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PhD defended april 9, 2013
Direction: Alain Sarniguet

Abstract:

The biocontrol rhizobacteria reduces plant disease impact through different mechanisms against pathogens. Their efficacy relies on their rhizospheric competence as their ability to colonize the different surfaces of roots, pathogens and soil aggregates. As many pathogenic bacteria, some of these rhizobacteria have different protein secretion systems like type III (T3SS) and VI (T6SS). Such secretion systems were searched in Pf29Arp genome and the role of T6SS in surface colonization was studied. The strain Pseudomonas fluorescens Pf29Arp reduces the severity of take-all of wheat, a root disease caused by the fungus Gaeumannomyces graminis var. tritici (Ggt). Pf29Arp belongs to the P. brassicacearum-like spp. group, has four T6SS gene clusters and one T3SS gene cluster and does not share most of the antibiotic synthesis genes involved in biopesticide ability of strain from this group. Some major genes encoding operating proteins of T6SS and T3SS are transcribed at the surface of wheat roots and their expression increases in the presence of Ggt. None of the obtained deletion mutants of each T6SS differed from the wild type for the ability to form biofilm. However, gene deletion in T6SS-IV cluster changes the colony morphology on agar medium, promotes phenotypic variation and increases the colonization ability of the mutant in wheat rhizosphere. This result is the first showing an implication of T6SS in the rhizosphere competence of a rhizobacterium.