Biosorption of heavy metals by dry biomass of metal tolerant bacterial biosorbents: an efficient metal clean-up strategy.

Heavy metals discharge at an unrestrained rate from various industries into the environment pose serious human health problems. Considering this, the present study aimed at exploring the metal biosorbing potentials of bacterial strains recovered from polluted soils. The bacterial strains (CPSB1, BM2 and CAZ3) belonging to genera Pseudomonas, Bacillus and Azotobacter expressing multi-metal tolerance ability were identified to species level as P. aeruginosa, B. subtilis and A. chroococcum, respectively, by 16S rRNA partial gene sequence analysis. The biosorption of cadmium, chromium, copper, nickel, lead and zinc by three dead bacterial genera were studied as a function of metal concentration, variable pH of the medium and reaction (contact) time. The three bacterial strains exhibited a tremendous metal removal ability which continued even at the highest tested concentration of some metals. Later, a decline in the percentage of biosorbed metals was recorded as the metal concentration was increased with the simultaneous generation of a driving force to overcome mass transfer resistance for movement of metal ions between the solution and the surface of adsorbent. Among test bacteria, B. subtilis biosorbed a maximum of 96% chromium at 25 µg mL-1 while the maximum percentage (91%) of biosorbed metals recorded at 400 µg Cd mL-1 was observed for P. aeruginosa. The sorption of metal ions by dead biomass of three bacterial genera at optimum conditions followed the order-(i) B. subtilis BM2: Pb > Cu > Ni > Cd > Cr, (ii) A. chroococcum CAZ3: Cr > Cd > Cu > Ni > Pb and (iii) P. aeruginosa CPSB1: Cd > Cr > Ni > Cu > Pb > Zn. It was found that the optimum pH for metal adsorption ranged between pH 8 and 9 which, however, declined substantially at pH 5.0 for all three bacterial strains. In general, the biosorption of Cd, Cr, Cu, Ni and Pb by B. subtilis and A. chroococcum and such metals along with Zn by P. aeruginosa occurred maximally up to 60 min of bacterial growth. The adsorption data with regard to five metals provide an outstanding fit to the Langmuir and Freundlich isotherms. The biosorptive ability of three bacterial genera correlated strongly (r2 > 0.9) with each metal. The bacteria belonging to two Gram-negative genera Pseudomonas (P. aeruginosa) and Azotobacter (A. chroococcum) and one Gram-positive genus Bacillus (B. subtilis) demonstrated exceptional metal removal efficiency and, hence, provides a comprehensive understanding of metal-bacteria sorption process which in effect paves the way for detoxifying/removing metals from contaminated environment.

Destruction of Cell Topography, Morphology, Membrane, Inhibition of Respiration, Biofilm Formation, and Bioactive Molecule Production by Nanoparticles of Ag, ZnO, CuO, TiO2, and Al2O3 toward Beneficial Soil Bacteria.

The unregulated discharge of nanoparticles (NPs) from various nanotechnology industries into the environment is expected to alter the composition and physiological functions of soil microbiota. Considering this knowledge gap, the impact of five NPs (Ag, ZnO, CuO, Al2O3, and TiO2) differing in size and morphology on growth behavior and physiological activity of Azotobacter chroococcum, Bacillus thuringiensis, Pseudomonas mosselii, and Sinorhizobium meliloti were investigated. Various biochemical and microscopic approaches were adopted. Interestingly, all bacterial strains were found sensitive to Ag-NPs and ZnO-NPs but showed tolerance toward CuO, Al2O3, and TiO2-NPs. The loss of cellular respiration due to NPs was coupled with a reduction in population size. ZnO-NPs at 387.5 µg mL-1 had a maximum inhibitory impact on A. chroococcum and reduced its population by 72%. Under Ag-NP stress, the reduction in IAA secretion by bacterial strains followed the order S. meliloti (74%) > P. mosselii (63%) > A. chroococcum (49%). The surface of bacterial cells had small- or large-sized aggregates of NPs. Also, numerous gaps, pits, fragmented, and disorganized cell envelopes were visible. Additionally, a treated cell surface appeared corrugated with depressions and alteration in cell length and a strong heterogeneity was noticed under atomic force microscopy (AFM). For instance, NPs induced cell roughness for P. mosselii followed the order 12.6 nm (control) > 58 nm (Ag-NPs) > 41 nm (ZnO-NPs). TEM analysis showed aberrant morphology, cracking, and disruption of the cell envelope with extracellular electron-dense materials. Increased permeability of the inner cell membrane caused cell death and lowered EPS production. Ag-NPs and ZnO-NPs also disrupted the surface adhering ability of bacteria, which varied with time and concentration of NPs. Conclusively, a plausible mechanism of NP toxicity to bacteria has been proposed to understand the mechanistic basis of ecological interaction between NPs and resourceful bacteria. These results also emphasize to develop strategies for the safe disposal of NPs.

Heavy metal induced stress on wheat: phytotoxicity and microbiological management.

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety. Due to these problems, soil biologists/agronomists in recent times have also raised concerns over heavy metal pollution, which indeed are unpleasantly affecting agro-ecosystems and crop production. The toxic heavy metals once deposited beyond certain permissible limits, obnoxiously affect the density, composition and physiological activities of microbiota, dynamics and fertility of soil leading eventually to reduction in wheat production and via food chain, human and animal health. Therefore, the metal induced phytotoxicity problems warrant urgent and immediate attention so that the physiological activities of microbes, nutrient pool of soils and concurrently the production of wheat are preserved and maintained in a constantly deteriorating environment. To mitigate the magnitude of metal induced changes, certain microorganisms have been identified, especially those belonging to the plant growth promoting rhizobacteria (PGPR) group endowed with the distinctive property of heavy metal tolerance and exhibiting unique plant growth promoting potentials. When applied, such metal-tolerant PGPR have shown variable positive impact on wheat production, even in soils contaminated with metals, by supplying macro and micro nutrients and secreting active biomolecules like EPS, melanins and metallothionein (MTs). Despite some reports here and there, the phytotoxicity of metals to wheat and how wheat production in metal-stressed soil can be enhanced is poorly explained. Thus, an attempt is made in this review to better understand the mechanistic basis of metal toxicity to wheat, and how such phytotoxicity can be mitigated by incorporating microbiological remediation strategies in wheat cultivation practices. The information provided here is likely to benefit wheat growers and consequently optimize wheat production inexpensively under stressed soils.

Effective Inhibition of Phytopathogenic Microbes by Eco-Friendly Leaf Extract Mediated Silver Nanoparticles (AgNPs).

ABSTRACT: This study was aimed at producing the eco-friendly, safe, and inexpensive silver (Ag) nanoparticles (NPs) and assessing its antimicrobial activity. Fungal pathogens isolated from diseased leaves and fruits of brinjal and bacterial pathogen obtained from a culture collection were used in this study. Green synthesis of AgNPs was performed and optimized using Azadirachta indica leaf extract. The newly synthesized AgNPs (λmax = 437 nm) showed isotropism in size (crystal size/diameter: 21/29 ± 5 nm) and morphology under transmission and scanning electron microscopy and energy dispersive X-ray analysis. The fourier transform infrared spectroscopy data suggested the role of various aliphatic/aromatic moieties and proteins in AgNPs stabilization. The AgNPs reduced the growth of Penicillium sp. maximally by 92% after 6 days. The sensitivity of test fungi towards AgNPs followed the order: Penicillium sp. (92%) > Fusarium sp. (89%) > Aspergillus sp. (69%). Exposure of Ralstonia solanacearum to AgNPs (MIC/MBC 200/400 µg ml-1) displayed damaged cellular envelopes, bulging of cells, and pit formation. The nucleic acid discharge showed a progressive increase from 8 to 34% (r2 = 0.97). The cellular metabolic activity and surface adhering ability of R. solanacearum were completely lost at 400 µgAgNPs ml-1. Results suggested that the AgNPs synthesized in this study had enough anti-pathogenic potential and could inexpensively and safely be used as a promising alternative to agrochemicals. Moreover, the findings observed in this study is likely to serve as an important indicator for the development of effective nano-control agents which in effect would help to manage some deadly phyto-pathogens capable of causing heavy losses to agricultural production systems. GRAPHICAL ABSTRACT: Effective inhibition of phytopathogenic microbes by eco-friendly neem leaf extract mediated silver nanoparticles (AgNPs).

In vitro investigation to explore the toxicity of different groups of pesticides for an agronomically important rhizosphere isolate Azotobacter vinelandii.

In this work, an attempt was made to evaluate the effect of pesticides on growth pattern, surface morphology, cell viability and growth regulators of nitrogen fixing soil bacterium. Pesticide tolerant Azotobacter vinelandii strain AZ6 (Accession no. MG028654) was found to tolerate maximum level of pesticide and displayed multifarious PGP activities. At higher concentrations, pesticides triggered cellular/structural damage and reduced the cell viability as clearly shown under SEM and CLSM. With increase in concentration, pesticides exhibited a significant (p < 0.05) decrease in PGP traits of strain AZ6. Among all three groups of pesticides, herbicides glyphosate and atrazine were most toxic. Kitazin, hexaconazole, metalaxyl, glyphosate, quizalofop, atrazine, fipronil, monocrotophos and imidacloprid at 2400, 1800, 1500, 900, 1200, 900, 1800, 2100 and 2700 µg mL-1, respectively, decreased the production of IAA by 19.5 ±â€¯1.9 (61%), 18.1 ±â€¯1.2 (64%), 36.4 ±â€¯3.4 (28%), 13.1 ±â€¯0.8 (74%), 15.6 ±â€¯1.0 (69%), 7.6 ±â€¯0.5 (83%), 11.9 ±â€¯0.8 (76%), 24.7 ±â€¯1.7 (51%) and 32 ±â€¯2.3 (37%) µg mL-1, respectively, over control (50.7 ±â€¯3.6 µg mL-1). A maximum reduction of 8.4 ±â€¯1.2 (46%), 5.8 ±â€¯0.6 (62%) and 4 ±â€¯0.2 (74%) µg mL-1 in 2, 3-DHBA at 300 (1×), 600 (2×) and 900 (3×) µg mL-1 glyphosate, respectively, While, 32.8 ±â€¯2.7 (19%), 27.2 ±â€¯2 (33%) and 21.5 ±â€¯1.3 (47%) µg mL-1, respectively in the production of SA was observed at 300 (1×), 600 (2×) and 900 (3×) µg mL-1 atrazine, respectively. Likewise, with increase in concentration of pesticides, decrease in P solubilization ability and change in pH of broth was detected. The order of pesticide toxicity to PSE (percent decline over control) at highest concentration was: atrazine (45) > kitazin (44) > metalaxyl (43) > monocrotophos (43) > glyphosate (41) > hexaconazole (39) > quizalofop (33) > imidacloprid (31) > fipronil (25). The present study undoubtedly suggests that even at higher doses of pesticides, A. vinelandii maintained secreting plant growth regulators and this property makes this strain agronomically important microbe for enhancing the growth of plants.

Bacterial toxicity of biomimetic green zinc oxide nanoantibiotic: insights into ZnONP uptake and nanocolloid-bacteria interface.

This study was aimed to fill the critical gap of knowledge regarding the interaction between green zinc oxide nanoparticles (ZnONPs) and bacterial interface. Wurtzite phase ZnONPs with a band gap energy of 3.28 eV were produced by exploiting a simple and green biosynthesis method using an inexpensive precursor of A. indica leaf extract and zinc nitrate. ZnONPs were characterized using UV-Vis spectroscopy, XRD, FTIR, SEM, EDX, DLS, TEM, and zeta-potential analysis. The primary size obtained was 26.3 nm (XRD) and 33.5 ± 6.5 nm (TEM), whereas, the secondary size was found to be 287 ± 5.2 nm with -32.8 ± 1.8 mV ζ-potential denoting the physical colloid chemistry of ZnONPs. Crystallinity and the spherical morphology of ZnONPs were also evident with some sort of particle agglomeration. ZnONPs retained plant functional groups endorsing their hydrophilic character. The antibacterial and antibiofilm activity of ZnONPs was significant (p ≤ 0.05) and the MIC/MBC against most frequent clinical isolates of Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus ranged from 0.5 to 1.0 (MIC)/1.0 to 1.5 mg ml-1 (MBC). The dissolution of ZnONPs to Zn2+ ions in a nutrient medium increased as a result of interaction with the bacterial surface and metabolites. Substantial surface binding of ZnONPs followed by intracellular uptake disrupted the cell morphology and caused obvious injury to the cell membrane. Interrupted bacterial growth kinetics, loss of cell respiration, enhanced production of intracellular ROS, and the leakage of the cytoplasmic content unequivocally suggested a strong interaction of ZnONPs with the exterior cell surface and intracellular components, eventually leading to cell death and destruction of biofilms. Overall, the results elucidated eco-friendly production of ZnONPs expressing a prominent interfacial correlation with bacteria and hence, prospecting the use of green ZnONPs as effective nanoantibiotics.

Bioreduction of toxicity influenced by bioactive molecules secreted under metal stress by Azotobacter chroococcum.

Heavy metal pollution destruct soil microbial compositions and functions, plant's performance and subsequently human health. Culturable microbes among many metal abatement strategies are considered inexpensive, viable and environmentally safe. In this study, nitrogen fixing bacterial strain CAZ3 recovered from chilli rhizosphere tolerated 100, 1000 and 1200 µg mL-1 of cadmium, chromium and nickel, respectively and was identified as Azotobacter chroococcum by 16S rDNA sequence analysis. Under metal stress, cellular morphology of A. chroococcum observed under SEM was found distorted and shrinkage of cells was noticed when grown with 50 µg mL-1 of Cd (cell size 1.7 µm) and 100 of µg mL-1 Ni (cell size 1.3 µm) compared to untreated control (cell size 1.8 µm). In the presence of 100 µg mL-1 of Cr, cells became elongated and measured 1.9 µm in size. Location of metals inside the cells was revealed by EDX. A dose dependent growth arrest and consequently the death of A. chroococcum cells was revealed under CLSM. A. chroococcum CAZ3 secreted 320, 353 and 133 µg EPS mL-1 when grown with 100 µg mL-1 each of Cd, Cr and Ni, respectively. The EDX revealed the presence of 0.4, 0.07 and 0.24% of Cd, Cr and Ni, respectively within EPS extracted from metal treated cells. Moreover, a dark brown pigment (melanin) secreted by A. chroococcum cells under metal pressure displayed tremendous metal chelating activity. The EDX spectra of melanin extracted from metal treated cells of A. chroococcum CAZ3 displayed 0.53, 0.22 and 0.12% accumulation of Cd, Cr and Ni, respectively. The FT-IR spectra of EPS and melanin demonstrated stretching vibrations and variations in surface functional groups of bacterial cells. The C-H stretching of CH3 in fatty acids and CH2 groups, stretching of N-H bond of proteins and O-H bond of hydroxyl groups caused the shifting of peaks in the EPS spectra. Similar stretching vibrations were recorded in metal treated melanin which involved CHO, alkyl, carboxylate and alkene groups resulting in significant peak shifts. Nuclear magnetic resonance (NMR) spectrum of EPS extracted from A. chroococcum CAZ3 revealed apparent peak signals at 4.717, 9.497, 9.369 and 9.242 ppm. However, 1H NMR peaks were poorly resolved due largely to the impurity/viscosity of the EPS. The entrapment of metals by EPS and melanin was confirmed by EDX. Also, the induction and excretion of variable amounts of metallothioneins (MTs) by A. chroococcum under metal pressure was interesting. Conclusively, the present findings establish- (i) cellular damage due to Cd, Cr and Ni and (ii) role of EPS, melanin and MTs in adsorption/complexation and concurrently the removal of heavy metals. Considering these, A. chroococcum can be promoted as a promising candidate for supplying N efficiently to plants and protecting plants from metal toxicity while growing under metal stressed environment.

Heavy metal mediated phytotoxic impact on winter wheat: oxidative stress and microbial management of toxicity by Bacillus subtilis BM2.

Heavy metals are toxic environmental contaminants, which severely affect microbial composition and functions and, concurrently, crop production. Due to these issues, the present study focussed on the selection of metal tolerant microbes endowed with metal detoxification abilities and their role in the management and remediation of metal contaminated soils. The metal tolerant bacterium BM2, identified as Bacillus subtilis by 16SrRNA gene sequencing, survived well under metal pressure and tolerated 1600 and 2000 µg mL-1 of Ni and Pb, respectively. The inhibitory impact of metals on wheat increased consistently with a progressive increase in metal concentration. Deposition of Ni and Pb within root and leaf and oxidative stress were validated by SEM, EDX and CLSM. The overall growth parameters of wheat grown under metal stress were improved following B. subtilis BM2 colonization. As an example, B. subtilis with 195 mg Pb kg-1 enhanced the length and dry biomass of shoots by 14% and 23%, respectively, over the control. Also, strain BM2 improved the grain yield significantly by 49% at 870 mg Ni kg-1 and by 50% at 585 mg Pb kg-1 compared to uninoculated plants. Moreover, B. subtilis BM2 relieved the metal stress on wheat and caused a significant drop in proline and malondialdehyde content and the activities of antioxidant enzymes, like catalase (CAT), superoxide dismutase (SOD) and glutathione reductase (GR). This study, therefore, provided solutions to the metal toxicity problems faced by winter wheat and clearly suggests that the metal detoxification potential of B. subtilis BM2 could be greatly useful in the management of metal polluted soils.

Understanding the phyto-interaction of heavy metal oxide bulk and nanoparticles: evaluation of seed germination, growth, bioaccumulation, and metallothionein production.

The fast-growing use of nano-based products without proper care has led to a major public health concern. Nanomaterials contaminating the environment pose serious threat to the productivity of plants and via food chain to human health. Realizing these, four vegetable crops, radish, cucumber, tomato, and alfalfa, were exposed to varying concentrations of heavy metal oxide (TiO2, ZnO, Al2O3 and CuO) submicron or bulk (BPs) and nanoparticles (NPs) to assess their impact on relative seed germination (RSG), seed surface adsorption, root/shoot tolerance index (RTI/STI), bioaccumulation, and metallothioneins (MTs) production. The results revealed a clear inhibition of RSG, RTI, and STI, which, however, varied between species of metal-specific nanoparticles and plants. SEM and EDX analyses showed significant adsorption of MONP agglomerates on seed surfaces. The concentration of metals detected by EDX differed among vegetables. Among the metals, Al, Cu, Ti, and Zn were found maximum in alfalfa (12.46%), tomato (23.2%), cucumber (6.32%) and radish (21.74%). Of the four metal oxides, ZnO was found most inhibitory to all vegetables and was followed by CuO. The absorption/accumulation of undesirable levels of MONPs in seeds and seedlings differed with variation in dose rates, and was found to be maximum (1748-2254 µg g-1 dry weight) in ZnO-NPs application. Among MONPs, the uptake of TiO2 was minimum (2 to 140 µg g-1) in radish seedlings. The concentration of MTs induced by ZnO-NPs, ZnO-BPs, and CuO-NPs ranged between 52 and 136 µ mol MTs g-1 FW in vegetal organs. Conclusively, the present findings indicated that both the nanosize and chemical composition of MONPs are equally dangerous for vegetable production. Hence, the accumulation of MONPs, specifically ZnO and CuO, in edible plant organs in reasonable amounts poses a potential environmental risk which, however, requires urgent attention to circumvent such toxic problems.

Toxicity of fungicides to Pisum sativum: a study of oxidative damage, growth suppression, cellular death and morpho-anatomical changes.

Considering the fungicidal threat to the sustainable agro-environment, the toxicological impacts of three fungicides, namely kitazin, hexaconazole and carbendazim, on the biological, chemical and morpho-anatomical changes of peas were assessed. Fungicide applications in general caused a slow but gradual reduction in growth, symbiosis and yields of peas, which, however, varied appreciably among species and concentrations of the three fungicides. Of the three fungicides, carbendazim had the most lethal effect, in which it delayed seed germination and also diminished the overall pea growth. Carbendazim at 3000 µg kg-1 maximally reduced the germination, SVI, size of roots and shoots and total dry matter accumulation in roots, shoots and whole plants distinctly by 40%, 84%, 72%, 73%, 68%, 75% and 73% (p ≤ 0.05), respectively. Hexaconazole at 120 µg kg-1 significantly (p ≤ 0.05) declined total chlorophyll, carotenoids, grain yields, grain protein, root P and shoot N by 19%, 28%, 46%, 69%, 48% and 51%, respectively, over the control. The synthesis of stress biomarkers and oxidative stress were increased with increasing dosage rates of fungicides. Proline content in roots, shoots, leaves and grains, MDA, electrolyte leakage and H2O2 of plants grown in soil treated with 288 µg kg-1 kitazin were increased significantly (p ≤ 0.05) by 73%, 52%, 41%, 24%, 59%, 40% and 27%, respectively, relative to the control. Antioxidant defence enzymes were greater in pea foliage. The SEM and CLSM images revealed an obvious alteration in root tips, enhanced cellular damage and cell death when plants were raised under fungicide stress. Also, morpho-anatomical variations in fungicide-treated foliage were visible in the SEM images. Overall, the present study suggests that a careful and secure strategy should be adopted before fungicides are chosen for enhancing pulse production in different agro-climatic regions.

Biosorption of heavy metals by Bacillus thuringiensis strain OSM29 originating from industrial effluent contaminated north Indian soil.

The study was navigated to examine the metal biosorbing ability of bacterial strain OSM29 recovered from rhizosphere of cauliflower grown in soil irrigated consistently with industrial effluents. The metal tolerant bacterial strain OSM29 was identified as Bacillus thuringiensis following 16S rRNA gene sequence analysis. In the presence of the varying concentrations (25-150 mgl(-1)) of heavy metals, such as cadmium, chromium, copper, lead and nickel, the B. thuringiensis strain OSM29 showed an obvious metal removing potential. The effect of certain physico-chemical factors such as pH, initial metal concentration, and contact time on biosorption was also assessed. The optimum pH for nickel and chromium removal was 7, while for cadmium, copper and lead, it was 6. The optimal contact time was 30 min. for each metal at 32 ± 2 °C by strain OSM29. The biosorption capacity of the strain OSM29 for the metallic ions was highest for Ni (94%) which was followed by Cu (91.8%), while the lowest sorption by bacterial biomass was recorded for Cd (87%) at 25 mgl(-1) initial metal ion concentration. The regression coefficients obtained for heavy metals from the Freundlich and Langmuir models were significant. The surface chemical functional groups of B. thuringiensis biomass identified by Fourier transform infrared (FTIR) were amino, carboxyl, hydroxyl, and carbonyl groups, which may be involved in the biosorption of heavy metals. The biosorption ability of B. thuringiensis OSM29 varied with metals and was pH and metal concentration dependent. The biosorption of each metal was fairly rapid which could be an advantage for large scale treatment of contaminated sites.

Antibacterial and cytotoxic efficacy of extracellular silver nanoparticles biofabricated from chromium reducing novel OS4 strain of Stenotrophomonas maltophilia.

Biofabricated metal nanoparticles are generally biocompatible, inexpensive, and ecofriendly, therefore, are used preferably in industries, medical and material science research. Considering the importance of biofabricated materials, we isolated, characterized and identified a novel bacterial strain OS4 of Stenotrophomonas maltophilia (GenBank: JN247637.1). At neutral pH, this Gram negative bacterial strain significantly reduced hexavalent chromium, an important heavy metal contaminant found in the tannery effluents and minings. Subsequently, even at room temperature the supernatant of log phase grown culture of strain OS4 also reduced silver nitrate (AgNO3) to generate nanoparticles (AgNPs). These AgNPs were further characterized by UV-visible, Nanophox particle size analyzer, XRD, SEM and FTIR. As evident from the FTIR data, plausibly the protein components of supernatant caused the reduction of AgNO3. The cuboid and homogenous AgNPs showed a characteristic UV-visible peak at 428 nm with average size of ~93 nm. The XRD spectra exhibited the characteristic Bragg peaks of 111, 200, 220 and 311 facets of the face centred cubic symmetry of nanoparticles suggesting that these nanoparticles were crystalline in nature. From the nanoparticle release kinetics data, the rapid release of AgNPs was correlated with the particle size and increasing surface area of the nanoparticles. A highly significant antimicrobial activity against medically important bacteria by the biofabricated AgNPs was also revealed as decline in growth of Staphylococcus aureus (91%), Escherichia coli (69%) and Serratia marcescens (66%) substantially. Additionally, different cytotoxic assays showed no toxicity of AgNPs to liver function, RBCs, splenocytes and HeLa cells, hence these particles were safe to use. Therefore, this novel bacterial strain OS4 is likely to provide broad spectrum benefits for curing chromium polluted sites, for biofabrication of AgNPs and ultimately in the nanoparticle based drug formulation for the treatment of infectious diseases.

Effects of heavy metal toxicity on growth, symbiosis, seed yield and metal uptake in pea grown in metal amended soil.

Soils contaminated with heavy metals present a major threat to sustainable agriculture. Understanding the effects of these metals on pea productivity will be useful. We studied the effects of cadmium, chromium and copper used both separately and as mixtures, on over all growth of pea plants inoculated with Rhizobium sp. Among the metals, copper was most toxic for pea plants and decreased the seed yield by 15% at 1,338 mg kg(-1) compared to control plants whereas cadmium and chromium in general, increased the measured parameters. The metal accumulation in roots and shoots at 90 d and in grains at 120 d differed among treatments.

Effect of metal-tolerant plant growth-promoting Rhizobium on the performance of pea grown in metal-amended soil.

The nickel- and zinc-tolerant plant growth-promoting (PGP) Rhizobium sp. RP5 was isolated from nodules of pea, grown in metal-contaminated Indian soils. The PGP potentials of strain RP5 was assessed under in vitro conditions. Strain RP5 displayed a high level of tolerance to nickel (350 microg ml(-1)) and zinc (1500 microg ml(-1)) and showed PGP activity under in vitro conditions. The PGP activity of this strain was further assessed with increasing concentrations of nickel and zinc, using pea as a test crop. The bio-inoculant enhanced the dry matter, nodule numbers, root N, shoot N, leghemoglobin, seed yield, and grain protein (GP) by 19%, 23%, 26%, 47%, 112%, 26%, and 8%, respectively, at 290 mg Ni kg(-1) while at 4890 mg Zn kg(-1) soil, it increased the dry matter, nodule numbers, leghemoglobin, seed yield, GP, and root and shoot N by 18%, 23%, 78%, 26%, 7%, 25%, and 42%, respectively, compared to plants grown in soil amended with metal only. The bio-inoculant increased the glutathione reductase activity of roots and nodules by 46% and 65% at 580 mg Ni kg(-1) and 47% and 54% at 9780 mg Zn kg(-1) soil, respectively, compared to uninoculated plants. The inoculated strain decreased the concentration of nickel and zinc in plant organs. The intrinsic abilities of nitrogen fixation, growth promotion, and the ability to reduce the toxicity of nickel and zinc of the tested strain could be of practical importance in augmenting the growth and yield of pea, in nickel- and zinc-polluted soils.

Chromium-reducing and plant growth-promoting Mesorhizobium improves chickpea growth in chromium-amended soil.

Mesorhizobium strain RC3, isolated from chickpea nodules, tolerated chromium up to 500 mug/ml and reduced it by 90% at pH 7 after 120 h. It produced plant growth-promoting substances, both in the presence and absence of chromium. Strain RC3 produced 35 mug indole acetic acid/ml in Luria Bertani broth with 100 mg tryptophan/ml, which decreased with an increase in chromium concentration. Chromium application to soil at 136 mg/kg was toxic to chickpea plants but when RC3 at 136 mg/kg was also added, it increased the dry matter accumulation, number of nodules, seed yield and grain protein by 71, 86, 36 and 16%, respectively, compared to non-inoculated plants. Nitrogen in roots and shoots were increased by 46 and 40%, respectively, at 136 mg Cr/kg. The bio-inoculant decreased the uptake of chromium by 14, 34 and 29% in roots, shoots and grains, respectively.

Effect of metal tolerant plant growth promoting Bradyrhizobium sp. (vigna) on growth, symbiosis, seed yield and metal uptake by greengram plants.

The nickel and zinc tolerant plant growth promoting Bradyrhizobium sp. (vigna) RM8 was isolated from nodules of greengram, grown in metal contaminated Indian soils. The plant growth promoting (PGP) potentials of strain RM8 was assessed both in the presence and absence of nickel and zinc under in vitro conditions. Strain RM8 tolerated a high level of nickel (300 microg ml(-1)) and zinc (1400 microg ml(-1)) on yeast extract mannitol agar medium. Bradyrhizobium sp. (vigna) strain RM8 produced 13.3 microg ml(-1) of indole acetic acid in Luria Bertani broth at 100 microg ml(-1) of tryptophan which increased to 13.6 microg ml(-1) at 50 microg Ni ml(-1) and 13.5 microg ml(-1) at 300 microg Zn ml(-1). Strain RM8 was positive for siderophore, HCN and ammonia both in the absence and presence of nickel and zinc. The PGP activity of this strain was further evaluated with increasing concentrations of nickel and zinc using greengram as a test crop. The bioinoculant enhanced the nodule numbers by 82%, leghaemoglobin by 120%, seed yield by 34%, grain protein by 13%, root N by 41% and shoot N by 37% at 290 mg Ni kg(-1) soil. At 4890 mg Zn kg(-1) soil, the bioinoculant increased the nodule numbers by 50%, leghaemoglobin by 100%, seed yield by 36%, grain protein by 13%, root N by 47% and shoot N by 42%. The bioinoculant strain RM8 reduced the uptake of nickel and zinc by plant organs compared to plants grown in the absence of bioinoculant. This study suggested that the bioinoculant due to its intrinsic abilities of growth promotion and attenuation of the toxic effects of nickel and zinc could be exploited for remediation of metal from nickel and zinc contaminated sites.

Chromium reduction, plant growth-promoting potentials, and metal solubilizatrion by Bacillus sp. isolated from alluvial soil.

The plant growth-promoting potentials, production of siderophore and solubilization of insoluble phosphorus (P) and zinc and lead by the chromium (vi) -reducing Bacillus species, PSB 1, PSB 7, and PSB 10, was assessed both in the presence and absence of chromium under in vitro conditions. The Bacillus strains tolerated chromium up to the concentration of 500 (PSB1), 400 (PSB7), and 550 microg ml(-1) (PSB10), respectively, on nutrient agar plates. Bacillus sp. PSB 10 reduced Cr (vi) by 87% at pH 7, which was followed by Bacillus sp. PSB 1 (83%) and PSB 7 (74%) in nutrient broth after 120 h of incubation. A concentration of 50 microg ml(-1) of Cr (vi) was completely reduced by Bacillus sp. PSB 1 and PSB 10 (after 100 h) and PSB 7 (after 120 h). The Bacillus strains PSB 1, PSB 7, and PSB 10 produced 19.3, 17.7, and 17.4 microg ml(-1) of indole acetic acid, respectively, in luria bertani broth at 100 microg ml(-1) of tryptophan, which consistently decreased with an increase in chromium concentration. The Bacillus strains were positive for siderophore, HCN, and ammonia both in the absence and presence of chromium. The Bacillus strains solubilized 375 (PSB 1), 340 (PSB 7), and 379 (PSB 10) microg ml(-1) P, respectively, in Pikovskaya broth devoid of chromium. In contrast, chromium at 150 microg ml(-1) reduced the amount of P solubilized by 17 (PSB 1), 15 (PSB 7), and 9% (PSB 10) compared to control. The tested bacterial strains solubilized a considerable amount of zinc and lead in nutrient broth both in the absence and presence of chromium. Generally, the chromium reduction and the plant growth-promoting potentials of chromium-reducing Bacillus were strongly correlated at the tested concentration of chromium. The present observations demonstrated that the chromium-reducing, metal-solubilizing, and plant growth-promoting potentials of the Bacillus strains PSB1, PSB 7, and PSB10 were not adversely affected by the chromium application and, hence, may be applied for raising the productivity of crops under metal-contaminated soils.

Fungicidal impact on chickpea--Mesorhizobium symbiosis.

The effects of carbendazim, captan, thiram, and mancozeb, on plant vitality, chlorophyll content, N uptake, protein content, nodulation, and seed yield in chickpea (Cicer arietinun) were assessed in a controlled environment. Seeds treated with fungicides at 1 and 1.5 g.a.i. kg seed had no significant adverse effect on plant vigor, seed yield, and N and protein contents. In contrast, fungicides applied at 2 g.a.i./kg of captan, thiram and mancozeb, significantly reduced the measured parameters. In general, the toxicity of fungicides in terms of seed yield increased in the following order: Control=carbendazim > thiram > captan > mancozeb. Total chlorophyll content in foliage declined consistently with fungicides dose rates and application days. Seeds treated with lower rates of fungicides significantly increased nodulation (nodule number per plant and its dry mass) and were compatible with chickpea inoculum used in this study. Although carbendazim at 2 g.a.i. kg seed had no phytotoxic effect assessed under greenhouse conditions, it significantly reduced the chlorophyll content, nodulation (60d) and N content in shoots.