Graduate Thesis Or Dissertation
 

Characteristics of bradyrhizobium sp. recovered from Oregon soils

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  • Although the biology of Bradyrhizobium has been studied for almost one hundred years, virtually nothing is known about the ecology and genetic variation in natural soil populations of this genus. The majority of information has been obtained from studies of one species, Bradyrhizobium japonicum, and the diversity within soil populations of the latter has been elucidated primarily by serotyping. With this background in mind, a project was initiated that has the long term goal of gaining a better understanding of soil populations of Bradyrhizobium in Oregon soils. Three specific objectives were formulated: (1) To evaluate if multi-locus allozyme electrophoresis (MLAE) has the potential for elucidating overall genetic relatedness among isolates of Bradyrhizobium. (2) To examine the diversity and genetic relatedness of bradyrhizobial isolates recovered from a western Oregon soil of the Abiqua series. (3) To determine if different bradyrhizobial plant hosts nodulate with different members of the soil population. Significant positive correlations (r²=0.59 to 0.95) were found between the relatedness among USDA strains of B. japonicum established by MLAE (based upon 10 enzyme loci examined), and the relatedness among the same strains determined by other workers using either DNA:DNA hybridization or determined from hybridization patterns with chromosomal gene probes. MLAE of the B. japonicum strains confirmed that the species can be divided into three genetically distinct lineages, and showed that USDA 3051, the 'type' strain of Bradyrhizobium sp. (Lupinus), was not closely related to any of the USDA strains of B. japonicum. A total of 95 isolates of Bradyrhizobium recovered from either Lupinus albus (white lupine), Lupinus angustifolius (blue lupine), Ornithopus compressus (serradella), or Macroptilium atropurpureum (siratro) were analyzed by MLAE at nine enzyme loci. Seventeen unique combinations of alleles were identified and are referred to as electrophoretic types (ETs). Using a cluster analysis, the ETs were grouped into six clusters. Cluster 1 contained the majority of the isolates (73%) which were represented predominantly by two closely related ETs (2 and 3). These ETs dominated the root nodules on white lupine, serradella, and siratro. In contrast, ET2 and ET3 were poorly represented in root nodules of blue lupine where isolates of ET1 were the major nodule occupants. Regardless of the host from whence they were recovered, or their ET, all of the bradyrhizobial isolates nodulated serradella poorly, inconsistently, and ineffectively. In contrast, the same isolates were shown to efficiently nodulate the lupine species. As a consequence, serradella cannot be used to provide an accurate estimate of the presence and size of lupine-nodulating bradyrhizobial populations in Oregon soils. Antisera were raised to three bradyrhizobial isolates taken from the first collection off white lupine. The bacteria were selected because of their different colony morphologies (rough or mucoid), and one isolate was consistently able to nodulate serradella. The three antisera were strainspecific within the group of three isolates. Based upon the results of gelimmune- diffusion analysis, however, many isolates of the same ET as the antisera strains were not antigenically related to the latter since they did not react with the antisera. In the case of antiserum LA(I)2-22, similar antigens were found on isolates of quite different ETs. Although serotyping has been the primary method used for developing concepts about the population ecology of B. japonicum in soil, further studies are required before serological techniques can be used with any confidence for studying the ecology of native Bradyrhizobium populations in Oregon soils.
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