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.

Insect pathogenicity in plant-beneficial pseudomonads: phylogenetic distribution and comparative genomics

“…This could also be the case for other virulence factors and might involve some of the many regulatory genes that were found to be specific to insecticidal strains. However, besides the Fit toxin (Péchy-Tarr et al, 2008;Péchy-Tarr et al, 2013;Ruffner et al, 2013), no other insecticidal toxin was identified. For most of the 207 genes unique to insecticidal strains, a prediction on the biological function of the encoded product as well as on a possible role during the infection of insects would be very speculative at the present stage.…”

“…How P. protegens and P. chlororaphis overcome these barriers remains unclear. However, an association with insecticidal activity has been demonstrated for a set of genes termed the fit genes (P. fluorescens insecticidal toxin) (Péchy-Tarr et al, 2008;Péchy-Tarr et al, 2013;Ruffner et al, 2013). The unique virulence cassette harbors the fitD gene encoding the proteinaceous Fit toxin as well as regulatory genes and a type I secretion system (Péchy-Tarr et al, 2008;Péchy-Tarr et al, 2013;Kupferschmied et al, 2014).…”

Life in earth – the root microbiome to the rescue?

“…To counter this second threat, and to prevent insect predation of their host plants, many Pseudomonas spp. produce insecticidal molecules such as the Mcf, IPD072Aa and Fit toxins [15 , 45,46]. Meanwhile, to fight against hostile bacteria, oomycetes and fungi, soil Pseudomonas spp.…”

Bacterial hybrid histidine kinases in plant–bacteria interactions

“…Some biocontrol pseudomonads belonging to species P. protegens and Pseudomonas chlororaphis exhibit insecticidal activity against larvae of major agricultural insect pests P echy-Tarr et al, 2008P echy-Tarr et al, , 2013Ruffner et al, 2015). This insecticidal activity depends to a significant extent on the production of a large protein, the Fit toxin.…”