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.     

Structural and functional peculiarities of the lipopolysaccharide of Azospirillum brasilense SR55, isolated from the roots of Triticum durum

Azospirillum brasilense SR55, isolated from the rhizosphere of Triticum durum, was classified as serogroup II on the basis of serological tests. Such serogroup affiliation is uncharacteristic of wheat-associated Azospirillum species. The lipid A of A. brasilense SR55 lipopolysaccharide contained 3-hydroxytetradecanoic, 3-hydroxyhexadecanoic, hexadecanoic and octadecenoic fatty acids. The structure of the lipopolysaccharide's O polysaccharide was established, with the branched octasaccharide repeating unit being represented by l-rhamnose, l-3-O-Me-rhamnose, d-galactose and d-glucuronic acid. The SR55 lipopolysaccharide induced deformations of wheat root hairs. The lipopolysaccharide was not involved in bacterial cell aggregation, but its use to pretreat wheat roots was conducive to cell adsorption. This study shows that Azospirillum bacteria can utilise their own lipopolysaccharide as a carbon source, which may give them an advantage in competitive natural environments.     

Mutants of Azospirillum brasilense resistant to methylammonium

One hundred and twenty-nine mutants of Azospirillum brasilense strain Sp6, resistant to methylammonium, were isolated. Three of the mutants were found to be able to reduce acetylene in the presence of 4 mM ammonium or 120mM methylammonium, concentrations which strongly reduced the nitrogenase activity of the parental strain. Under N₂-fixing conditions, two mutants failed to switch off nitrogenase when NH₄Cl was added. Moreover, the three mutants showed a reduced capacity to incorporate [¹⁴C]methylammonium. The level of glutamine synthetase activity found in the mutants was not reduced as compared to that of the parental strain. All of the data indicate an impairement in the mechanism of ammonium uptake by the bacterial cell.Abbreviations MEA Methylammonium - MSP minimal medium (ammonium free) - PY complete medium - GS glutamine synthetase     

Production of vitamins by Azospirillum brasilense in chemically-defined media

The production of vitamins by Azospirillum brasilense was studied in chemically-defined media amended with malate, gluconate and fructose. The liberation of vitamins was significantly affected by the presence of different carbon sources and the age of the culture. Thiamine, niacin and pantothenic acid were produced in large amounts. Thiamine and riboflavin were produced only in culture containing malate or fructose. Biotin was not detected in the supernatants of the culture media.     

Structure of the O-specific polysaccharide of the lipopolysaccharide of Azospirillum brasilense Sp245

An O-specific polysaccharide was isolated from the lipopolysaccharide of a plant-growth-promoting bacterium Azospirillum brasilense Sp245 and studied by sugar analyses along with one- and two-dimensional 1H and 13C NMR spectroscopy, including NOESY. The polysaccharide was found to be a new rhamnan with a pentasaccharide repeating unit having the following structure:-->2)-beta-D-Rhap-(1-->3)-alpha-D-Rhap-(1-->3)-alpha-D-Rhap-(1-->2)-alpha-D-Rhap-(1-->2)-alpha-D-Rhap-(1-->     

Effect of Azospirillum inoculation on some growth parameters and n-content of wheat,sorghum and panicum

The potential of the nitrogen fixing bacteriumAzospirillum brasilense to enhance development and increase growth of several gramineae was investigated. In both sterilized and non-sterilized systems heading and flowering occurred earlier in the inoculated plants as compared to the noninoculated ones. Total shoot and root weights, total-N content, plant height and leaf length were significantly increased by inoculation.From the Department of Agricultural Botany.     

Analysis of DNA,lipopolysaccharide structure,and some cultural and morphological properties in closely related strains of Azospirillum brasilense

We studied closely related Azospirillum brasilense strains Sp7 and Cd. For probing of their genomes, the fragments of 85 MDa (p85) and 120 MDa (p120) from A. brasilense Sp245 plasmids were hybridized with 115-MDa (p115) and 90-MDa (p90) plasmids of strain Sp7, respectively. Strain Cd was found to lose the 115-MDa plasmid and one of the two EcoRI restriction fragments of the total DNA (localized within p115 and the chromosome) that was homologous to an EcoRI-generated p85 fragment of 2.4 kb. On the contrary, in the total DNA of strain Sp7-S, in spite of the previously established disappearance of the 115-MDa replicon, two fragments homologous to p85 were revealed, as with strain Sp7. It is suggested that the Sp7-S genome contains the total p115 DNA or at least a certain part of it. Strains Sp7 and Cd were found to differ in size and morphology of colonies on solid and semisolid media, in the levels of resistance to the cation surfactant cetavlon, and in the antigen structure of lipopolysaccharides.__________Translated from Mikrobiologiya, Vol. 74, No. 2, 2005, pp. 224–230.Original Russian Text Copyright © 2005 by Petrova, Matora, Burygin, Borisov, Katsy.     

Changes in Azospirillum brasilense motility and the effect of wheat seedling exudates

The rhizobacterium Azospirillum brasilense Sp245 swims, swarms (Swa⁺ phenotype) or, very rarely, migrates with the formation of granular macrocolonies (Gri⁺ phenotype). Our aims were (i) to identify Sp245 mutants that swarm faster than the parent strain or differ from it in the mode of spreading and (ii) to compare the mutants’ responses to wheat seedling exudates. In isotropic liquid media, the swimming speeds of all motile A. brasilense strains were not influenced by the exudates. However, the exudates significantly stimulated the swarming of Sp245. In several Sp245 mutants, the superswarming phenotype was insensitive to local colonial density and to the presence of wheat seedling exudates. An A. brasilense polar-flagellum-defective Gri⁺ mutant BK759.G gave rise to stable Swa⁺⁺ derivatives with restored flagellum production. This transition was concurrent with plasmid rearrangements and was stimulated in the presence of wheat seedling exudates. The swarming rate of the Swa⁺⁺ derivatives of BK759.G was affected by the local density of their colonies but not by the presence of the exudates.     

Structural analysis of the O-polysaccharide from the lipopolysaccharide of Azospirillum brasilense S17

A mixture of two structurally distinct neutral O-polysaccharides was obtained by mild acid degradation of the lipopolysaccharide isolated by the phenol/water extraction from the asymbiotic diazotrophic rhizobacterium Azospirillum brasilense S17. The following structures of the O-polysaccharides were established by composition and methylation analyses, Smith degradation, and 1H and 13C NMR spectroscopy, including a 2D NOESY experiment: [Formula: see text] where L-Rha2Me stands for 2-O-methyl-L-rhamnose and SHb for the (S)-3-hydroxybutanoyl group. The occurrence of two distinct polysaccharides is reported for the first time in Azospirillum spp.     

Electrophysical characteristics of Azospirillum brasilense Sp245 during interaction with antibodies to various cell surface epitopes

This work was undertaken to examine the electrooptical characteristics of cells of Azospirillum brasilense Sp245 during their interaction with antibodies developed to various cell surface epitopes. We used the dependences of the cell suspension optical density changes induced by electroorientation on the orienting field frequency (740, 1000, 1450, 2000, and 2800kHz). Cell interactions with homologous strain-specific antibodies to the A. brasilense Sp245 O antigen and with homologous antibodies to whole bacterial cells brought about considerable changes in the electrooptical properties of the bacterial suspension. When genus-specific antibodies to the flagellin of the Azospirillum sheathed flagellum and antibodies to the serologically distinct O antigen of A. brasilense Sp7 were included in the A. brasilense Sp245 suspension, the changes caused in the electrooptical signal were slight and had values close to those for the above changes. These findings agree well with the immunochemical characteristics of the Azospirillum O antigens and with the data on the topographical distribution of the Azospirillum major cell surface antigens. The obtained results can serve as a basis for the development of a rapid test for the intraspecies detection of microorganisms.     

Atypical R–S Dissociation in Azospirillum brasilense

It was found that atypical R–S dissociation in the type strain A. brasilense Sp7 is not accompanied by drastic changes in the polysaccharide moieties of bacterial lipopolysaccharides but is rather due to different contributions of two O-specific polysaccharides (found in both R and S dissociants) to the age-dependent architectonics of the cell surface.     

Encapsulation as a response of Azospirillum brasilense sp7 to zinc stress

Azospirillum brasilense sp7 was exposed to 2mm Zn²⁺ in minimal medium upon which the cells turned black and non-motile within 24 h. A streptomycin-resistant variant did not exhibit this phenomenon and is sensitized to zinc. A prelude to encystation was the elution of a melanin-like pigment into the medium.The authors are with the Department of Genetics, University of Delhi South Campus, Benito Juarez Road, New Delhi-110021, India