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Erwinia amylovora  is the causative agent of the devastating disease fire blight of pome fruit trees. After infection of host plant leaves at apple shoot tips,  E. amylovora  cells form biofilms in xylem vessels, restrict water flow, and cause wilting symptoms. Although  E. amylovora  is well known to be able to cause systemic infection, how biofilm cells of  E. amylovora  transit from the sessile mode of growth in xylem to the planktonic mode of growth in cortical parenchyma remains unknown. Increasing evidence has suggested the important modulatory roles of Hfq‐dependent small RNAs (sRNAs) in the pathogenesis of  E. amylovora . Here, we demonstrate that the sRNA RprA acts as a positive regulator of amylovoran exopolysaccharide production, the type III secretion system (T3SS), and flagellar‐dependent motility, and as a negative regulator of levansucrase activity and cellulose production. We also show that RprA affects the promoter activity of multiple virulence factor genes and regulates  hrpS , a critical T3SS regulator, at the posttranscriptional level. We determined that  rprA  expression can be activated by the Rcs phosphorelay, and that expression is active during T3SS‐mediated host infection in an immature pear fruit infection model. We further showed that overexpression of  rprA  activated the in vitro dispersal of  E. amylovora  cells from biofilms. Thus, our investigation of the varied role of RprA in affecting  E. amylovora  virulence provides important insights into the functions of this sRNA in biofilm control and systemic infection.

molecular plant pathology

Orchestration of virulence factor expression and modulation of biofilm dispersal in  Erwinia amylovora  through activation of the Hfq‐dependent small RNA RprA