Activation of Ralfuranone/Ralstonin Production by Plant Sugars Functions in the Virulence of <i>Ralstonia solanacearum</i>

Abstract

Plant pathogenic bacteria possess sophisticated mechanisms to detect the presence of host plants by sensing host-derived compounds. <i>Ralstonia solanacearum</i>, the causative agent of bacterial wilt on solanaceous plants, employs quorum sensing to control the production of the secondary metabolite ralfuranones/ralstonins, which have been suggested to be involved in virulence. Here, we report that d-galactose and d-glucose, plant sugars, activate the production of ralfuranones/ralstonins in <i>R. solanacearum</i>. As a result, two new derivatives, ralfuranone M (<b>1</b>) and ralstonin C (<b>2</b>), were found in the culture extracts, and their structures were elucidated by spectroscopic and chemical methods. Ralstonin C (<b>2</b>) is a cyclic lipopeptide containing a unique fatty acid, (2<i>S</i>,3<i>S</i>,<i>Z</i>)-3-amino-2-hydroxyicos-13-enoic acid, whereas ralfuranone M (<b>1</b>) has a common aryl-furanone structure with other ralfuranones. d-Galactose and d-glucose activated the expression of the biosynthetic ralfuranone/ralstonin genes and in part became the biosynthetic source of ralfuranones/ralstonins. Ralfuranones and ralstonins were detected from the xylem fluid of the infected tomato plants, and their production-deficient mutants exhibited reduced virulence on tomato and tobacco plants. Taken together, these results suggest that activation of ralfuranone/ralstonin production by host sugars functions in <i>R. solanacearum</i> virulence.