title: Control and host-dependent activation of insect toxin expression in a root-associated biocontrol pseudomonad.
abstract: Pseudomonas fluorescens CHA0 is a root-associated biocontrol agent that suppresses soil-borne fungal diseases of crops. Remarkably, the pseudomonad is also endowed with systemic and oral activity against pest insects which depends on the production of the insecticidal Fit toxin. The toxin gene (fitD) is part of a virulence cassette encoding three regulators (FitF, FitG, FitH) and a type I secretion system (FitABC-E). Immunoassays with a toxin-specific antibody and transcriptional analyses involving fitG and fitH deletion and overexpression mutants identified LysR family regulator FitG and response regulator FitH as activator and repressor, respectively, of Fit toxin and transporter expression. To visualize and quantify toxin expression in single live cells by fluorescence microscopy, we developed reporters which in lieu of the native toxin protein express a fusion of the Fit toxin with red fluorescent mCherry. In a wild-type background, expression of the mCherry-tagged Fit toxin was activated at high levels in insect hosts, i.e. when needed, yet not on plant roots or in batch culture. By contrast, a derepressed fitH mutant expressed the toxin in all conditions. P. fluorescens hence can actively induce insect toxin production in response to the host environment, and FitH and FitG are key regulators in this mechanism.
authors.affiliation: Department of Fundamental Microbiology, University of Lausanne, Lausanne, Switzerland.
authors.forename: Maria
authors.initials: M
authors.lastname: Péchy-Tarr
authors.forename: Naomi
authors.initials: N
authors.lastname: Borel
authors.forename: Peter
authors.initials: P
authors.lastname: Kupferschmied
authors.forename: Vincent
authors.initials: V
authors.lastname: Turner
authors.forename: Olivier
authors.initials: O
authors.lastname: Binggeli
authors.forename: Dragica
authors.initials: D
authors.lastname: Radovanovic
authors.forename: Monika
authors.initials: M
authors.lastname: Maurhofer
authors.forename: Christoph
authors.initials: C
authors.lastname: Keel
mesh_terms: D000818:Animals; D001427:Bacterial Toxins / Q000235:genetics*; D004783:Environmental Microbiology; D015964:Gene Expression Regulation, Bacterial; D007313:Insecta / Q000235:genetics / Q000378:metabolism; D007814:Larva; D009154:Mutation; D010572:Pest Control, Biological; D018517:Plant Roots / Q000382:microbiology*; D011551:Pseudomonas fluorescens / Q000235:genetics* / Q000378:metabolism*
chemical_list: D001427:Bacterial Toxins