Involvement of ralfuranones in the quorum sensing signalling pathway and virulence of Ralstonia solanacearum strain OE1‐1. (9th March 2017) - Mori, Yuka
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 (Δ ralA ) 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. Δ ralA expressed phcB, which is associated with 3‐OH MAME production, and phcA at levels similar to those in strain OE1‐1. In addition, Δ ralA exhibited down‐regulated expression of more than 90% of the QS positively regulated genes, and up‐regulated 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 Δ ralA cells to aggregate. In addition, ralfuranones A and B restored the swimming motility of Δ ralA to wild‐type levels. However, the application of exogenous ralfuranones did not affect the production of the major exopolysaccharide, EPS I, in Δ ralA . Quantitative real‐time polymerase chain reaction assays revealed that the deletion of ralA results in the down‐regulated 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 .