P-364 - Nodule-isolated Pseudomonas promote Lotus growth and suppress cheater Rhizobium

Leguminous plants form symbiosis with nitrogen-fixing rhizobia bacteria in root nodules, converting atmospheric nitrogen into ammonia, a key nutrient for plant growth. While rhizobia dominate these nodules, non-rhizobial bacteria also colonize them, though their roles remain largely unknown. Evidence suggests non-rhizobial bacteria can influence symbiosis; for instance, nodule-isolated Pseudomonas reduced ineffective nodulation in Lotus plants caused by the cheater Rhizobium sp. BW8-2, though the mechanism was unclear. To investigate this, we isolated 14 Pseudomonas strains from Lotus nodules, identifying five species, including the novel one, Pseudomonas monachiensis. All isolates were non-pathogenic and promoted Lotus burttii growth, with variable effects on reducing ineffective nodulation. We selected Pseudomonas sp. Qb1D2, which strongly suppressed BW8-2 colonization and nodulation, and inhibited BW8-2 growth in vitro, to perform Tn5 mutagenesis for identifying genes involved in BW8-2 inhibition. In 67% of the mutants, Tn5 insertions were found in genes related to a two-component signaling pathway, potentially controlling non-ribosomal peptide biosynthesis, as supported by antiSMASH predictions. These findings indicate that Pseudomonas can promote Lotus plants growth and reduce the ineffective nodules potentially by suppressing cheater Rhizobium growth, providing insights into potential applications for sustainable agriculture.