Functional Characterisation of the Quorum-Sensing Regulator ExpR_Ecz in Modulation of Dickeya oryzae Motility and...

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.