Identification of traits implicated in the rhizosphere competence of fluorescent pseudomonads: description of a strategy based on population and model strain studies

The lack of consistency of the beneficial effects of inoculated fluorescent pseudomonads has often been related to their bad survival in the rhizosphere. In this review, we describe the strategy followed over the last decade to study traits involved in the rhizosphere competence of these bacteria. The diversity of indigenous populations associated with plant roots was first compared to that of populations associated with uncultivated soils in order to identify traits that discriminate these populations. The involvement of these bacterial traits in the rhizosphere competence was then assessed by comparing the competitiveness of a wild-type strain to that of mutants affected in the corresponding phenotypes. Finally, traits shared by populations adapted to the rhizosphere were identified by comparing both the competitiveness in the rhizosphere and the metabolism of a collection of bacterial strains. The data yielded indicated that rhizosphere-competent pseudomonads show a specific metabolism especially characterized by the efficiency of the pyoverdine-mediated iron uptake and by the ability to reduce nitrogen oxides.