ISOLATION AND CHARACTERIZATION OF THE RHIZOBIUM-MELILOTI TN5-MUTANTS WITH IMPAIRED NODULATION COMPETITIVENESS

Two Tn5-mutants, T37 and T107, with decreased nodulation competitivene ss (Cmp(-)) were isolated in Rhizobium meliloti strain CXM1-105. Their Cmp(-) phenotype was repeatedly displayed in sterile plant tests, whe re competitiveness was measured against a Fix(-) reference strain CXM1 -48 and the parental strain CXM1-105. In addition mutant T37 showed a decreased nodulation rate (Nra(-)) at the early stages of alfalfa deve lopment (9-18 days after germination), Its mutant phenotype was demons trated also under greenhouse conditions when it was evaluated against reference strain CXM1-48, and against indigenous rhizobial strains per sisting in the soil. Tn5 insertions from mutants T37 and T107 were tra nsduced using phage phi M12 to the parental strain CXM1-105. All teste d transductants showed Cmp(-) and Nra(-) phenotype in the case of T37 and Cmp(-) phenotype in the case of T107. By means of conjugations wit h avirulent R. meliloti mutant ZB121 and plasmidless Agrobacterium tum efaciens UBAPF2 we located both Tn5-insertions on the R. meliloti Sym- plasmid.