Functional Characterisation of the Quorum‐Sensing Regulator ExpR _Ecz in Modulation of Dickeya oryza

ABSTRACT

The AHL (acyl‐homoserine lactone)‐mediated quorum‐sensing (QS) regulatory mechanism is ubiquitously distributed in gram‐negative pathogens. In Dickeya oryzae , N ‐(3‐oxo‐hexanoyl)‐L‐homoserine lactone (OHHL), synthesised by ExpI _Ecz , regulates bacterial swimming motility and virulence. However, the role of its putative cognate receptor ExpR _Ecz has not yet been characterised. In this study, we showed that deletion of expR _Ecz alone did not cause significant changes in various phenotypes associated with D. oryzae physiology and pathogenesis. However, in the absence of a functional OHHL synthase ExpI _Ecz , ExpR _Ecz was a potent repressor modulating the production of cellulases, polygalacturonases and zeamines, biofilm formation and pathogenicity of D. oryzae EC1, and it was a positive regulator modulating bacterial swimming motility. Further analysis showed that the regulatory activity of ExpR _Ecz was abolished by OHHL, and that the QS signal might relieve the regulation of ExpR _Ecz by interacting with two conserved AHL‐binding residues of ExpR _Ecz . Moreover, we found that ExpR _Ecz repressed its own expression through a region in the 5′ noncoding region of expR _Ecz that lacks a canonical lux box. These findings suggest that the mechanism may enable D. oryzae to relieve ExpR _Ecz suppression and initiate infection at high cell density when the QS signal reaches a threshold level.