The genetic diversity of rhizobia associated withDalea purpureaVent. in fragmented grasslands of west-central Minnesota

The increase in human population and the spread of agriculture over the past 150 years have transformed the landscape in west-central Minnesota into a mosaic of agricultural fields and urban land, leaving only remnants of the once dominant prairie ecosystem. Limited natural habitat in this fragmented landscape threatens the diversity and abundance of native legumes and could impact the size and function of associated belowground microbial populations. In this study, BOXA1R PCR and 16S rRNA gene sequence analyses were used to assess the genetic diversity of rhizobia associated with Dalea purpurea (Vent.) in nine prairie remnants ranging in size from 0.04 to 3.5 ha. The variation in soil properties was also determined. While 53 different genotypes of rhizobia were identified, four of these accounted for 84% of the 1029 rhizobia characterized using BOXA1R PCR. Representatives from three of the four dominant genotypes had a 16S rRNA gene sequence similar to that of Rhizobium gallicum , with two of these genotypes recovered at all sites. The fourth genotype was similar to that of Rhizobium etli and occurred with frequency at only two sites. Rhizobium genotype richness and site area were positively correlated. The implications of these results are discussed.

Characteristics of the rhizobia associated with Dalea spp. in the Ordway, Kellogg–Weaver Dunes, and Hayden prairies

Habitat fragmentation affects the biodiversity and function of aboveground organisms in natural ecosystems but has not been studied for effects on belowground species. In this paper, we consider the diversity of the rhizobia associated with the indigenous legume Dalea purpurea in 3 residual prairie areas in Minnesota and Iowa. Using Dalea purpurea as a trap host, 218 rhizobia were recovered from these soils then characterized using BOXA1R PCR. Three major and 13 minor groups were distinguished based on a similarity of greater than 75% in fingerprint patterns. Each major group consisted almost exclusively of rhizobia from a single prairie, with the diversity of Dalea rhizobia recovered from the Hayden Prairie less than that obtained with rhizobia from the other prairies. Based on 16S rRNA gene sequence analysis, isolates from the Hayden, Ordway, and Kellogg–Weaver Dunes prairies were most similar to Rhizobium etli and Rhizobium leguminosarum, Rhizobium gallicum, and Mesorhizobium amorphae and Mesorhizobium huakuii, respectively. This variation in the dominant microsymbiont species across the 3 prairies studied was unexpected but could have been influenced by the limited number of samples that we were allowed to take, by unanticipated cross-nodulation between native legumes, and by variation in the range of legume species present in each residual prairie area. While some of the rhizobia from Dalea nodulated Phaseolus vulgaris, Macroptilium atropurpureum, Leucaena leucocephala, and Onobrychis viciifolia in addition to the Dalea species tested, others nodulated Astragalus canadensis or Amorpha canescens.Key words: Rhizobium, Dalea, prairie, diversity, fragmentation, rep PCR.

Characteristics of rhizobia nodulating beans in the central region of Minnesota

Until recently, beans (Phaseolus vulgaris L.) grown in Minnesota were rarely inoculated. Because of this, we hypothesized that bean rhizobia collected in Minnesota would either share characteristics identifiable with Rhizobium etli of Mesoamerican or Andean origin, introduced into the region as seed-borne contaminants, or be indigenous rhizobia from prairie species, such as Dalea spp. The latter organisms have been shown to nodulate and fix N2with Phaseolus vulgaris. Rhizobia recovered from the Staples, Verndale, and Park Rapids areas of Minnesota were grouped according to the results of BOXA1R–PCR fingerprint analysis into 5 groups, with only one of these having banding patterns similar to 2 of 4 R. etli reference strains. When representative isolates were subject to fatty acid - methyl ester analysis and 16S rRNA gene sequence analysis, the results obtained differed. 16S rRNA gene sequences of half the organisms tested were most similar to Rhizobium leguminosarum. Rhizobia from Dalea spp., an important legume in the prairie ecosystem, did not play a significant role as the microsymbiont of beans in this area. This appears to be due to the longer time needed for them to initiate infection in Phaseolus vulgaris. Strains of Rhizobium tropici IIB, including UMR1899, proved tolerant to streptomycin and captan, which are commonly applied as seed treatments for beans. Local rhizobia appeared to have very limited tolerance to these compounds.Key words: Rhizobium diversity, Phaseolus vulgaris, seed treatment, taxonomy.