E. Velázquez

Growth promotion of chickpea and barley by a phosphate solubilizing strain of Mesorhizobium mediterraneum under growth chamber conditions

The efficacy of a strain of Mesorhizobium mediterraneum to enhance the growth and phosphorous content in chickpea and barley plants was assessed in a soil with and without the addition of phospates in a growth chamber. The results obtained show that the strain PECA21 was able to mobilize phosphorous efficiently in both plants when tricalcium phosphate was added to the soil. In barley and chickpea growing in soils treated with insoluble phosphates and inoculated with strain PECA21 the phosphorous content was significantly increased in a 100 and 125%, respectively. Also, the dry matter, nitrogen, potassium, calcium and magnesium content in both plants was significantly increased in inoculated soil added with insoluble phosphate. These results show that the inoculation of a soil with rhizobia should not be based only on the effectiveness of the strains with respect to their nitrogen fixation potential, since these microorganisms can increase the growth of plants by means of other mechanisms, for example the phosphate solubilization. q 2001 Elsevier Science Ltd. All rights reserved.

Characterization of xylanolytic bacteria present in the bract phyllosphere of the date palm Phoenix dactylifera

Aims:  Despite the interest of phyllosphere microbiology, no studies have addressed the bacteria present in bract phyllosphere, an ecosystem that has special characteristics in palm trees because the dry bracts remain on the plant until pruning and may contain polymer‐degrading bacteria involved in plant degradation. Therefore, the aim of this work was to characterize xylanolytic bacteria isolated from palm bract phyllosphere.

Biodiversity of populations of phosphate solubilizing rhizobia that nodulates chickpea in different Spanish soils

Within rhizobia, two species nodulating chickpea, Mesorhizobium ciceri and Mesorhizobium mediterraneum, are known as good phosphate solubilizers. For this reason, we have analysed the ability to solubilize phosphate of a wide number of strains isolated from Cicer arietinum growing in several soils in Spain. The aim of this work was to analyse microbial populations nodulating chickpea, that are able to solubilize phosphates, using molecular techniques. In the present work we analyzed 19 strains isolated from effective nodules of C. arietinum growing in three soils from the North of Spain. Nineteen strains showed ability to solubilize phosphate in YED-P medium. These strains were separated into 4 groups according to the results obtained by 879F-RAPD fingerprinting. The 16S rDNA sequencing of a representative strain from each group allowed the identification of strains as belonging to the genus Mesorhizobium. Strains from groups I and II showed a 99.4% and 99.2% similarity with M. mediterraneum UPM-CA142T, respectively. The strains from group III were related to M. tianshanense USDA 3592T at a 99.4% similarity level. Finally, the strain from group IV was related to M. ciceri USDA 3383T with a 99.3% similarity. The LMW RNA profiles confirmed these results. Strains from groups I and II showed an identical LMW RNA profile to that of M. mediterraneum UPM-CA142T; the profile of strains from group III was identical to that of M.␣tianshanense USDA 3592T and the profile of strains from group IV was identical to that of M. ciceri USDA 3383T. Different 879F-RAPD patterns were obtained for strains of the group I, group II and the M.␣mediterraneum type strain (UPM-CA142T). The 879-RAPD patterns obtained for group III also differed from the pattern shown by M.tianshanense USDA 3592T. Finally, the patterns between group IV and M. ciceri USDA 3383T were also different. These results suggest that groups I and II may be subspecies of M. mediterraneum, group III a subspecies of M. tianshanense and group IV a subspecies of M. ciceri. Nevertheless, more studies are needed to establish the taxonomic status of strains isolated in this study.

Strains of Mesorhizobium amorphae and Mesorhizobium tianshanense, carrying symbiotic genes of common chickpea endosymbiotic species, constitute a novel biovar (ciceri) capable of nodulating Cicer arietinum

Aims:  To identify several strains of Mesorhizobium amorphae and Mesorhizobium tianshanense nodulating Cicer arietinum in Spain and Portugal, and to study the symbiotic genes carried by these strains.

Growth promotion of common bean (Phaseolus vulgaris L.) by a strain of Burkholderia cepacia under growth chamber conditions

We isolated a strain of Burkholderia cepacia (SAOCV2), which solubilizes inorganic phosphates and antagonizes Fusarium oxysporum f. sp. phaseoli and Fusarium solani in vitro, from soil. However, this strain does not have antibiotic activity against the bacteria tested in this study, which included bacterial plant pathogens and rhizobia. The efficacy of this strain to enhance the growth and P content in common bean was assesed in a soil traditionally cultivated with this species. In this soil, the common bean is affected by Fusarium. Our results show that the strain SAOCV2 was able to mobilize P efficiently in the common bean, so that its P content was increased by 44% with respect to uninoculated plants, whereas it was not significantly different with respect to the plants treated with fungicide. Also, the N content in plants inoculated with the strain SAOCV2 was significantly higher than in uninoculated plants. This result is correlated with a larger number of nodules in plants inoculated with SAOCV2 and in plants treated with fungicide and indicates that the inhibition of fungal growth enhances the bacterial community in the plant rhizosphere, including rhizobia. Our results suggest that the inoculation with strain SAOCV2 promotes the growth of common bean by several mechanisms, that include P mobilization, antagonism towards pathogenic species of Fusarium and, indirectly, by an increase in nodulation that may lead to an increase in N2 fixation.

Role of Rhizobium endoglucanase CelC2 in cellulose biosynthesis and biofilm formation on plant roots and abiotic surfaces

BackgroundThe synthesis of cellulose is among the most important but poorly understood biochemical processes, especially in bacteria, due to its complexity and high degree of regulation. In this study, we analyzed both the production of cellulose by all known members of the Rhizobiaceae and the diversity of Rhizobium celABC operon predicted to be involved in cellulose biosynthesis. We also investigated the involvement in cellulose production and biofilm formation of celC gene encoding an endoglucanase (CelC2) that is required for canonical symbiotic root hair infection by Rhizobium leguminosarum bv. trifolii.ResultsANU843 celC mutants lacking (ANU843ΔC2) or overproducing cellulase (ANU843C2+) produced greatly increased or reduced amounts of external cellulose micro fibrils, respectively. Calcofluor-stained cellulose micro fibrils were considerably longer when formed by ANU843ΔC2 bacteria rather than by the wild-type strain, in correlation with a significant increase in their flocculation in batch culture. In contrast, neither calcofluor-stained extracellular micro fibrils nor flocculation was detectable in ANU843C2+ cells. To clarify the role of cellulose synthesis in Rhizobium cell aggregation and attachment, we analyzed the ability of these mutants to produce biofilms on different surfaces. Alteration of wild-type CelC2 levels resulted in a reduced ability of bacteria to form biofilms both in abiotic surfaces and in planta.ConclusionsOur results support a key role of the CelC2 cellulase in cellulose biosynthesis by modulating the length of the cellulose fibrils that mediate firm adhesion among Rhizobium bacteria leading to biofilm formation. Rhizobium cellulose is an essential component of the biofilm polysaccharidic matrix architecture and either an excess or a defect of this “building material” seem to collapse the biofilm structure. These results position cellulose hydrolytic enzymes as excellent anti-biofilm candidates.

Molecular methods for biodiversity analysis of phosphate solubilizing microorganisms (PSM)

Although the phosphate solubilizing potential is not a very common characteristic among microorganisms, phosphate solubilizers belonging to diverse groups of microorganisms, especially bacteria, are known. The ecological role of these microorganisms in soil is very important, as they take part in the biogeochemical cycles of the main nutrient elements in the ecosystems. Thus, it is necessary to study the composition and dynamics of these microbial populations to reach a better understanding of soil microbial diversity and nutrient uptake by plants. The study of populations of microorganisms, which share the common characteristic of phosphate solubilization has great complexity, because they belong to very diverse groups sometimes not closely related under a phylogenetic point of view. Therefore good techniques are needed to perform the analysis and identification of PSM populations. The molecular techniques based on nucleic acid composition are excellent tools for this purpose, as they are precise, reproducible and not dependent on culture media composition or growth phase of microorganisms. In this paper main molecular methods based on electrophoresis of nucleic acids as LMW RNA profiling and PCR-based techniques specially DNA involving approaches are reviewed and discussed, highlighting the main advantages and drawbacks of the different methods.

Strained silicon modulation field-effect transistor as a new sensor of terahertz radiation

In this paper, we report on room temperature detection of terahertz radiation from strained-Si modulation-doped field-effect transistors. A non-resonant signal was observed with a maximum around the threshold voltage. The signal was interpreted due to the plasma wave nonlinearities in the channel. The intensity of the signal increases for the higher applied drain-to-source current. We also observed a dependence of the signal on the polarization of the incoming radiations.

Effect of inoculation with a strain of Pseudomonas fragi in the growth and phosphorous content of strawberry plants

Within genus Pseudomonas, several species are able to solubilize phosphate in plates and some of these species are also able to mobilize phosphorous to plants. In this work we isolated a strain, SAPA2, from the rhizosphere of barley plants growing in a soil from Northern Spain. This strain was able to solubilize phosphates in plates forming great halos of solubilization in 24 h. Moreover, this strain retained its ability to solubilize phosphate after five culture passes. The 16S rRNA sequence of this strain showed a similarity of 99.9% with that of Pseudomonas fragi. The inoculation of strawberry plants with this strain was carried out in growth chamber applying 10 ml of a suspension containing 108 UFC/ml to each plant. According to the results obtained, the plants inoculated with this strain growing in a soil amended with insoluble phosphate had a phosphorous content significantly higher than uninoculated plants growing in soil with or without insoluble phosphates. Therefore, the strain SAPA2 promotes phosphorous mobilization to strawberry plants. Therefore, the inoculation of plants with suitable phosphate solubilizing bacteria can increase the crop yield and allows a better exploitation of natural soil resources.

Characterization of a strain of Pseudomonas fluorescens that solubilizes phosphates in vitro and produces high antibiotic activity against several microorganisms

The genus Pseudomonas is known as a P-solubilizer and antibiotic producer. In this work we have characterized a strain, PFBV1, which actively solubilizes phosphate in vitro and produces several antibiotics with high activity against several microorganisms. Using LMW RNA profile analysis we identified this strain as Pseudomonas fluorescens. Antimicrobial activity was detected against a wide spectrum of microorganisms, which included Gram negative bacteria such as Proteus vulgaris; Gram positive bacteria such as Bacillus subtilis and yeasts such as Candida albicans.