Towards synthetic nitrogen-fixing symbioses in grasses

Michael Udvardi1, Evangelia Kouri1, John Peters2, Amaya Garcia Costas2, Florence Mus2, Jean-Michel Ane3, Kevin Garcia3, Chris Voigt4, Min-Hyung Ryu4, Giles Oldroyd5, Ponraj Paramasivian5, Ramakrishnan Karunakaran5, Barney Geddes6, and Philip Poole6.

1The Samuel Roberts Noble Foundation, USA

2Montana State University, USA

3University of Wisconsin-Madison, USA;

4Massachusetts Institute of Technology, USA;

5John Innes Center, UK;

6University of Oxford, UK. 



Too much nitrogen (N)-fertilizer is used in many agricultural systems, at great environmental cost, while too little is used in the poorest systems, jeopardising food security.  As a step towards solving these contrasting N-related problems, we aim to build synthetic nitrogen-fixing symbioses between bacteria and grasses, based on knowledge gained from decades of research on natural nitrogen-fixing symbioses in legumes.  Key steps in this synthetic biology project include engineering of: signal compound production in bacteria and signal recognition in plants; concomitant biosynthesis of a specialized C-source by the plant for use by the bacteria; catabolism of this specialized C-source for energy production, as well as nitrogen fixation, respiratory protection of nitrogenase, and conditional suppression of ammonia assimilation in bacteria; and, finally, ammonium uptake by plant cells.  Chassis’ for the bacterial synthetic biology are natural endophytes or epiphytes of grasses, while the target model and crop species are barley and maize.  Significant progress has been made in each of these areas. Ultimately, substantial synthetic associative nitrogen-fixation in staple food crops could increase yields of resource-poor farmers and decrease the need for industrial N-fertilizers in resource-rich agricultural systems.