Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/19/2009
Publication Date: 6/1/2009
Citation: Hagen, M., Stockwell, V., Whistler, C., Johnson, K.B., Loper, J.E. 2009. Stress tolerance and environmental fitness of Pseudomonas fluorescens A506, which has a mutation in rpoS. Phytopathology. 99:689-695. Interpretive Summary: The biological control agent Pseudomonas fluorescens A506 is registered for use on pear and apple trees to manage fire blight, an important disease that constrains pome fruit production throughout the United States and many parts of the world. The biological control agent is typically sprayed on pear and apple trees during the bloom period, and it colonizes blossom surfaces thereby preventing the infection of those blossoms by the fire blight pathogen. To be an effective biological control agent, A506 must survive on plant surfaces, even when the weather is severe or rapidly changing. Because most bacteria require an rpoS gene to survive environmental stress, we evaluated the importance of rpoS in the survival of A506 on pear and apple flowers and leaves in the field. To our surprise, we found that A506 has a defective rpoS gene and rpoS was not required for the survival of A506. Although A506 can survive environmental stress, it does so in a way that differs from other bacteria.
Technical Abstract: Bacteria living epiphytically on aerial plant surfaces encounter severe and rapidly fluctuating environmental conditions, and their capacity to withstand stress 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 A506, a well-studied bacterial epiphyte of plants. We detected a frameshift mutation in the rpoS of A506 and demonstrated that the mutation resulted in a truncated, non-functional RpoS. We repaired the rpoS mutation in the genome of A506 using PCR followed by recombinational gene replacement. We then compared the stress response and epiphytic fitness of A506 to derivative strains having a full-length repaired RpoS or a disrupted RpoS. No consistent influence of RpoS on stress response or epiphytic population size of A506 of pear or apple leaves or blossoms was observed. Although the capacity of strain A506 to withstand exposure to environmental stress was similar to that of other fluorescent pseudomonads, this capacity was largely independent of rpoS.