Four unnamed species of nonsymbiotic rhizobia isolated from the rhizosphere of Lotus corniculatus

Previously, me found that genetically diverse rhizobia nodulating Lotus corniculatus at a held site devoid of naturalized rhizobia had symbiotic 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 isolates from the rhizosphere oft. corniculatus. These included two from plants 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 nodulate Lotus species or hybridize to symbiotic gene probes but did hybridize to genomic DNA probes from Rhizobium loti. Their genetic relationships with symbiotic isolates obtained from the same sites, with inoculant strain ICMP3153, and with R. loti NZP2213(T) were determined by three methods. Genetic distance estimates based on genomic DNA-DNA hybridization 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 the site belonged to one of these species, The inoculant strain ICMP3153 belonged to a fish genomic species that was more closely related to Rhizobium huakuii. 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.