A roadmap towards engineered nitrogen-fixing nodule symbiosis

Abstract In the late nineteenth century, it was discovered that legumes can establish a root nodule endosymbiosis with nitrogen-fixing rhizobia. Soon after, the question was raised whether it is possible to transfer this trait to non-leguminous crops? In the past century, an ever-increasing amount of knowledge provided unique insights into the cellular, molecular and genetic processes controlling this endosymbiosis. Additionally, recent phylogenomic studies uncovered several genes that evolved to function specifically to control nodule formation and bacterial infection. However, despite this massive body of knowledge, the long-standing objective to engineer the nitrogen-fixing nodulation trait on non-leguminous crop plants hasn’t been achieved yet. In this review, the unsolved questions and engineering strategies will be highlighted.