22 January 2026 | 16:30 - 19:30
Xanthomonas translucens (Xt) is a plant-pathogenic bacterium responsible for the bacterial leaf streak (BLS), a disease threatening cereals worldwide. Among xanthomonads, two contact-dependent systems involved in interbacterial competition have been identified, the Xanthomonadales-like type IV (X-T4SS) and type VI (T6SS) secretion systems.
These molecular weapons enable bacteria to inject toxic effectors into nearby bacterial cells, thereby killing them or inhibiting their growth. Despite their widespread occurrence, both systems remain largely uncharacterised in the Xanthomonas genus.
This thesis investigates the function and evolution of X-T4SS and T6SSs of Xt sensu lato. While X-T4SS and T6SS-i4 loci are mutually exclusive in Xt strains, we demonstrated that both systems play overlapping roles in killing cereal-associated bacteria.
Our comparative genetics and phylogenetic analyses revealed the functional replacement of the ancestral X-T4SS by T6SS-i4 killing strategy in several xanthomonads lineages. We identified a broad diversity of T6SS effectors within the Xt complex, revealing a relationship between the T6SS effector repertoire and the vascular lifestyle. Bacteriolytic activity was confirmed for all Xt core T6SS effectors. Effector-immunity interactions have been predicted and experimentally confirmed.
Finally, we showed that Xt T6SS-i4 can impact the spatial distribution of a co-infected Xt strain in planta. To conclude, X-T4SS and T6SS-i4 are key antibacterial mechanisms promoting survival and providing competitive advantage to Xt in cereal phytobiomes.
Prof. Claude Bragard (UCLouvain) (Supervisor)
Dr. Ralf Koebnik (IRD) (Supervisor)
Prof. Stephan Declerck (UCLouvain) (Chairperson)
Prof. Jacques Mahillon (UCLouvain) (Secretary)
Prof. Jonathan M. Jacobs (OSU)
Dr. Roland Kölliker (ETH Zürich)