The second messenger
cyclic di-GMP (
c-di-GMP) is a nearly ubiquitous intracellular signal molecule known to regulate various cellular processes, including biofilm formation, motility, and virulence. The intracellular concentration of
c-di-GMP is inversely governed by
diguanylate cyclase (DGC)
enzymes and
phosphodiesterase (PDE)
enzymes, which synthesize and degrade
c-di-GMP, respectively. The role of
c-di-GMP in the plant pathogen and causal agent of fire blight disease Erwinia amylovora has not been studied previously. Here we demonstrate that three of the five predicted DGC genes in E. amylovora (edc genes, for Erwinia
diguanylate cyclase), edcA, edcC, and edcE, are active diguanylate cyclases. We show that
c-di-GMP positively regulates the secretion of the main exopolysaccharide in E. amylovora,
amylovoran, leading to increased biofilm formation, and negatively regulates flagellar swimming motility. Although
amylovoran secretion and biofilm formation are important for the colonization of plant xylem tissues and the development of systemic
infections, deletion of the two biofilm-promoting DGCs increased tissue
necrosis in an immature-pear
infection assay and an apple shoot
infection model, suggesting that
c-di-GMP negatively regulates virulence. In addition,
c-di-GMP inhibited the expression of hrpA, a gene encoding the major structural component of the type III secretion pilus. Our results are the first to describe a role for
c-di-GMP in E. amylovora and suggest that downregulation of motility and type III secretion by
c-di-GMP during
infection plays a key role in the coordination of pathogenesis.