Cell-cell interactionsand LuxR solos in plant associated bacteria

In the last decade, plant microbiome studies have evidenced that bacteria live as part of complex multispecies communities. Plant health heavily depends on its microbiome and cell-cell signaling among beneficial bacteria as well as pathogens and harmless bacteria are likely to be very important for the establishment and maintenance of microbial communities. This research is now a major challenge in microbiology as cell-cell signaling has thus far been mainly studied in the laboratory in pure cultures. In this thesis, three experimental chapters are presented that are focused on the mechanisms of interspecies signaling in plant associated bacteria and how these contribute in creating a stable multispecies community. The first chapter uses the rice foot rot disease caused by Dickeya zeaeas a model to decipher the possible interactions between the pathogen and the commensal members of the microbiome. 16S rRNA gene amplicon-based community profiling showed that the pathogen significantly alters the resident bacterial community and its presence is positively correlated with several bacterial species which are likely to team-up with the pathogen and be involved in the disease process. The second chapter focuses on the role of a sub-family of quorum sensing regulators called LuxR solos in bacterial cell-cell interactions. The distribution, frequency and functional role of the LuxR solos regulators is investigated in the ubiquitous plant associated group of fluorescent Pseudomonas spp.; nine different sub-groups have been identified and the majority of them are likely to respond to novel exogenous (or possibly endogenous) signals, suggesting that these regulators could play a role in inter-species/inter-kingdom signaling. The last experimental chapter investigates a novel cell-cell communication system, in which a LuxR solo responds to and regulates the biosynthesis of a pigment molecule. In summary, this thesis highlights cell-cell signaling in the microbiome and 4the emerging role played by the LuxR solo regulators in providing different ways of bacterial signaling.