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The Type III Secretion System of Biocontrol Pseudomonas fluorescens KD Targets the Phytopathogenic Chromista Pythium ultimum and Promotes Cucumber Protection

Abstract : The type III secretion system (TTSS) is used by Proteobacteria for pathogenic or symbiotic interaction with plant and animal hosts. Recently, TTSS genes thought to originate from the phytopathogen Pseudomonas syringae were evidenced in Pseudomonas fluorescens KD, which protects cucumber from the oomycete Pythium ultimum (kingdom Chromista/Stramenopila). However, it is not known whether the TTSS contributes to plant protection by the bacterium and, if so, whether it targets the plant or the phytopathogen. Inactivation of TTSS gene hrcV following the insertion of an omega cassette strongly reduced the biocontrol activity of the pseudomonad against P. ultimum on cucumber when compared with the wild type, but had no effect on its root-colonization ability. Analysis of a plasmid-based transcriptional hrpJ ′-inaZ reporter fusion revealed that expression in strain KD of the operon containing hrcV was strongly stimulated in vitro and in situ by the oomycete and not by the plant. In vitro, both strain KD and its hrcV mutant reduced the activity level of the pectinase polygalacturonase (a key pathogenicity factor) from P. ultimum, but the reduction was much stronger with the wild type. Together, these results show that the target range of bacterial TTSS is not restricted to plants and animals but also can include members of Chromista/Stramenopila, and suggest that virulence genes acquired horizontally from phytopathogenic bacteria were functionally recycled in biocontrol saprophytic Pseudomonas spp., resulting in enhanced plant protection by the latter.
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https://hal.archives-ouvertes.fr/halsde-00354196
Contributor : Christine Delorme Connect in order to contact the contributor
Submitted on : Monday, January 19, 2009 - 11:30:37 AM
Last modification on : Thursday, September 1, 2022 - 11:08:43 AM

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Fabio Rezzonico, Christian Binder, Geneviève Défago, Yvan Moënne-Loccoz. The Type III Secretion System of Biocontrol Pseudomonas fluorescens KD Targets the Phytopathogenic Chromista Pythium ultimum and Promotes Cucumber Protection. Molecular Plant-Microbe Interactions, 2005, 18 (9), pp.991-1001. ⟨10.1094/MPMI⟩. ⟨halsde-00354196⟩

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