Biosorption and biotransformation of hexavalent chromium [Cr(VI)]: A comprehensive review

The biodiversity of wheat-associated bacteria from the northern hills zone of India was deciphered. A total of 247 bacteria was isolated from five different sites. Analysis of these bacteria by amplified ribosomal DNA restriction analysis (ARDRA) using three restriction enzymes, AluI, MspI and HaeIII, led to the grouping of these isolates into 19–33 clusters for the different sites at 75 % similarity index. 16S rRNA gene based phylogenetic analysis revealed that 65 %, 26 %, 8 % and 1 % bacteria belonged to four phyla, namely Proteobacteria, Firmicutes, Actinobacteria and Bacteroidetes, respectively. Overall, 28 % of the total morphotypes belonged to Pseudomonas followed by Bacillus (20 %), Stenotrophomonas (9 %), Methylobacterium (8 %), Arthrobacter (7 %), Pantoea (4 %), Achromobacter, Acinetobacter, Exiguobacterium and Staphylococcus (3 %), Enterobacter, Providencia, Klebsiella and Leclercia (2 %), Brevundimonas, Flavobacterium, Kocuria, Kluyvera and Planococcus (1 %). Representative strains from each cluster were screened in vitro for plant growth promoting traits, which included solubilisation of phosphorus, potassium and zinc; production of ammonia, hydrogen cyanide, indole-3-acetic acid and siderophore; nitrogen fixation, 1-aminocyclopropane-1-carboxylate deaminase activity and biocontrol against Fusarium graminearum, Rhizoctonia solani and Macrophomina phaseolina. Cold-adapted isolates may have application as inoculants for plant growth promotion and biocontrol agents for crops growing under cold climatic conditions.

Assessment of genetic diversity and plant growth promoting attributes of psychrotolerant bacteria allied with wheat ( Triticum aestivum ) from the northern hills zone of India

Endophytes are microorganisms that live within host plants for at least part of their life and do not cause apparent symptoms of diseases. In general, beneficial endophytes promote host plant growth, increase plant nutrient uptake, inhibit plant pathogen growth, reduce disease severity, and enhance tolerance to environmental stresses. As sustainable and renewable agricultural production (including current biofuel and bioenergy crops) increases in prominence, endophytic microorganisms will play important roles and offer environmentally-friendly methods to increase productivity while reducing chemical inputs. This review discusses various aspects of beneficial fungal and bacterial endophyte interactions with plants, including the physiological and molecular mechanisms by which they benefit plant performance. We also discuss the potential for genetic modification of endophytes with useful genes, which could be used to impart additional traits following inoculation with the genetically engineered endophytes. Finally, we review US-issued patents over the past decade which relate to the use of fungal and bacterial endophytes for plant growth and stress tolerance improvement.

The use of beneficial microbial endophytes for plant biomass and stress tolerance improvement.

Gluconacetobacter diazotrophicus Pal5 is an endophytic diazotrophic bacterium that lives in association with sugarcane plants. It has important biotechnological features such as nitrogen fixation, plant growth promotion, sugar metabolism pathways, secretion of organic acids, synthesis of auxin and the occurrence of bacteriocins. Gluconacetobacter diazotrophicus Pal5 is the third diazotrophic endophytic bacterium to be completely sequenced. Its genome is composed of a 3.9 Mb chromosome and 2 plasmids of 16.6 and 38.8 kb, respectively. We annotated 3,938 coding sequences which reveal several characteristics related to the endophytic lifestyle such as nitrogen fixation, plant growth promotion, sugar metabolism, transport systems, synthesis of auxin and the occurrence of bacteriocins. Genomic analysis identified a core component of 894 genes shared with phylogenetically related bacteria. Gene clusters for gum-like polysaccharide biosynthesis, tad pilus, quorum sensing, for modulation of plant growth by indole acetic acid and mechanisms involved in tolerance to acidic conditions were identified and may be related to the sugarcane endophytic and plant-growth promoting traits of G. diazotrophicus. An accessory component of at least 851 genes distributed in genome islands was identified, and was most likely acquired by horizontal gene transfer. This portion of the genome has likely contributed to adaptation to the plant habitat. The genome data offer an important resource of information that can be used to manipulate plant/bacterium interactions with the aim of improving sugarcane crop production and other biotechnological applications.

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Complete genome sequence of the sugarcane nitrogen-fixing endophyte Gluconacetobacter diazotrophicus Pal5

This study uses a set of PCR-based methods to examine the putative microbiota associated with lichen thalli. In initial experiments, generalized oligonucleotide-primers for the 16S rRNA gene resulted in amplicon pools populated almost exclusively with fragments derived from lichen photobionts (i.e., Cyanobacteria or chloroplasts of algae). This effectively masked the presence of other lichen-associated prokaryotes. In order to facilitate the study of the lichen microbiota, 16S ribosomal oligonucleotide-primers were developed to target Bacteria, but exclude sequences derived from chloroplasts and Cyanobacteria. A preliminary microbiotic survey of lichen thalli using these new primers has revealed the identity of several bacterial associates, including representatives of the extremophilic Acidobacteria, bacteria in the families Acetobacteraceae and Brucellaceae, strains belonging to the genus Methylobacterium, and members of an undescribed lineage in the Rhizobiales. This new lineage was investigated and characterized through molecular cloning, and was found to be present in all examined lichens that are associated with green algae. There is evidence to suggest that members of this lineage may both account for a large proportion of the lichen-associated bacterial community and assist in providing the lichen thallus with crucial nutrients such as fixed nitrogen.

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A microbiotic survey of lichen-associated bacteria reveals a new lineage from the Rhizobiales

Gluconacetobacter diazotrophicus has a long-standing history of bacterial-plant interrelationship as a symbiotic endophyte capable of fixing atmospheric nitrogen. In low nitrogen fertilized sugarcane fields it plays a significant role and its occurrence was realised in most of the sugarcane growing countries. In this mini review, the association of G. diazotrophicus with sugarcane, other crop plants and with various hosts is discussed. The factors affecting survival in the rhizosphere and the putative soil mode of transmission are emphasized. In addition, other N2-fixing Acetobacteraceae members, including Gluconacetobacter azotocaptans, Gluconacetobacter johannae and Swaminathania salitolerans, occurring in coffee, corn and rice plants are also covered. Lastly, the plant-growth-promoting traits identified in this group of bacteria, including N2 fixation, phytohormone synthesis, P and Zn solubilization and biocontrol, are analysed.

Ecological occurrence of Gluconacetobacter diazotrophicus and nitrogen-fixing Acetobacteraceae members: their possible role in plant growth promotion.

Marine algae are rich sources of bioactive compounds capable of harboring secondary metabolites which are structurally and biologically active. In our study, the methanolic extract of marine algae Caulerpa racemosa (green algae) was employed to determine the antibacterial and larvicidal activity. The antibacterial activity showed effective inhibition against five pathogenic bacteria. A significant zone size of 16mm was observed for Pseudomonas aeruginosa. The methanolic extract of Caulerpa racemosa showed effective larvicidal activity against Culex tritaeniorhynchus and the histopathological studies revealed the rupture in mid gut of larvae. The bioactive compounds in the crude extract were further identified as 2-(-3-bromo-1-adamantyl) acetic acid methyl ester and Chola-5, 22-dien-3-ol by GC-MS. Hence the bioactive compounds obtained from the methanolic extracts could be used for the bactericidal and larvicidal activity which will overcome the harmful impact of synthetic insecticide on environment.

Bioactive compounds from Caulerpa racemosa as a potent larvicidal and antibacterial agent

Reactive dyes are pernicious pollutants in textile effluent, which are to be treated passably before discharging into the environment. In the present study, a potential dye degrading bacterial strain Pseudomonas guariconensis was isolated from paddy rhizosphere and was characterized by 16S rRNA gene sequencing. The biodegradation ability of the strain was evaluated by time-based study with immobilized bacterial cells in calcium alginate biocarrier matrix and also with free cells. The results indicated that the strain exhibited maximum degradation of 91% when immobilized in the biocarrier matrix. The enzymatic study revealed the production of oxidoreductase enzymes. The degraded products were identified as 2-amino-3-phenylpropanoic acid and benzoquinone by gas chromatography-mass spectroscopy (GC-MS) analysis, and a degradative pathway was derived based on the enzymatic profile. A packed bed column was designed using P. guariconensis VITSAJ5 immobilized in calcium alginate beads as a biosorbent for the removal of Reactive Red 120. The immobilized bacterial cells exhibited 87% uptake of RR120, whereas the nonimmobilized bacterial cells exhibited a maximum uptake of 37%. The phytotoxicity analysis by seed germination assay revealed an enhanced plumule and radicle length, indicating the nontoxic byproducts after the treatment of Reactive Red 120 by VITSAJ5 compared to the untreated Reactive Red 120 solution. PRACTITIONER POINTS: Current study is the first report on Pseudomonas guariconensis capable of degrading reactive dyes (Reactive Red 120) It was observed that the degradation potential was maximum when cells were immobilized with Ca-Ag biocarrier matrix Breakdown metabolism of Reactive Red 120 was derived through pathway prediction Employing immobilized bacteria in a packed bed column found to possess a prominent biosorption ability on the matrix enhancing the degradation process Toxic reactive dye was converted into nontoxic compounds, evidenced by phytotoxicity studies.

Biodegradation and biosorption of Reactive Red 120 dye by immobilized Pseudomonas guariconensis: Kinetic and toxicity study

Gluconacetobacter diazotrophicus strain PAl5 exhibited a minimum inhibitory concentration value of 11 mM in an LGI medium amended with ZnCl2. When an LGI medium was amended with Zn metal, solubilization halos were observed in a plate assay, and further solubilization was confirmed in a broth assay. The maximum solubilization was recorded after 120 h with a 0.1% Zn metal amendment. During solubilization, the culture growth and pH of the broth were indirectly correlated. Using a Fourier Transform Infrared Spectroscopy analysis, one of the agents solubilizing the Zn metal was identified as gluconic acid. When the Zn-amended broth was observed under a bright field microscope, long involution cells were observed, and further analysis with Atomic Force Microscopy revealed highly deformed, pleomorphic, aggregate-like cells.

Zinc metal solubilization by Gluconacetobacter diazotrophicus and induction of pleomorphic cells.

The present study was focused on the biodegradation of monocrotophos (MCP) by indigenous bacteria isolated from sugarcane cultivable soil. The isolates were tested for pesticide tolerance, and the strain VITNNDJ5 showed maximum MCP tolerance and highest degradation potential. The presence of candidate gene for pesticide degradation and plant growth promoting rhizobacterial traits confirmed VITNNDJ5 to be the effective strain. Biodegradation of MCP (1000 mg L−1) was monitored using UV spectrophotometer and HPLC; the degradation products were identified by GCMS. The degradation kinetics was analyzed, and the rate constant (k) and half-life (t1/2) were calculated. Maximum degradation of up to 93% was observed within 5 days. The 16S rRNA gene sequencing revealed VITNNDJ5 to be the closest neighbor of Bacillus aryabhattai (GenBank accession number—KU598848). The augmentation of VITNNDJ5 to the rhizosphere of Liriope muscari enhanced the plant growth and degradation of MCP in soil. The fate of MCP in soil was analyzed, and a metabolic pathway was proposed.

Jabez W. Osborne

Papers

Ecological occurrence of Gluconacetobacter diazotrophicus and nitrogen-fixing Acetobacteraceae members: their possible role in plant growth promotion.

Bioactive compounds from Caulerpa racemosa as a potent larvicidal and antibacterial agent

Biodegradation and biosorption of Reactive Red 120 dye by immobilized Pseudomonas guariconensis: Kinetic and toxicity study

Zinc metal solubilization by Gluconacetobacter diazotrophicus and induction of pleomorphic cells.

Biodegradation of monocrotophos by a plant growth promoting Bacillus aryabhattai (VITNNDJ5) strain in artificially contaminated soil