Studies on the Inter-Cellular Communication Mechanisms in the Rice Bacterial Pathogens Xanthomonas oryzae pv. oryzae and Pseudomonas fuscovaginae

Ferluga, Sara (2008). Studies on the Inter-Cellular Communication Mechanisms in the Rice Bacterial Pathogens Xanthomonas oryzae pv. oryzae and Pseudomonas fuscovaginae. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f260

Abstract

The most common quorum sensing (QS) system in Gram-negative bacteria employs N-acyl homoserine lactones (AHLs) as signal molecule. AHLs allow bacteria to monitor their cell density being commonly used to synchronize/coordinate the expression of virulence-associated factors in a community. An AHL QS system is most commonly mediated by two proteins belonging to the LuxI-AHL synthase and to LuxR-AHL response regulator protein families. AHLs interact directly at quorum concentration with the cognate LuxR-type protein which then binds at QS target gene promoters affecting their transcription. The purpose of this thesis was to investigate the QS systems based on AHL signal molecules in two important bacterial rice pathogens: Xanthomonas oryzae pv. oryzae (Xoo) and Pseudomonas fuscovaginae. Studies revealed that Xoo does not produce AHLs and does not possess a luxI AHL synthase gene; it does have however an unpaired luxR-homolog gene closely related to QS luxR family genes which was designated oryR. OryR was demonstrated to be involved in inter-kingdom signalling by binding an unknown rice signal molecule (RSM) and affecting bacterial gene expression. The concentration of the RSM increases in rice when it is infected with Xoo possibly meaning that it is involved in a response to pathogen attack. RSM does not bind canonical LuxR-family proteins and is not related to AHLs. It was concluded that OryR is not involved in bacterial QS but in inter-kingdom signalling by recognizing and responding to a molecule present in rice. Studies in P. fuscovaginae revealed that it possesses a typical AHL QS system, designated Pfvl/R, highly conserved within the species and highly similar to the Lasl/R and Ppul/R QS systems present in Pseudomonas aeruginosa and Pseudomonas putida respectively. The Pfvl/R QS system was shown to be involved in virulence, to be important for the hypersensitivity response in non-host plants and for bacterial motility.