Involvement of ralfuranones in the quorum sensing signalling pathway and virulence of Ralstonia solanacearum strain OE1‐1

SUMMARY The soil-borne plant pathogenic Ralstonia solanacearum strain OE1-1 produces and secretes methyl 3-hydroxymyristate (3-OH MAME) as a quorum sensing (QS) signal, which contributes to its virulence. A global virulence regulator, PhcA, functioning through the QS system positively regulates the expression of ralA, which encodes furanone synthase, to produce aryl-furanone secondary metabolites, ralfuranones. A ralfuranone-deficient mutant (ΔralA) is weakly virulent when directly inoculated into tomato xylem vessels. To investigate the functions of ralfuranones, we analysed R. solanacearum transcriptome data generated by RNA sequencing technology. ΔralA expressed phcB, which is associated with 3-OH MAME production, and phcA at levels similar to those in strain OE1-1. Additionally, ΔralA exhibited downregulated expression of more than 90% of the QS-positively regulated genes, and upregulated expression of more than 75% of the QS-negatively regulated genes. These results suggest that ralfuranones affect the QS feedback loop. Ralfuranone supplementation restored the ability of ΔralA cells to aggregate. Additionally, ralfuranones A and B restored the swimming motility of ΔralA to wild-type levels. However, the application of exogenous ralfuranones did not affect the production of the major exopolysaccharide, EPS I, in ΔralA. Quantitative real-time PCR assays revealed that the deletion of ralA results in downregulated expression of vsrAD and vsrBC, which encode a sensor kinase and a response regulator, respectively, in the two-component regulatory systems that influence EPS I production. The application of ralfuranone B restored the expression of these two genes. Overall, our findings indicate that integrated signalling via ralfuranones influences the QS and virulence of R. solanacearum. This article is protected by copyright. All rights reserved.