The enterobacterium Erwinia amylovora is a devastating plant pathogen causing necrotrophic fire blight disease of apple, pear, and other rosaceous plants. In this study, we used a modified in vivo expression technology system to identify E. amylovora genes that are activated during
infection of immature pear tissue, a process that requires the major
pathogenicity factors of this organism. We identified 394 unique pear fruit-induced (pfi) genes on the basis of sequence similarity to known genes and separated them into nine putative function groups including host-microbe interactions (3.8%), stress response (5.3%), regulation (11.9%), cell surface (8.9%), transport (13.5%), mobile elements (1.0%), metabolism (20.3%), nutrient acquisition and synthesis (15.5%), and unknown or hypothetical
proteins (19.8%). Known virulence genes, including hrp/hrc components of the
type III secretion system, the major effector gene
dspE, type II secretion,
levansucrase (lsc), and regulators of
levansucrase and
amylovoran biosynthesis, were upregulated during pear tissue
infection. Known
virulence factors previously identified in E. (Pectobacterium) carotovora and Pseudomonas syringae were identified for the first time in E. amylovora and included HecA
hemagglutinin family adhesion, Peh
polygalacturonase, new effector HopPtoC(EA), and membrane-bound lytic
murein transglycosylase MltE(EA). An insertional mutation within hopPtoC(EA) did not result in reduced virulence; however, an mltE(EA) knockout mutant was reduced in virulence and growth in immature pears. This study suggests that E. amylovora utilizes a variety of strategies during plant
infection and to overcome the stressful and poor nutritional environment of its plant hosts.