Phosphate solubilizing rhizobia originating from Medicago, Melilotus and Trigonella grown in a Spanish soil

Although phosphate solubilization is a character known to be present in species of Mesorhizobium, this property has not been described before in species of Sinorhizobium. The type strains of the three species that nodulate Medicago species, Sinorhizobium meliloti, S. medicae and Rhizobium mongolense, do not solubilize phosphate from bicalcium phosphate in plate culture. We observed phosphate solubilization among isolates we obtained from nodules of Medicago sativa, Melilotus and Trigonella growing in a Spanish soil. Phenotypic and genetic analyses of these isolates led to the conclusion that they were placed within the genus Sinorhizobium with characteristics in common with S. meliloti and S. medicae. The group of strains solubilizing phosphate is distinguishable to strains from S. meliloti and S. medicae basing on LMW RNA profiles, TP-RAPD patterns and SDS-PAGE profiles.