Abstract of Dissertation by Jose Rodolfo Valdez Barillas.  Completed Summer, 2008.

Swainsonine producing Oxytropis can establish mutualisms with dinitrogen fixing bacteria and endophytic fungi. Dinitrogen fixation facilitates the growth of Oxytropis species in low nitrogen soil, while sustaining the fungal-plant symbiosis. Contributions from dinitrogen fixation in Oxytropis sericea development and swainsonine synthesis were studied in a greenhouse experiment. The role of Oxytropis mutualisms in swainsonine production was tested beyond O. sericea by studying swainsonine producing Oxytropis from Asia. For this study it was hypothesized that fixed dinitrogen is used by the fungal endophyte during the synthesis of swainsonine when soil nitrogen is low. It was also hypothesized that dinitrogen fixing Oxytropis can allocate fixed nitrogen toward plant biomass and still sustain swainsonine synthesis by the fungal endophyte in low nitrogen soil. In a second study it was hypothesized that Oxytropis from Asia produce dinitrogen fixing nodules and host swainsonine producing fungal endophytes. It was also hypothesized that alkaloid similarities in Oxytropis from both continents could be explained by similar fungal endophyte hosted by North American and Asian Oxytropis.  15N-enrichment on dinitrogen fixing and non dinitrogen fixing O. sericea was detected in 15N-swainsonine produced by non-dinitrogen fixers. Low 15N-swainsonine was detected in dinitrogen fixers as a result of 14N incorporation. These results suggest the fungal endophyte is a nitrogen sink. Non-dinitrogen fixing O. sericea with no fungal endophytes had greater biomass than non-dinitrogen fixers with fungal endophytes. Non-dinitrogen fixers with fungal endophytes produced similar levels of swainsonine, but no increase in biomass. Dinitrogen fixers with fungal endophytes had greater biomass than non-dinitrogen fixers with fungal endophyte. Older dinitrogen fixers produced more swainsonine than non dinitrogen fixers, increased plant growth and fungal biomass. Results from the second study suggest that dinitrogen fixing Oxytropis from Asia produced swainsonine in association with a fungal endophyte that is 99% similar to the fungal endophyte in Oxytropis sericea from North America. These studies suggest that dinitrogen fixation increases the ecological niche of Oxytropis in both continents and sustains the plant-fungal symbiosis, as well as swainsonine production. Fungal symbiosis and dinitrogen fixation are old and conserved mutualisms that have benefited the fitness of Oxytropis populations.