GENOTYPIC DIVERSITY OF SINORHIZOBIUM (FORMERLY RHIZOBIUM) MELILOTI STRAINS ISOLATED DIRECTLY FROM A SOIL AND FROM NODULES OF ALFALFA (MEDICAGO-SATIVA) GROWN IN THE SAME SOIL

The genetic structure of a population of Sinorhizobium meliloti (forme rly Rhizobium meliloti) isolated from nodules of alfalfa (Medicago sat iva cv. Europe) or isolated directly from the soil using an elective m edium and colony hybridization was assessed by three molecular methods . Sixty-seven S. meliloti isolates were obtained from alfalfa nodules and 61 directly from soil. Plasmid profiles of S. meliloti isolates we re analyzed as well as insertion sequence (IS) fingerprints using prob es corresponding to the two indigenous IS elements: ISRm1 and ISRm5. T he presence of a gene involved in nodule forming efficiency (nfeA gene ) was also determined by PCR using specific primers, among S. meliloti isolates. Plasmid profile analysis revealed the presence of 23 distin ct plasmid types among the 128 isolates tested and the distribution of plasmid types was significantly different between isolates obtained f rom nodules and isolates obtained from soil. The distributions of plas mid types among S. meliloti nodule isolates were similar to those in a collection of nodule isolates obtained from the same field 8 years pr eviously. We have thus demonstrated that the distribution of plasmid t ypes in alfalfa nodules has not varied over long periods in the absenc e of the host plant. IS fingerprinting was more discriminative than pl asmid profiling as a total of 65 distinct IS genotypes were found amon g the same isolates. The distribution of IS genotypes was different be tween isolates obtained from nodules and those isolated from soil. The se results suggest that sampling S. meliloti populations using the hos t plant may not be representative of existing soil populations. Genes like nfeA were detected in neither the nodule nor soil isolates. The d ominant plasmid type in nodule isolates (30% of the population), carry ing a large plasmid of 230 kb, is poorly represented among soil isolat es (8%). This result suggests that this plasmid might be involved in t he competitiveness of S. meliloti strains. (C) 1998 Federation of Euro pean Microbiological Societies. Published by Elsevier Science B.V.