitrogen source, and identified as homologous to Achromobacter piechaudii based on morphological and biochemical characteristics, partial 16S rDNA sequence and phylogenetic analysis. Strain E6S showed high level of resistance to various metals (Cd, Zn, and Pb). Besides utilizing ACC, strain E6S exhibited plant beneficial traits, such as solubilization of phosphate and production of indole-3-acetic acid. Inoculation with E6S significantly increased the bioavailability of Cd, Zn, and Pb in soil. In addition, bacterial cells bound considerable amounts of metal ions in the following order: Zn > Cd >Pb. Inoculation of E6S significantly stimulated plant biomass, uptake and bioaccumulation of Cd, Zn, and Pb. However, E6S greatly reduced the root to shoot translocation of Cd and Zn, indicating that bacterial inoculation assisted the host plant to uptake and store heavy metals in its root system. Inoculation with the endophytic bacterium E6S homologous to A. piechaudii can improve phytostabilization of metalliferous soils due to its effective ability to enhance in situ metal rhizoaccumulation in plants.

The hyperaccumulator Sedum plumbizincicola harbors metal-resistant endophytic bacteria that improve its phytoextraction capacity in multi-metal contaminated soil

Endophyte-assisted phytoremediation has recently been suggested as a successful approach for ecological restoration of metal contaminated soils, however little information is available on the influence of endophytic bacteria on the phytoextraction capacity of metal hyperaccumulating plants in multi-metal polluted soils. The aims of our study were to isolate and characterize metal-resistant and 1-aminocyclopropane-1-carboxylate (ACC) utilizing endophytic bacteria from tissues of the newly discovered Zn/Cd hyperaccumulator Sedum plumbizincicola and to examine if these endophytic bacterial strains could improve the efficiency of phytoextraction of multi-metal contaminated soils. Among a collection of 42 metal resistant bacterial strains isolated from the tissues of S. plumbizincicola grown on Pb/Zn mine tailings, five plant growth promoting endophytic bacterial strains (PGPE) were selected due to their ability to promote plant growth and to utilize ACC as the sole nitrogen source. The five isolates were identified as Bacillus pumilus E2S2, Bacillus sp. E1S2, Bacillus sp. E4S1, Achromobacter sp. E4L5 and Stenotrophomonas sp. E1L and subsequent testing revealed that they all exhibited traits associated with plant growth promotion, such as production of indole-3-acetic acid and siderophores and solubilization of phosphorus. These five strains showed high resistance to heavy metals (Cd, Zn and Pb) and various antibiotics. Further, inoculation of these ACC utilizing strains significantly increased the concentrations of water extractable Cd and Zn in soil. Moreover, a pot experiment was conducted to elucidate the effects of inoculating metal-resistant ACC utilizing strains on the growth of S. plumbizincicola and its uptake of Cd, Zn and Pb in multi-metal contaminated soils. Out of the five strains, B. pumilus E2S2 significantly increased root (146%) and shoot (17%) length, fresh (37%) and dry biomass (32%) of S. plumbizincicola as well as plant Cd uptake (43%), whereas Bacillus sp. E1S2 significantly enhanced the accumulation of Zn (18%) in plants compared with non-inoculated controls. The inoculated strains also showed high levels of colonization in rhizosphere and plant tissues. Results demonstrate the potential to improve phytoextraction of soils contaminated with multiple heavy metals by inoculating metal hyperaccumulating plants with their own selected functional endophytic bacterial strains.

Phosphate-solubilizing bacteria-assisted phytoremediation of metalliferous soils: a review

Heavy metal pollution of soils is of great concern. The presence of the toxic metal species above critical concentration not only harmfully affects human health but also the environment. Among existing strategies to remediate metal contaminates in soils, phytoremediation approach using metal accumulating plants is much convincing in terms of metal removal efficiency, but it has many limitations because of slow plant growth and decreased biomass owing to metal-induced stress. In addition, constrain of metal bioavailability in soils is the prime factor to restrict its applicability. Phytoremediation of metals in association with phosphate-solubilizing bacteria (PSB) considerably overcomes the practical drawbacks imposed by metal stress on plants. This review is an effort to describe mechanism of PSB in supporting and intensifying phytoremediation of heavy metals in soils and to address the developmental status of the current trend in application of PSB in this context.

Inoculation with Metal-Mobilizing Plant-Growth-Promoting Rhizobacterium Bacillus sp. SC2b and Its Role in Rhizoremediation

A plant growth-promoting bacterial (PGPB) strain SC2b was isolated from the rhizosphere of Sedum plumbizincicola grown in lead (Pb)/zinc (Zn) mine soils and characterized as Bacillus sp. based on (1) morphological and biochemical characteristics and (2) partial 16S ribosomal DNA sequencing analysis. Strain SC2b exhibited high levels of resistance to cadmium (Cd) (300 mg/L), Zn (730 mg/L), and Pb (1400 mg/L). This strain also showed various plant growth-promoting (PGP) features such as utilization of 1-aminocyclopropane-1-carboxylate, solubilization of phosphate, and production of indole-3-acetic acid and siderophore. The strain mobilized high concentration of heavy metals from soils and exhibited different biosorption capacity toward the tested metal ions. Strain SC2b was further assessed for PGP activity by phytagar assay with a model plant Brassica napus. Inoculation of SC2b increased the biomass and vigor index of B. napus. Considering such potential, a pot experiment was conducted to assess the effects of inoculating the metal-resistant PGPB SC2b on growth and uptake of Cd, Zn and Pb by S. plumbizincicola in metal-contaminated agricultural soils. Inoculation with SC2b elevated the shoot and root biomass and leaf chlorophyll content of S. plumbizincicola. Similarly, plants inoculated with SC2b demonstrated markedly higher Cd and Zn accumulation in the root and shoot system, indicating that SC2b enhanced Cd and Zn uptake by S. plumbizincicola through metal mobilization or plant-microbial mediated changes in chemical or biological soil properties. Data demonstrated that the PGPB Bacillus sp. SC2b might serve as a future biofertilizer and an effective metal mobilizing bioinoculant for rhizoremediation of metal polluted soils.

Pyruvic oxime nitrification and copper and nickel resistance by a Cupriavidus pauculus, an active heterotrophic nitrifier-denitrifier

Heterotrophic nitrifiers synthesize nitrogenous gasses when nitrifying ammonium ion. A Cupriavidus pauculus, previously thought an Alcaligenes sp. and noted as an active heterotrophic nitrifier-denitrifier, was examined for its ability to produce nitrogen gas (N₂) and nitrous oxide (N₂O) while heterotrophically nitrifying the organic substrate pyruvic oxime [CH₃–C(NOH)–COOH]. Neither N₂ nor N₂O were produced. Nucleotide and phylogenetic analyses indicated that the organism is a member of a genus (Cupriavidus) known for its resistance to metals and its metabolism of xenobiotics. The microbe (a Cupriavidus pauculus designated as C. pauculus strain UM1) was examined for its ability to perform heterotrophic nitrification in the presence of Cu²⁺ and Ni²⁺ and to metabolize the xenobiotic phenol. The bacterium heterotrophically nitrified well when either 1 mM Cu²⁺ or 0.5 mM Ni²⁺ was present in either enriched or minimal medium. The organism also used phenol as a sole carbon source in either the presence or absence of 1 mM Cu²⁺ or 0.5 mM Ni²⁺. The ability of this isolate to perform a number of different metabolisms, its noteworthy resistance to copper and nickel, and its potential use as a bioremediation agent are discussed.

Integrated metabolomic and proteomic approaches dissect the effect of metal-resistant bacteria on maize biomass and copper uptake

Marginal soils arise due to various industrial and agricultural practices reducing crop productivity. Pseudomonas sp. TLC 6-6.5-4 is a free-living multiple-metal-resistant plant-growth-promoting bacteria (PGPB) isolated from Torch Lake sediment that promotes maize growth and nutrient uptake. In this study, we examined both PGPB-soil and PGPB-plant interactions. PGPB inoculation resulted in significant increase in maize biomass. Soil inoculation before sowing seeds and coating seeds with the PGPB resulted in higher copper uptake by maize compared to other methods. The PGPB-soil interaction improved phosphorus uptake by maize and led to significant decrease in organic bound copper in marginal soil and a notable increase in exchangeable copper. PGPB improved soil health based on soil enzyme activities. Metabolomic analysis of maize revealed that PGPB inoculation upregulated photosynthesis, hormone biosynthesis, and tricarboxylic acid cycle metabolites. Proteomic analysis identified upregulation of proteins related to plant development and stress response. Further, the activity of antioxidant enzymes and total phenolics decreased in plants grown in marginal soil suggesting alleviation of metal stress in presence of PGPB. The ability of PGPB to modulate interconnected biochemical pathways could be exploited to increase crop productivity in marginal soils, phytoremediation of metal contaminated soils, and organic agriculture.

Improvement of Ni phytostabilization by inoculation of Ni resistant Bacillus megaterium SR28C

The use of metal tolerant plants for the phytostabilization of metal contaminated soil is an area of extensive research and development. In this study the effects of inoculation of Ni-resistant bacterial strains on phytostabilization potential of various plants, including Brassica juncea, Luffa cylindrica and Sorghum halepense, were studied. A Ni-resistant bacterial strain SR28C was isolated from a nickel rich serpentine soil and identified as Bacillus megaterium based on the morphological features, biochemical characteristics and partial 16S rDNA sequence analysis. The strain SR28C tolerated concentrations up to 1200 mg Ni L(-1) on a Luria-Bertani (LB) agar medium. Besides, it showed high degree of resistance to various metals (Cu, Zn, Cd, Pb and Cr) and antibiotics (ampicillin, tetracycline, streptomycin, chloramphenicol, penicillin and kanamycin) tested. In addition, the strain bound considerable amounts of Ni in their resting cells. Besides, the strain exhibited the plant growth promoting traits, such as solubilization of phosphate and production of indole-3-acetic acid (IAA) in modified Pikovskayas medium and LB medium, respectively in the absence and presence of Ni. Considering such potential, the effects of SR28C on the growth and Ni accumulation of B. juncea, L. cylindrica and S. halepense, were assessed with different concentrations of Ni in soil. Inoculation of SR28C stimulated the biomass of the test plants grown in both Ni contaminated and non-contaminated soils. Further, SR28C alleviated the detrimental effects of Ni by reducing its uptake and translocation to the plants. This study suggested that the PGPB inoculant due to its intrinsic abilities of growth promotion and attenuation of the toxic effects of Ni could be exploited for phytostabilization of Ni contaminated site.

Phytoextraction of heavy metal polluted soils using Sedum plumbizincicola inoculated with metal mobilizing Phyllobacterium myrsinacearum RC6b

The aim of this study was to investigate the effects of metal mobilizing plant-growth beneficial bacterium Phyllobacterium myrsinacearum RC6b on plant growth and Cd, Zn and Pb uptake by Sedum plumbizincicola under laboratory conditions. Among a collection of metal-resistant bacteria, P. myrsinacearum RC6b was specifically chosen as a most favorable metal mobilizer based on its capability of mobilizing high concentrations of Cd, Zn and Pb in soils. P. myrsinacearum RC6b exhibited a high degree of resistance to Cd (350 mg L(-1)), Zn (1000 mg L(-1)) and Pb (1200 mg L(-1)). Furthermore, P. myrsinacearum RC6b showed multiple plant growth beneficial features including the production of 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophore and solubilization of insoluble phosphate. Inoculation of P. myrsinacearum RC6b significantly increased S. plumbizincicola growth and organ metal concentrations except Pb, which concentration was lower in root and stem of inoculated plants. The results suggest that the metal mobilizing P. myrsinacearum RC6b could be used as an effective inoculant for the improvement of phytoremediation in multi-metal polluted soils.

Plant growth promoting properties of Halobacillus sp. and Halomonas sp. in presence of salinity and heavy metals

Salinity and heavy metal stress are challenging problems in agriculture. Here we report the plant growth promoting ability of three moderate halophiles, Halobacillus sp. ADN1, Halomonas sp. MAN5, and Halobacillus sp. MAN6, in presence of both salinity and heavy metal stress. Halobacillus sp. ADN1, Halomonas sp. MAN5, and Halobacillus sp. MAN6 can tolerate 25, 21, and 29% NaCl, respectively and grow in presence of 1 mM cobalt, cadmium, and nickel and 0.04 mM mercury and 0.03 mM silver. Halobacillus sp. ADN1, Halomonas sp. MAN5, and Halobacillus sp. MAN6 produced 152.5, 95.3, and 167.3 µg/ml indole acetic acid (IAA) and could solubilize 61, 53, and 75 parts per million (ppm) phosphate, respectively in the presence of 15% NaCl. The production of IAA and solubilization of phosphate was well retained in the presence of salinity and heavy metals like 1 mM cadmium, 0.7 mM nickel, 0.04 mM mercury, and 0.03 mM silver. Besides, the strains showed amylase and protease activities and could produce hydrogen cyanide and ammonia in presence of salinity and heavy metals. A mixture of three strains enhanced the root growth of Sesuvium portulacastrum under saline and heavy metal stress, where the root length increased nearly 4.5 ± 0.6 times and root dry weight increased 5.4 ± 0.5 times as compared to control. These strains can thus be useful in microbial assisted phytoremediation of polluted saline soils.

Inoculating Helianthus annuus (sunflower) grown in zinc and cadmium contaminated soils with plant growth promoting bacteria--effects on phytoremediation strategies

Plant growth promoting bacteria (PGPR) may help reducing the toxicity of heavy metals to plants in polluted environments. In this work the effects of inoculating metal resistant and plant growth promoting bacterial strains on the growth of Helianthus annuus grown in Zn and Cd spiked soils were assessed. The PGPR strains Ralstonia eutropha (B1) and Chrysiobacterium humi (B2) reduced losses of weight in metal exposed plants and induced changes in metal bioaccumulation and bioconcentration - with strain B2 decreasing up to 67% Zn accumulation and by 20% Zn bioconcentration factor (BCF) in the shoots, up to 64% Zn uptake and 38% Zn BCF in the roots, and up to 27% Cd uptake and 27% Cd BCF in plant roots. The impact of inoculation on the bacterial communities in the rhizosphere of the plant was also assessed. Bacterial community diversity decreased with increasing levels of metal contamination in the soil, but in rhizosphere soil of plants inoculated with the PGPR strains, a higher bacterial diversity was kept throughout the experimental period. Inoculation of sunflower, particularly with C. humi (B2), appears to be an effective way of enhancing the short term stabilization potential of the plant in metal contaminated land, lowering losses in plant biomass and decreasing aboveground tissue contamination.

Intrinsic characteristics of Cr⁶⁺-resistant bacteria isolated from an electroplating industry polluted soils for plant growth-promoting activities

The Cr(6+)-resistant plant growth-promoting bacteria was isolated from soil samples that were collected from an electroplating industry at Coimbatore, India, that had tolerated chromium concentrations up to 500 mg Cr(6+)/L in Luria-Bertani medium. Based on morphology, physiology, and biochemical characteristics, the strain was identified as Bacillus sp. following the Bergey's manual of determinative bacteriology. Evaluation of plant growth-promoting parameters has revealed the intrinsic ability of the strain for the production of indole-3-acetic acid (IAA), siderophore, and solubilization of insoluble phosphate. Bacillus sp. have utilized tryptophan as a precursor for their growth and produced IAA (122 μg/mL). Bacillus sp. also exhibited the production of siderophore that was tested qualitatively using Chrome Azurol S (CAS) assay solution and utilized the insoluble tricalcium phosphate as the sole source of phosphate exhibiting higher rate of phosphate solubilization after 72 h of incubation (1.45 μg/mL). Extent of Cr(6+) uptake and accumulation of Cr(6+) in the cell wall of Bacillus sp. was investigated using atomic absorption spectrophotometer and scanning electron microscope-energy dispersive spectroscopy, respectively. The congenital capability of this Cr(6+)-resistant plant growth-promoting Bacillus sp. could be employed as bacterial inoculum for the improvement of phytoremediation in heavy metal contaminated soils.

Assessment of mineral phosphate-solubilizing properties and molecular characterization of zinc-tolerant bacteria

Plant growth-promoting bacteria with the ability to tolerate heavy metals have importance both in sustainable agriculture and phytoremediation. The present study reports on the isolation and characterization of mineral phosphate-solubilizing (MPS) bacteria associated with the Achyranthes aspera L. plant (prickly chaff, flower plant). Out of 35 bacterial isolates, 6 isolates, namely RS7, RP23, EPR1, RS5, RP11 and RP19, with high MPS activity were selected and subjected to the assessment of MPS activity under various stress conditions, viz. ZnSO(4) (0.30-1.5 M), NaCl and temperature. MPS activity by the selected isolates was observed at concentrations of as high as >1.2 M ZnSO(4). Significant improvement in plant growth was observed on bacterization of seeds (pearl millet) with all of the six selected isolates. Plant growth was measured in terms of root length, shoot length, fresh weight and % increase in root biomass. The molecular diversity among the phosphate-solubilizing bacteria was studied employing enterobacterial repetitive intergenic sequence-PCR (ERIC-PCR). Representative strains from each ERIC type were identified, on the basis of a partial sequence of the 16S rRNA gene, as members of the genera Pseudomonas, Citrobacter, Acinetobacter, Serratia, and Enterobacter. Among all the isolates, RP19 was the best in terms of phosphate-solubizing activity and its response to various stresses. The ability of RP19 and other isolates to exhibit MPS activity at high ZnSO(4) concentrations suggests their potential as efficient biofertilizer for growing plants in metal (ZnSO(4))-contaminated soil.

Effect of multiple metal resistant bacteria from contaminated lake sediments on metal accumulation and plant growth

Naturally occurring bacteria play an important role in bioremediation of heavy metal pollutants in soil and wastewater. This study identified high levels of resistance to zinc, cesium, lead, arsenate and mercury in eight copper resistant Pseudomonas strains previously isolated from Torch Lake sediment. These strains showed variable susceptibility to different antibiotics. Furthermore, these metal resistant strains were capable of bioaccumulation of multiple metals and solubilization of copper. Bacterial strains TLC 3-3.5-1 and TLC 6-6.5-1 showed high bioaccumulation ability of Zn (up to 15.9 mg/g dry cell) and Pb (80.7 mg/g dry cell), respectively. All the strains produced plant growth promoting indole-3-acetic acid (IAA), iron chelating siderophore and solubilized mineral phosphate and metals. The effect of bacterial inoculation on plant growth and copper uptake by maize (Zea mays) and sunflower (Helianthus annuus) was investigated using one of the isolates (Pseudomonas sp. TLC 6-6.5-4) with higher IAA production and phosphate and metal solubilization, which resulted in a significant increase in copper accumulation in maize and sunflower, and an increase in the total biomass of maize. The multiple metal-resistant bacterial isolates characterized in our study have potential applications for remediation of metal contaminated soils in combination with plants and metal contaminated water.

Making phytoremediation work better: maximizing a plant's growth potential in the midst of adversity

While a number of different plants can either breakdown a variety of organic contaminants or hyperaccumulate metals from the environment, even the most efficient of those plants is typically inhibited by the presence of the toxicant(s). The plant stress that is induced by the presence of various environmental toxicants typically limits a plant's growth and ultimately its ability to phytoremediate the toxicant(s). Here, it is argued that the simple strategy of adding plant growth-promoting bacteria (preferably endophytes) that reduce plant ethylene levels by ACC deaminase activity and have the ability to synthesize the phytohoromone IAA, and are used to phytoremediate various toxicants can significantly (and often dramatically) increase both plant growth and phytoremediation activity in the presence of those toxicants.

Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils

Technogenic activities (industrial-plastic, textiles, microelectronics, wood preservatives; mining-mine refuse, tailings, smelting; agrochemicals-chemical fertilizers, farm yard manure, pesticides; aerosols-pyrometallurgical and automobile exhausts; biosolids-sewage sludge, domestic waste; fly ash-coal combustion products) are the primary sources of heavy metal contamination and pollution in the environment in addition to geogenic sources. During the last two decades, bioremediation has emerged as a potential tool to clean up the metal-contaminated/polluted environment. Exclusively derived processes by plants alone (phytoremediation) are time-consuming. Further, high levels of pollutants pose toxicity to the remediating plants. This situation could be ameliorated and accelerated by exploring the partnership of plant-microbe, which would improve the plant growth by facilitating the sequestration of toxic heavy metals. Plants can bioconcentrate (phytoextraction) as well as bioimmobilize or inactivate (phytostabilization) toxic heavy metals through in situ rhizospheric processes. The mobility and bioavailability of heavy metal in the soil, particularly at the rhizosphere where root uptake or exclusion takes place, are critical factors that affect phytoextraction and phytostabilization. Developing new methods for either enhancing (phytoextraction) or reducing the bioavailability of metal contaminants in the rhizosphere (phytostabilization) as well as improving plant establishment, growth, and health could significantly speed up the process of bioremediation techniques. In this review, we have highlighted the role of plant growth promoting rhizo- and/or endophytic bacteria in accelerating phytoremediation derived benefits in extensive tables and elaborate schematic sketches.

Biotechnological applications of serpentine soil bacteria for phytoremediation of trace metals

Serpentine or ultramafic soils are produced by weathering and pedogenesis of ultramafic rocks that are characterized by high levels of Ni, Cr, and sometimes Co, but contain low levels of essential nutrients such as N, P, K, and Ca. A number of plant species endemic to serpentine soils are capable of accumulating exceptionally high concentrations of Ni, Zn, and Co. These plants are known as metal "hyperaccumulators." The function of hyperaccumulation depends not only on the plant, but also on the interaction of the plant roots with rhizosphere microbes and the concentrations of bioavailable metals in the soil. The rhizosphere provides a complex and dynamic microenvironment where microorganisms, in association with roots, form unique communities that have considerable potential for the detoxification of hazardous materials. The rhizosphere bacteria play a significant role on plant growth in serpentine soils by various mechanisms, namely, fixation of atmospheric nitrogen, utilization of 1-aminocyclopropane-1-carboxylic acid (ACC) as the sole N source, production of siderophores, or production of plant growth regulators (hormones). Further, many microorganisms in serpentine soil are able to solubilize "unavailable" forms of heavy metal-bearing minerals by excreting organic acids. In addition, the metal-resistant serpentine isolates increase the efficiency of phytoextraction directly by enhancing the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass of hyperaccumulators. Hence, isolation of the indigenous and stress-adapted beneficial bacteria serve as a potential biotechnological tool for inoculation of plants for the successful restoration of metal-contaminated ecosystems. In this study, we highlight the diversity and beneficial features of serpentine bacteria and discuss their potential in phytoremediation of serpentine and anthropogenically metal-contaminated soils.