AbstractsBiology & Animal Science

The interactions between phytopathogenic bacteria and their host plants can be characterized as an intricate web of signals and appropriate responses. Phytopathogenic soft rot bacteria occur globally, causing disease in Solanum tuberosum (potato) and other tubular staple foods in both the field and storage. One widely studied soft rot bacterium is Pectobacterium wasabiae, which has been identified in Eutrema wasabi (wasabi) plants in Japan and in potatoes in Finland. Generally, the interactions between this type of bacterium and host plants are characterized by maceration of plant tissue, due to the actions of secreted plant cell wall degrading enzymes (PCWDE), and the induction of phytohormone dependent defenses in the plants. The maceration of plant tissue involves the release of pectic oligogalacturonides (OGs) from plant cell walls. OGs have been identified as important signaling compounds, inducing the expression of a variety of defense-related genes. As the bacterial infection advances, the bacteria coordinate the production of virulence factors by utilizing regulatory proteins that modulate the transcriptome. Transcriptomic analyses have been used extensively in past studies to identify regulatory networks and signaling pathways, and these studies have provided insights into the processes underlying plant-pathogen interactions. The novel scientific results of this dissertation are derived from a combination of transcriptomic, genomic, genetic, and phenotypic analyses. This study analyzed various aspects of plant-pathogen interactions. The central bacterial model used was P. wasabiae, and the model plant of interest was Arabidopsis thaliana. This study characterized the genome of P. wasabiae via sequencing and bioinformatics analysis. Various virulence associated genes and operons, such as two distinct type 6 secretion systems, were identified and annotated. The bacterium was found to in fact be more related to P. wasabiae than Pectobacterium carotovorum, which the strain originally had been named after. Furthermore, a combination of functional genetics and transcriptomic methods, such as reverse transcription quantitative PCR (RT-qPCR) and microarrays, were used to determine the regulons controlled by the proteins ExpA and RsmA in P. wasabiae. These two proteins have been identified as important for the virulence of several γ-proteobacterial pathogens. This study analyzed the regulons via the use of three mutants: expA, rsmA, and an expA rsmA double mutant (DM). Overlapping and independently regulated targets were identified between ExpA and RsmA. Phenotypic assays for motility, growth, PCWDE activity, and virulence confirmed the transcriptomic data for the mutant strains. Novel findings included reduction of swimming motility in agar medium for P. wasabiae expA and rsmA mutants. In addition, the DM exhibited enhanced virulence and fitness in planta compared to either single mutant. Via analysis of transcriptomic data, a subset of genes was identified as affected in expression by an expA mutation…