Involvement of nitrate reductase and pyoverdine in competitiveness of Pseudomonas fluorescens strain C7R12 in soil

Involvement of nitrate reductase and pyoverdine in the competitiveness of t he biocontrol strain Pseudomonas fluorescens C7R12 was determined, under gn otobiotic conditions, in two soil compartments (bulk and rhizosphere soil), with the soil being kept at two different values of matric potential (-1 a nd -10 kPa), Three mutants affected in the synthesis of either the nitrate reductase (Nar(-)), the pyoverdine (Pvd(-)), or both (Nar Pvd(-)) were used . The Nar(-) and Nar(-) Pvd- mutants were obtained by site-directed mutagen esis of the wildtype strain and of the Pvd(-) mutant, respectively. The sel ective advantage given by nitrate reductase and pyoverdine to the wild-type strain was assessed by measuring the dynamic of each mutant-to-total-inocu lant (wild-type strain plus mutant) ratio. All three mutants showed a lower competitiveness than the wild-type strain, indicating that both nitrate re ductase and pyoverdine are involved in the fitness of P, fluorescens C7R12, The double mutant presented the lowest competitiveness. Overall, the compe titive advantages given to C7R12 by nitrate reductase and pyoverdine were s imilar. However, the selective advantage given by nitrate reductase was mor e strongly expressed under conditions of lower aeration (-1 kPa), In contra st, the selective advantage given by nitrate reductase and pyoverdine did n ot differ in bulk and rhizosphere soil, indicating that these bacterial tra its are not specifically involved in the rhizosphere competence but rather in the saprophytic ability of C7R12 in soil environments.