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Poster

Session: Session 2 Presenting Authors at Posters (Evens)

P-330 - Evolutionary switch from T4SS- to T6SS-mediated bacterial killing strategy in xanthomonads

Thursday, July 17, 2025
13:30 - 15:15  
Location: Confex Hall 3
Plant microbiota functions and intermicrobial interactions

Diverse antibacterial weapons have arisen in bacteria over evolution. Two distinct contact-dependent competitive strategies have been identified within the Xanthomonas genus, the type IV (X-T4SS) and the type VI (T6SS) secretion systems. These systems deliver toxic effectors into rival bacterial cells, promoting survival within their ecological niches. However, the ecological relevance for the evolution of two functionally convergent secretion systems in xanthomonads remains unclear. Here, we investigated their evolution and function in xanthomonads including the Bacterial Leaf Streak pathogen, Xanthomonas translucens (Xt). We show that X-T4SS and T6SS-i4 loci display mutually exclusive distributions in Xt strains. Using genetic and fluorescence approaches, we demonstrated that T6SS-i4 and X-T4SS in Xt play overlapping roles in killing of neighboring bacteria from cereals. Antibacterial T6SS-i4 activity was further observed by real-time confocal microscopy. From comparative genetic and phylogenetic analyses, we propose an evolutionary model in which X-T4SS evolved first and was subject to degradation and loss events, followed by gain of horizontally transferred T6SS-i4 among Xanthomonas lineages. Comparison of X-T4SS and T6SS-i4 effector arsenals in xanthomonads further support those results. Overall, this research offers new insights into the evolutionary trajectories of X-T4SS and T6SS, and how bacteria adapt their competitive strategies to thrive in dynamic environments.