Cell-density regulated adhesins contribute to early disease development and adhesion inRalstonia solanacearum

Abstract

Adhesins (adhesive proteins) help bacteria stick to and colonize diverse surfaces and often contribute to virulence. The genome of the bacterial wilt pathogenRalstonia solanacearum(Rs) encodes dozens of putative adhesins, some of which are upregulated during plant pathogenesis. Little is known about the role of these proteins in bacterial wilt disease. During tomato colonization, three putative Rs adhesin genes were upregulated in a ΔphcAquorum sensing mutant that cannot respond to high cell densities:radA(Ralstonia adhesin),rcpA(Ralstonia collagen-like protein), andrcpB. Based on this differential gene expression, we hypothesized that adhesins negatively regulated by PhcA contribute to early disease stages when Rs experiences a low cell density. During root colonizationRsupregulatedrcpAandrcpBrelative to bacteria in the stem at mid-disease, but notradA. Root attachment assays and confocal microscopy withΔrcpA/BandΔradArevealed that all three adhesins helpRsattach to tomato seedling roots. Biofilm assays on abiotic surfaces found thatRsdoes not require RadA, RcpA, or RcpB for interbacterial attachment (cohesion), but these proteins are essential for anchoring aggregates to a surface (adhesion). However,Rsdid not require the adhesins for later disease stagesin planta, including colonization of the root endosphere and stems. Interestingly, all three adhesins were essential for full competitive fitnessin planta. Together, these infection stage-specific assays identified three proteins that contribute to adhesion and the critical first host-pathogen interaction in bacterial wilt disease.