title: Relative involvement of nitrate and nitrite reduction in the competitiveness of Pseudomonas fluorescens in the rhizosphere of maize under non-limiting nitrate conditions.
abstract: Competition between different isogenic mutants of Pseudomonas fluorescens unable to carry out the first steps of the denitrification pathway was compared in soil micro-columns non-planted or planted with maize. A new isogenic mutant of P. fluorescens YT101 affected in both nitrate and nitrite respirations was constructed and used as a model of non-denitrifying strain (FM69MS strain). The outcome of the selection exerted by the plant after co-inoculation of FM69MS at the same ratio either with an isogenic denitrifier unable to reduce nitrate (Nar(-) mutant) or with an isogenic NO2 (-) accumulator (Nir(-) mutant) was investigated in non-limiting NO3 (-) conditions. Regardless of the inoculated mixture, both strains were able to grow in both rhizosphere and non-planted soil. The proportion of Nar(-) or Nir(-) strain in the Nar(-)+FM69MS or Nir(-)+FM69MS total introduced population remained stable in non-planted soil. In the rhizosphere, we observed a higher competitiveness of the Nir(-) mutant compared with FM69MS, whereas the latter showed the same competitiveness as the Nar(-) mutant. These results provide the first demonstration that NO3 (-) reduction is the main nitrogen-dissimilating step controlling the competitiveness of P. fluorescens in the rhizosphere.
authors.affiliation: Ecologie Microbienne, UMR CNRS 5557, Université Claude Bernard-Lyon1, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France.
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Ghiglione
authors.forename: Agnès
authors.initials: A
authors.lastname: Richaume
authors.forename: Laurent
authors.initials: L
authors.lastname: Philippot
authors.forename: Robert
authors.initials: R
authors.lastname: Lensi