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

Abstract

AbstractAdhesins (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 putativeRsadhesin genes were upregulated in a ΔphcAquorum sensing mutant that cannot respond to high cell densities:radA(Ralstoniaadhesin),rcpA(Ralstoniacollagen-likeprotein), andrcpB. Based on this differential gene expression, we hypothesized that adhesins repressed by PhcA contribute to early disease stages whenRsexperiences a low cell density. During root colonizationRsupregulatedrcpAandrcpB, but notradA, relative to bacteria in the stem at mid-disease. Root attachment assays and confocal microscopy with ΔrcpA/Band ΔradArevealed 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.ImportanceEvery microbe must balance its need to attach to surfaces with the biological imperative to move and spread. The high-impact plant pathogenic bacteriumRalstonia solanacearumcan stick to biotic and abiotic substrates, presumably using some of the dozens of putative adhesins encoded in its genome. We confirmed the functions and identified the biological roles of several afimbrial adhesins. By assaying the competitive fitness and the success of adhesin mutants in three individual plant compartments, we identified the specific disease stages and host tissues where three previously cryptic adhesins contribute to bacterial success. Combined with tissue-specific regulatory data, this work indicates thatR. solanacearumdeploys distinct adhesins that help it succeed at different stages of plant pathogenesis.Research AreasPlant Microbiology, Host-Microbial Interactions, Microbial Pathogenesis