ISOLATION FROM SOIL OF SYMBIOTIC AND NONSYMBIOTIC RHIZOBIUM-LEGUMINOSARUM BY DNA HYBRIDIZATION

A procedure based upon DNA hybridization was developed for the specific detection of Rhizobium leguminosarum and its different biovars among bacteria isolated from soil. DNA colony hybridization and restriction fragment length polymorphism analysis with a R. leguminosarum chromosomal probe were found to be species specific for R. leguminosarum and Rhizobium etli. By using R. leguminosarum nod gene probes, biovar specificity was obtained. Of 302 soil isolates screened for their inability to grow on Luria-Bertani agar medium, 13 strains could be assigned to the R. leguminosarum species on the basis of DNA homology to the chromosomal probe and antibiotic resistance tests. Of these strains, three and two were assigned by colony hybridization and subsequent plant host specificity tests, respectively, to R. leguminosarum biovars viciae and trifolii. The eight other R. leguminosarum soil isolates lacked symbiotic information but were able to gain nodulation capacity with the acquisition of a conjugative symbiotic plasmid. They were thus considered as nonsymbiotic R. leguminosarum.