Non-nodulating Rhizobium-like ACO-34A fixes nitrogen in pure cultures and has a nif plasmid

Abstract Rhizobia fix nitrogen in plant nodules. Notably, Rhizobium sp. ACO-34A (which could be reclassified as Paenirhizobium ), recovered from the rhizosphere of Agave americana , is capable of fixing nitrogen in a defined medium in microaerobic conditions and carries nifHDKENBV genes in a 213 kb plasmid. ACO-34A failed to induce nodules in several leguminous hosts and does not have nod genes. ACO-34A NifH mutant did not fix nitrogen in pure cultures and did not promote stem growth in Lotus japonicum plants as the wild strain did. The plasmid harboring the nif genes contains repABC replication genes, genes for homocitrate synthesis, for toxin-antitoxin production and for plant colonization. Comparative phylogenomic analyses revealed that strain ACO-34A is close to Ciceribacter sichuanensis S101, which was isolated from soybean nodules and should be reclassified. According to ANI, AAI and dDDH parameters, ACO-34A may represent a novel species within the Rhizobiacea family.