|Van Berkum, Peter|
Submitted to: Journal of Applied & Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/1996
Publication Date: N/A
Interpretive Summary: Rhizobia are important soil bacteria which establish symbioses with legume crop plants. The symbiosis benefits the plant because the bacteria fix inert gaseous nitrogen into a form the plant uses for growth and production. Crop plants are inoculated with rhizobia to enhance production while minimizing the use of nitrogen fertilizers. A problem in the management of rhizobia is that there is considerable genetic variation in the capacity for nitrogen fixation. Our work has show that soils devoid of Rhizobium which nodulate Lotus corniculatus, may harbor non-symbiotic bacteria with the capacity to obtain the symbiotic genes by genetic transformation from an inoculant strain. This lateral transfer of symbiotic genes from an inoculant to other soil bacteria could be an important mechanism contributing to rhizobia diversity in soils when legumes are cultivated. It is hypothesized that the recipients could become competitors with the inoculants for nodulation of the crop and for niches important for bacterial saprophytic proliferation. This information is useful to scientists with interest in rhizobial ecology and symbiotic nitrogen fixation.
Technical Abstract: Previously, we found that genetically diverse rhizobia nodulating Lotus corniculatus at a field site devoid of naturalized rhizobia had identical symbiotic DNA regions to ICMP3153, the inoculant strain used at the site (Sullivan, J.T., H.N. Patrick, W.L. LLowther, D.B. Scott and C.W. Ronson. 1995. Proc. Natl. Acad. Sci. USDA 92:8985-8989). In this study, we characterized seven nonsymbiotic rhizobial isolates from the rhizosphere o L. corniculatus. These included two from plants from the field site sampled by Sullivan et. al. and five from plants from a new field plot adjacent to that site. The isolates neither nodulated Lotus species nor hybridized to symbiotic gene probes, but hybridized to genomic DNA probes from Rhizobium lot. Their genetic relationships with symbiotic isolates obtained from the same sites, with inoculant strain ICM3153, and with R. loti strain NZP2213T were determined by three methods. Genetic distance estimates based on DNA-DNA hybridization and mutilocus 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 R. loti; the diverse symbiotic isolates from the site belonged to one of these species. The inoculant strain ICMP3153 belonged to a fifth genomic species that was more closely related to R. 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.