Enriquecimiento diferencial de Pseudomonas spp. en el rizoplano de distintas especies cultivadas / Selective enrichment of Pseudomonas spp. in the rhizoplane of different plant species

En contraste con la simbiosis entre rizobios y leguminosas, la especificidad de las Pseudomonas en la colonización radicular parece menos estricta. Sin embargo, estudios sobre la diversidad bacteriana del nicho rizosférico resaltan la influencia de la especie vegetal en la selección específica de ciertos microorganismos a partir de la flora residente del suelo. Para evaluar el efecto que los cultivos extensivos de nuestro país tienen sobre la estructura de las comunidades de Pseudomonas, se realizaron experimentos con plantas trampa, partiendo de semillas de trigo, maíz y soja desinfectadas superficialmente y sembradas en un mismo suelo prístino. A partir de las suspensiones representativas de la microflora del rizoplano, se realizaron recuentos en placa en medio selectivo para Pseudomonas. El conjunto de colonias originado a partir de los distintos rizoplanos se utilizó como fuente de ADN para analizar la estructura de comunidad a través del perfil de restricción de amplicones de los genes oprF y gacA. El análisis comparativo de estos perfiles agrupó a las muestras por especie de planta y las distinguió del patrón obtenido a partir del suelo prístino. La secuenciación parcial del gen 16S ADNr de aislamientos bacterianos representativos confirmó la existencia de genotipos enriquecidos diferencialmente en el rizoplano de cada especie vegetal. Estos resultados apoyan la hipótesis de la existencia de mecanismos de selección específica de estirpes de Pseudomonas a partir de la flora nativa del suelo en la interacción cooperativa entre estas PGPR y las raíces de diferentes cultivos como trigo, soja y maíz

In contrast to rhizobia-legume symbiosis, the specificity for root colonization by pseudomonads seems to be less strict. However, several studies about bacterial diversity in the rhizosphere highlight the influence of plant species on the selective enrichment of certain microorganisms from the bulk soil community. In order to evaluate the effect that different crops have on the structure of pseudomonad community on the root surface, we performed plant trap experiments, using surface-disinfected maize, wheat or soybean seeds that were sown in pots containing the same pristine soil as substrate. Rhizoplane suspensions were plated on a selective medium for Pseudomonas, and pooled colonies served as DNA source to carry out PCR-RFLP community structure analysis of the pseudomonads-specific marker genes oprF and gacA. PCR-RFLP profiles were grouped by plant species, and were distinguished from those of bulk soil samples. Partial sequencing of 16S rDNA genes of some representative colonies of Pseudomonas confirmed the selective enrichment of distinctive genotypes in the rhizoplane of each plant species. These results support the idea that the root systems of agricultural crops such as soybean, maize and wheat, select differential sets of pseudomonads from the native microbial repertoire inhabiting the bulk soil

What stories can the Frankia genomes start to tell us.

Among the Actinobacteria, the genus Frankia is well known for its facultative lifestyle as a plant symbiont of dicotyledonous plants and as a free-living soil dweller. Frankia sp. strains are generally classified into one of four major phylogenetic groups that have distinctive plant host ranges. Our understanding of these bacteria has been greatly facilitated by the availability of the first three complete genome sequences, which suggested a correlation between genome size and plant host range. Since that first report, eight more Frankia genomes have been sequenced. Representatives from all four lineages have been sequenced to provide vital baseline information for genomic approaches toward understanding these novel bacteria. An overview of the Frankia genomes will be presented to stimulate discussion on the potential of these organisms and a greater understanding of their physiology and evolution.