Variability of isolated colonies in bean nodulating Rhizobium strains before and after exposure to high temperature

Irregular response to bean plants to Rhizobium inoculation has been attributed to among other factors, low competitive ability, low N2 fixation efficiency and genetic instability of the symbiont. This genetic instability caused by high rates of genomic rearrangements and/or plasmid deletions can be accentuated by high temperatures. This fact may limit the utilization of these strains as inoculants, especially in tropical soils. In this study, the variability of isolated colonies derived from effective R. leguminosarum bv. phaseoli (SLP1.3 and BR 10.026) and R tropici (SLA2.2 and BR322) strains was evaluated before and after exposure to high temperatures (four consecutive thermal shocks at 45ºC). This evaluation involved plant dry matter analysis of inoculated plants and genotypic (plasmid profile and genomic patterns via RAPD) analysis of the Rhizobium strains. The results evidenced that high temperature improve the natural performance variability especially between isolated colonies from R. leguminosarum bv. phaseoli strains. The plasmid profile of isolated colonies from R. tropici strains were identical regardless of temperature treatment whereas isolated colonies from R. leguminosarum bv. phaseoli alterations were detected especially after the thermal treatment. The genomic patterns generated by AP-PCR showed more alterations and genetic variation in isolated colonies from R. leguminosarum bv. phaseoli strains indicating that R. tropici strains are more stable and lower affected by high temperature.

Variability of isolated colonies in bean nodulating Rhizobium strains before and after exposure to high temperature

Irregular response to bean plants to Rhizobium inoculation has been attributed to among other factors, low competitive ability, low N2 fixation efficiency and genetic instability of the symbiont. This genetic instability caused by high rates of genomic rearrangements and/or plasmid deletions can be accentuated by high temperatures. This fact may limit the utilization of these strains as inoculants, especially in tropical soils. In this study, the variability of isolated colonies derived from effective R. leguminosarum bv. phaseoli (SLP1.3 and BR 10.026) and R tropici (SLA2.2 and BR322) strains was evaluated before and after exposure to high temperatures (four consecutive thermal shocks at 45ºC). This evaluation involved plant dry matter analysis of inoculated plants and genotypic (plasmid profile and genomic patterns via RAPD) analysis of the Rhizobium strains. The results evidenced that high temperature improve the natural performance variability especially between isolated colonies from R. leguminosarum bv. phaseoli strains. The plasmid profile of isolated colonies from R. tropici strains were identical regardless of temperature treatment whereas isolated colonies from R. leguminosarum bv. phaseoli alterations were detected especially after the thermal treatment. The genomic patterns generated by AP-PCR showed more alterations and genetic variation in isolated colonies from R. leguminosarum bv. phaseoli strains indicating that R. tropici strains are more stable and lower affected by high temperature.

A irregularidade de resposta do feijoeiro à inoculação com rizóbio tem sido atribuída, entre outros fatores, à baixa capacidade competitiva, baixa eficiência em fixar N2 e instabilidade genética do simbionte. Esta instabilidade genética decorrente de frequências elevadas de rearranjos genômicos e/ou deleções plasmidiais pode ser acentuada após exposição a temperaturas elevadas, comprometendo a utilização dessas estirpes como inoculantes, especialmente nos solos tropicais. Nesse estudo, foi avaliada a variabilidade de colônias isoladas derivadas de estirpes efetivas de R. leguminosarum bv. phaseoli (SLP1.3 e BR10.026) e R. tropici (SLA2.2 e BR322), antes e após exposição à temperatura elevada (4 choques térmicos sucessivos a 45ºC). Essa avaliação foi efetuada através da análise de características fenotípicas (medida pela produção de matéria seca das plantas inoculadas) e genotípicas (perfil plasmidial e padrões genômicos via RAPD). Os resultados obtidos indicaram que à temperatura elevada acentuou a variabilidade na performance natural entre as colônias isoladas das estirpes testadas, especialmente da espécie R. leguminosarum bv. phaseoli. Os perfis plasmidiais das colônias derivadas das estirpes de R. tropici antes e após exposição à temperatura elevada apresentaram-se idênticos entre si e em relação a estirpe original. Os padrões genômicos determinados via RAPD mostraram mais alterações e variação genética nas colônias isoladas das estirpes de R. leguminosarum bv. phaseoli, indicando que as estirpes de R. tropici são mais estáveis e foram menos afetadas pela ação da temperatura.

Effectes of high temperature on survival, symbiotic performance and genomic modifications of bean nodulating Rhizobium strains

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 were analysed. After successive thermal shocks at 45ºC for four hours, survival capacity appeared to be strain-specifc, independent of thermo-tolerance and was more apparent in "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 innoculation in plants.