Molecular Mechanisms of Pathogenesis and Resistance to the Bacterial Pathogen Erwinia amylovora, Causal Agent of Fire Blight Disease in Rosaceae

Fire blight, caused by the necrogenic Gram-negative bacterium Erwinia amylovora, is one of the most destructive bacterial diseases of apple (Malus x domestica) and pear (Pyrus communis), among other members of the Rosaceae family. This disease poses a major economic threat to pome production as there are no available effective control measures. Genetic enhancement of fire blight resistance in apples is the best alternative for averting disease damage, loss of crop, and loss of whole trees. In this review, current knowledge of the molecular mechanisms of E. amylovora pathogenesis will be presented, especially those of effector proteins during bacterial-host interactions, as well as assessment of current understanding of the molecular controls of plant host resistance. Recent studies are elucidating how type III effectors modulate plant susceptibility and promote growth and dissemination of the pathogen. The large multidomain protein DspE is essential for E. amylovora pathogenesis and plays an additional role(s) in inhibiting salicylic acid-mediated innate immunity. On the other hand, the apple host defends itself against E. amylovora invasion by relying on quantitative resistance genes that likely respond to and/or complex with E. amylovora effectors. Thus far, a total of 27 quantitative trait loci (QTL) linked to fire blight resistance have been identified in different apple genetic backgrounds and in response to different E. amylovora strains. In addition to quantitative genetic approaches, microarray analysis of E. amylovora-challenged apple genotypes identified differential transcriptional expression in susceptible and resistant apples. Mechanisms of bacterial pathogenicity and plant host resistance offer intriguing scenarios as to how effector proteins in E. amylovora interact with groups of genes for resistance in the apple host, particularly when considering that these quantitative genes have small effects in plant defense against the invading bacterial pathogen. This collective knowledge will provide insights into bacterial pathogenesis and plant host resistance, as well as highlight implications and opportunities for developing fire blight-resistant apple cultivars.