Genetic Structure and Symbiotic Characteristics of a Bradyrhizobium Population Recovered from a Pasture Soil

Abstract

We examined the genetic structure and symbiotic characteristics of Bradyrhizobium isolates recovered from four legume species (Lupinus albus [white lupine], Lupinus angustifolius [blue lupine], Ornithopus compressus [yellow serradella], and Macroptilium atropurpureum [siratro]) grown in an Oregon soil. We established that multilocus enzyme electrophoresis (MLEE) can provide insights into the genetic relatedness among Bradyrhizobium strains by showing a positive correlation (r2 = .0.90) between the relatedness of Bradyrhizobium japonicum strains determined by MLEE at 13 enzyme loci and that determined by other workers using either DNA-DNA hybridization or DNA sequence divergence estimates. MLEE identified 17 electrophoretic types (ETs) among 95 Bradyrhizobium isolates recovered from the four hosts. Although the overall genetic diversity among the ETs (H = 0.69) is one of the largest measured to date in a local population of any soilborne bacterial species, there was no evidence of multilocus structure (linkage disequilibrium) within the population. The majority of the isolates (73%) were represented by two closely related ETs (2 and 3) which dominated the root nodules of white lupine, serradella, and siratro. In contrast, ETi dominated nodules of blue lupine. Although representative isolates from all of the 17 ETs nodulated siratro, white lupine, blue lupine, and big trefoil (Lotus pedunculatus), they were either completely ineffective or poorly effective at fixing nitrogen on these hosts. Despite the widespread use of serradella as a surrogate host for lupine-nodulating bradyrhizobia, 7 of the 17 ETs did not nodulate this host, and the remaining 10 ETs were ineffective at fixing nitrogen.

Genetic Structure of Acetobacter diazotrophicus Populations and Identification of a New Genetically Distant Group

“…In this study, we report that there is a lower genetic diversity in A. diazotrophicus recovered from different host species collected in widely separated regions of the world in comparison to the majority of other bacterial species studied (6,9,10,13,14,19,29,35,44). The results confirm previous data (8) on genetic diversity among 21 Mexican and 3 Brazilian isolates exclusively recovered from sugarcane plants.…”

“…Since a low number of isolates recovered from Cameroon grass and sweet potato were analyzed, it was not possible to determine if certain ETs of A. diazotrophicus are predominantly associated with a particular host species, as observed, for instance, with the pathogen of mammals Bordetella bronchiseptica (28) or with the legume-nodulating Bradyrhizobium sp. (6). However, the results clearly demonstrated that ET 1 was extensively distributed among all host species analyzed.…”

Divergence in Symbiotic Compatibility in a Legume-Bradyrhizobium Mutualism

“…5) was very closely related to an isolate from the IL-2 population that conferred exceptionally high growth on its native hosts (see experiment 2 of Table 3). Surveys of bacteria associated with domesticated legumes typically find very wide heterogeneity in plant growth with different isolates, including some isolates that nodulate plants without providing any benefit at all to their hosts (e.g., Jordan 1974;Fuhrmann 1990;Bottomley et al 1994). The few studies on undomesticated legumes have reported similar patterns (e.g., Batzli et al 1992).…”

Presence of a novel 16S–23S rRNA gene intergenic spacer insert in Bradyrhizobium canariense strains

“…It was shown that Bradyrhizobium strains isolated from Lupinus spp. can form an effective (nitrogen fixing) symbiosis with Ornithopus plants, but the latter plant genus has more stringent nodulation requirements (Bottomley et al, 1994;Ballard, 1996;Stepkowski et al, 2005). At present, strains infecting Ornithopus species have been poorly classified taxonomically.…”