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Ralfuranones feedback-regulate the quorum sensing, contributing to virulence of Ralstonia solanacearum strain OE1-1

Yasufumi Hikichi: Kochi University

<div><em>Ralstonia solanacearum </em>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 positively regulates the expression of <em>ralA</em>, which encodes furanone synthase, to produce aryl-furanone secondary metabolites, ralfuranones. A ralfuranone-deficient mutant (Δ<em>ralA</em>) is weakly virulent on tomato plants when directly inoculated into tomato xylem vessels. However, when inoculated using the root dipping, Δ<em>ralA</em> did not systemically infect tomato plants and lost its virulence. To investigate the functions of ralfuranones, we analysed <em>R. solanacearum</em> transcriptome data generated by RNA sequencing technology. Δ<em>ralA</em> 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. Quantitative real-time PCR assays revealed that <em>phcB</em>, which is associated with 3-OH MAME production, and <em>phcA</em> in Δ<em>ralA</em> were expressed at levels similar to those in strain OE1-1. Additionally, Ralfuranone supplementation restored the ability of Δ<em>ralA</em> cells to aggregate dependent on the QS. Ralfuranones A and B restored the swimming motility of Δ<em>ralA</em> to wild-type levels. Additionally, the deletion of <em>ralA</em> resulted in downregulated expression of <em>epsB</em>, which is induced by the QS and involved in major exopolysaccharide, EPS I production, and upregulated expression of <em>fliC</em>, which is suppressed by the QS and involved in swimming motility. Overall, our findings indicate that integrated signalling <em>via</em> ralfuranones influences the QS and virulence of <em>R. </em><em>solanacearum</em>.</div>