Quaternary ammonium cationic surfactants target Vfm quorum sensing to suppress the virulence of rice pathogen Dickeya oryzae.

The rice foot rot disease caused by Dickeya oryzae is an important bacterial disease that could cause tremendous economic losses. The virulence factor modulating cluster (Vfm) quorum sensing (QS) system, a major virulence regulatory mechanism conserved in the Dickeya genus, controls the production of zeamines and various extracellular cell wall degradation enzymes in D. oryzae. In this study, we targeted the Vfm QS system of D. oryzae strain EC1 to screen for chemical compounds capable of attenuating its virulence. Using a high-throughput vfmE-lux reporter assay, we screened over 2000 compounds and identified a series of long aliphatic chain compounds - quaternary ammonium cationic surfactants (QACs) that substantially reduced the vfmE promoter activity. Further analysis revealed that QACs blocked strain EC1 response to Vfm signals. Quantitative PCR results showed that the transcriptional expression of the vfm gene cluster and its downstream virulence genes, including those encoding production of phytotoxin zeamines and CWDEs were significantly downregulated. Consistent with their role in blocking Vfm signaling, QACs markedly inhibited the production of virulence determinant zeamines and suppressed the secretion of cellulase and pectinase in strain EC1. Infection assays demonstrated that these compounds substantially reduced the virulence of D. oryzae EC1 toward rice seeds and alleviated the soft rot symptoms in potato tubers. This is the first report that QACs are potent inhibitors of the D. oryzae Vfm QS system, illustrating a promising QS-targeted approach for the control of bacterial rice foot rot disease.