A mutant of Rhizobium meliloti that elicited the formation of inactive nodules in alfalfa was found not to form beta-(1----2) glucan in vivo or in vitro. It was nonmotile because it lacks flagella. The 235-kilodalton protein which acts as an intermediate in beta-(1----2) glucan synthesis was undetectable in the mutant. These properties of the mutant are common to those of chvB mutants of Agrobacterium tumefaciens. Exopolysaccharide formation by the R. meliloti mutant was about double that by the wild type.
The nitrate reductase (NR, EC 1.6.6.1) activity in root nodules formed by hydrogenase positive (Hup+) and hydrogenase negative (Hup−) Rhizobium leguminosarum strains was examined in symbioses with the pea cultivar Alaska (Pisum sativum L.), Rates of activity were determined by the in vivo assay in nodules from plants that were only N2‐dependent or grown in the presence of 2 mM KNO3. The rates varied widely among strains, regardless of the Hup phenotype of the R. leguminosarum strain used for inoculation, but the overall results indicated that nodules formed by Hup− strains accumulated more nitrite in the incubation medium than did those with Hup− phenotypes. Total plant dry weight and reduced nitrogen content of pea plants grown in the presence of 2 mM KNO3 and inoculated with single Hup+ and Hup−R. leguminosarum strains were statistically different among some strains. These observations suggest that the possible advantages derived from the presence of the Hup system on whole plant growth may be counteracted by the higher rates of NR activity in the Hup− strains in the R. leguminosarum‐pea symbiosis.
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