Global Regulation of Virulence Determinants During Plant Colonization in the Bacterial Phytopathogen, Pantoea stewartii subsp. stewartii
Pantoea stewartii subsp. stewartii, the etiological agent of Stewart's wilt, is a bacterial pathogen of sweet corn which colonizes both the apoplast and xylem tissues. During the initial stages of the infection process, the pathogen forms water-soaked lesions through lysis of the plant cells, followed by colonization of the xylem tissue where it can grow to high cell densities and form biofilms. Biofilm formation within the xylem vessels can block water flow, causing the characteristic wilting symptoms of P. stewartii infection. Throughout the infection process, P. stewartii must contend with a variety of stresses inherent to its changing environment, such as production of reactive oxygen species (ROS) in the host, and nutrient limitation. Global regulatory mechanisms that control gene expression in response to environmental conditions therefore play a crucial role in success of the pathogen. We found that ROS exposure initiates an induced stress response through the OxyR and SoxR transcription factors which plays an important biological role in adaptation of the pathogen to different niches in the host during infection. OxyR is essential for the wilting symptoms characteristic of the later stages of infection, likely due to its role in optimal exopolysaccharide (EPS) production. The initial stages of infection (water-soaked lesion formation) rely on SoxR-dependent gene expression in order to colonize to wild type levels. In addition to the oxidative stress response, we found that OxyR indirectly regulates iron transport through a siderophore-dependent mechanism. We found that siderophore-mediated iron acquisition, in addition to its primary role of enhancing bacterial growth under iron-limiting conditions, regulates motility and bacterial movement in planta. As a result, siderophore production is required for full virulence of P. stewartii. In order to investigate a link between OxyR regulation and optimal EPS production, we characterized the Rcs phosphorelay signal transduction system, a known regulator of EPS and biofilm formation. Rcs regulation is crucial for virulence in P. stewartii due to its regulation of motility and EPS production. These studies unravel some of the complexities of global regulation required for P. stewartii and, likely, other bacterial pathogens to adapt to different environments such as the xylem.