4 UNNAMED SPECIES OF NONSYMBIOTIC RHIZOBIA ISOLATED FROM THE RHIZOSPHERE OF LOTUS-CORNICULATUS

Previously, me found that genetically diverse rhizobia nodulating Lotu s corniculatus at a held site devoid of naturalized rhizobia had symbi otic DNA regions identical to those of ICMP3153, the inoculant strain used at the site (J. T. Sullivan, H. N. Patrick, W. L. Lowther, D. B. Scott, and C. W. Ronson, Proc. Natl. Acad. Sci. USA 92:8985-8989, 1995 ), In this study, we characterized seven nonsymbiotic rhizobial isolat es from the rhizosphere oft. corniculatus. These included two from pla nts at the field site sampled by Sullivan et al, and five from plants at a new field plot adjacent to that site, The isolates did not nodula te Lotus species or hybridize to symbiotic gene probes but did hybridi ze to genomic DNA probes from Rhizobium loti. Their genetic relationsh ips with symbiotic isolates obtained from the same sites, with inocula nt strain ICMP3153, and with R. loti NZP2213(T) were determined by thr ee methods. Genetic distance estimates based on genomic DNA-DNA hybrid ization and moltilocus enzyme electrophoresis were correlated but were not consistently reflected by 16S rRNA nucleotide sequence divergence . The nonsymbiotic isolates represented four genomic species that were related to X. loti; the diverse symbiotic isolates from tile site bel onged to one of these species, The inoculant strain ICMP3153 belonged to a fish genomic species that was more closely related to Rhizobium h uakuii. These results support the proposal that nonsymbiotic rhizobia persist in soils in the absence of legumes and acquire symbiotic genes from inoculant strains upon introduction of host legumes.