Quorum sensing-dependent invasion of Ralstonia solanacearum into Fusarium oxysporum chlamydospores

Strains of Ralstonia solanacearum species complex (RSSC), though known as the causative agent of bacterial wilt disease in plants, induce the chlamydospores of many fungi species and invade them through the spores. The lipopeptide ralstonins are the chlamydospore inducers produced by RSSC and are essential for this invasion. However, no mechanistic investigation of this interaction has been conducted. In this study, we report that quorum sensing (QS), which is bacterial cell-cell communication, is important for RSSC to invade the fungus Fusarium oxysporum (Fo). {Delta}phcB, a deletion mutant of QS signal synthase, lost the ability to both produce ralstonins and invade Fo chlamydospores. The QS signal methyl 3-hydroxymyristate rescued these disabilities. In contrast, exogenous ralstonin A, while inducing Fo chlamydospores, failed to rescue the invasive ability. Gene-deletion and -complementation experiments revealed that the QS-dependent production of extracellular polysaccharide (EPS I) is essential for this invasion. The RSSC cells adhered to Fo hyphae and formed biofilms there before inducing chlamydospores. This biofilm formation was not observed in the EPS I- or the ralstonin-deficient mutant. Microscopic analysis showed that RSSC infection resulted in the death of Fo chlamydospores. Altogether, we reported that the RSSC QS system is important for this lethal endoparasitism. Among the factors regulated by the QS system, ralstonins, EPS I, and biofilm are important parasitic factors. SignificanceRSSC strains are Gram-negative bacteria that infect both plants and fungi. The phc QS system of RSSC is important for parasitism in plants because it allows them to invade and increase within the host by causing appropriate system activation at each infection step. In this study, we confirmed not only the importance of ralstonins as Fo chlamydospore inducers, but also that of biofilm formation on the hyphae. In addition to ralstonins, EPS I turned out to be important for biofilm formation. The QS system comprehensively controls the production of these factors in the interaction with Fo. Due to RSSC infection, the cell membranes and organelles of Fo chlamydospores were destroyed, showing that RSSC cells are not endosymbionts but lethal invaders. This result advocates a new QS-dependent mechanism for the process by which a bacterium invades a fungus.