A Hierarchical OhrR-EmrR Regulatory Circuit Fine-Tunes the Vfm Quorum- Sensing System and Virulence in Dickeya oryzae.

Rice foot rot, caused by Dickeya oryzae, is a destructive bacterial disease. The Vfm quorum-sensing (QS) system is a key virulence regulator of this pathogen. However, the mechanisms by which it perceives and integrates upstream environmental signals for precise transcriptional control are poorly understood. Here, we identify a cascade regulatory network involving the MarR-family transcription factors OhrR and EmrR. Deletion of ohrR in D. oryzae EC1 significantly reduced the Vfm signalling activity, diminished the production of virulence factors including phytotoxin zeamines and plant cell wall-degrading enzymes, and attenuated its pathogenicity. Molecular analyses revealed that OhrR directly bound to the key promoters in the vfm cluster (PvfmA, PvfmK, PvfmE and PvfmF) and activated their transcription. Intriguingly, OhrR also directly activated the expression of emrR, which encodes another MarR-family transcriptional regulator. In contrast to the activator role of OhrR, EmrR acted as a feedback inhibitor, directly bound to the same vfm promoter regions but repressed their transcription. OhrR-EmrR thus acts as a two-tiered 'activation-inhibition' regulatory circuit in modulation of the Vfm QS system, in which OhrR functions as a master switch for environmental signal response and initiation of the Vfm system, whereas EmrR acts as a precise brake to prevent overexpression of virulence determinants. The findings from this study unveil the fascinating interaction between OhrR-EmrR and the Vfm QS system, and depict a novel mechanism for precise virulence control in D. oryzae via complex, dynamic transcriptional reprogramming.