Relative involvement of nitrate and nitrite reduction in the competitiveness of Pseudomonas fluorescens in the rhizosphere of maize under non-limiting nitrate conditions

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 NO 2 − accumulator (Nir − mutant) was investigated in non-limiting NO 3 − 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 NO 3 − reduction is the main nitrogen-dissimilating step controlling the competitiveness of P. fluorescens in the rhizosphere.