Microbial populations and activities in the rhizoplane of rock-weathering desert plants. II. Growth promotion of cactus seedlings

Four bacterial species isolated from the rhizoplane of cacti growing in bare lava rocks were assessed for growth promotion of giant cardon cactus seedlings (Pachycereus pringlei). These bacteria fixed N(2), dissolved P, weathered extrusive igneous rock, marble, and limestone, and significantly mobilized useful minerals, such as P, K, Mg, Mn, Fe, Cu, and Zn in rock minerals. Cardon cactus seeds inoculated with these bacteria were able to sprout and grow normally without added nutrients for at least 12 months in pulverized extrusive igneous rock (ancient lava flows) mixed with perlite. Cacti that were not inoculated grew less vigorously and some died. The amount of useful minerals (P, K, Fe, Mg) for plant growth extracted from the pulverized lava, measured after cultivation of inoculated plants, was significant. This study shows that rhizoplane bacteria isolated from rock-growing cacti promote growth of a cactus species, and can help supply essential minerals for a prolonged period of time.     

A major chemotaxis gene cluster in Azospirillum brasilense and relationships between chemotaxis operons in alpha-proteobacteria

Azospirillum brasilense shows chemotaxis to a variety of nutrients and oxygen. Genes encoding the central signal transduction pathway in chemotaxis were identified by phenotypic complementation of generally non-chemotactic mutants. Sequencing of a DNA fragment, which complemented two different mutants, revealed a region of five open reading frames translated in one direction and encoding homologs of known genes comprising excitation and adaptation pathways for chemotaxis in other bacterial species. The major chemotaxis gene cluster appears to be essential for all known behavioral responses that direct swimming motility in A. brasilense. Phylogenetic and genomic analysis revealed three groups of chemotaxis operons in alpha-proteobacterial species and assigned the A. brasilense operon to one of them. Interestingly, operons that are shown to be major regulators of behavior in several alpha-proteobacterial species are not orthologous.     

Nitrogenase activity of Herbaspirillum seropedicae grown under low iron levels requires the products of nifXorf1 genes

Herbaspirillum seropedicae strains mutated in the nifX or orf1 genes showed 90% or 50% reduction in nitrogenase activity under low levels of iron or molybdenum respectively. Mutations in nifX or orf1 genes did not affect nif gene expression since a nifH::lacZ fusion was fully active in both mutants. nifX and the contiguous gene orf1 are essential for maximum nitrogen fixation under iron limitation and are probably involved in synthesis of nitrogenase iron or iron-molybdenum clusters.     

Phylogeny of sulfate-reducing bacteria(1)

Three starvation regimes (a deficient culture medium, a saline buffer solution and distilled water) were evaluated for their possible effect on cell surface characteristics of Azospirillum lipoferum 1842 related to the initial adsorption of the bacterium to surfaces. The bacteria survived for 7 days in all media although they did not multiply. Upon transfer from a rich growth medium (nutrient agar) to starvation conditions, cell surface hydrophobicity dropped sharply but recovered its initial value within 24 to 48 h, except in phosphate-buffered saline, the length of the recovery period depending on the starvation medium. Starvation affected the sugar affinity of the A. lipoferum cell surface mainly towards p-aminophenyl-alpha-D-mannopyranoside, to a lesser extent to glucose, but not to other monosaccharides tested. Starvation changed the concentration of several cell surface proteins but did not induce the synthesis of new ones. The cell surface hydrophobic protein (43 kDa) of A. lipoferum 1842 was unaffected by any starvation treatment for a period of up to 48 h, but later disappeared. These data showed that starvation is not a major factor in inducing changes in the cell surface which lead to the primary phase of attachment of Azospirillum to surfaces.     

Composted sawdust as a carrierfor Bradyrhizobium,Rhizobium and Azospirillum in crop inoculation

Sawdust was composted by inoculation with a cellulose-decomposing fungus (Cephalosporium sp.) and an N₂-fixing bacterium (Azospirillum brasilense). The product was investigated as a possible carrier for Bradyrhizobium, Rhizobium and Azospirillum. The simple technology and composition of the carrier supported good growth and survival of the investigated strains. Yield increases following crop inoculation with the carrier containing the Bradyrhizobium/Rhizobium/Azospirillum mixture were observed with soybean (34–62%), groundnuts (4–39%), lucerne (24–82%) and a grass mixture of bird's foot trefoil and ryegrass (20–21%).     

Relationship between survival rates of associative nitrogen-fixers on roots and yield response of plants to inoculation

Abstract: The population dynamics of associative nitrogen-fixers Azospirillum lipoferum 137, Arthrobacter mysorens 7, Flavobacterium sp. L30 and phosphate-solubilizing strain Agrobacterium radiobacter 10 in soil and the rhizoplane of inoculated plants was studied in pot and field experiments. All of the present strains were able to actively colonize the rhizoplane of barley, wheat, oat, tomatoes, rape, and alfalfa. For the most part the population size and dynamics of introduced bacteria depended only slightly on the plant genotype and soil conditions. The overall pictures of survival of the strains in soil and on plant roots were similar. The reliable effect of inoculation on plants was observed only in individual cases. No correlation was established between survival of introduced bacteria and their effect on plant development. It was concluded that the influence of plants on survival of bacteria was not specific. In contrast, the plant response to inoculation was conditioned to a greater extent by the plant genotype.     

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.