Cyclic di-GMP modulates sessile-motile phenotypes and virulence in Dickeya oryzae via two PilZ domain receptors.

Dickeya oryzae is a bacterial pathogen causing the severe rice stem rot disease in China and other rice-growing countries. We showed recently that the universal bacterial second messenger c-di-GMP plays an important role in modulation of bacterial motility and pathogenicity, but the mechanism of regulation remains unknown. In this study, bioinformatics analysis of the D. oryzae EC1 genome led to the identification of two proteins, YcgR and BcsA, both of which contain a conserved c-di-GMP receptor domain, known as the PilZ-domain. By deleting all the genes encoding c-di-GMP-degrading enzymes in D. oryzae EC1, the resultant mutant 7ΔPDE with high c-di-GMP levels became nonmotile, formed hyperbiofilm, and lost the ability to colonize and invade rice seeds. These phenotypes were partially reversed by deletion of ycgR in the mutant 7ΔPDE, whereas deletion of bcsA only reversed the hyperbiofilm phenotype of mutant 7ΔPDE. Significantly, double deletion of ycgR and bcsA in mutant 7ΔPDE rescued its motility, biofilm formation, and virulence to levels of wild-type EC1. In vitro biochemical experiments and in vivo phenotypic assays further validated that YcgR and BcsA proteins are the receptors for c-di-GMP, which together play a critical role in regulating the c-di-GMP-associated functionality. The findings from this study fill a gap in our understanding of how c-di-GMP modulates bacterial motility and biofilm formation, and provide useful clues for further elucidation of sophisticated virulence regulatory mechanisms in this important plant pathogen.