Isolation of phosphate solubilizing rhizobia nodulating Medicago, Melilotus and Trigonella from a Spanish soil

María del Villar Igea¹, Encarna Velázquez¹, Raúl Rivas-González¹, Anne Willems², Peter van Berkum³, Martha E. Trujillo¹, Pedro F. Mateos¹, Monique Gillis², and Eustoquio Martínez-Molina¹.

¹Departamento de Microbiología y Genética. Edificio Departamental. Campus Miguel de Unamuno. Universidad de Salamanca. SPAIN.

²Laboratorium voor Microbiologie. Vakgroep Biochemie, Fysiologie en Microbiologie. K.L. Ledeganckstraat 35. B-9000 Gent. BELGIUM

³U.S. Department of Agriculture, ARS, Soybean and Alfalfa Research Laboratory, Beltsville, MD 20705, USA.

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The type strains of Sinorhizobium meliloti and S. medicae do not to solubilize phosphate from bicalcium phosphate in plate. However, some strains nodulating alfalfa have been described as phosphate solubilizers. Therefore, we tested this ability in several strains isolated from several plants of the cross-inoculation group of Medicago sativa. LMW RNA analysis was used for the identification of strains isolated from the root nodules of diverse species from Medicago, Melilotus and Trigonella. These strains were also studied by TP-RAPD, plasmid profiles, SDS-PAGE of total cellular proteins, determination of the DNA base composition, DNA-DNA reassociation experiments and physiological and biochemical tests. The whole 16S rRNA gene of isolate RTM17 was sequenced and showed more than 99% similarity with S. meliloti. The results of the genotypic and phenotypic characterization showed that the strains isolated suggest that they belong to the species Sinorhizobium meliloti. Nevertheless, the results obtained by the different techniques used showed showed differences with respect to the type strain of S. meliloti and suggest the existence of several genomic groups within this species that can be considered as a subspecies of S. meliloti. Although only in one species of rhizobia, Mesorhizobium huakuii, a subspecies has been recently described, the possibility of the existence of subspecies within more rhizobial species should be considered in the future.