Symbiotic Nitrogen Fixation and Challenges to Extending it to Non-Legumes 1 2

2 Florence Mus, Matthew B. Crook, Kevin Garcia, Amaya Garcia Costas, Barney A. 3 Geddesc, Evangelia-Diamanto Kouri, Ponraj Paramasivan, Min-Hyung Ryu, Giles E. D. 4 Oldroyd, Philip S. Poole, Michael K. Udvardi, Christopher A. Voigt, Jean-Michel Ané*, 5 and John W. Peters* 6 7 Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, USA 8 Departments of Bacteriology and Agronomy, University of Wisconsin–Madison, Madison, WI 9 53706, USA 10 Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom 11 d Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401, USA 12 John Innes Centre, Norwich Research Park, Norwich NR4 7UH, UK 13 Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA 14 02139, USA 15 16