Osmoregulated periplasmic glucans (OPGs) are general constituents of many proteobacteria. OPGs are important factors required for full virulence in many pathogens includingDickeya dadantii.D. dadantiicauses the soft-rot disease in a wide range of plant species. The pleiotropic phenotype ofopg-negative strains includes total loss of virulence and motility, and is linked to the constitutive activation of the RcsCDB phosphorelay, deduced from expression analysis of genes of the RcsCDB regulon. The constitutive activation of the RcsCDB phosphorelay in anopg-negative strain was demonstrated by direct analysis of the phosphorylation level of the RcsB regulator proteinin vivoby using a Phos-tag retardation gel approach, and was correlated with the phenotype and the expression of motility genes. Data revealed a low level of RcsB phosphorylated form in the wild-type strain and a slight increase of phosphorylation inopgGmutant strains sufficient to induce the pleiotropic phenotype observed.