Towards Synthetic Nitrogen-Fixing Symbioses in Grasses

Nitrogen fertilizers fuelled the Green Revolution and today inject over 100 million tons of reactive-N per year into agricultural systems.  Without N-fertilizers there would be 2 billion fewer people alive today, yet massive use of fertilizers over much of the globe is compromising human health and natural ecosystems, and challenging the sustainability of modern agriculture.  In contrast, millions of resource-poor farmers lack sufficient N-fertilizer to ensure good harvests, especially in Africa where yields are often only 10-20% of yield potential for staples like maize. 

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 for several of these steps and will be presented at the NUE workshop.  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.