Advances in the application of plant growth-promoting rhizobacteria in phytoremediation of heavy metals

Bio-mediated detoxification of heavy metal contaminated soil and phytotoxicity reduction using novel strain of Brevundimonas vancanneytii SMA3

Heavy metals pose a serious environmental threat on a global scale due to their toxicity towards livings. Therefore, removing harmful metals from the environment has become more challenging in recent years. The objective of this study is to isolate, examine, and characterize naturally existing bacteria that possess the ability to mitigate and detoxify heavy metals such as cadmium, mercury, and lead. The selected bacteria SMA3 actively demonstrated metal tolerance during screening and was then employed for biosorption study using a lab-scale technique. The bacterium belonged to Brevundimonas sp., according to 16 S rRNA analysis. To enhance the removal efficiency of SMA3, response surface methodology (RSM) was employed, resulting in the identification of optimized conditions (pH 7, temperature 30 °C and shaking speed 120 rpm) for achieving maximum removal percentage (69.5 % of Cd, 58.6 % of Hg, and 85.1 % of Pb) within 72 h. The structural changes induced by microbial treatment were demonstrated by comparing the findings of FESEM images and FTIR spectra confirming the disappearance of C ^ C, C]O peaks along with C]O, C-O-C, C-H, and O-H bond destabilization following bioaccumulation. Moreover, in terms of phytotoxicity evaluation, it was observed that the treated soil, containing both heavy metals and the selected potent bacterial strain, exhibited reduced toxicity, resulting in improved germination and growth parameters for the seeds of Solanum lycopersicum (tomato plant). Overall, the selected bacterial strain demonstrated its potential for effectively removing multiple metals from the metal contaminated environment.

Comprehensive spatio-temporal benchmarking of surface water quality of Hindon River, a tributary of river Yamuna, India: Adopting multivariate statistical approach

The Hindon River is the main tributary of river Yamuna and it is a significant source of surface water, which flows through the major cities of western Uttar Pradesh, India. The indiscriminate development of industries and urbanization along river basin coupled with rapid population growths contribute various amounts of pollutant in the river. Therefore, the present study was conducted to assess the spatial-temporal variability of river water quality (seventeen physicochemical parameters and eight heavy metals) during pre- and post-monsoon seasons for 5 years data at 19 sampling sites along the river stretch. Indices associated with water quality and heavy metals were computed to scale the accurate state of risk associated to its use for drinking and irrigation. During the pre- and post-monsoon seasons, only four sites were found having safe water quality index (WQI) values. The mean heavy metal concentrations are found in order of Zn > Fe > Pb > Cu > Cr > Cd > Ni > Mn. Considering the spatial and temporal distribution, the study benchmarked the water quality of Hindon River for priority attention.

Plant growth promoting rhizobacteria and biochar production from Parthenium hysterophorus enhance seed germination and productivity in barley under drought stress

Drought stress can significantly affect plant growth and development. Biochar (BC) and plant growth-promoting rhizobacteria (PGPR) have been found to increase plant fertility and development under drought conditions. The single effects of BC and PGPR in different plant species have been widely reported under abiotic stress. However, there have been relatively few studies on the positive role of PGPR, BC, and their combination in barley (Hordeum vulgare L.). Therefore, the current study investigated the effects of BC from Parthenium hysterophorus, drought tolerant PGPR (Serratia odorifera), and the combination of BC + PGPR on the growth, physiology, and biochemical traits of barley plants under drought stress for two weeks. A total of 15 pots were used under five treatments. Each pot of 4 kg soil comprised the control (T0, 90% water), drought stress alone (T1, 30% water), 35 mL PGPR/kg soil (T2, 30% water), 2.5%/kg soil BC (T3, 30% water), and a combination of BC and PGPR (T4, 30% water). Combined PGPR and BC strongly mitigated the negative effects of drought by improving the shoot length (37.03%), fresh biomass (52%), dry biomass (62.5%), and seed germination (40%) compared to the control. The PGPR + BC amendment treatment enhanced physiological traits, such as chlorophyll a (27.9%), chlorophyll b (35.3%), and total chlorophyll (31.1%), compared to the control. Similarly, the synergistic role of PGPR and BC significantly (p< 0.05) enhanced the antioxidant enzyme activity including peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) to alleviate the toxicity of ROS. The physicochemical properties (N, K, P, and EL) of the soils were also enhanced by (85%, 33%, 52%, and 58%) respectively, under the BC + PGPR treatment compared to the control and drought stress alone. The findings of this study have suggested that the addition of BC, PGPR, and a combination of both will improve the soil fertility, productivity, and antioxidant defense systems of barley under drought stress. Therefore, BC from the invasive plant P. hysterophorus and PGPR can be applied to water-deficient areas to improve barley crop production.

A review of the effects of environmental hazards on humans, their remediation for sustainable development, and risk assessment

In the race for economic development and prosperity, our earth is becoming more polluted with each passing day. Technological advances in agriculture and rapid industrialization have drastically polluted the two pillars of natural resources, land and water. Toxic chemicals and microbial contaminants/agents created by natural and anthropogenic activities are rapidly becoming environmental hazards (EH) with increased potential to affect the natural environment and human health. This review has attempted to describe the various agents (chemical, biological, and physical) responsible for environmental contamination, remediation methods, and risk assessment techniques (RA). The main focus is on finding ways to mitigate the harmful effects of EHs through the simultaneous application of remediation methods and RA for sustainable development. It is recommended to apply the combination of different remediation methods using RA techniques to promote recycling and reuse of different resources for sustainable development. The report advocates for the development of site-specific, farmer-driven, sequential, and plant-based remediation strategies along with policy support for effective decontamination. This review also focuses on the fact that the lack of knowledge about environmental health is directly related to public health risks and, therefore, focuses on promoting awareness of effective ways to reduce anthropological burden and pollution and on providing valuable data that can be used in environmental monitoring assessments and lead to sustainable development.

A Green Approach Used for Heavy Metals 'Phytoremediation' Via Invasive Plant Species to Mitigate Environmental Pollution: A Review

Heavy metals (HMs) normally occur in nature and are rapidly released into ecosystems by anthropogenic activities, leading to a series of threats to plant productivity as well as human health. Phytoremediation is a clean, eco-friendly, and cost-effective method for reducing soil toxicity, particularly in weedy plants (invasive plant species (IPS)). This method provides a favorable tool for HM hyperaccumulation using invasive plants. Improving the phytoremediation strategy requires a profound knowledge of HM uptake and translocation as well as the development of resistance or tolerance to HMs. This review describes a comprehensive mechanism of uptake and translocation of HMs and their subsequent detoxification with the IPS via phytoremediation. Additionally, the improvement of phytoremediation through advanced biotechnological strategies, including genetic engineering, nanoparticles, microorganisms, CRISPR-Cas9, and protein basis, is discussed. In summary, this appraisal will provide a new platform for the uptake, translocation, and detoxification of HMs via the phytoremediation process of the IPS.

Toxic metals in the regulation of epithelial-mesenchymal plasticity: demons or angels?

Epithelial cells can trans-differentiate into motile mesenchymal cells through a dynamic process known as epithelial-mesenchymal transition (EMT). EMT is crucial in embryonic development and wound healing but also contributes to human diseases such as organ fibrosis and cancer progression. Heavy metals are environmental pollutants that can affect human health in various ways, including causing cancers. The cytotoxicity and carcinogenicity of heavy metals are complex, and studies have demonstrated that some of these metals can affect the progress of EMT. Here, we focus on reviewing the roles of six environmentally common toxic metals concerning EMT: arsenic (AS), cadmium (Cd), cobalt (Co), chromium (Cr), nickel (Ni), and copper (Cu). Noteworthily, the effects of these elements on EMT may vary according to the form, dose, and exposure time; the dual role of heavy metals (e.g., AS, Cd, and Cu) on EMT is also observed, in which, sometimes they can promote while sometimes inhibit the EMT process. Given the vast number of toxicologically relevant metals that exist in nature, we believe a comprehensive understanding of their effects on EMT is required to dictate in what circumstances these metals act more likely as demons or angels.

Novel strategies and advancement in reducing heavy metals from the contaminated environment

The most contemporary ecological issues are the dumping of unprocessed factories' effluent. As a result, there is an increasing demand for creative, practical, environmentally acceptable, and inexpensive methodologies to remediate inorganic metals (Hg, Cr, Pb, and Cd) liquidated into the atmosphere, protecting ecosystems. Latest innovations in biological metals have driven natural treatment as a viable substitute for traditional approaches in this area. To eliminate pesticide remains from soil/water sites, technologies such as oxidation, burning, adsorption, and microbial degradation have been established. Bioremediation is a more cost-effective and ecologically responsible means of removing heavy metals than conventional alternatives. As a result, microorganisms have emerged as a necessary component of methyl breakdown and detoxification via metabolic reactions and hereditary characteristics. The utmost operative variant for confiscating substantial metals commencing contaminated soil was A. niger, which had a maximum bioaccumulation efficiency of 98% (Cd) and 43% (Cr). Biosensor bacteria are both environmentally sustainable and cost-effective. As a result, microbes have a range of metal absorption processes that allow them to have higher metal biosorption capabilities. Additionally, the biosorption potential of bacterium, fungus, biofilm, and algae, inherently handled microorganisms that immobilized microbial cells for the elimination of heavy metals, was reviewed in this study. Furthermore, we discuss some of the challenges and opportunities associated with producing effective heavy metal removal techniques, such as those that employ different types of nanoparticles.

Phosphate-Solubilizing Pseudomonas sp. Strain WS32 Rhizosphere Colonization-Induced Expression Changes in Wheat Roots

Rhizosphere colonization is a pre-requisite for the favorable application of plant growth-promoting rhizobacteria (PGPR). Exchange and mutual recognition of signaling molecules occur frequently between plants and microbes. Here, the luciferase luxAB gene was electrotransformed into the phosphate-solubilizing strain Pseudomonas sp. WS32, a type of plant growth-promoting rhizobacterium with specific affinity for wheat. A labeled WS32 strain (WS32-L) was applied to determine the temporal and spatial traits of colonization within the wheat rhizosphere using rhizoboxes experimentation under natural condition. The effects of colonization on wheat root development and seedling growth were evaluated, and RNA sequencing (RNA-seq) was performed to explore the transcriptional changes that occur in wheat roots under WS32 colonization. The results showed that WS32-L could survive in the wheat rhizosphere for long periods and could expand into new zones following wheat root extension. Significant increases in seedling fresh and dry weight, root fresh and dry weight, root surface area, number of root tips, and phosphorus accumulation in the wheat leaves occurred in response to WS32 rhizosphere colonization. RNA-seq analysis showed that a total of 1485 genes in wheat roots were differentially expressed between the inoculated conditions and the uninoculated conditions. Most of the transcriptional changes occurred for genes annotated to the following functional categories: "phosphorus and other nutrient transport," "hormone metabolism and organic acid secretion," "flavonoid signal recognition," "membrane transport," and "transcription factor regulation." These results are therefore valuable to future studies focused on the molecular mechanisms underlying the growth-promoting activities of PGPR on their host plants.

Helping plants to deal with heavy metal stress: the role of nanotechnology and plant growth promoting rhizobacteria in the process of phytoremediation

Heavy metals (HMs) are not destroyable or degradable and persist in the environment for a long duration. Thus, eliminating and counteracting the HMs pollution of the soil environment is an urgent task to develop a safe and sustainable environment. Plants are in close contact with the soil and can play an important role in soil clean-up, and the process is known as phytoremediation. However, under HM contaminated conditions, plants suffer from several complications, like nutrient and mineral deficiencies, alteration of various physiological and biological processes, which reduces the plant's growth rate. On the other hand, the bioavailability of HMs is another factor for reduced phytoremediation, as most of the HMs are not bioavailable to plants for efficient phytoremediation. The altered plant growth and reduced bioavailability of HMs could be overcome and enhance the phytoremediation efficiency by incorporating either nanotechnology, i.e., nanoparticles (NPs) or plant growth promoting rhizobacteria (PGPR) along with phytoremediation. Single incorporation of NPs and PGPR might improve the growth rate in plants by enhancing nutrient availability and uptake and also by regulating plant growth regulators under HM contaminated conditions. However, there are certain limitations, like a high dose of NPs that might have toxic effects on plants. Thus, the combination of two techniques such as PGPR and NPs-based remediation can conquer the limitations of individual techniques and consequently enhance phytoremediation efficiency. Considering the negative impacts of HMs on the environment and living organisms, this review is aimed at highlighting the concept of phytoremediation, the single or combined integration of NPs and PGPR to help plants deal with HMs and their basic mechanisms involved in the process of phytoremediation. Additionally, the complications of using NPs and PGPR in the phytoremediation process are discussed to determine future research questions and this will assist to stimulate further research in this field and increase its effectiveness in practical application.

Insight Into Microbes and Plants Ability for Bioremediation of Heavy Metals

Contamination of ground and surface water, soil, and air by harmful and carcinogenic chemicals is one of the most prevalent problems in the modern industrialized world. Heavy metal toxicity has demonstrated to be paramount hazardous and there are various risks associated with it. In addition, these heavy metals have adverse effects on human health and plant physiology. The field of bioremediation has undergone an impactful revolution in recent years due to an exponential increase in various issues related to soil and water pollution. Bioremediation is an advanced and efficient technology, which involves the use of biological means such as microorganisms and plants to degrade heavy metal contaminants. Among the millions of microbes present in the ecosystem, the highest metal adsorption ability is possessed by species belonging to genus Penicillium, Streptomyces, Bacillus, Rhizopus, Chlorella, Ascophyllum, Sargassum, and Aspergillus. Among different plant species, Allium, Eucalyptus, Helianthus, and Hibiscus are the main heavy metal absorbers. The present review concentrates on the research in the bioremediation of important heavy metals through the use of plants and microbes.

Heteroauxin-producing bacteria enhance the plant growth and lead uptake of Miscanthus floridulus (Lab.)

Soil lead (Pb) contamination has caused severe environmental threats and is in urgent need of remediation. This study was aimed to explore the feasibility of using the Miscanthus-microbe combination to reduce Pb pollution in the farmland surrounding a lead-zinc mining area. We have screened three heteroauxin (IAA)-producing microbes (Lelliottia jeotgali MR2, Klebsiella michiganensis TS8, and Klebsiella michiganensis ZR1) with high Pb tolerance. The IAA-producing ability of the mixed-species was stronger than that of the single bacterium. In pot experiments, the mixed-species of MR2-ZR1 and MR2-TS8 had better performance in enhancing the weight of Miscanthus grass (increased by 22.2-53.6% compared to the control group without inoculating microbes). The remediation efficiency of Pb was significantly higher in the MR2 (30.79%), MR2-TS8 (24.96%), and TS8-ZR1 (21.10%) groups than that in the control group (6.75%). We speculated that MR2 and mixed species of MR2-TS8 and TS8-ZR1 could promote the percentages of activated Pb fractions in soils and increase the Pb uptake of M. floridulus (Lab.). These results implied that the MR2-TS8 mixed-species might be selected as the effective microbial agent to simultaneously enhance the remediation efficiency of Pb-contaminated soils and the biomass of M. floridulus (Lab.).

Plant Growth-Promoting Bacteria (PGPB) integrated phytotechnology: A sustainable approach for remediation of marginal lands

Land that has little to no utility for agriculture or industry is considered marginal land. This kind of terrain is frequently found on the edge of deserts or other arid regions. The amount of land that can be used for agriculture continues to be constrained by increasing desertification, which is being caused by climate change and the deterioration of agriculturally marginal areas. Plants and associated microorganisms are used to remediate and enhance the soil quality of marginal land. They represent a low-cost and usually long-term solution for restoring soil fertility. Among various phytoremediation processes (viz., phytodegradation, phytoextraction, phytostabilization, phytovolatilization, phytofiltration, phytostimulation, and phytodesalination), the employment of a specific mechanism is determined by the state of the soil, the presence and concentration of contaminants, and the plant species involved. This review focuses on the key economically important plants used for phytoremediation, as well as the challenges to plant growth and phytoremediation capability with emphasis on the advantages and limits of plant growth in marginal land soil. Plant growth-promoting bacteria (PGPB) boost plant development and promote soil bioremediation by secreting a variety of metabolites and hormones, through nitrogen fixation, and by increasing other nutrients' bioavailability through mineral solubilization. This review also emphasizes the role of PGPB under different abiotic stresses, including heavy-metal-contaminated land, high salinity environments, and organic contaminants. In our opinion, the improved soil fertility of marginal lands using PGPB with economically significant plants (e.g., Miscanthus) in dual precession technology will result in the reclamation of general agriculture as well as the restoration of native vegetation.

Study on metal binding capacity of the freshwater crab Sinopotamon henanense's recombinant copper specific binding metallothionein expressed in Escherichia coli

The copper specific binding metallothionein (CuMT) is a type of cysteine-rich, metal-binding, small protein which plays an important role in Cu²⁺ metabolism in vertebrates. In this study, we investigated the metal tolerance and removing ability of recombinant strains harboring CuMT obtained in vivo from the freshwater crab Sinopotamon henanense (ShCuMT) in order to study its physiological functions and metal binding capacity. We performed a 3D modeling of ShCuMT and created its structural and functional models using the I-TASSER program. The shCumt gene was inserted into a pGEX-4t-1 vector and recombinant soluble ShCuMT was expressed in Escherichia coli. In addition, in order to characterize the tolerance and removing ability of heavy metals in E. coli with ShCuMT expression, the recombinant strains harboring ShCuMT were exposed to various concentrations of Cd²⁺, Cu²⁺ and Zn²⁺, respectively. The results showed that ShCuMT contains transition metal binding sites. In addition, E. coli cells expressing ShCuMT exhibited enhanced metal tolerance and higher removing ability of metal ions than control cells. However, compared with Cd²⁺ and Zn²⁺, E. coli cells expressing ShCuMT have stronger tolerance and higher removing ability of Cu²⁺. In general, ShCuMT contains multiple transition metal binding sites, and it could enhance tolerance and removing ability of metal ions. Therefore, ShCuMT can provide potential candidates for heavy metal bioremediation. This research on the metal binding properties of ShCuMT provides a scientific basis for bioremediation of heavy metal pollution by the recombinant strains.

Utilization of Microbial Consortia as Biofertilizers and Biopesticides for the Production of Feasible Agricultural Product

Farmers are now facing a reduction in agricultural crop yield, due to the infertility of soils and poor farming. The application of chemical fertilizers distresses soil fertility and also human health. Inappropriate use of chemical fertilizer leads to the rapid decline in production levels in most parts of the world, and hence requires the necessary standards of good cultivation practice. Biofertilizers and biopesticides have been used in recent years by farmers worldwide to preserve natural soil conditions. Biofertilizer, a replacement for chemical fertilizer, is cost-effective and prevents environmental contamination to the atmosphere, and is a source of renewable energy. In contrast to chemical fertilizers, biofertilizers are cost-effective and a source of renewable energy that preserves long-term soil fertility. The use of biofertilizers is, therefore, inevitable to increase the earth's productivity. A low-input scheme is feasible to achieve farm sustainability through the use of biological and organic fertilizers. This study investigates the use of microbial inoculants as biofertilizers to increase crop production.

Cross-Talks Between Macro- and Micronutrient Uptake and Signaling in Plants

In nature, land plants as sessile organisms are faced with multiple nutrient stresses that often occur simultaneously in soil. Nitrogen (N), phosphorus (P), sulfur (S), zinc (Zn), and iron (Fe) are five of the essential nutrients that affect plant growth and health. Although these minerals are relatively inaccessible to plants due to their low solubility and relative immobilization, plants have adopted coping mechanisms for survival under multiple nutrient stress conditions. The double interactions between N, Pi, S, Zn, and Fe have long been recognized in plants at the physiological level. However, the molecular mechanisms and signaling pathways underlying these cross-talks in plants remain poorly understood. This review preliminarily examined recent progress and current knowledge of the biochemical and physiological interactions between macro- and micro-mineral nutrients in plants and aimed to focus on the cross-talks between N, Pi, S, Zn, and Fe uptake and homeostasis in plants. More importantly, we further reviewed current studies on the molecular mechanisms underlying the cross-talks between N, Pi, S, Zn, and Fe homeostasis to better understand how these nutrient interactions affect the mineral uptake and signaling in plants. This review serves as a basis for further studies on multiple nutrient stress signaling in plants. Overall, the development of an integrative study of multiple nutrient signaling cross-talks in plants will be of important biological significance and crucial to sustainable agriculture.

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

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

The Influence of Soil Fertilization on the Distribution and Diversity of Phosphorus Cycling Genes and Microbes Community of Maize Rhizosphere Using Shotgun Metagenomics

Biogeochemical cycling of phosphorus in the agro-ecosystem is mediated by soil microbes. These microbes regulate the availability of phosphorus in the soil. Little is known about the response of functional traits of phosphorus cycling microbes in soil fertilized with compost manure (derived from domestic waste and plant materials) or inorganic nitrogen fertilizers at high and low doses. We used a metagenomics investigation study to understand the changes in the abundance and distribution of microbial phosphorus cycling genes in agricultural farmlands receiving inorganic fertilizers (120 kg N/ha, 60 kg N/ha) or compost manure (8 tons/ha, 4 tons/ha), and in comparison with the control. Soil fertilization with high level of compost (Cp8) or low level of inorganic nitrogen (N1) fertilizer have nearly similar effects on the rhizosphere of maize plants in promoting the abundance of genes involved in phosphorus cycle. Genes such as ppk involved in polyphosphate formation and pstSABC (for phosphate transportation) are highly enriched in these treatments. These genes facilitate phosphorus immobilization. At a high dose of inorganic fertilizer application or low compost manure treatment, the phosphorus cycling genes were repressed and the abundance decreased. The bacterial families Bacillaceae and Carnobacteriaceae were very abundant in the high inorganic fertilizer (N2) treated soil, while Pseudonocardiaceae, Clostridiaceae, Cytophagaceae, Micromonosporaceae, Thermomonosporaceae, Nocardiopsaceae, Sphaerobacteraceae, Thermoactinomycetaceae, Planococcaceae, Intrasporangiaceae, Opitutaceae, Acidimicrobiaceae, Frankiaceae were most abundant in Cp8. Pyrenophora, Talaromyces, and Trichophyton fungi were observed to be dominant in Cp8 and Methanosarcina, Methanobrevibacter, Methanoculleus, and Methanosphaera archaea have the highest percentage occurrence in Cp8. Moreover, N2 treatment, Cenarchaeum, Candidatus Nitrososphaera, and Nitrosopumilus were most abundant among fertilized soils. Our findings have brought to light the basis for the manipulation of rhizosphere microbial communities and their genes to improve availability of phosphorus as well as phosphorus cycle regulation in agro-ecosystems.

Bioaugmented Phytoremediation of Metal-Contaminated Soils and Sediments by Hemp and Giant Reed

We assessed the effects of EDTA and selected plant growth-promoting rhizobacteria (PGPR) on the phytoremediation of soils and sediments historically contaminated by Cr, Ni, and Cu. A total of 42 bacterial strains resistant to these heavy metals (HMs) were isolated and screened for PGP traits and metal bioaccumulation, and two Enterobacter spp. strains were finally selected. Phytoremediation pot experiments of 2 months duration were carried out with hemp (Cannabis sativa L.) and giant reed (Arundo donax L.) grown on soils and sediments respectively, comparing in both cases the effects of bioaugmentation with a single PGPR and EDTA addition on plant and root growth, plant HM uptake, HM leaching, as well as the changes that occurred in soil microbial communities (structure, biomass, and activity). Good removal percentages on a dry mass basis of Cr (0.4%), Ni (0.6%), and Cu (0.9%) were observed in giant reed while negligible values (<100‰) in hemp. In giant reed, HMs accumulated differentially in plant (rhizomes > > roots > leaves > stems) with largest quantities in rhizomes (Cr 0.6, Ni 3.7, and Cu 2.2 g plant^–1). EDTA increased Ni and Cu translocation to aerial parts in both crops, despite that in sediments high HM concentrations in leachates were measured. PGPR did not impact fine root diameter distribution of both crops compared with control while EDTA negatively affected root diameter class length (DCL) distribution. Under HM contamination, giant reed roots become shorter (from 5.2 to 2.3 mm cm^–3) while hemp roots become shorter and thickened from 0.13 to 0.26 mm. A consistent indirect effect of HM levels on the soil microbiome (diversity and activity) mediated by plant response (root DCL distribution) was observed. Multivariate analysis of bacterial diversity and activity revealed not only significant effects of plant and soil type (rhizosphere vs. bulk) but also a clear and similar differentiation of communities between control, EDTA, and PGPR treatments. We propose root DCL distribution as a key plant trait to understand detrimental effect of HMs on microbial communities. Positive evidence of the soil-microbe-plant interactions occurring when bioaugmentation with PGPR is associated with deep-rooting perennial crops makes this combination preferable over the one with chelating agents. Such knowledge might help to yield better bioaugmented bioremediation results in contaminated sites.

Soil Texture, Sampling Depth and Root Hairs Shape the Structure of ACC Deaminase Bacterial Community Composition in Maize Rhizosphere

Preservation of the phytostimulatory functions of plant growth-promoting bacteria relies on the adaptation of their community to the rhizosphere environment. Here, an amplicon sequencing approach was implemented to specifically target microorganisms with 1-aminocyclopropane-1-carboxylate deaminase activity, carrying the acdS gene. We stated the hypothesis that the relative phylogenetic distribution of acdS carrying microorganisms is affected by the presence or absence of root hairs, soil type, and depth. To this end, a standardized soil column experiment was conducted with maize wild type and root hair defective rth3 mutant in the substrates loam and sand, and harvest was implemented from three depths. Most acdS sequences (99%) were affiliated to Actinobacteria and Proteobacteria, and the strongest influence on the relative abundances of sequences were exerted by the substrate. Variovorax, Acidovorax, and Ralstonia sequences dominated in loam, whereas Streptomyces and Agromyces were more abundant in sand. Soil depth caused strong variations in acdS sequence distribution, with differential levels in the relative abundances of acdS sequences affiliated to Tetrasphaera, Amycolatopsis, and Streptomyces in loam, but Burkholderia, Paraburkholderia, and Variovorax in sand. Maize genotype influenced the distribution of acdS sequences mainly in loam and only in the uppermost depth. Variovorax acdS sequences were more abundant in WT, but Streptomyces, Microbacterium, and Modestobacter in rth3 rhizosphere. Substrate and soil depth were strong and plant genotype a further significant single and interacting drivers of acdS carrying microbial community composition in the rhizosphere of maize. This suggests that maize rhizosphere acdS carrying bacterial community establishes according to the environmental constraints, and that root hairs possess a minor but significant impact on acdS carrying bacterial populations.

Ameliorative effect of co-application of Bradyrhizobium japonicum EI09 and Se to mitigate chromium stress in Capsicum annum L

The present study was conducted to explore the potential of Bradyrhizobium japonicum EI09 (EI09) and selenium (Se) alone or in combination to mitigate hexavalent chromium (Cr⁶⁺) stress in Capsicum annum L. Chromium stressed plants exhibited significant reduction in biomass, chlorophyll content and gas exchange characteristics. The inoculated seedlings subjected to Cr⁶⁺stress showed improvement in growth, proline content, gas exchange attributes and total soluble proteins. Likewise, inoculated C. annum seedlings exhibited augmented activity of ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) under Cr⁶⁺ stress. The Cr⁶⁺ stress mitigation in inoculated seedlings was ascribed to reduction in malondialdehyde (MDA) content, hydrogen peroxide (H₂O₂) besides increase activity of flavonoids, proline, phenolic content along with modulation of antioxidative enzymes. The growth-enhancing attributes of bacteria such as indole acetic acid (IAA) content and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity enhanced growth in Cr⁶⁺-stressed plants. Moreover, co-treatment of EI09 and 5 µM Se effectively mitigated Cr (VI) stress in C. annum plants. Current studies provide a novel insight into potential of B. japonicum EI09 and Se in reduction of Cr⁶⁺ toxicity in C. annum plants.

Assessment of spatial and temporal variations in water quality by the application of multivariate statistical methods in the Kali River, Uttar Pradesh, India

The Kali River is a significant source of surface water as well as the main tributary of River Hindon that flows through major cities of western Uttar Pradesh, India. It flows throughout the urban and industrial regions; hence, it carries various amounts of pollutant. Therefore, a study was conducted to examine spatial-temporal variations in river water quality by determining physicochemical variables and heavy metal concentrations at seventeen sampling stations (S₁-S₁₇) throughout the river stretch. Various physicochemical variables, namely pH, EC, TDS, turbidity, BOD, COD, TH, TA, Ca, Mg, Na, K, HCO₃^-, Cl^-, SO₄^2-, NO₃^-, and PO₄^3- were higher in summer than in winter. The order of mean metal concentrations was Fe > Pb > Mn > Ni > Zn > Cu > Cr > Cd. The relationships among measured physicochemical variables and pollution index were examined. Furthermore, multivariate statistical methods were used to assess spatial-temporal variation in water quality to identify current pollution sources and validate results. Water quality index and comprehensive pollution index indicated that the Kali River was less polluted from S₁ to S₈. However, downstream sampling sites were polluted. Pollution starts from S₉ and drastically increases at and beyond S₁₃ because of effluents from industries and sugar mills in Muzaffarnagar. The study suggests cleaning the downstream region of river to restore human health and flora and fauna in the river ecosystem.

Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils

Accumulation of heavy metals in agricultural soils due to human production activities-mining, fossil fuel combustion, and application of chemical fertilizers/pesticides-results in severe environmental pollution. As the transmission of heavy metals through the food chain and their accumulation pose a serious risk to human health and safety, there has been increasing attention in the investigation of heavy metal pollution and search for effective soil remediation technologies. Here, we summarized and discussed the basic principles, strengths and weaknesses, and limitations of common standalone approaches such as those based on physics, chemistry, and biology, emphasizing their incompatibility with large-scale applications. Moreover, we explained the effects, advantages, and disadvantages of the combinations of common single repair approaches. We highlighted the latest research advances and prospects in phytoremediation-chemical, phytoremediation-microbe, and phytoremediation-genetic engineering combined with remediation approaches by changing metal availability, improving plant tolerance, promoting plant growth, improving phytoextraction and phytostabilization, etc. We then explained the improved safety and applicability of phytoremediation combined with other repair approaches compared to common standalone approaches. Finally, we established a prospective research direction of phytoremediation combined with multi-technology repair strategy.

Lead Toxicity: Health Hazards, Influence on Food Chain, and Sustainable Remediation Approaches

Lead (Pb) toxicity has been a subject of interest for environmental scientists due to its toxic effect on plants, animals, and humans. An increase in several Pb related industrial activities and use of Pb containing products such as agrochemicals, oil and paint, mining, etc. can lead to Pb contamination in the environment and thereby, can enter the food chain. Being one of the most toxic heavy metals, Pb ingestion via the food chain has proven to be a potential health hazard for plants and humans. The current review aims to summarize the research updates on Pb toxicity and its effects on plants, soil, and human health. Relevant literature from the past 20 years encompassing comprehensive details on Pb toxicity has been considered with key issues such as i) Pb bioavailability in soil, ii) Pb biomagnification, and iii) Pb- remediation, which has been addressed in detail through physical, chemical, and biological lenses. In the review, among different Pb-remediation approaches, we have highlighted certain advanced approaches such as microbial assisted phytoremediation which could possibly minimize the Pb load from the resources in a sustainable manner and would be a viable option to ensure a safe food production system.

Genome Sequencing of Pantoea agglomerans C1 Provides Insights into Molecular and Genetic Mechanisms of Plant Growth-Promotion and Tolerance to Heavy Metals

Distinctive strains of Pantoea are used as soil inoculants for their ability to promote plant growth. Pantoea agglomerans strain C1, previously isolated from the phyllosphere of lettuce, can produce indole-3-acetic acid (IAA), solubilize phosphate, and inhibit plant pathogens, such as Erwinia amylovora. In this paper, the complete genome sequence of strain C1 is reported. In addition, experimental evidence is provided on how the strain tolerates arsenate As (V) up to 100 mM, and on how secreted metabolites like IAA and siderophores act as biostimulants in tomato cuttings. The strain has a circular chromosome and two prophages for a total genome of 4,846,925-bp, with a DNA G+C content of 55.2%. Genes related to plant growth promotion and biocontrol activity, such as those associated with IAA and spermidine synthesis, solubilization of inorganic phosphate, acquisition of ferrous iron, and production of volatile organic compounds, siderophores and GABA, were found in the genome of strain C1. Genome analysis also provided better understanding of the mechanisms underlying strain resistance to multiple toxic heavy metals and transmission of these genes by horizontal gene transfer. Findings suggested that strain C1 exhibits high biotechnological potential as plant growth-promoting bacterium in heavy metal polluted soils.

Functionalized magnetic particles for water treatment

In this study, we have taken the concept of water treatment by functionalized magnetic particles one step forward by integrating the technology into a complete proof of concept, which included the preparation of surface modified beads, their use as highly selective absorbents for heavy metals ions (Zinc, Nickel), and their performance in terms of magnetic separation. The separation characteristics were studied both through experiments and by simulations. The data gathered from these experimental works enabled the elaboration of various scenarios for Life Cycle Analysis (LCA). The LCA showed that the environmental impact of the system is highly dependent on the recovery rate of the magnetic particles. The absolute impact on climate change varied significantly among the scenarios studied and the recovery rates. The results support the hypothesis that chelation specificity, magnetic separation and bead recovery should be optimized to specific targets and applications.

Safe Cultivation of Medicago sativa in Metal-Polluted Soils from Semi-Arid Regions Assisted by Heat- and Metallo-Resistant PGPR

Soil contamination with heavy metals is a constraint for plant establishment and development for which phytoremediation may be a solution, since rhizobacteria may alleviate plant stress under these conditions. A greenhouse experiment was conducted to elucidate the effect of toxic metals on growth, the activities of ROS (reactive oxygen species)-scavenging enzymes, and gene expression of Medicago sativa grown under different metal and/or inoculation treatments. The results showed that, besides reducing biomass, heavy metals negatively affected physiological parameters such as chlorophyll fluorescence and gas exchange, while increasing ROS-scavenging enzyme activities. Inoculation of M. sativa with a bacterial consortium of heat- and metallo-resistant bacteria alleviated metal stress, as deduced from the improvement of growth, lower levels of antioxidant enzymes, and increased physiological parameters. The bacteria were able to effectively colonize and form biofilms onto the roots of plants cultivated in the presence of metals, as observed by scanning electron microscopy. Results also evidenced the important role of glutathione reductase (GR), phytochelatin synthase (PCS), and metal transporter NRAMP1 genes as pathways for metal stress management, whereas the gene coding for cytochrome P450 (CP450) seemed to be regulated by the presence of the bacteria. These outcomes showed that the interaction of metal-resistant rhizobacteria/legumes can be used as an instrument to remediate metal-contaminated soils, while cultivation of inoculated legumes on these soils is still safe for animal grazing, since inoculation with bacteria diminished the concentrations of heavy metals accumulated in the aboveground parts of the plants to below toxic levels.

Impact of Salicylic Acid and PGPR on the Drought Tolerance and Phytoremediation Potential of Helianthus annus

The present study was aimed to isolate and characterize plant growth promoting rhizobacteria (PGPR) from the rhizosphere of rainfed area (Karak) in Pakistan. The influence of isolated rhizobacteria, in association with salicylic acid (SA), physiological attributes, drought tolerance potential, and phytoremediation in drought-stressed sunflower exposed was investigated. The isolated bacteria were named P1 and P2 and characterized on the basis of colony morphology and biochemical traits. Both PGPR P1 and P2 were identified on the basis of 16S-rRNA gene sequencing as Planomicrobium chinense strain P1 (Accession No. MF616408) and Bacillus cereus strain P2 (Accession No. MF616406). The fresh cultures (24 h old) of isolates were used to soak the seeds pre-sowing. SA was foliar applied at three-leaf-stage. Likewise, the 30-days-old seedlings (three leaf stage) were exposed to drought stress. Drought stress was imposed to 30-days-old plants (three-leaf stage) by withholding water supply for the next 15 days until the soil water content reached 10%. The PGPR and/or SA treatment resulted in significant accumulation of Cd (84%), Pb (66%), and Ni (65%) in the rhizosphere. PGPR also induced accumulation of Cd and Ni in plant shoot. Combined treatment of PGPR and SA increased the Cu (21%), Co (11%), and Zn (8%) accumulation but decreased (12%) the Fe accumulation as compared to coinoculation of PGPR P1 and P2. Inoculation of plants with PGPR significantly increased shoot length (60%), root length (68%), root fresh (61%), and dry (63%) biomass under water stress. The inoculated plants had increased chlorophyll (67%), carotenoid (70%), leaf protein (64%), sugar (64%), and phenolic (62%) contents while lower leaf proline (62%) content, malondialdehyde (MDA) (64%), and antioxidant enzymes (67%) which suggest their role in drought tolerance. It is concluded that integrative use of PGPR in combination with SA found to be an efficacious strategy to improve the phytoremediation of heavy metals and plant growth under stressed conditions particularly under water-deficient conditions.

Inoculation of plant growth-promoting bacteria Bacillus sp. YM-1 alleviates the toxicity of Pb to pakchoi

Heavy metal accumulation in plants may imperil human health. Inoculation of plant growth-promoting bacteria can alleviate the toxicity of heavy metal and promote plant growth. In this study, Bacillus sp. YM-1, a heavy metal resistant and plant growth-promoting bacterium, was immobilized with spent substrate of mushroom and applied to alleviate the toxicity of Pb to pakchoi. The results indicated that the biomass of pakchoi in inoculation group was increased by 17.45 to 27.05% compared with that in non-inoculated group (p < 0.05). The root and shoot were lengthened by 13.45% to 39.17% and 20.23% to 42.36%, respectively. The content of Pb in root and shoot obviously reduced and that in edible part (shoot) was less than 0.2 mg kg^-1 in the low concentration of Pb. Other indicators such as superoxide dismutase (SOD), peroxidase (POD), chlorophyll, and protein all testified that YM-1 inoculation was conducive to the alleviation of Pb toxicity to pakchoi.

Enzymatic detoxification of azo dyes by a multifarious Bacillus sp. strain MR-1/2-bearing plant growth-promoting characteristics

This study was conducted to elucidate the inherent potential of Bacillus sp. MR-1/2, which was isolated from root zone of maize crop grown on a textile wastewater-irrigated soil. The isolated strain was identified through its ribosomal RNA sequence. Under in vitro conditions, the strain demonstrated its tolerance for high concentrations of various heavy metal ions as determined by minimum inhibitory concentration. Moreover, the strain MR-1/2 exhibited many important phytobeneficial traits such as inorganic P solubilization and 1-aminocyclopropane-1-carboxylate (ACC) deaminase ability even under high metal and salt stress. Results showed that the strain proficiently decolorizes various azo dye compounds, e.g., reactive black-5, reactive red-120, and direct blue-1 and congo red, in broth culture. The bioremediation potential of the strain MR-1/2 was further confirmed by analyzing the retrieved azoreductase gene sequence through bioinformatics tools, whereby a subsequent prediction revealed that the azoreductase enzyme activity was involved in decolorization process. When mung bean seeds were grown in pots under various concentrations of decolorized and non-decolorized azo dye, the Bacillus sp. MR-1/2 not only alleviated the azo dye toxicity, but also increased the plant growth parameters. In conclusion, the strain MR-1/2 efficiently decolorized the azo dyes and helped in mung bean plant growth by alleviating azo dye toxicity.

The influence of plant growth-promoting rhizobacteria in plant tolerance to abiotic stress: a survival strategy

Action is needed to face the global threat arising from inconsistent rainfall, rise in temperature, and salinization of farm lands which may be the product of climate change. As crops are adversely affected, man and animals may face famine. Plants are severely affected by abiotic stress (drought, salinity, alkalinity, and temperature), which impairs yield and results in loss to farmers and to the nation at large. However, microbes have been shown to be of great help in the fight against abiotic stress, via their biological activities at the rhizosphere of plants. The external application of chemical substances such as glycine betaine, proline, and nutrients has helped in sustaining plant growth and productive ability. In this review, we tried to understand the part played by bioinoculants in aiding plants to resist the negative consequences arising from abiotic stress and to suggest better practices that will be of help in today’s farming systems. The fact that absolute protection and sustainability of plant yield under stress challenges has not been achieved by microbes, nutrients, nor the addition of chemicals (osmo-protectants) alone suggests that studies should focus on the integration of these units (microbes, nutrients, chemical stimulants, and osmo-protectants) into a strategy for achieving a complete tolerance to abiotic stress. Also, other species of microbes capable of shielding plant from stress, boosting yield and growth, providing nutrients, and protecting the plants from harmful invading pathogens should be sought.

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.

Bacterial Rhizoplane Colonization Patterns of Buchloe dactyloides Growing in Metalliferous Mine Tailings Reflect Plant Status and Biogeochemical Conditions

Plant establishment during phytostabilization of legacy mine tailings in semiarid regions is challenging due to low pH, low organic carbon, low nutrients, and high toxic metal(loid) concentrations. Plant-associated bacterial communities are particularly important under these harsh conditions because of their beneficial services to plants. We hypothesize that bacterial colonization profiles on rhizoplane surfaces reflect deterministic processes that are governed by plant health and the root environment. The aim of this study was to identify associations between bacterial colonization patterns on buffalo grass (Buchloe dactyloides) rhizoplanes and both plant status (leaf chlorophyll and plant cover) and substrate biogeochemistry (pH, electrical conductivity, total organic carbon, total nitrogen, and rhizosphere microbial community). Buffalo grass plants from mesocosm- and field-scale phytostabilization trials conducted with tailings from the Iron King Mine and Humboldt Smelter Superfund Site in Dewey-Humboldt, Arizona, were analyzed. These tailings are extremely acidic and have arsenic and lead concentrations of 2-4 g kg^-1 substrate. Bacterial communities on rhizoplanes and in rhizosphere-associated substrate were characterized using fluorescence in situ hybridization and 16S rRNA gene amplicon sequencing, respectively. The results indicated that the metabolic status of rhizoplane bacterial colonizers is significantly related to plant health. Principal component analysis revealed that root-surface Alphaproteobacteria relative abundance was associated most strongly with substrate pH and Gammaproteobacteria relative abundance associated strongly with substrate pH and plant cover. These factors also affected the phylogenetic profiles of the associated rhizosphere communities. In summary, rhizoplane bacterial colonization patterns are plant specific and influenced by plant status and rhizosphere biogeochemical conditions.

Double genetically modified symbiotic system for improved Cu phytostabilization in legume roots

Excess copper (Cu) in soils has deleterious effects on plant growth and can pose a risk to human health. In the last decade, legume-rhizobium symbioses became attractive biotechnological tools for metal phytostabilization. For this technique being useful, metal-tolerant symbionts are required, which can be generated through genetic manipulation.In this work, a double symbiotic system was engineered for Cu phytostabilization: On the one hand, composite Medicago truncatula plants expressing the metallothionein gene mt4a from Arabidopsis thaliana in roots were obtained to improve plant Cu tolerance. On the other hand, a genetically modified Ensifer medicae strain, expressing copper resistance genes copAB from Pseudomonas fluorescens driven by a nodulation promoter, nifHp, was used for plant inoculation. Our results indicated that expression of mt4a in composite plants ameliorated plant growth and nodulation and enhanced Cu tolerance. Lower levels of ROS-scavenging enzymes and of thiobarbituric acid reactive substances (TBARS), such as malondialdehyde (a marker of lipid peroxidation), suggested reduced oxidative stress. Furthermore, inoculation with the genetically modified Ensifer further improved root Cu accumulation without altering metal loading to shoots, leading to diminished values of metal translocation from roots to shoots. The double modified partnership is proposed as a suitable tool for Cu rhizo-phytostabilization.

A New Strategy for Heavy Metal Polluted Environments: A Review of Microbial Biosorbents

Persistent heavy metal pollution poses a major threat to all life forms in the environment due to its toxic effects. These metals are very reactive at low concentrations and can accumulate in the food web, causing severe public health concerns. Remediation using conventional physical and chemical methods is uneconomical and generates large volumes of chemical waste. Bioremediation of hazardous metals has received considerable and growing interest over the years. The use of microbial biosorbents is eco-friendly and cost effective; hence, it is an efficient alternative for the remediation of heavy metal contaminated environments. Microbes have various mechanisms of metal sequestration that hold greater metal biosorption capacities. The goal of microbial biosorption is to remove and/or recover metals and metalloids from solutions, using living or dead biomass and their components. This review discusses the sources of toxic heavy metals and describes the groups of microorganisms with biosorbent potential for heavy metal removal.

Burkholderia phytofirmans Inoculation-Induced Changes on the Shoot Cell Anatomy and Iron Accumulation Reveal Novel Components of Arabidopsis-Endophyte Interaction that Can Benefit Downstream Biomass Deconstruction

It is known that plant growth promoting bacteria (PGPB) elicit positive effects on plant growth and biomass yield. However, the actual mechanism behind the plant-PGPB interaction is poorly understood, and the literature is scarce regarding the thermochemical pretreatability and enzymatic degradability of biomass derived from PGPB-inoculated plants. Most recent transcriptional analyses of PGPB strain Burkholderia phytofirmans PsJN inoculating potato in literature and Arabidopsis in our present study have revealed the expression of genes for ferritin and the biosynthesis and transport of siderophores (i.e., the molecules with high affinity for iron), respectively. The expression of such genes in the shoots of PsJN-inoculated plants prompted us to propose that PsJN-inoculation can improve the host plant's iron uptake and accumulation, which facilitates the downstream plant biomass pretreatment and conversion to simple sugars. In this study, we employed B. phytofirmans PsJN to inoculate the Arabidopsis thaliana plants, and conducted the first investigation for its effects on the biomass yield, the anatomical organization of stems, the iron accumulation, and the pretreatment and enzymatic hydrolysis of harvested biomass. The results showed that the strain PsJN stimulated plant growth in the earlier period of plant development and enlarged the cell size of stem piths, and it also indeed enhanced the essential metals uptake and accumulation in host plants. Moreover, we found that the PsJN-inoculated plant biomass released more glucose and xylose after hot water pretreatment and subsequent co-saccharification, which provided a novel insight into development of lignocellulosic biofuels from renewable biomass resources.

Copper-resistant bacteria reduces oxidative stress and uptake of copper in lentil plants: potential for bacterial bioremediation

For effective microbe-assisted bioremediation, metal-resistant plant growth-promoting bacteria (PGPB) must facilitate plant growth by restricting excess metal uptake in plants, leading to prevent its bio-amplification in the ecosystem. The aims of our study were to isolate and characterize copper (Cu)-resistant PGPB from waste water receiving contaminated soil. In addition, we investigated the phytotoxic effect of copper on the lentil plants inoculated with copper-resistant bacteria Providencia vermicola, grown in copper-contaminated soil. Copper-resistant P. vermicola showed multiple plant growth promoting characteristics, when used as a seed inoculant. It protected the lentil plants from copper toxicity with a considerable increase in root and shoot length, plant dry weight and leaf area. A notable increase in different gas exchange characteristics such as A, E, C i , g s , and A/E, as well as increase in N and P accumulation were also recorded in inoculated plants as compared to un-inoculated copper stressed plants. In addition, leaf chlorophyll content, root nodulation, number of pods, 1,000 seed weight were also higher in inoculated plants as compared with non-inoculated ones. Anti-oxidative defense mechanism improved significantly via elevated expression of reactive oxygen species -scavenging enzymes including ascorbate peroxidase, superoxide dismutase, catalase, and guaiacol peroxidase with alternate decrease in malondialdehyde and H2O2 contents, reduced electrolyte leakage, proline, and total phenolic contents suggesting that inoculation of P. vermicola triggered heavy metals stress-related defense pathways under copper stress. Overall, the results demonstrated that the P. vermicola seed inoculation confer heavy metal stress tolerance in lentil plant which can be used as a potent biotechnological tool to cope with the problems of copper pollution in crop plants for better yield.

Endophytic Cultivable Bacteria of the Metal Bioaccumulator Spartina maritima Improve Plant Growth but Not Metal Uptake in Polluted Marshes Soils

Endophytic bacterial population was isolated from Spartina maritima tissues, a heavy metal bioaccumulator cordgrass growing in the estuaries of Tinto, Odiel, and Piedras River (south west Spain), one of the most polluted areas in the world. Strains were identified and ability to tolerate salt and heavy metals along with plant growth promoting and enzymatic properties were analyzed. A high proportion of these bacteria were resistant toward one or several heavy metals and metalloids including As, Cu, and Zn, the most abundant in plant tissues and soil. These strains also exhibited multiple enzymatic properties as amylase, cellulase, chitinase, protease and lipase, as well as plant growth promoting properties, including nitrogen fixation, phosphates solubilization, and production of indole-3-acetic acid (IAA), siderophores and 1-aminocyclopropane-1-carboxylate (ACC) deaminase. The best performing strains (Micrococcus yunnanensis SMJ12, Vibrio sagamiensis SMJ18, and Salinicola peritrichatus SMJ30) were selected and tested as a consortium by inoculating S. maritima wild plantlets in greenhouse conditions along with wild polluted soil. After 30 days, bacterial inoculation improved plant photosynthetic traits and favored intrinsic water use efficiency. However, far from stimulating plant metal uptake, endophytic inoculation lessened metal accumulation in above and belowground tissues. These results suggest that inoculation of S. maritima with indigenous metal-resistant endophytes could mean a useful approach in order to accelerate both adaption and growth of this indigenous cordgrass in polluted estuaries in restorative operations, but may not be suitable for rhizoaccumulation purposes.

Biochemistry and genetics of ACC deaminase: a weapon to "stress ethylene" produced in plants

1-aminocyclopropane-1-carboxylate deaminase (ACCD), a pyridoxal phosphate-dependent enzyme, is widespread in diverse bacterial and fungal species. Owing to ACCD activity, certain plant associated bacteria help plant to grow under biotic and abiotic stresses by decreasing the level of “stress ethylene” which is inhibitory to plant growth. ACCD breaks down ACC, an immediate precursor of ethylene, to ammonia and α-ketobutyrate, which can be further metabolized by bacteria for their growth. ACC deaminase is an inducible enzyme whose synthesis is induced in the presence of its substrate ACC. This enzyme encoded by gene AcdS is under tight regulation and regulated differentially under different environmental conditions. Regulatory elements of gene AcdS are comprised of the regulatory gene encoding LRP protein and other regulatory elements which are activated differentially under aerobic and anaerobic conditions. The role of some additional regulatory genes such as AcdB or LysR may also be required for expression of AcdS. Phylogenetic analysis of AcdS has revealed that distribution of this gene among different bacteria might have resulted from vertical gene transfer with occasional horizontal gene transfer (HGT). Application of bacterial AcdS gene has been extended by developing transgenic plants with ACCD gene which showed increased tolerance to biotic and abiotic stresses in plants. Moreover, distribution of ACCD gene or its homolog's in a wide range of species belonging to all three domains indicate an alternative role of ACCD in the physiology of an organism. Therefore, this review is an attempt to explore current knowledge of bacterial ACC deaminase mediated physiological effects in plants, mode of enzyme action, genetics, distribution among different species, ecological role of ACCD and, future research avenues to develop transgenic plants expressing foreign AcdS gene to cope with biotic and abiotic stressors. Systemic identification of regulatory circuits would be highly valuable to express the gene under diverse environmental conditions.

Prospecting metal-tolerant rhizobia for phytoremediation of mining soils from Morocco using Anthyllis vulneraria L

The aim of this work was using the legume plant Anthyllis vulneraria L. (ecotype metallicolous) as a trap plant, in order to isolate metal-tolerant rhizobial strains from metal-contaminated soils from Morocco, with pollution indexes spanning three orders of magnitude. As bioindicator, soil bacterial density was inversely correlated to the pollution index. Forty-three bulk soil bacteria and sixty two bacteria from nodules were isolated. The resistance of bacteria from nodules to heavy metals was four to ten times higher than that of bulk soil bacteria, reaching high maximum tolerable concentrations for Cd (2 mM), Cu (2 mM), Pb (7 mM), and Zn (3 mM). Besides, some strains show multiple metal-tolerant abilities and great metal biosorption onto the bacterial surface. Amplification and restriction analysis of ribosomal 16S rDNA (ARDRA) and 16S ribosomal DNA (rDNA) sequencing were used to assess biodiversity and phylogenetic position among bacteria present in nodules. Our results suggest that a great diversity of non-rhizobial bacteria (alpha- and gamma-proteobacteria) colonize nodules of Anthyllis plants in contaminated soils. Taking together, our results evidence that, in polluted soils, rhizobia can be displaced by non-rhizobial (and hence, non-fixing) strains from nodules. Thus, the selection of metal-resistant rhizobia is a key step for using A. vulneraria symbioses for in situ phytoremediation.

The role of dark septate endophytic fungal isolates in the accumulation of cesium by chinese cabbage and tomato plants under contaminated environments

Following the 2011 Fukushima Daiichi Nuclear Power Plant accident, the preservation of the food chain from radionuclides contamination has become of crucial importance. The potential of Dark septate endophytic fungi in the management of Cs accumulation in plants under contaminated environments was investigated using Chinese cabbage and tomato plants. Four endophytic fungal isolates of different species, i.e. Pseudosigmoidea ibarakiensis I.4-2-1, Veronaeopsis simplex Y34, Helminthosporium velutinum 41-1, and as yet unidentified taxon 312-6 were tested In Vitro in two levels of Cs (5ppm and 10ppm). On the plant growth, the inoculation of the selected DSEs to both Chinese cabbage and tomato resulted in an increased biomass of up to 82% and 122%, respectively compared to control (non-inoculated) plants. With regards to the Cs accumulation, it varied with the host plant considered. In Chinese cabbage, DSEs inoculation caused higher Cs accumulation in above ground plant parts, whereas in tomato, Cs accumulation decreased significantly with three of the isolates tested, i.e., V. simplex Y34, P. ibarakiensis I.4-2-1, and the as yet unidentified taxon 312-6 suggesting low-risk transfer on the above ground plants parts as a result of high and negative plant reactions rather than high and positive reactions as it is the case with Chinese cabbage. These results suggested that DSEs can be recommended for use with Chinese cabbage to enhance phytoremediation of Cs in surrounding contaminated areas. With tomato, DSEs can be recommended for decreasing the accumulation of Cs in plants under contaminated environments.

Culturable heavy metal-resistant and plant growth promoting bacteria in V-Ti magnetite mine tailing soil from Panzhihua, China

To provide a basis for using indigenous bacteria for bioremediation of heavy metal contaminated soil, the heavy metal resistance and plant growth-promoting activity of 136 isolates from V-Ti magnetite mine tailing soil were systematically analyzed. Among the 13 identified bacterial genera, the most abundant genus was Bacillus (79 isolates) out of which 32 represented B. subtilis and 14 B. pumilus, followed by Rhizobium sp. (29 isolates) and Ochrobactrum intermedium (13 isolates). Altogether 93 isolates tolerated the highest concentration (1000 mg kg⁻¹) of at least one of the six tested heavy metals. Five strains were tolerant against all the tested heavy metals, 71 strains tolerated 1,000 mg kg⁻¹ cadmium whereas only one strain tolerated 1,000 mg kg⁻¹ cobalt. Altogether 67% of the bacteria produced indoleacetic acid (IAA), a plant growth-promoting phytohormone. The concentration of IAA produced by 53 isolates was higher than 20 µg ml⁻¹. In total 21% of the bacteria produced siderophore (5.50–167.67 µg ml⁻¹) with two Bacillus sp. producing more than 100 µg ml⁻¹. Eighteen isolates produced both IAA and siderophore. The results suggested that the indigenous bacteria in the soil have beneficial characteristics for remediating the contaminated mine tailing soil.

Plant growth-promoting rhizobacteria and root system functioning

The rhizosphere supports the development and activity of a huge and diversified microbial community, including microorganisms capable to promote plant growth. Among the latter, plant growth-promoting rhizobacteria (PGPR) colonize roots of monocots and dicots, and enhance plant growth by direct and indirect mechanisms. Modification of root system architecture by PGPR implicates the production of phytohormones and other signals that lead, mostly, to enhanced lateral root branching and development of root hairs. PGPR also modify root functioning, improve plant nutrition and influence the physiology of the whole plant. Recent results provided first clues as to how PGPR signals could trigger these plant responses. Whether local and/or systemic, the plant molecular pathways involved remain often unknown. From an ecological point of view, it emerged that PGPR form coherent functional groups, whose rhizosphere ecology is influenced by a myriad of abiotic and biotic factors in natural and agricultural soils, and these factors can in turn modulate PGPR effects on roots. In this paper, we address novel knowledge and gaps on PGPR modes of action and signals, and highlight recent progress on the links between plant morphological and physiological effects induced by PGPR. We also show the importance of taking into account the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations in soil will be a prerequisite to develop novel management strategies for sustainable agriculture.