Oral insecticidal activity of plant‐associated pseudomonads

Abstract

Biocontrol pseudomonads are most known to protect plants from fungal diseases and to increase plant yield, while intriguing aspects on insecticidal activity have been discovered only recently. Here, we demonstrate that Fit toxin producing pseudomonads, in contrast to a naturally Fit-deficient strain, exhibit potent oral activity against larvae of Spodoptera littoralis, Heliothis virescens and Plutella xylostella, all major insect pests of agricultural crops. Spraying plant leaves with suspensions containing only 1000 Pseudomonas cells per ml was sufficient to kill 70-80% of Spodoptera and Heliothis larvae. Monitoring survival kinetics and bacterial titres in parallel, we demonstrate that Pseudomonas fluorescens CHA0 and Pseudomonas chlororaphis PCL1391, two bacteria harbouring the Fit gene cluster colonize and kill insects via oral infection. Using Fit mutants of CHA0 and PCL1391, we show that production of the Fit toxin contributes substantially to oral insecticidal activity. Furthermore, the global regulator GacA is required for full insecticidal activity. Our findings demonstrate the lethal oral activity of two root-colonizing pseudomonads so far known as potent antagonists of fungal plant pathogens. This adds insecticidal activity to the existing biocontrol repertoire of these bacteria and opens new perspectives for applications in crop pest control and in research on their ecological behaviour.

Heavy metal tolerant Pseudomonas protegens isolates from agricultural well water in northeastern Algeria with plant growth promoting, insecticidal and antifungal activities

“…Currently, genomic analysis of the two isolates is in progress, which will demonstrate which biosynthetic gene clusters for antibiotic production and other antagonistic activities are present in the two well water isolates from Algeria. Like the reference strain P. protegens CHA0 T and related strains [41,47], the isolates S4LiBe and S5LiBe also harbor insecticidal activities, as could be demonstrated by the positive Galleria mellonella injection assay and the presence of the fitD gene (Figure 3). Since the infection and growth within insect larvae constitutes a second growth cycle of these bacteria -apart from plant roots , it may be hypothesized, that these types of bacteria were enriched in the well water also harboring insect larvae.…”

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

“…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). Nevertheless, fitD deletion mutants retain substantial toxicity, indicating the existence of additional virulence factors (Péchy-Tarr et al, 2008;Ruffner et al, 2013). Mutational analyses provide evidence that some of them are regulated by the global regulator GacA (Olcott et al, 2010;Ruffner et al, 2013).…”

“…When injected into the hemocoel of Galleria mellonella or Manduca sexta larvae, they rapidly multiply and cause larval death within a few hours (Péchy-Tarr et al, 2008). Ecologically more relevant, these strains are also able to infect and kill insect larvae, such as Drosophila melanogaster and the agricultural pests Spodoptera littoralis or Plutella xylostella, after oral uptake (Olcott et al, 2010;Ruffner et al, 2013). Oral insecticidal activity is considered a rare trait amongst bacteria and requires specific mechanisms to cope with host immune responses and to breach the gut epithelium in order to access the hemocoel (Vallet-Gely et al, 2008;Opota et al, 2011;Herren and Lemaitre, 2012).…”

“…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).…”

“…On top of plant-beneficial activity, genomics has revealed unexpected and broader ecological versatility for these bacteria (Paulsen et al, 2005;Loper et al, 2012). Three of the best-characterized biocontrol strains, Pseudomonas protegens strains CHA0 and Pf-5 and Pseudomonas chlororaphis PCL1391, were shown to have potent insecticidal activity (Péchy-Tarr et al, 2008;Ruffner et al, 2013). When injected into the hemocoel of Galleria mellonella or Manduca sexta larvae, they rapidly multiply and cause larval death within a few hours (Péchy-Tarr et al, 2008).…”