Genetic Diversity of Rhizobium Leguminosarum From Pea Fields In Washington State.

Rhizobial-mediated biological nitrogen (N) fixation in legumes contributes to yield potential in these crops and also provides residual fertilizer to subsequent small grain crops. Diverse areas of study including plant-microbe interactions and soil microbiology would benefit from an improved understanding of the genetic structure of rhizobia populations. Our objectives were to collect isolates of Rhizobium leguminsarum from several pea fields in Washington, examine genetic diversity among these isolates and several commercial isolates of R. leguminosarum, and compare genetically distinct isolates for their ability to fix N in a range of pea hosts. 79 isolates were collected from pea root nodules (cv. Banner) collected from four non-inoculated pea fields in the state of Washington. Sequence-Related Amplified Polymorphisms (SRAP) markers generated by PCR were used to discriminate among isolates. Isolates collected from a field in Colfax that had not been previously cultivated in over a decade tended to be the most diverse field isolates and fell into distinct subgroups. The majority of commercial isolates examined were quite distinct from field isolates, although two commercial isolates did group in a cluster containing the majority of field isolates.  Four genetically distinct isolates were compared for their efficiency in fixing nitrogen in greenhouse experiments that included two yellow and three green pea cultivars. Host plant variety effects were significant for biomass due to N fixation and also for the quantity of N fixed per variety. Significant effects of R. leguminosarum isolates were observed for the quantity of N fixed per isolate, biomass due to N fixation, and quantity of N per plant. Patterns of genetic diversity between field isolates and commercial isolates suggest that selection may be operating on field populations over several years in the absence of pea hosts.