A series of Tn5-induced mutants which showed enhanced symbiotic effectiveness (Eff) was isolated from strains CXM1-105 and CXM1-188. Alfalfa plants inoculated by the Eff mutants had significantly higher shoot dry mass than plants inoculated by the parental strains. In the greenhouse, the most effective mutants increased the shoot dry mass of the host plants by 23-26% and plant total nitrogen by 23-27%. Interestingly, the frequency of the Eff mutants in strain CXM1-188 was higher than in the strain CXM1-105 (1.1% versus 0.4%); this was also the case for the auxotrophic mutants (1.2% in CXM1-188 versus 0.3% in CXM1-105). Genetic analysis of the mutants showed that the enhanced symbiotic effectiveness was cotransducible with Tn5. By the use of Southern hybridization and plasmid transfer, it was found that ten Tn5 insertions were located in the chromosome, five in megaplasmid 1, and six in megaplasmid 2.


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