-The objective of this work was to identify the most competitive and effective Rhizobium strains in order to increase common bean yield by nitrogen fixation as alternative or complementation to the nitrogen fertilization. Competitiveness tests were lead in axenic conditions, in Cerrado soil pots and in three field experiments, with native Rhizobium strains that were previously identified, according to their effectiveness and genetic variability. The identification of strains in nodules was performed using serological tests (axenic conditions) -agglutination and enzyme linked immunosorbent (Elisa) assays -and random amplified polymorfic DNA (RAPD) (Cerrado soil). Plant yield was determined using the dry weight (greenhouse conditions), total N and grain yield (field experiments). Among the analyzed Rhizobium strains, native strain SLA 2.2 and commercial strain CIAT 899 were the dominant nodules in plants of the most productive plots, presenting yield productivity similar or higher to those obtained in treatments where 20 kg ha -1 of N were applied.Index terms: Phaseolus vulgaris, nodule occupancy, Elisa, RAPD. Capacidade competitiva de estirpes de Rhizobium na nodulação do feijoeiro em solos de CerradoResumo -O objetivo deste trabalho foi identificar as estirpes de Rhizobium mais efetivas e competitivas, a fim de maximizar a produtividade do feijoeiro por meio da fixação de nitrogênio, como alternativa à adubação nitrogenada. Foram conduzidos testes de competitividade em condições axênicas, em vasos com solo do Cerrado e em três experimentos de campo, com estirpes de Rhizobium nativas, previamente selecionadas quanto à efetividade e à variabilidade genética. A identificação das estirpes nos nódulos foi efetuada por meio das técnicas de aglutinação e ensaio imunoabsorvente de ligação de enzimas (Elisa), em condições de casa de vegetação, e pela técnica de DNA polimórfico amplificado ao acaso (RAPD), em solo de Cerrado. A produtividade das plantas foi determinada pela produção de matéria seca, teor de N e produção de grãos (condições de campo). A estirpe nativa SLA 2.2 e a estirpe comercial CIAT 899 foram dominantes nos nódulos das plantas das parcelas mais produtivas, com índices de produtividade iguais ou superiores aos obtidos nos tratamentos em que foram aplicados 20 kg ha -1 de N.Termos para indexação: Phaseolus vulgaris, ocupação nodular, Elisa, RAPD.
The genus Chromobacterium encompasses free-living Gram-negative bacteria. Until 2007, the genus consisted of only one species but six species are now recognized. Chromobacterium violaceum is the type species of the genus and is commonly found in soil and water in tropical and sub-tropical regions. We have investigated a collection of 111 isolates displaying violet pigmentation from undisturbed aquatic and soil environments from Brazilian Cerrado ecosystem. The 16S rRNA gene phylogeny revealed that all isolates were allocated in a monophyletic cluster inside the Chromobacterium genus and formed few clusters related most closely with Chromobacterium piscinae. The two sets of isolates from water and soil were analyzed by the repetitive extragenic palindromic (rep)-PCR genomic fingerprinting technique using a BOX-AR1 primer. The antimicrobial susceptibility and the different carbon sources utilized by these isolates were also investigated. Physiological profiles of the isolates generated by BIOLOG GN2 plates showed great versatility in the substrate utilization, much higher than the C. violaceum ATCC 12472. All isolates exhibited a high minimum inhibitory concentration (MIC) to ampicillin (MIC > 512 μg/ml) and were inhibited by ciprofloxacin, tetracycline and mercury at the lowest concentration tested (MIC < 2 μg/ml). Thirteen BOX-PCR band patterns were identified from 33 individual fingerprints. Eleven patterns provided evidence for endemic distributions. Antimicrobial susceptibility and BOX-PCR fingerprint clustering showed a clear distinction between Chromobacterium isolates from the water and soil. The results suggested that microenvironment barriers such as water and soil can play an important role in the periodic selection and diversification of Chromobacterium population ecotypes.
High temperatures can affect the survival, establishment and symbiotic properties of Rhizobium strains. Bean nodulating Rhizobium strains are considered particularly sensitive because on this strains genetic recombinations and/or deletions occur frequently, thus compromising the use of these bacteria as inoculants. In this study R. tropici and R. leguminosarum bv. phaseoli strains isolated from Cerrado soils were exposed to thermal stress and the strains’ growth, survival and symbiotic relationships as well as alterations in their genotypic and phenotypic characteristics were analyzed. After successive thermal shocks at 45ºC for four hours, survival capacity appeared to be strain-specific, independent of thermo-tolerance and was more apparent in R. tropici strains. Certain R. leguminosarum bv. phaseoli strains had significant alterations in plant dry weight and DNA patterns obtained by AP-PCR method. R. tropici strains (with the exception of FJ2.21) were more stable than R. leguminosarum bv. phaseoli strains because no significant phenotypic alterations were observed following thermal treatments and they maintained their original genotypic pattern after inoculation in plants.
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