Phosphodiesterase Genes Regulate Amylovoran Production, Biofilm Formation, and Virulence in Erwinia amylovora.

Cyclic di-GMP (c-di-GMP) is a ubiquitous bacterial second messenger molecule that is an important virulence regulator in the plant pathogen Intracellular levels of c-di-GMP are modulated by diguanylate cyclase (DGC) enzymes that synthesize c-di-GMP and by phosphodiesterase (PDE) enzymes that degrade c-di-GMP. The regulatory role of the PDE enzymes in has not been determined. Using a combination of single, double, and triple deletion mutants, we determined the effects of each of the four putative PDE-encoding genes (, , , and ) in on cellular processes related to virulence. Our results indicate that and are the two phosphodiesterases most active in virulence regulation in Ea1189. The deletion of resulted in a measurably significant increase in the intracellular pool of c-di-GMP, and the highest intracellular concentrations of c-di-GMP were observed in the Ea1189 Δ and Ea1189 Δ mutants. The regulation of virulence traits due to the deletion of the genes showed two patterns. A stronger regulatory effect was observed on amylovoran production and biofilm formation, where both Ea1189 Δ and Ea1189 Δ mutants exhibited significant increases in these two phenotypes In contrast, the deletion of two or more genes was required to affect motility and virulence phenotypes. Our results indicate a functional redundancy among the genes in for certain traits and indicate that the intracellular degradation of c-di-GMP is mainly regulated by and , but they also suggest a role for in regulating motility and virulence. Precise control of the expression of virulence genes is essential for successful infection of apple hosts by the fire blight pathogen, The presence and buildup of a signaling molecule called cyclic di-GMP enables the expression and function of some virulence determinants in , such as amylovoran production and biofilm formation. However, other determinants, such as those for motility and the type III secretion system, are expressed and functional when cyclic di-GMP is absent. Here, we report studies of enzymes called phosphodiesterases, which function in the degradation of cyclic di-GMP. We show the importance of these enzymes in virulence gene regulation and the ability of to cause plant disease.