The genetic diversity of rhizobia associated with Dalea purpurea Vent. in fragmented grasslands of West-Central Minnesota.

Authors: MARTIR, MAINA - UNIVERSITY OF MINNESOTA; TLUSTY, BECKI - UNIVERSITY OF MINNESOTA; Van Berkum, Peter; GRAHAM, PETER - UNIVERSITY OF MINNESOTA

Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2007
Publication Date: 7/15/2007
Citation: Martir, M., Tlusty, B., Van Berkum, P.B., Graham, P.H. 2007. The genetic diversity of rhizobia associated with Dalea purpurea Vent. in fragmented grasslands of West-Central Minnesota. Canadian Journal of Microbiology. 53:351-363.

Interpretive Summary: Soil bacteria known as rhizobia form a symbiosis with legume crops such as soybean and alfalfa. In symbiosis, these rhizobia extract nitrogen gas from the atmosphere and convert it into a form that the plants use for growth. This results in efficient crop production since growers need not apply fertilizer, which saves money and prevents pollution of the environment. Natural ecosystems, such as Mid-Western prairies, also depend upon wild legume species that form symbioses with highly diverse rhizobia to ensure nitrogen accretion in the soil. However, loss of the prairies through urban development and increased agriculture may threaten to reduce the abundance of native legume species and the diversity of their associated rhizobia. To investigate this, rhizobia were isolated from the legume purple prairie clover growing on 9 prairie remnants that were of a different size. The rhizobia were then tested for their genetic diversity to determine the effect of the growing area of the legume on the variability of the rhizobia. Data were obtained revealing that there was more diversity on the larger areas. The cause may have been the effect of area size on the number of plants and their variability. Another contributing factor may have been that there was more variation across each area within the larger areas. These results are important to ecologists. Also, these result are of interest to the Minnesota Department of Transportation with their goal to preserve the Midwestern Prairies.

Technical Abstract: Increase in human population and the spread of agriculture over the past 150 years has transformed the landscape in West-Central Minnesota into a mosaic of agricultural fields and urban land, and have left only remnants of the once dominant prairie ecosystem. Limited natural habitats in this fragmented landscape threatens the diversity and abundance of native legumes, and could impact the size and function of associated below-ground microbial populations. In this study BOXA1R PCR and 16S rRNA gene-sequence analysis 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. Variation in soil properties and in the %N derived from fixation in D. purpurea were 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 R. gallicum, with two of these recovered at all sites. The fourth genotype was similar to R. etli, and was common at only two sites. Rhizobium genotype richness and site area were positively correlated. Variability in conditions at the larger sites, coupled with events that allowed greater legume abundance affected rhizobial genotype richness and diversity.