Etude des mécanismes d'induction de la pathogénicité chez Streptomyces scabies

Abstract

The research topic investigated during this master thesis focused on the induction of the pathogenic behaviour in Streptomyces scabies. The Streptomyces genus, well-known for its extensive production of valuable bioactive compounds, includes – among many saprophytic species – a few pathogens with S. scabies 87-22 as model species. This Gram-positive bacterium is responsible for the common scab disease affecting potatoes and other tuber plants in the fields, causing substantial economic losses. The signalling pathway leading to the production of the main phytotoxin – namely thaxtomin A – produced by S. scabies upon cello-oligosaccharides (cellobiose and cellotriose) feeding was recently described. However, the model is likely more complex than previously thought, and some key features of this model still need to be confirmed. We decided to investigate the genetic adaptations – either at the gene level or more subtle genetic mutations – responsible for the development of a pathogenic lifestyle in the Streptomyces genus. This work was divided in two parts: • The first aspect that we studied was the role of the beta-glucosidase BglC in the induction pathway of thaxtomin A biosynthesis. The mutant of S. scabies defective in bglC (ΔbglC) displayed two unexpected phenotypes regarding thaxtomin A production, i.e. reduced and increased production levels in the presence and absence of cello-oligosaccharides, respectively. Our study of the cello-oligosaccharides consumption and beta-glucosidase activity in the mutant allowed us to propose the awakening of alternative beta-glucosidase(s) as explanation for the reduced thaxtomin production observed in presence of cello-oligosaccharides. Our virulence bioassays confirmed that the overproduction of thaxtomin A by the bglC null mutant results in hypervirulence in media devoid of cello-oligosaccharides. • The second aspect of our research focused on the impact of point mutations on the DNA-binding ability of CebR – the master repression of thaxtomin A production – when different cis-acting elements are encountered. In this work, we confirmed that single-nucleotide mutations caused a substantial and meaningful decrease of the affinity of CebR. Our results strengthen the idea that cellotriose, which is not the best allosteric effector of CebR, is likely able to remove CebR from its cis-acting binding sites located in the thaxtomin biosynthesis gene cluster and induce the pathogenic lifestyle of S. scabies. Finally, our work brought new insights into the understanding of the mechanisms governing the induction of pathogenicity in S. scabies and opened new horizons for this research project.