Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/19/2009
Publication Date: 6/1/2009
Citation: Stockwell, V., Hockett, K., Loper, J.E. 2009. Role of RpoS in stress tolerance and environmental fitness of the phyllosphere bacterium Pseudomonas fluorescens 122. Phytopathology. 99:689-695. Interpretive Summary: The ability of biological control agents to survive on plant surfaces is key to their success in managing plant diseases. In this study, we evaluated the survival of a beneficial strain of Pseudomonas fluorescens on flowers and leaves of pear and apple in orchards in the Pacific Northwest region of the United States. Because the capacity to withstand exposure to severe and rapidly changing weather conditions is important to the survival of bacteria on plants, we evaluated the importance of a gene that controls stress response of bacteria. Our hypothesis was that this stress control gene would be important to the survival of the bacterial strain on plants in the field. Contrary to our hypothesis, we saw no effect of that gene on the survival of the bacteria in the field, even though the gene was important in other bacteria that we studied earlier. This research shows that strains of bacteria differ in the ways that they respond to the environment. This finding is important because it gives us new avenues to follow in our quest to improve biological control by improving the survival of beneficial bacteria on plant surfaces.
Technical Abstract: Bacteria living epiphytically on aerial plant surfaces encounter severe and rapidly fluctuating environmental conditions, and their capacity to withstand stresses is considered to be a crucial factor contributing to epiphytic fitness. The stationary phase sigma factor RpoS is a key determinant in stress response of Gram-negative bacteria, including many strains of Pseudomonas spp. that inhabit plant surfaces. This study focused on the role of RpoS in stress response and epiphytic fitness of Pseudomonas fluorescens strain 122, a bacterium isolated from plant surfaces. We then compared the stress response and epiphytic fitness of strain 122 to a mutant deficient in RpoS. No consistent influence of RpoS on stress response or epiphytic population size of 122 on pear or apple leaves or blossoms was observed. Although the capacity of strain 122 to withstand exposure to environmental stresses was similar to that of other fluorescent pseduomonads, this capacity was largely independent of rpoS.