Polyphasic characterization of phosphate-solubilizing bacteria isolated from rhizospheric soils of the north-eastern region of Portugal

Angel Valverde, José M. Igual and Emilio Cervantes.

Instituto de Recursos Naturales y Agrobiología de Salamanca. IRNA-CSIC. Salamanca. SPAIN.

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Phosphate-solubilizing microorganisms are often used as plant growth promoters. In the frame of a project focused on the isolation of endosymbionts and ectorhizospheric bacteria associated with Phaseolus vulgaris plants growing in a mountain soil at the north-east of Portugal, we obtained six bacterial strains with high phosphate-solubilizing capacity, as demonstrated by the formation of “halos” when plated in YED medium supplemented with tri-calcium phosphate.

These bacterial strains were characterized by a polyphasic approach using both phenotypic (API 20 NE, API ZYM) and molecular assays (RAPD, with M 13 primer, TP-RAPD, 16S rDNA sequencing).

TP-RAPD yielded an identical band patterns in the six strains indicating that they belong to the same bacterial species. The 16S rRNA sequence analysis of a group-representative strain revealed a sequence similarity value of 99.48% with Pseudomonas pavonaceae. Moreover, the RAPD fingerprints of these six strains did not show differences, indicating that they form a highly homogenous group. This high homogeneity could be a consequence of the recurring agronomical practices used in this region from the antiquity that include organic fertilization and monoculture of Phaseolus vulgaris.