[PhD defence] 26/07/2024 - Emma Caullireau: "Induction or restriction of bacterial virulence: the role of time and environment in the outcome of plant-pathogen interaction" (UPR 407 - PV - Plant pathology)

Emma CAULLIREAU will submit her thesis, under joint supervision with the University of Verona, on 26 July 2024 in Italy, on the theme: "Induction or restriction of bacterial virulence: the role of time and the environment in the outcome of plant-pathogen interaction".

Date and place

Oral defense scheduled on Friday 26 July 2024 at 2pm
Location: Strada Le Grazie, 15 37134 VERONA (VR) Italia
Room : ate1.02

Discipline

AGRONOMIC SCIENCES

Laboratory

UPR 407 - PV - Plant pathology

Composition of the jury

Ms CINDY MORRIS	UR0407 Plant Pathology	Thesis co-director
Ms Emilia LóPEZ SOLANILLA	Universidad Politécnica de Madrid	Examiner
Ms Elodie VANDELLE	Università degli Studi di Verona	Thesis co-director
Ms Chiaraluce MORETTI	Università degli studi di Perugia	Examiner

Summary

Crop losses due to pests threaten food safety. To predict and control epidemics, it is essential to understand the interactions between plants, pests and the environment. Among plant pathogens, Pseudomonas syringae is a gram-negative bacterium that causes disease in many of the world's agronomically important crops. P. syringae also serves as a model organism for the study of interactions between plants and pathogens. The bacteria rely on various virulence mechanisms to successfully infect their host, one of the most important being the type III secretion system (SST3), a needle-like structure that injects virulence factors (effectors) into the host cell to overcome the plant's defences. This thesis focused on the study of SST3 activation in response to environmental factors and host signals, addressing little-studied aspects of the time course of events that frame the molecular dialogue between the pathogen and the plant. The study focused on a set of genetically diverse strains of P. syringae. Firstly, the strains were compared in terms of their capacity and speed of activation of their SST3, across a range of temperatures.

To this end, strains were transformed to express the effector AvrB, known to trigger a robust immune response leading to cell death (hypersensitive response, HR) in Arabidopsis thaliana Col-0. Monitoring of HR induction by AvrB-expressing mutants revealed diverse behaviours in terms of temperature dependence or independence for SST3 efficacy, even among closely related strains. The strains were then compared for their growth dynamics at different temperatures in a dozen plant species. Four distinct growth scenarios were identified on the basis of the trend observed during the first two days post-inoculation and whether or not colonisation was ultimately successful. The analyses revealed that an increase in temperature generally accelerates bacterial growth and the appearance of symptoms, and that the efficacy behaviour of SST3 in response to temperature does not fully predict the outcome of the interaction.

In addition, the regulation of SST3-related gene expression by various in vitro signals was studied. The results showed that plant extracts generally enhance SST3 induction in a non-host-specific manner, and highlighted that pH plays a central role in the regulation of SST3 expression. Once again, a wide range of behaviour was observed between strains. We also explored the effect of priming SST3 expression prior to interaction with the host, and observed cases where this treatment was disadvantageous for bacterial virulence. The origin of this detrimental effect in relation to the effector repertoires of each strain was studied. Finally, preliminary studies were carried out to assess the moment of activation of the plant's defence during recognition of the non-self. Further work on these research questions is needed to elucidate whether differences between the timing of perception and activation of SST3 are the main driver of the outcome of the interaction. In conclusion, this work highlights the importance of taking environmental factors into account in the study of plant-pathogen interactions, particularly in the early phases of the interaction. It also recommends exploring the diversity of the P. syringae species complex in order to understand and prevent the risk of new epidemics.

Keywords  Pseudomonas syringae, T3SS, temperature, environmental signals, plant-pathogen interaction