A proteomic network for symbiotic nitrogen fixation efficiency in Bradyrhizobium elkanii

Authors: Cooper, Bret; Campbell, Kimberly; Beard, Hunter; Garrett, Wesley; Mowery, Joseph; Bauchan, Gary; Elia, Patrick

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2017
Publication Date: 5/7/2018
Citation: Cooper, B., Campbell, K., Beard, H.S., Garrett, W.M., Mowery, J.D., Bauchan, G.R., Elia, P.E. 2018. A proteomic network for symbiotic nitrogen fixation efficiency in Bradyrhizobium elkanii . Molecular Plant-Microbe Interactions. 31(3):334-343. https://doi.org/10.1094/MPMI-10-17-0243-R.
DOI: https://doi.org/10.1094/MPMI-10-17-0243-R

Interpretive Summary: Inefficient symbiotic nitrogen fixation is prevalent in agriculture and is a waste of natural resources. This study describes the differentially regulated proteins from a nitrogen-fixing rhizobium bacterium isolated from soybean root nodules where symbiotic nitrogen fixation is efficient compared to root nodules where nitrogen fixation is inefficient. 3,900 proteins, almost half of all proteins encoded in the rhizobium genome, were identified by mass spectrometry, an analytical technique. The proteins include enzymes that produce signaling molecules that establish and maintain nodulation, that generate energy needed to fix nitrogen, and that assimilate fixed nitrogen into chemical forms that can be utilized by both the bacterium and the soybean. The results suggest a model for the metabolism of symbiotic nitrogen fixation in soybean that is different than resolved in other plants. It is expected that a better understanding of symbiotic nitrogen fixation efficiency will lead to sustainable soybean production. These results will be useful to scientists at private companies, universities, and government laboratories who research biological nitrogen fixation with the goal to improve crop productivity and reduce energy costs.

Technical Abstract: Rhizobia bacteroids colonize legumes and reduce N2 to NH3 in root nodules. The current model is that bacteroids avoid assimilating this NH3. Instead, the legume forms glutamine from it, the nitrogen of which is returned to the bacteroid as leucine, isoleucine, valine, dicarboxylates, and peptides. In soybean cells surrounding bacteroids, it is thought that the glutamine also is converted to ureides for systemic transport. One problem for soybean cultivation is N2 fixation inefficiency, the biochemical basis of which is unknown. Here, the proteomes of Bradyrhizobium elkanii bacteroids isolated from N2 fixation-efficient Peking and inefficient Williams 82 soybean nodules were analyzed by quantitative mass spectrometry. Nearly half of the encoded proteins were interrogated. The results reveal that efficient bacteroids from Peking produced greater amounts of enzymes to form Nod factors to maintain symbiosis. Bacteroids from Peking and Williams 82 had no significantly altered accumulations of nitrogenase, but efficient bacteroids had increased signaling proteins, transporters, and enzymes needed to generate ATP to power nitrogenase, to acquire resources, and to sustain their metabolisms. Parallel investigation of nodules revealed that Peking had no greater accumulation of enzymes needed to assimilate nitrogen. Instead, efficient bacteroids had increased amounts of enzymes to produce all amino acids, including glutamine, and to form ureide precursors. These results support a model for efficient symbiotic N2 fixation in soybean where the bacteroid assimilates NH3 for itself and its partner