Project Description:
            Pseudomonas syringae pv. tomato strain DC3000(PstDC3000) is a plant pathogen that causes disease on both the model plant Arabidopsis thaliana and tomato, an agriculturally important plant. The phytotoxin coronatine (COR) is a crucial virulence factor that promotes PstDC3000 pathogenesis in at least three different ways. COR is required for entry into host plants, suppression of host defenses, and promotion of disease symptoms. COR suppresses plant host bacterial defenses by mimicking the plant hormone jasmonic acid (JA), to suppress the hormone defense pathway effective against bacterial pathogens, the salicylic acid (SA) pathway. COR may promote disease symptom development by manipulating a third plant hormone, auxin (indole acetic acid, IAA). IAA levels increase during PstDC3000 infection, indicating that IAA may have some role in disease. Furthermore, IAA applied exogenously to plants infected with PstDC3000 enhances disease symptom development, and during infection there is a COR dependent induction of genes required for IAA synthesis. To address how COR may be manipulating IAA levels in the plant, I am measuring IAA levels in the plant during infection with bacteria that either produce COR or fail to produce COR. I am also using transgenic Arabidopsis lines to increase or decrease IAA levels during infection and address the role that IAA plays in susceptibility to PstDC3000. The results of these experiments will help us determine the role that IAA plays during infection and how COR may be influencing IAA levels during infection.

            I am also interested in identifying novel virulence factors of PstDC3000. One of the novel virulence factors we have identified is an enzyme in the citric acid cycle, malate quinone oxidoreductase (MQO). PstDC3000 without functional MQO fail to grow to the same levels as wild-type bacteria in planta, but have similar growth to wild-type bacteria in rich media.  MQO mutants also have growth defects in culture when C4-dicarboxylates (such as malate and succinate) are the only carbon source, whereas growth is similar to wild-type on sucrose and other citric acid cycle intermediates. We believe this may indicate that a C4-dicarboxylate may be an important carbon source for PstDC3000 when it is growing in planta.