Biological nitrogen fixation in poplar trees
S. DOTY (1), A. Sher (2), Z. Khan (1), M. Khorasani (1), N. Fleck (1), R. Bumgarner (1), S. Kim (1), T. DeLuca (1) (1) University of Washington, U.S.A.; (2) University of California, San Diego, U.S.A.

Recent evidence points to symbiosis with internal microorganisms, termed endophytes, as a mechanism for non-nodulating plants to survive in N-limited environments. Poplar is an early successional tree species able to colonize such areas. N-fixing (diazotrophic) endophytes isolated from these wild plants were added to cultivated hybrid poplar, resulting in increased biomass, N-fixation, and drought tolerance. N-fixation within wild poplar was directly assessed using the 15N incorporation assay and indirectly using the acetylene reduction assay. Both methods indicated that N-fixation in poplar occurs in an unevenly distributed fashion. Sequencing of the nifH genes revealed a diversity of diazotrophic bacterial species within poplar. Assessment of culturable endophytes from adjacent tissue sections demonstrated significant differences in microbial density and composition. Using fluorescent microscopy, differential colonization patterns of the endophyte strains were visualized. These data support the hypothesis that N-fixation is variable in this non-legume due to a requirement for specific communities and/or densities. Experiments involving FISH to measure nifH expression and NanoSIMS to directly visualize N transfer are underway. Since poplar was the first tree to have its genome sequenced and members of the poplar microbiome are undergoing full genomic sequencing, it can serve as a model system for studying N-fixation in non-nodulating trees at the molecular level.

Abstract Number: P2-21
Session Type: Poster