ISOLATION AND CHARACTERIZATION OF RHIZOBIUM-MELILOTI TN5 MUTANTS SHOWING ENHANCED SYMBIOTIC EFFECTIVENESS

A series of Tn5-induced mutants which showed enhanced symbiotic effect iveness (Eff(++)) was isolated from Rhizobium meliloti strains CXM1-10 5 and CXM1-188. Alfalfa plants inoculated by the Eff(++) mutants had s ignificantly higher shoot dry mass than plants inoculated by the paren tal strains. In the greenhouse, the most effective mutants increased t he shoot dry mass of the host plants by 23-26% and plant total nitroge n by 23-27%. interestingly, the frequency of the Eff(++) mutants in st rain 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 CXM1-18 8 versus 0.3% in CXM1-105). Genetic analysis of the mutants showed tha t 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 megapl asmid 1, and six in megaplasmid 2.