Bacterial virulence induction or restriction: does timing matter to make a plant a good host?

Global crop losses due to pests jeopardize food security. Effective prediction and control of epidemic outbreaks require a thorough understanding of plant-pest-environment interactions. Among the plant pathogens, Pseudomonas syringae is a gram-negative bacterium causing disease on a wide range of agronomically important crops worldwide. Moreover, P. syringae serves as a model organism for studying plant-pathogen interactions. Bacteria rely on various virulence mechanisms to successfully infect their host, one of the most important of which is the Type III Secretion System (T3SS), a needle-like structure that injects virulence factors (effectors, T3SEs) into the host cell to overcome plant defenses. This thesis focused on investigating the activation of the T3SS in response to environmental factors and host signals, addressing the overlooked aspects of the time course of the events that frame the molecular dialogue between the pathogen and the plant. The study involved a set of genetically diverse P. syringae strains. Firstly, the strains were compared for their ability and velocity to activate their T3SS, across a range of temperatures. For this purpose, strains were transformed to express the AvrB effector, known to trigger a robust immune response leading to cell death (hypersensitive response, HR) in Arabidopsis thaliana Col-0. Monitoring HR induction by the AvrB-expressing mutants revealed diverse behaviors in temperature dependency or independency for T3SS efficiency, even among closely related strains. Secondly, the strains were compared for their growth dynamics at different temperatures in about ten plant species. Four distinct growth scenarios were identified based on the trend observed for the first two days post-inoculation and the final success of colonization or not. Analyses revealed that an increase in temperature generally accelerates bacterial growth and symptom appearance, and that T3SS efficiency behaviors in response to temperature do not fully predict the outcome of the interaction. Furthermore, the regulation of T3SS-related gene expression by various in-vitro signals was explored. The findings showed that plant extracts generally enhance T3SS induction in a host-unspecific manner, and highlighted that pH plays a pivotal role in regulating T3SS expression. Once again, a significant diversity of behaviors among the strains was observed. In addition, we also explored the effect of priming T3SS expression before the interaction with the host. Surprisingly, cases where this treatment was disadvantageous for bacterial virulence were observed. The origin of this detrimental effect with regard to the effector repertoire of each strain was investigated. Finally, preliminary studies have been carried out to assess the timing of plant defense activation upon recognition of non-self. Further efforts in these research questions are needed to elucidate whether the differences in timing of perception and that of T3SS activation are the major driver of the interaction outcome. In conclusion, this work emphasizes the importance of deeply considering environmental factors in the study of plant-pathogen interactions, especially in the early phases of the interaction. Furthermore, it advocates for exploring the diversity of the P. syringae species complex to comprehensively understand and prevent the risk of new outbreaks.