The bacterium Pantoea stewartii subsp. stewartii causes Stewart’s wilt disease in corn and serves as a model for other xylem-dwelling phytopathogens. P. stewartii is transmitted to plants via corn flea beetles where it first colonizes the apoplast causing water-soaked lesions, then migrates to the xylem and forms a biofilm that blocks water transport. P. stewartii uses quorum sensing to insure the exopolysaccharide production necessary for biofilm formation occurs only at high cell density during growth in the xylem. To gain broader insights into what genes are important for plant pathogens like P. stewartii during in planta growth, the bacteria were grown inside the xylem followed by genomic-level analyses in vitro.  RNA purified from in planta grown P. stewartii was used to determine the most highly expressed genes via RNA-Seq through Next-generation Illumina sequencing of cDNA. Analysis of the transcriptome data via gene ontology revealed that bacterial transporters and systems important for oxidation-reduction processes are highly expressed in response to the environment inside the plant.  A subsequent genomic-level Tn-Seq analysis was performed to identify bacterial genes essential for survival in planta. A mariner transposon library of approximately 40,000 mutants was constructed and used to inoculate corn seedlings through a xylem infection model. The Tn-Seq analysis suggested that over 400 genes are important for xylem survival. In planta competition assays and virulence assays have so far revealed important roles for outer membrane proteins and Lon protease in bacterial colonization, growth and/or virulence in the corn plant. These studies are giving us new insights into the lifestyle of a pathogenic plant bacterium when living inside its plant host.
 

Learning Objectives: 

1. Explain the role of quorum sensing during plant disease caused by the phytopathogen Pantoea stewartii subsp. stewartii.
2. Design genome-levels experiments to study the expression and role in survival of bacterial genes during in planta growth.