Sym plasmid genes of Rhizobium trifolii expressed in Lignobacter and Pseudomonas strains

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Abstract
A 14-kilobase (kb) fragment of R. trifolii Sym plasmid containing nodulation (nod) genes or the pSym plasmid of R. trifolii cointegrated with a broad-host-range vector R68.45 (pPN1) were transferred to Lignobacter strain K17 and P. aeruginosa strain PAO5 by conjugation. Lignobacter transconjugants carrying Sym plasmid pPN1 formed nodules on white, red and subterranean clover plants. Lignobacter transconjugants containing a 14-kb fragment of nod genes cloned into a multicopy plasmid nodulated only white and subterranean clover plants, whereas transcojugants carrying the same fragment cloned into a low-copy plasmid vector modulated only white clover plants. All nodules formed by Lignobacter transconjugants showed bacterial release from the infection threads into the host cytoplasm. Pseudomonas transconjugants with plasmid pPN1 formed nodule-like structures on white clover plants. These structures were not invaded by bacteria; a few bacteria were found within the intercellular spaces of the outermost cells of the structures. Pseudomonas transconjugants carrying the 14-kb fragment of R. trifolii nod genes did not form nodules on tested clover plants. All clover plants inoculated with Pseudomonas or Lignobacter transconjugants containing a 14-kb fragment of nod genes (but not entire Sym plasmid) showed the thick- and -short-root response when compared to the control plants inoculated with the R. trifolii wild-type strain.