The <scp>MarR</scp> Family Transcriptional Regulator <scp>EmrR</scp> Negatively Regulates the Type <scp>III</scp> Secretion System ( <scp>T3SS</scp> ) and Positively Modulates Pathogenicity in <i>Dickeya oryzae</i>

The pathogen Dickeya oryzae is responsible for soft rot disease in both monocotyledonous and dicotyledonous plants, resulting in substantial yield losses across a variety of important crops. However, the molecular mechanisms and signalling pathways that regulate bacterial virulence remain poorly understood. In this study, we identified a conserved transcriptional regulator, EmrR (multidrug efflux transporter EmrAB transcriptional repressor), in Dickeya species that plays a critical role in controlling cellular motility. Phenotypic analysis demonstrated that EmrR is also involved in the regulation of cellulase, as well as biofilm formation. emrRdeletion mutants displayed markedly decreased virulence in rice seed germination and potato tubes. Further investigation revealed that EmrR negatively regulates the expression of genes encoding the type III secretion system by decreasing the intracellular concentration of c-di-GMP (bis-(3'-5')-cyclic dimeric guanosine monophosphate) and also modulates the key regulatory gene hrpL within this system. DNase I footprinting confirmed that EmrR binds specifically to a sequence (5'-CATTCGTTACTATGCAGGTTG-3') in the emrR promoter region. This finding highlights EmrR as a conserved and essential regulator of virulence in Dickeya, providing novel insights into the complex signalling and regulatory networks that regulate physiological processes, biochemical mechanisms and virulence development in this pathogen.