P-201 - Engineering nodulation into cereal root-associated free-living diazotrophs

Engineering nitrogen fixation into non-legume crops has long been a goal for researchers as an alternative to highly polluting nitrogen-based fertilisers. As our understanding of the genetic regulation of nodule formation increases, we are moving closer to inducing their formation on cereal crop roots. Identifying bacteria that could enter and fix within these nodules is essential—however, the highly specific partnership between legumes and rhizobia presents a complex challenge. 

This research investigated whether bacterial strains that exhibit high levels of N₂ fixation in free-living conditions can be converted into “symbiotic” or nodule-forming bacteria in pea plants. Two N₂-fixing strains from the genus Ciceribacter were isolated from barley roots; these strains fix N₂ efficiently in free-living conditions but lack genes for nodule formation. Conjugation of the mobile symbiotic plasmid pJB5JI into these strains resulted in small, white nodules on pea roots. Sections through these nodules showed the presence of the engineered strains, but which were unable to fix N₂ using the transferred plasmid.

Future work will focus on maintaining the native nif-fix cluster and facilitating ammonia release from these strains to determine whether pea plants can form functional nodules to sustain the engineered strains. These findings have significant implications for engineering “synthetic symbioses” and advancing our understanding of what it takes for bacteria to become symbionts.