Isolation and characterization of the Rhizobium meliloti Tn5-mutants with impaired nodulation competitiveness

Abstract

Two Tn5-mutants, T37 and T107, with decreased nodulation competitiveness (Cmp-) were isolated in Rhizobium meliloti strain CXM1-105. Their Cmp-phenotype was repeatedly displayed in sterile plant tests, where competitiveness was measured against a Fix-reference strain CXM1-48 and the parental strain CXMI-105. In addition mutant T37 showed a decreased nodulation rate (Nra-) at the early stages of alfalfa development (9-18 days after germination). Its mutant phenotype was demonstrated also under greenhouse conditions when it was evaluated against reference strain CXM1-48, and against indigenous rhizobial strains persisting in the soil.Tn5 insertions from mutants T37 and T107 were transduced using phage ~bM12 to the parental strain CXM1-105. All tested transductants showed Cmp-and Nra-phenotype in the case of T37 and Cmp-phenotype in the case of T107. By means of conjugations with avirulent R. meliloti mutant ZB 121 and plasmidless Agrobacterium tumefaciens UBAPF2 we located both Tn5-insertions on the R. meliloti Sym-plasmid.

Identification of Sinorhizobium meliloti Genes Influencing Synthesis of Surface Polysaccharides and Competitiveness

“…This agrees with the earlier data on the correlation between the rate of nodule formation and NC [19]. It should also be noted that symbiotic effectiveness (evaluated in respect to the dry mass of plants) was the same in mutants and the parental strain.…”

“…Symbiotic traits of mutants were examined in the sterile plant tube test with alfalfa Medicago sativa L. (cultivar Vega). Seeds were planted into sterile tubes containing vermiculite to determine adsorption abilities and nodulation competitiveness (NC) or into tubes containing 0.7% Krasilnikov-Korenyako agar medium to count nodules [19,20].…”

“…Nodulation competitiveness was tested by the direct method as the ratio between the numbers of generated nodules [19] using tester strains: M4gus and the parental strain SXM1-105. Suspensions of these strains and of the analyzed mutant that had the same optical density were mixed, and the mixture (approximately 10 8 cells/ml) was used to inoculate two-day alfalfa seedlings (the experiment duration was 45 days).…”

Selection of Rhizobium leguminosarum bv. viceae strains for inoculation of Pisum sativum L. cultivars: Analysis of symbiotic efficiency and nodulation competitiveness

“…An effective strain was considered to be lowcompetitive if plants inoculated with the strain mixture did not differ significantly in their mass or N accumulation from those inoculated with the tester strain only (C-phenotype). An effective strain is highlycompetitive (C + phenotype) if the plants inoculated with the strain mixture had the same mass (N accumulation) as the plants inoculated with the effective strain only and significantly exceeded plants inoculated with the ineffective tester strain (Onischuk et al, 1994). All tests for competitiveness were performed with the pea cultivar Orlovchanin.…”

Factors Influencing Nodule Occupancy by Inoculant Rhizobia

“…Based on these observations, nodulation characteristics, such as the average position of the uppermost nodule and the number of nodules induced above the RTM, have been assigned as indicators of a strain's ability to initiate and sustain an infection, the so-called "nodule-forming efficiency" (McDermott and Graham, 1990). Strains with high noduleforming efficiency (i.e., initiating and sustaining an infection rapidly and efficiently) are often found to have a competitive advantage over those that are slower and less efficient (Sargent et al, 1987;Hahn and Hennecke, 1988;Stephens and Cooper, 1988;de Oliveira and Graham, 1990b;McDermott and Graham, 1990;Onishchuk et al, 1994). This correlation between competitiveness and nodule-forming efficiency of a strain can be partially explained by the autoregulatory response exerted by the host.…”