Exposure to Umbelliferone Reduces Ralstonia solanacearum Biofilm Formation, Transcription of Type III Secretion System Regulators and Effectors and Virulence on Tobacco

<i>Ralstonia solanacearum</i> is one of the most devastating phytopathogens and causes bacterial wilt, which leads to severe economic loss due to its worldwide distribution and broad host range. Certain plant-derived compounds (PDCs) can impair bacterial virulence by suppressing pathogenic factors of <i>R. solanacearum</i>. However, the inhibitory mechanisms of PDCs in bacterial virulence remain largely unknown. In this study, we screened a library of coumarins and derivatives, natural PDCs with fused benzene and α-pyrone rings, for their effects on expression of the type III secretion system (T3SS) of <i>R. solanacearum</i>. Here, we show that umbelliferone (UM), a 7-hydroxycoumarin, suppressed T3SS regulator gene expression through <i>HrpG-HrpB</i> and <i>PrhG-HrpB</i> pathways. UM decreased gene expression of six type III effectors (<i>RipX</i>, <i>RipD</i>, <i>RipP1</i>, <i>RipR</i>, <i>RipTAL</i>, and <i>RipW</i>) of 10 representative effector genes but did not alter T2SS expression. In addition, biofilm formation of <i>R. solanacearum</i> was significantly reduced by UM, though swimming activity was not affected. We then observed that UM suppressed the wilting disease process by reducing colonization and proliferation in tobacco roots and stems. In summary, the findings reveal that UM may serve as a plant-derived inhibitor to manipulate <i>R. solanacearum</i> T3SS and biofilm formation, providing proof of concept that these key virulence factors are potential targets for the integrated control of bacterial wilt.