The sigma factor RpoS is required for stress tolerance and environmental fitness of Pseudomonas fluorescens Pf-5

1 Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, USA 2 United States Department of Agriculture, Agricultural Research Service, Horticultural Crops Research Laboratory, 3420 NW Orchard Avenue, Corvallis, OR 97330, USA Correspondence Joyce Loper loperj{at}science.oregonstate.edu Many micro-organisms exist in natural habitats that are subject to severe or dramatically fluctuating environmental conditions. Such is the case for bacteria inhabiting plant surfaces, where they are exposed to UV irradiation, oxygen radicals, and large fluctuations in temperature and moisture. This study focuses on the role of RpoS, a central regulator of stationary-phase gene expression in bacterial cells, in stress response and environmental fitness of Pseudomonas fluorescens Pf-5. Strain Pf-5 is a rhizosphere-inhabiting bacterium that suppresses plant diseases caused by several plant-pathogenic fungi and oomycetes. Previous studies demonstrated that rpoS was required for osmotic and oxidative stress resistance of Pf-5. The results of this study demonstrate a role for rpoS in tolerance of Pf-5 to freezing, starvation, UV irradiation and desiccation stress. In field studies, an rpoS mutant was compromised in rhizosphere colonization of plants in dry soil, whereas similar rhizosphere populations were established by Pf-5 and an rpoS mutant in well-irrigated soils. RpoS is a key determinant in stress response and environmental fitness of the rhizosphere bacterium P. fluorescens Pf-5. Abbreviations: AUPC, area under the population curve; RAUPC, relative area under the population curve