Quorum sensing-dependent invasion ofRalstonia solanacearumintoFusarium oxysporumchlamydospores

Abstract

Strains ofRalstonia solanacearumspecies 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 fungusFusarium oxysporum(Fo). ∆phcB, a deletion mutant of QS signal synthase, lost the ability to both produce ralstonins and invadeFochlamydospores. The QS signal methyl 3-hydroxymyristate rescued these disabilities. In contrast, exogenous ralstonin A, while inducingFochlamydospores, 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 toFohyphae 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 ofFochlamydospores. 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.

The phc Quorum-Sensing System in Ralstonia solanacearum Species Complex

“…Ralstonins (aka ralsolamycins) exhibit chlamydospore-inducing activity on a diverse range of fungi and are thought to be key molecules in RSSC parasitism in fungi (7,61,80). Although it is a preprint article at this time, Tsumori et al (86) reported that the phc QS system is important for this phenomenon. phcB lost the ability both to produce ralstonins and to invade Fusarium oxysporum chlamydospores.…”