The in planta role of the global regulator Lrp in the bacterial phytopathogen Pantoea stewartii subsp. stewartii

Pantoea stewartii subsp. stewartii is a bacterial phytopathogen that causes the disease Stewart's wilt in corn. The insect vector Chaetocnema pulicaria, the corn flea beetle, transmits P. stewartii into corn plants through wounds in the leaves. The bacteria can then move to the xylem of the plant where they form a biofilm that inhibits the flow of water. A previous in planta RNA-Seq study resulted in the selection of lrp as a gene of interest for further analyses. A reverse genetics approach was used for the creation of a strain containing the in-frame deletion of lrp, as well as a revertant strain. The strain with the deletion of the lrp gene showed reduced motility and capsule formation when in vitro assays were conducted. It has previously been demonstrated that these characteristics are both important for the bacteria's ability to form a biofilm in the xylem of corn plants and produce disease symptoms. The in planta virulence and competition assays demonstrated that the lrp gene deletion also results in reduced disease symptoms in infected corn plants, as well as an inability to outcompete wildtype P. stewartii in xylem colonization.

In a bioinformatics approach, the transcriptional regulator Lrp of P. stewartii was present in the same node of the phylogeny as homologues from other closely related phytopathogens. This demonstrates that Lrp from P. stewartii and such homologues have evolved from a recent common ancestral gene. Examining the genomic islands present in P. stewartii, it is possible to begin to predict where some of the genes which have functions involved in plant colonization may have originated.

Overall, the results collected from the studies in this thesis contribute to improving understanding of how P. stewartii is successful at colonizing the xylem of corn plants and cause disease. This research could result in the development of methods to decrease crop susceptibility to infection with P. stewartii.