Identification and analysis of induced genes from Erwinia amylovora and Malus x domestica during fire blight infection

Fire blight, caused by the bacterial pathogen Erwinia amylovora , is a difficult disease to manage due to the virulent nature of the pathogen, disease susceptibility of most popular apple varieties, and the lack of known resistance genes in Malus spp. The goals of my research are to identify and characterize genes from E. amylovora that are induced during infection and to identify genes in Malus spp. that are associated with disease resistance. I utilized an in vivo expression technology screen to identify in planta upregulated E. amylovora genes, two of which had homology to pseudopilins from a type II secretion operon and an endo-polygalacturonase (peh). Deletion mutants of both the peh gene and outDEA were constructed to further analyze the contribution of type II secretion to the E. amylovora-Malus spp. interaction. This work resulted in the first report of a polygalacturonase enzyme influencing virulence of E. amylovora . Some apple cultivars, such as Red Delicious, exhibit tolerance to fire blight. Because resistance to fire blight appears to be a quantitative trait in apple, I chose to identify genes associated with resistance using suppressive subtractive hybridization. I generated 183 unique expressed sequence tags (ESTs) from cultivar Red Delicious during infection that are absent or repressed in the fire blight sensitive cultivar Gala. Temporal expression analysis identified 21 of the ESTs which are induced in Red Delicious of which eight show differential expression in other apple cultivars of varying resistance to fire blight. Further functional characterization of these genes will shed new light on signaling cascades that lead to a successful resistance response; in addition these genes can be converted into molecular markers for mapping onto linkage maps of apple for use in marker assisted selection of E. amylovora resistance in apple breeding projects.