Bragard, ClaudePeduzzi, ChloéHeylens, SimonSimonHeylens2025-07-012025-07-012025-06-0820252025-06-10https://hdl.handle.net/2078.2/43270Xanthomonas translucens is a bacterial phytopathogen responsible for Bacterial Leaf Streak (BLS), a disease affecting wheat and other cereals. Like many Gram-negative bacteria, X. translucens possesses a Type VI Secretion System (T6SS), a “nanomachine” used to deliver toxic effectors into neighboring cells. While the T6SS is well characterized in other bacterial species, its functional role in X. translucens, particularly in microbial competition within plant-associated environments, remains poorly understood. This master thesis aimed to investigate the diversity and activity of T6SS effectors in X. translucens pv. undulosa strain UPB513 and to assess their ecological relevance both in vitro and in planta. First, a subset of predicted T6SS effectors was cloned and expressed in Escherichia coli, revealing toxicity profiles, especially when effectors were targeted to the periplasm. Then, co-expression of the effector proteins with their cognate immunity proteins successfully neutralized toxicity in some cases, confirming specific effector– immunity (E-I) interactions. These findings support the hypothesis that E-I systems contribute to interbacterial competition and may enhance future antimicrobial strategies research. In planta assays using co-inoculations of different strains of X. translucens with Pseudomonas sivasensis did not provide conclusive proof regarding the role of the T6SS in X. translucens colonization under the tested conditions, due to variability and absence of statistical significance between replicates. Moreover, the impact of P. sivasensis on X. translucens growth should not be excluded. For further interpretations, suggestions are proposed to optimize the experimental protocol, to better assess the ecological interactions between those two species. Finally, broader competition assays against 17 wheat-associated bacterial strains demonstrated strain-dependent inhibitory effects of T6SS on many strains. The gene virD4, associated with the Type IV Secretion System (T4SS), was also found to be essential for competition activity against this bacterial set. Overall, this work reinforces the hypothesis that X. translucens uses its T6SS not primarily for virulence, but rather to shape microbial communities in its environment. These insights offer new perspectives for microbiome-informed biocontrol strategies in cereal cropping systems.Xanthomonas translucensType VI Secretion SystemMicrobiomeT6SSCompetitionEffectorImmunity proteintext::thesis::master thesis