ARS | Publication request: INFLUENCE OF THE STATIONARY PHASE SIGMA FACTOR RPOS ON FITNESS OF PSEUDOMONAS FLUORESCENS

Submitted to: International Congress of Plant Pathology Abstracts and Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 31, 1997
Publication Date: N/A

Technical Abstract: We evaluated the role of the RpoS in environmental fitness and stress response of two bacterial biological control agents, P. fluorescens Pf- 5 (which suppresses diseases caused by soilborne fungi) and A506 (which suppresses fire blight).In culture, stationary-phase cells of an RpoS- mutant of Pf-5 were more sensitive than the wildtype to desiccation, oxidative stress, and freezing. In replicated field trials, Pf-5 established greater population sizes than the RpoS- mutant in the rhizosphere of bean grown in dry soils, whereas the strains established similar rhizosphere population sizes in moist soils. These data provide indirect evidence that, during at least a portion of its life in the rhizosphere, P. fluorescens is in a physiological state where the stationary-phase sigma factor RpoS is operative. Furthermore, RpoS contributes significantly to the fitness of Pf-5 in the rhizosphere of of bean grown in dry soils, which may be due to its role in desiccation tolerance.Stock cultures of P. fluorescens A506 had an rpoS gene with a frameshift mutation that resulted in a truncated RpoS, as determined by western analysis. We repaired the rpoS mutation in the genome of A506 through a combination of PCR and marker exchange techniques, and compared the repaired strain to the original A506 for stress response in culture and survival on apple leaves in the field. In field experiments, near-isogenic strains with truncated and full-length RpoS proteins established similar population sizes on apple leaf surfaces. Therefore, RpoS played a less significant role in the survival of A506 than of Pf-5 in the field environments evaluated in this study.