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

A procedure based upon DNA hybridization was developed for the specifi c 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 chromos omal probe were found to be species specific for R. leguminosarum and Rhizobium etli. By using R. leguminosarum nod gene probes, biovar spec ificity was obtained. Of 302 soil isolates screened for their inabilit y to grow on Luria-Bertani agar medium, 13 strains could be assigned t o the R. leguminosarum species on the basis of DNA homology to the chr omosomal probe and antibiotic resistance tests. Of these strains, thre e and two were assigned by colony hybridization and subsequent plant h ost specificity tests, respectively, to R. leguminosarum biovars vicia e and trifolii. The eight other R. leguminosarum soil isolates lacked symbiotic information but were able to gain nodulation capacity with t he acquisition of a conjugative symbiotic plasmid. They were thus cons idered as nonsymbiotic R. leguminosarum.