Opportunities and challenges to optimise symbiotic nitrogen fixation

Legumes are not only major cash crops but also contribute valuable nitrogen to cropping systems due to their ability to form a symbiotic relationship with nitrogen-fixing rhizobia in specialised root organs called nodules. To balance the cost of carbon provision to the rhizobia, nodulation is finely regulated in legumes across various spatiotemporal levels, including host-microbe signalling within the rhizosphere, infection of the legume host, and nodule initiation, function, and senescence. Since symbiotic nitrogen fixation (SNF) evolved in natural ecosystems which lack resemblance to modern agricultural systems, opportunities present themselves to genetically improve SNF. Based on recent findings and the opportunities arising with new breeding technologies, we review here the many opportunities to optimise SNF and highlight the key challenges associated with these approaches.