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

Abstract

Previously, we found that genetically diverse rhizobia nodulating Lotus corniculatus at a field 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 of L. 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 multilocus 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 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.

Slow-growing and oligotrophic soil bacteria phylogenetically close to Bradyrhizobium japonicum

“…The 11 DNB bacteria exhibited total DNA similarities of 26^40% with B. japonicum USDA110 (Table 3), the levels of which corresponded to the similarities among diverse symbiotic and non-symbiotic R. loti isolates [31,32], and also ranged within the natural diversity of the genus Bradyrhizobium [33]. When 16S rRNA phylogenetic trees were constructed from all published data of Bradyrhizobium and the four DNB bacteria (G14003, G14109, G14127, and G14130), the Bradyrhizobium cluster included the four DNB bacteria (data not shown).…”

Characterization of soybean Bradyrhizobium strains adapted to the Brazilian savannas

“…The second hypothesis is that the stressful conditions of the Cerrados, characterized by environmentally inhospitable conditions, especially subjection to long periods of water stress and high temperatures ( s 40³C), as well as soil fertility problems, such as low pH ( 6 5.0) and aluminum toxicity, caused considerable genomic rearrangements in the parental genotypes, resulting in an enormous genetic variation in the variant strains. The third hypothesis would be raised in light of the results obtained in New Zealand soils [25,26] with Lotus corniculatus and Mesorhizobium loti, showing that non-symbiotic rhizobia persist in soils in the absence of host legume, acquiring the symbiotic genes from an inoculant strain upon the introduction of the host legume. Therefore, diverse strains arose by transfer of chromosomal symbiotic genes from the inoculant M. loti strain ICMP3153 [25,26].…”

“…The third hypothesis would be raised in light of the results obtained in New Zealand soils [25,26] with Lotus corniculatus and Mesorhizobium loti, showing that non-symbiotic rhizobia persist in soils in the absence of host legume, acquiring the symbiotic genes from an inoculant strain upon the introduction of the host legume. Therefore, diverse strains arose by transfer of chromosomal symbiotic genes from the inoculant M. loti strain ICMP3153 [25,26]. The transfer of the chromosomal symbiotic genes, later termed symbiosis island, to non-symbiotic mesorhizobia was con¢rmed under laboratory conditions [24].…”

Effects of soil biota from different ranges onRobiniainvasion: acquiring mutualists and escaping pathogens

“…88B, a strain isolated from Lotus corniculatus. Based on the 16S rRNA sequences, strain R88 had five nucleotide mismatches with that of M. loti type strain NZP2213 (Sullivan et al 1996). The closest relative species to strains RP1 (together with isolate RP52 of the same REP-PCR group), RP8 (together with isolate RP3 of the same REP-PCR group), RP18 (together with isolates RP55 and RP59 of the same REP-PCR group), and RP26 (together with isolates RP11, RP57, and RP65 of the same REP-PCR group) was M. amorphae, with 99.6%, 98.8%, 98.4%, and 99.7% similarity, respectively.…”