Selenite reduction by the rhizobacterium Azospirillum brasilense, synthesis of extracellular selenium nanoparticles and their characterisation.

Microbial reduction of selenium oxyanions has attracted attention in recent years. In this study, an original and simple method for the synthesis of extracellular selenium nanoparticles (Se NPs) of relatively uniform size has been developed using strains Sp7 and Sp245 of the ubiquitous plant-growth promoting rhizobacterium Azospirillum brasilense, both capable of selenite (SeO32-) reduction. In addition, a reliable purification protocol for the recovery of the Se NPs has been perfected, which could be applied with minor modifications to cultures of other microbial species. Importantly, it was found that, by changing the conditions of bacterial reduction of selenite, extracellularly localised Se NPs can be obtained using bacteria which would otherwise produce intracellular Se NPs. In particular, bacterial cultures grown up to the end of the logarithmic growth phase, washed free of culture medium and then incubated with selenite, were used to obtain extracellular Se NPs. Their sizes depended on the initial selenite concentration (∼25-80 nm in diameter at 50-10 mM selenite, respectively). The Se NPs obtained were characterised by transmission electron microscopy (TEM), dynamic light scattering, as well as Raman and UV-vis spectroscopies. Their zeta potential was found to be negative (ca. minus 21-24 mV). Bacterial selenite reduction was also studied in the presence of the efflux pump inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP). In this case, TEM indicated the formation only of intracellular selenium crystallites. The data show that the formation of extracellular Se NPs requires normal bacterial metabolic activity, while CCCP evidently blocks the membrane export of Se0 nuclei.

Multiple CheY Homologs Control Swimming Reversals and Transient Pauses in Azospirillum brasilense.

Chemotaxis, together with motility, helps bacteria foraging in their habitat. Motile bacteria exhibit a variety of motility patterns, often controlled by chemotaxis, to promote dispersal. Motility in many bacteria is powered by a bidirectional flagellar motor. The flagellar motor has been known to briefly pause during rotation because of incomplete reversals or stator detachment. Transient pauses were previously observed in bacterial strains lacking CheY, and these events could not be explained by incomplete motor reversals or stator detachment. Here, we systematically analyzed swimming trajectories of various chemotaxis mutants of the monotrichous soil bacterium, Azospirillum brasilense. Like other polar flagellated bacterium, the main swimming pattern in A. brasilense is run and reverse. A. brasilense also uses run-pauses and putative run-reverse-flick-like swimming patterns, although these are rare events. A. brasilense mutant derivatives lacking the chemotaxis master histidine kinase, CheA4, or the central response regulator, CheY7, also showed transient pauses. Strikingly, the frequency of transient pauses increased dramatically in the absence of CheY4. Our findings collectively suggest that reversals and pauses are controlled through signaling by distinct CheY homologs, and thus are likely to be functionally important in the lifestyle of this soil organism.

Distinct Domains of CheA Confer Unique Functions in Chemotaxis and Cell Length in Azospirillum brasilense Sp7.

Chemotaxis is the movement of cells in response to gradients of diverse chemical cues. Motile bacteria utilize a conserved chemotaxis signal transduction system to bias their motility and navigate through a gradient. A central regulator of chemotaxis is the histidine kinase CheA. This cytoplasmic protein interacts with membrane-bound receptors, which assemble into large polar arrays, to propagate the signal. In the alphaproteobacterium Azospirillum brasilense, Che1 controls transient increases in swimming speed during chemotaxis, but it also biases the cell length at division. However, the exact underlying molecular mechanisms for Che1-dependent control of multiple cellular behaviors are not known. Here, we identify specific domains of the CheA1 histidine kinase implicated in modulating each of these functions. We show that CheA1 is produced in two isoforms: a membrane-anchored isoform produced as a fusion with a conserved seven-transmembrane domain of unknown function (TMX) at the N terminus and a soluble isoform similar to prototypical CheA. Site-directed and deletion mutagenesis combined with behavioral assays confirm the role of CheA1 in chemotaxis and implicate the TMX domain in mediating changes in cell length. Fluorescence microscopy further reveals that the membrane-anchored isoform is distributed around the cell surface while the soluble isoform localizes at the cell poles. Together, the data provide a mechanism for the role of Che1 in controlling multiple unrelated cellular behaviors via acquisition of a new domain in CheA1 and production of distinct functional isoforms.IMPORTANCE Chemotaxis provides a significant competitive advantage to bacteria in the environment, and this function has been transferred laterally multiple times, with evidence of functional divergence in different genomic contexts. The molecular principles that underlie functional diversification of chemotaxis in various genomic contexts are unknown. Here, we provide a molecular mechanism by which a single CheA protein controls two unrelated functions: chemotaxis and cell length. Acquisition of this multifunctionality is seemingly a recent evolutionary event. The findings illustrate a mechanism by which chemotaxis function may be co-opted to regulate additional cellular functions.

Azospirillum brasilense Chemotaxis Depends on Two Signaling Pathways Regulating Distinct Motility Parameters.

UNLABELLED: The genomes of most motile bacteria encode two or more chemotaxis (Che) systems, but their functions have been characterized in only a few model systems. Azospirillum brasilense is a motile soil alphaproteobacterium able to colonize the rhizosphere of cereals. In response to an attractant, motile A. brasilense cells transiently increase swimming speed and suppress reversals. The Che1 chemotaxis pathway was previously shown to regulate changes in the swimming speed, but it has a minor role in chemotaxis and root surface colonization. Here, we show that a second chemotaxis system, named Che4, regulates the probability of swimming reversals and is the major signaling pathway for chemotaxis and wheat root surface colonization. Experimental evidence indicates that Che1 and Che4 are functionally linked to coordinate changes in the swimming motility pattern in response to attractants. The effect of Che1 on swimming speed is shown to enhance the aerotactic response of A. brasilense in gradients, likely providing the cells with a competitive advantage in the rhizosphere. Together, the results illustrate a novel mechanism by which motile bacteria utilize two chemotaxis pathways regulating distinct motility parameters to alter movement in gradients and enhance the chemotactic advantage. IMPORTANCE: Chemotaxis provides motile bacteria with a competitive advantage in the colonization of diverse niches and is a function enriched in rhizosphere bacterial communities, with most species possessing at least two chemotaxis systems. Here, we identify the mechanism by which cells may derive a significant chemotactic advantage using two chemotaxis pathways that ultimately regulate distinct motility parameters.

Rapid quantification of rice root-associated bacteria by flow cytometry.

UNLABELLED: To understand the mechanism of plant-bacterium interaction, it is critical to enumerate epiphytic bacteria colonizing the roots of the host. We developed a new approach, based on flow cytometry, for enumerating these bacteria and used it with rice plants, 7 and 20 days after colonization with Herbaspirillum rubrisubalbicans and Azospirillum brasilense. The results were compared with those obtained with the traditional plate count method. Both methods gave similar numbers of H. rubrisubalbicans associated with rice roots (c. 10(9) CFU g(-1) ). However, flow cytometry gave a number of viable cells of rice-associated A. brasilense that was approx. 10-fold greater than that obtained with the plate count method. These results suggest that the plate count method can underestimate epiphytic populations. Flow cytometry has the additional advantage that it is more precise and much faster than the plate count method. SIGNIFICANCE AND IMPACT OF THE STUDY: Determination of precise number of root-associated bacteria is critical for plant-bacteria interaction studies. We developed a flow cytometry approach for counting bacteria and compared it with the plate count method. Our flow cytometry assay solves two major limitations of the plate count method, namely that requires long incubation times of up to 48 h and only determines culturable cells. This flow cytometry assay provides an efficient, precise and fast tool for enumerating epiphytic cells.

Cellular responses during morphological transformation in Azospirillum brasilense and Its flcA knockout mutant.

FlcA is a response regulator controlling flocculation and the morphological transformation of Azospirillum cells from vegetative to cyst-like forms. To understand the cellular responses of Azospirillum to conditions that cause morphological transformation, proteins differentially expressed under flocculation conditions in A. brasilense Sp7 and its flcA knockout mutant were investigated. Comparison of 2-DE protein profiles of wild-type (Sp7) and a flcA deletion mutant (Sp7-flcAΔ) revealed a total of 33 differentially expressed 2-DE gel spots, with 22 of these spots confidently separated to allow protein identification. Analysis of these spots by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and MASCOT database searching identified 48 proteins (≥10% emPAI in each spot). The functional characteristics of these proteins included carbon metabolism (beta-ketothiolase and citrate synthase), nitrogen metabolism (Glutamine synthetase and nitric oxide synthase), stress tolerance (superoxide dismutase, Alkyl hydroperoxidase and ATP-dependent Clp protease proteolytic subunit) and morphological transformation (transducer coupling protein). The observed differences between Sp7 wild-type and flcA- strains enhance our understanding of the morphological transformation process and help to explain previous phenotypical observations. This work is a step forward in connecting the Azospirillum phenome and genome.

A CheR/CheB fusion protein is involved in cyst cell development and chemotaxis in Azospirillum brasilense Sp7.

We here report the sequence and functional analysis of cstB of Azospirillum brasilense Sp7. The predicted cstB contains C-terminal two PAS domains and N-terminal part which has similarity with CheB-CheR fusion protein. cstB mutants had reduced swarming ability compared to that of A. brasilense wild-type strain, implying that cstB was involved in chemotaxis in A. brasilense. A microscopic analysis revealed that cstB mutants developed mature cyst cells more quickly than wild type, indicating that cstB is involved in cyst formation. cstB mutants were affected in colony morphology and the production of exopolysaccharides (EPS) which are essential for A. brasilense cells to differentiate into cyst-like forms. These observations suggested that cstB was a multi-effector involved in cyst development and chemotaxis in A. brasilense.

ADP-ribosylation of dinitrogenase reductase in Azospirillum brasilense is regulated by AmtB-dependent membrane sequestration of DraG.

Nitrogen fixation in some diazotrophic bacteria is regulated by mono-ADP-ribosylation of dinitrogenase reductase (NifH) that occurs in response to addition of ammonium to the extracellular medium. This process is mediated by dinitrogenase reductase ADP-ribosyltransferase (DraT) and reversed by dinitrogenase reductase glycohydrolase (DraG), but the means by which the activities of these enzymes are regulated are unknown. We have investigated the role of the P(II) proteins (GlnB and GlnZ), the ammonia channel protein AmtB and the cellular localization of DraG in the regulation of the NifH-modification process in Azospirillum brasilense. GlnB, GlnZ and DraG were all membrane-associated after an ammonium shock, and both this membrane sequestration and ADP-ribosylation of NifH were defective in an amtB mutant. We now propose a model in which membrane association of DraG after an ammonium shock creates a physical separation from its cytoplasmic substrate NifH thereby inhibiting ADP-ribosyl-removal. Our observations identify a novel role for an ammonia channel (Amt) protein in the regulation of bacterial nitrogen metabolism by mediating membrane sequestration of a protein other than a P(II) family member. They also suggest a model for control of ADP-ribosylation that is likely to be applicable to all diazotrophs that exhibit such post-translational regulation of nitrogenase.

Arabinose content of extracellular polysaccharide plays a role in cell aggregation of Azospirillum brasilense.

Extracellular polysaccharides play an important role in aggregation and surface colonization of plant-associated bacteria. In this work, we report the time course production and monomer composition of the exopolysaccharide (EPS) produced by wild type strain and several mutants of the plant growth promoting rhizobacterium (PGPR) Azospirillum brasilense. In a fructose synthetic medium, wild type strain Sp7 produced a glucose-rich EPS during exponential phase growth and an arabinose-rich EPS during stationary and death phase growth. D-glucose or L-arabinose did not support cell growth as sole carbon sources. However, glucose and arabinose-rich EPSs, when used as carbon source, supported bacterial growth. Cell aggregation of Sp7 correlated with the synthesis of arabinose-rich EPS. exoB (UDP-glucose 4'-epimerase), exoC (phosphomannomutase) and phbC (poly-beta-hydroxyburyrate synthase) mutant strains, under tested conditions, produced arabinose-rich EPS and exhibited highly cell aggregation capability. A mutant defective in LPS production (dTDP 4-rhamnose reductase; rmlD) produced glucose-rich EPS and did not aggregate. These results support that arabinose content of EPS plays an important role in cell aggregation. Cell aggregation appears to be a time course phenomenon that takes place during reduced metabolic cell activity. Thus, aggregation could constitute a protected model of growth that allows survival in a hostile environment. The occurrence of exoC and rmlD was detected in several species of Azospirillum.

Monitoring of cobalt(II) uptake and transformation in cells of the plant-associated soil bacterium Azospirillum brasilense using emission Mössbauer spectroscopy.

Interaction of cobalt(II) at micromolar concentrations with live cells of the plant-growth-promoting rhizobacterium Azospirillum brasilense (strain Sp245) and further transformations of the metal cation were monitored using 57Co emission Mössbauer spectroscopy (EMS). Cell suspensions of the bacterial culture (2.4 x 10(8) cells ml(-1)) were doped with radioactive 57CoCl2 (1 mCi; final concentration 2 x 10(-6) M 57Co2+), kept under physiological conditions for various periods of time (from 2 min up to 1 hour) and then rapidly frozen in liquid nitrogen. Analysis of emission Mössbauer spectra of the frozen aqueous suspensions of the bacterial cell samples shows that the primary absorption of cobalt(II) at micromolar concentrations by the bacterial cells is rapid and virtually complete, giving at least two major forms of cobalt(II) species bound to the cells. Within an hour, the metal is involved in further metabolic transformations reflected by changes occurring in the spectra. The Mössbauer parameters calculated from the EMS data by statistical treatment were different for suspensions of live and dead (thermally killed) bacterial cells that had been in contact with 57Co2+ for 1 h, as well as for the cell-free culture medium containing the same concentration of 57Co2+. Chemical after-effects of the nuclear transition (57Co --> 5 7Fe), which provide additional information on the chemical environment of metal ions, are also considered. The data presented demonstrate that EMS is a valuable tool for monitoring the chemical state of cobalt species in biological matter providing information at the atomic level in the course of its uptake and/or metabolic transformations.

Monitoring Azospirillum-wheat interactions using the gfp and gusA genes constitutively expressed from a new broad-host range vector.

To monitor the colonization of wheat roots by Azospirillum brasilense, we constructed several plasmids based on the pBBR1 replicon expressing the gfp and gusA genes constitutively. Both genes were placed under control of the gentamycin resistance gene promoter resulting in high levels of expression in Escherichia coli and A. brasilense. The constructed plasmids were stably maintained in A. brasilense strains even in the absence of selective pressure. The colonization of wheat plants grown under controlled conditions in sterilized vermiculite by A. brasilense strain FP2 (a Sp7-derivative) transconjugants containing these plasmids was monitored. Bacteria expressing GFP were easily observed in fresh plant material by fluorescence microscopy. Cell aggregates and single bacteria were visualized on the surfaces of young root zones, such as roots hairs and lateral roots. Large cellular clumps were observed at the points of lateral root emergence or at intercellular spaces of root epidermal cells 30 days after inoculation. Although we failed to detected bacteria in internal cortical and xylem tissues of wheat roots, the initial stage of endophytic colonization by A. brasilense may involve the sites detected in this work.

Identification and characterization of a periplasmic nitrate reductase in Azospirillum brasilense Sp245.

The Azospirillum brasilense Sp245 napABC genes, encoding nitrate reductase activity, were isolated and sequenced. The derived protein sequences are very similar throughout the whole Nap segment to the NapABC protein sequences of Escherichia coli, Pseudomonas sp. G-179, Ralstonia eutropha, Rhodobacter sphaeroides, and Paracoccus denitrificans. Based on whole-cell nitrate reductase assays with the artificial electron donors benzyl viologen and methyl viologen, and assays with periplasmic cell-free extracts, it was concluded that the napABC-encoded enzyme activity in Azospirillum brasilense Sp245 corresponds to a periplasmic dissimilatory nitrate reductase, which was expressed under anoxic conditions and oxic conditions. A kanamycin-resistant Azospirillum brasilense Sp245 napA insertion mutant was constructed. The mutant still expressed assimilatory nitrate reductase activity, but was devoid of its periplasmic dissimilatory nitrate reductase activity.

Extracellular polysaccharide composition of Azospirillum brasilense and its relation with cell aggregation.

The exopolysaccharide (EPS) and capsular polysaccharide (CPS) composition of four Azospirillum brasilense strains differing in their aggregation capacity was analyzed by high performance anion exchange chromatography. When growing the different strains in an aggregation inducing medium containing a high carbon:nitrogen (C:N) ratio, both EPS and CPS showed a positive correlation between aggregation and the relative amount of arabinose. Arabinose was not detected in polysaccharides from Sp72002, a pleiotrophic Tn5 mutant strain impaired in aggregation. Arabinose was also not detected in extracellular polysaccharides of bacteria grown in a low C:N ratio, non-inducing aggregation medium, with exception for a relatively small amount found in the CPS of FAJ0204, a super-aggregating mutant strain. The only monosaccharides able to significantly inhibit aggregation at low sugar concentration when tested in a bioassay were arabinose (at a higher extent) and galactose. The possibility that residues of arabinose present in the extracellular polysaccharides are involved in the aggregation of A. brasilense is discussed.

Auxins upregulate expression of the indole-3-pyruvate decarboxylase gene in Azospirillum brasilense.

Transcription of the Azospirillum brasilense ipdC gene, encoding an indole-3-pyruvate decarboxylase involved in the biosynthesis of indole-3-acetic acid (IAA), is induced by IAA as determined by ipdC-gusA expression studies and Northern analysis. Besides IAA, exogenously added synthetic auxins such as 1-naphthaleneacetic acid, 2,4-dichlorophenoxypropionic acid, and p-chlorophenoxyacetic acid were also found to upregulate ipdC expression. No upregulation was observed with tryptophan, acetic acid, or propionic acid or with the IAA conjugates IAA ethyl ester and IAA-L-phenylalanine, indicating structural specificity is required for ipdC induction. This is the first report describing the induction of a bacterial gene by auxin.

Cloning, sequencing, and phenotypic analysis of laf1, encoding the flagellin of the lateral flagella of Azospirillum brasilense Sp7.

Azospirillum brasilense can display a single polar flagellum and several lateral flagella. The A. brasilense Sp7 gene laf1, encoding the flagellin of the lateral flagella, was isolated and sequenced. The derived protein sequence is extensively similar to those of the flagellins of Rhizobium meliloti, Agrobacterium tumefaciens, Bartonella bacilliformis, and Caulobacter crescentus. An amino acid alignment shows that the flagellins of these bacteria are clustered and are clearly different from other known flagellins. A laf1 mutant, FAJ0201, was constructed by replacing an internal part of the laf1 gene by a kanamycin resistance-encoding gene cassette. The mutant is devoid of lateral flagella but still forms the polar flagellum. This phenotype is further characterized by the abolishment of the capacities to swarm on a semisolid surface and to spread from a stab inoculation in a semisolid medium. FAJ0201 shows a normal wheat root colonization pattern in the initial stage of plant root interaction.

[Effect of plasmid composition on the chemotaxis reaction of Azospirillum brasilense Sp245 associated with gramineae]. / Vliianie plazmidnogo sostava na reaktsii khemotaksisa assotsiirovannykh so zlakami bakterii Azospirillum brasilense Sp245.

Correlation between the loss of the 120 and/or 130 kb plasmids by the spontaneous Azospirillum brasilense mutant and appearance of the Che(-)-phenotype towards 9-mono- and disaccharides has been established. The plasmid rearrangement is concomitant with the appearance of a new 44 Kda protein among the membrane proteins of azospirilli. Chemotaxis to alanine, glutamic acid, and to organic acids is not impaired.

Identification and mapping of loci involved in motility, adsorption to wheat roots, colony morphology, and growth in minimal medium on the Azospirillum brasilense Sp7 90-MDa plasmid.

We have constructed a cosmid library of the Azospirillum brasilense Sp7 90-MDa plasmid (p90) and established the EcoRI restriction map of this plasmid. The central regions of cloned p90 DNA fragments from several recombinant cosmids were deleted by restriction endonuclease digestion and replaced by a DNA cassette encoding kanamycin resistance. Using these in vitro constructed deletions for marker exchange in Sp7, we made six different p90 deletion derivatives spanning all together 50% of the total length of p90. Comparison of the deletion derivatives with Sp7 for several properties revealed p90 loci involved in colony morphology, growth on minimal medium, motility, and adsorption to wheat roots. In analogy with the rhizobial symbiotic plasmids (pSym), we propose to denote the p90 plasmid as a rhizocoenotic plasmid (pRhico), carrying several genes involved in the A. brasilense-plant root interaction.