[Show abstract][Hide abstract] ABSTRACT: Recently, nanoscience has become one of the most promising fields of research with greater impact on economy and environment health. The research on nanomaterials: materials of 100 nm in at least one dimension, is likely to result in the production of huge number of new nano-products in the coming years. Considering the importance of nanotechnology, a greater attention has been paid on this industry which is expected to reach a market size of approximately 2.6 trillion dollars by 2015 . In addition, nanotechnology is also likely to influence agricultural research especially in (i) the conversion of agricultural and food wastes to energy and other useful by-products through enzymatic nano-bio-processing (ii) disease prevention and treatment of plants using various nanomaterials  and (iii) reproductive science and technology. Despite these benefits, the increasing numbers of commercial products, from cosmetics to medicine and fertilizers to crop products are adding sufficient amounts of nanomaterials ultimately to soils. Such nanoparticles have however, been found highly resistant to degradation and persist in soil or water bodies. Nanomaterials for example carbon nanotubes [3, 4], graphene-based nanomaterials , iron-based nanoparticles , silver  and copper, zinc and titanium oxide nanoparticles [8, 9] have been reported to cause biologically undesirable toxic effects on both deleterious and beneficial rhizosphere microorganisms [10-12] including Escherichia coli, Bacillus subtilis, and Streptococcus aureus , Pseudomonas chlororaphis [14-18], Pseudomonas putida  and Campylobacter jejuni . However, the reports on the effect of nanoparticles on secondary metabolites of microbes are conflicting. For example, Dimkpa et al.  in a recent study found that sub-lethal levels of CuONPs reduced the secretion of plant growth promoting substance siderophore in P. chlororaphis O6 whereas ZnO NPs increased the production of the fluorescent siderophore pyoverdine. Similarly, a contrasting effect of CuO and ZnO NPs on siderophores and IAA has also been reported by Dimpka et al.  suggesting that the effect of NPs on secondary metabolite production by bacterial populations cannot be generalized rather it is highly metabolite/nano specific and may vary from Symbiotic nitrogen fixing rhizobia besides fixing atmospheric nitrogen also produces plant growth promoting substances such as indole acetic acids, siderophores, and cyanogenic compounds etc. However, the effects of nanomaterials on plant growth regulating substances synthesized by these bacteria are not reported. In this paper we have examined the impact of varying concentration of three metal oxide nanoparticles (MONPs) namely copper oxide (CuO), iron oxide (Fe2O3) and zinc oxide (ZnO) on growth behaviour and plant growth promoting activities of nodule forming bacterium Rhizobium sp. strain OS1. The three MONPs tested in this study differentially affected the levels of plant growth regulating substances in a dose dependent manner which varied with species of each nanoparticle. A maximum reduction in indole acetic acid, hydrogen cyanide, ammonia and siderophores, expressed by Rhizobium sp. OS1 was observed at 150 µgml-1 each of CuO, Fe2O3 and ZnO. Iron oxide did not show any toxicity to siderophores. At 50 µgml
[Show abstract][Hide abstract] ABSTRACT: Plant growth promoting rhizobacteria affects the overall performance of plants by one or combination of mechanisms. However, little information is available on how ACC deaminase secreting bacteria enhance crop production. The present study aimed at identifying ACC deaminase producing and phosphate solubilizing bacterial strains and to assess their plant growth promoting activities. Additionally, the effect of two ACC deaminase positive bacterial strains Pseudomonas putida and Rhizobium leguminosarum on pea plants was determined to find a novel and compatible bacterial pairing for developing efficient inoculants for enhancing legume production and reducing dependence on chemical fertilizers. The isolated bacterial cultures were characterized biochemically and by 16S rRNA sequence analysis. The plant growth promoting activities was determined using standard microbiological methods. The impact of P. putida and R. leguminosarum, on pea plants was determined both in pots and in field environments. Of the total 40 bacterial strains, strain PSE3 isolated from Mentha arvenss rhizosphere and RP2 strain from pea nodules produced ACC deaminase, solubilized insoluble phosphate, synthesized indole acetic acid, ammonia, cyanogenic compounds, exopolysaccharides and had antifungal activity. The dual inoculation of P. putida strain PSE3 and R. leguminosarum strain RP2 had largest positive effect and markedly increased the growth, symbiotic characteristics, nutrient pool and quantity and quality of pea seeds. The measured parameters were further augmented when inoculated pea plants were grown in soils treated with urea or DAP. A significant variation in the measured parameters of pea plants was observed under both pot and field trials following microbial inoculation but the bacterial cultures did not differ significantly in growth promoting activities. The results suggest that ACC deaminase positive bacterial cultures endowed with multiple potential can be targeted to develop mixed inoculants for enhancing pea production and hence, to reduce dependence on synthetic fertilizers.
[Show abstract][Hide abstract] ABSTRACT: Pseudomonas aeruginosa strain OSG41, isolated from the heavy metal contaminated water irrigated to rhizospheric soil of mustard crop, tolerated chromium up to the concentration of 1800 μg ml−1 and reduced it by 100% at pH 6–8 after 120 h incubation at 30–40 °C. P. aeruginosa produced plant growth-promoting substances, both in the presence and absence of chromium; it produced 32 μg ml−1 indole acetic acid ml−1, in Luria Bertani broth with 100 mg tryptophan ml−1, solubilized tri-calcium phosphate (417 μg ml−1) and secreted 20.8 μg ml−1 exopolysaccharides (EPS) which decreased with increasing concentration of chromium added to growth medium. While investigating the impact of hexavalent chromium on chickpea, chromium application to soil had a phytotoxic effect. The application of P. aeruginosa strain OSG41 even with three times concentration of chromium increased the dry matter accumulation, symbiotic attributes (like nodule formation), grain yield and protein of chickpea compared to non-inoculated plants. The bio-inoculant decreased the uptake of chromium by 36, 38 and 40% in roots, shoots and grains, respectively. The present finding suggests that the bioinoculant effectively reduced the toxicity of hexavalent chromium to chickpea plants and concurrently enhanced the biological and chemical characteristics of chickpea, when grown in chromium treated soils.
European Journal of Soil Biology 05/2013; · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Saudi Journal of Biological Sciences 04/2013; 20(2):121-129. · 0.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
PLoS ONE 03/2013; 8(3):e59140. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Phosphorus (P) is one of the major plant nutrients whose deficiency results
in severe losses to crop yields. To achieve optimum crop production, P is,
therefore, consistently required. The use of chemical fertilizers in contrast
is discouraged for two basic reasons: one, the repeated and injudicious
application may alter soil fertility by adversely affecting microbial composition
and functions and, second, it is expensive. To address these problems, scientists
have identified soil-borne microorganisms belonging to a specific functional
group generally referred to as phosphate-solubilizing microorganisms (PSM)
which play many ecophysiological roles, especially in providing plants with
P. They can be found in any environment from conventional to contaminated
ones and are able to express their activity both in vitro and under field
conditions. The solubilization of P by bacteria including even some of the strict
nitrogen fixers, for example, rhizobia (symbiotic) or Azotobacter (asymbiotic),
is a multifactor process. The ability to release bound P from both organic
(enzymatic) and inorganic (acidification) sources by this functionally diverse
group of organisms and to provide growth regulators (phytohormones) to plants
or protecting plants from various diseases through other mechanisms (such as
synthesizing antibiotics, siderophores, cyanogenic compounds, etc.) is indeed
some of the most fascinating biological traits that have resulted in increased
crop yields. Here, we highlight the functional aspects of PS bacteria especially
their role in crop improvement particularly legumes and cereals grown in varied
agro-ecological regions. The discussion attempted here is likely to serve as
a low-cost prospective option for sustainable agriculture and also to solve
economic constraint to considerable extent faced by the farming communities.
01/2013: chapter Functional Aspect of Phosphate-Solubilizing Bacteria: Importance in Crop Production; Springer-Verlag Berlin Heidelberg.
Toxicity of Heavy Metals to Legumes and Bioremediation, 01/2012: chapter Heavy Metal Toxicity to Symbiotic Nitrogen-Fixing Microorganism and Host Legumes. In: Toxicity of Heavy Metals to Legumes: pages 29-44; Springer-Verlag/Wien.
[Show abstract][Hide abstract] ABSTRACT: Heavy metal contamination resulting from rapid industrialization and other sources is a growing problem worldwide. Increasing pollution of soils with heavy metals disturbs the microbial biodiversity, soil fertility, and plant production and may cause significant human health problems. The excessive accumulation of heavy metals within plant tissues can modify protein structure or replace an essential element causing chlorosis, growth impairment, browning of roots, and photosystems dysfunction. To circumvent metal toxicity, bioremediation, a process that involves the use of biological materials to detoxify the contaminated sites and brings the environment to its contaminant free (original) state, has emerged as a promising alternative to widely practiced physicochemical methods used to clean up contaminated lands. Biological materials used to remediate contaminated sites are inexpensive, are easy to operate, do not produce hazardous by-products, and can be effective even if metals are present in low concentrations. Here, we integrate the knowledge obtained so far on the removal of metals and metalloids employing bioremediation strategies for contaminated soils. The information regarding different types of bioremediation and the challenges facing bioremediation are highlighted. The role and impacts of plant-growth-promoting rhizobacteria on bioremediation efficiency are addressed.
Toxicity of Heavy Metals to Legumes and Bioremediation, First Edition edited by Almas Zaidi, Parvaze Ahmad Wani, Mohammad Saghir Khan, 01/2012: chapter Bioremediation: A Natural Method for the Management of Polluted Environment: pages 101-114; SPRINGER-VERLAG., ISBN: 978-3-7091-0730-0
Biomanagement of Metal Contaminated Soil., 01/2011: chapter Importance of free living fungi in heavy metal remediation. In: Biomanagement of Metal Contaminated Soil.: pages 479-494; Springer The Netherlands.
[Show abstract][Hide abstract] ABSTRACT: Soils contaminated with heavy metals present a major threat to nodule-forming rhizobia, legumes, and symbiosis formed by the
interacting symbionts. The symbiotic relation, as it occurs generally in economically important legumes, has deep impact on
human interest. However, in legume–Rhizobium symbiosis, maximum yield is possible only when there is suitable condition for both symbiotic partners. Thus, understanding
the effects of heavy metals on rhizobia–legume symbiosis will be useful. Although mechanical and chemical processes have been
used to clean up metal-contaminated soils, most traditional remediation technologies do not provide acceptable solutions for
the removal of metal from soils. The use of metal tolerant/detoxifying microbes offers a viable and inexpensive alternative
technology to clean up polluted soils. Metal-tolerant microbes not only help to remediate the contaminated soils, but also
provide elements essential to the growing legumes. Given the importance of legumes in animal and human consumption and their
role in maintaining soil fertility, attention is paid to understand how rhizobia develops resistance to various heavy metals.
Possible role of symbiotic nitrogen fixers in the metal-contaminated soils and how these microbes influence the productivity
of various legumes in metal-contaminated soils across different geographical regions are discussed.
[Show abstract][Hide abstract] ABSTRACT: Nitrogen-fixing symbioses between legumes and rhizobia over the years have played a major role in sustainable agricultural
ecosystems. Owing to specific interactions with rhizobia, the leguminous plants form specialized nitrogen-fixing organ called
as nodule, wherein rhizobia dwell and bring out the conversion of atmospheric nitrogen (N) to its usable form. This symbiosis
in turn may abate the demand for external application of nitrogenous fertilizers while growing legumes under natural soil
environment. Contemporary genomic research has provided a better understanding of the Rhizobium–legume interaction at molecular level. Several genomic approaches have been employed to define and demonstrate the involvement
of rhizobial genomes in the symbiotic events. The genomes of two rhizobial species namely Mesorhizobium loti, the symbiont of several Lotus species, and Sinorhizobium meliloti, the symbiont of alfalfa, have now been completely sequenced, which have revealed interesting information about the genome
evolution and structure, plant–microbes communication, and physiological diversity among the microsymbionts of legumes. While
for legumes, numerous expressed sequence tags representing tens of thousands of different genes involved in root nodule formation
and nitrogen fixation from three major legume species, Glycine max, Medicago truncatula, and Lotus japonicus have been deposited in the public domain. Currently, biological research is directed to understand gene expression and function
involved in rhizobia–legume interaction. In this context, proteomics with continually evolving set of novel techniques to
study all facets of protein structure and function is being considered as a promising and effective tool in the postgenomic
era to explore further the intricacies of symbiotic process. It is likely that the proteomics approach may reveal the newer
possibilities for better understanding the complex interactions of rhizobia and legumes, and also the mechanisms as to how
rhizobia survive under stressed environment. The major breakthroughs from the contemporary proteome-level investigations into
legume–rhizobia interactions are discussed.
[Show abstract][Hide abstract] ABSTRACT: Microbial communities inhabiting soil or rhizosphere play important roles in growth and development of plants. Of these, phosphate-solubilizing
bacteria (PSB) play fundamental roles in biogeochemical phosphorus cycling in agro-ecosystems. Phosphate-solubilizing microbes
transform the insoluble phosphorus to soluble forms by acidification, chelation, exchange reactions, and polymeric substances
formation. The use of phosphate-solubilizing microbes in agronomic practices helps not only to offset the high cost of phosphatic
fertilizers but also to mobilize insoluble phosphorus in the fertilizers and soils to which they are applied. And hence, application
of such naturally occurring organisms possessing multiple growth-promoting activities hold greater promise for increasing
the productivity of crops including legumes. Another agronomically promising organism is rhizobia which are known exclusively
for its ability to form symbiosis with legumes and enrich nitrogen pool of soil, can also facilitate plant growth by synthesizing
plant growth regulators and solubilizing insoluble phosphorus besides providing nitrogen to plants. In addition, under low
nitrogen fertilizer inputs, availability of phosphorus is a major factor restricting the rate of N2-fixation in legumes. The combined inoculation of N2-fixers, PSB, and mycorrhizal fungi could be more effective than single organism for providing a more balanced nutrition for
legume plants under conditions of reduced nutrient inputs. In this chapter, the strategic and applied research conducted so
far to understand as to how PSB along with other symbionts enhance nutrient availability to legumes and concomitantly improve
yield are reviewed and discussed. The application of synergically interacting yet phylogenetically diverse microbes is likely
to help sustaining the legume productivity in different agricultural production systems.
[Show abstract][Hide abstract] ABSTRACT: Phosphorus is abundant in soils in both organic and inorganic forms; nevertheless, it is unavailable to plants. Accordingly, soil becomes phosphorus (P)-deficient, making P one of the most important nutrient elements limiting crop productivity. To circumvent the P deficiency, phosphate-solubilizing microorganisms could play an important role in making P available for plants by dissolving insoluble P. The dissolution of inorganic P by microbial communities including fungi is though common under in vitro conditions; the performance of phosphate-solubilizing microbes in situ has been contradictory. Fungi exhibit traits such as mineral solubilization, biological control, and production of secondary metabolites. As such, their potential to enhance plant growth when present in association with the roots is clear. The challenge is how to make use of such biological resources to maintain soil health while increasing the crop productivity by providing P to plants through the application of phosphate-solubilizing fungi. The present review focuses on the mechanisms of phosphate solubilization, development and mode of fungal inoculants application and mechanisms of growth promotion by phosphate-solubilizing fungi for crop productivity under a wide range of agro-ecosystems, and the understanding and management of P nutrition of plants through the application of phosphate-solubilizing fungi will be addressed and discussed.
[Show abstract][Hide abstract] ABSTRACT: Most agronomic soils contain large reserves of total phosphorus [P], but the fixation and precipitation of P cause P deficiency, and in turn, restrict the growth of crops severely. Phosphorus replenishment, especially in sustainable production systems, remains a major challenge as it is mainly fertilizer-dependent. Though the use of chemical P fertilizers is obviously the best means to circumvent P deficiency in different agro-ecosystems, their use is always limited due to its spiralling cost. A greater interest has, therefore, been generated to find an alternative yet inexpensive technology that could provide sufficient P to plants while reducing the dependence on expensive chemical P fertilizers. Among the heterogeneous and naturally abundant microbes inhabiting the rhizosphere, the phosphate solubilizing microorganisms (PSM) including bacteria have provided an alternative biotechnological solution in sustainable agriculture to meet the P demands of plants. These organisms in addition to providing P to plants also facilitate plant growth by other mechanisms. Despite their different ecological niches and multiple functional properties, P-solubilizing bacteria have yet to fulfil their promise as commercial bio-inoculants. Current developments in our understanding of the functional diversity, rhizosphere colonizing ability, mode of actions and judicious application are likely to facilitate their use as reliable components in the management of sustainable agricultural systems.
[Show abstract][Hide abstract] ABSTRACT: Chromium-reducing and plant growth–promoting potential, including production of siderophores by chromium(VI)-resistant Mesorhizobium species RC1 and RC4, isolated from chickpea nodules, was assessed both in the presence and absence of chromium(VI) under in vitro conditions. The Mesorhizobium strains displayed a high level of tolerance to chromium (400 μg ml), and showed a varied sensitivity to antibacterial drugs, on yeast extract mannitol (YEM) agar plates. Mesorhizobium strains RC1 and RC4 reduced chromium(VI) by 84% and 83%, respectively at pH 7 in YEM broth after 120 h of incubation. Mesorhizobial strains RC1 and RC4 produced 27 and 35 μg ml of indole acetic acid (IAA), respectively, in Luria-Bertani broth with 100 μg ml of tryptophan. The IAA production by the mesorhizobial strains did not differ significantly (p ≤ .05) under chromium stress and showed a positive reaction for siderophore, HCN, and ammonia, both in the absence and presence of chromium(VI).The present observations suggest that the chromium reducing and plant growth promoting activities of the Mesorhizobium strains could be exploited for bioremediation of chromium(VI) and to enhance the legume productivity for chromium-contaminated soils.