Abstract
Pseudomonas syringae pv. actinidiae (Psa) is an emerging phytopathogen causing bacterial canker disease in kiwifruit plants worldwide. Quorum sensing (QS) gene regulation plays important roles in many different bacterial plant pathogens. In this study we analyzed the presence and possible role of N-acyl homoserine lactone (AHL) quorum sensing in Psa. It was established that Psa does not produce AHLs and that a typical complete LuxI/R QS system is absent in Psa strains. Psa however possesses three putative luxR solos designated here as PsaR1, PsaR2 and PsaR3. PsaR2 belongs to the sub-family of LuxR solos present in many plant associated bacteria (PAB) that binds and responds to yet unknown plant signal molecules. PsaR1 and PsaR3 are highly similar to LuxRs which bind AHLs and are part of the canonical LuxI/R AHL QS systems. Mutation in all the three luxR solos of Psa showed reduction of in planta survival and also showed additive effect if more than one solo was inactivated in double mutants. Gene promoter analysis revealed that the three solos are not auto-regulated and investigated their possible role in several bacterial phenotypes.
Highlights
Quorum sensing (QS) is an intercellular communication system in bacteria that links bacterial cell density to gene expression via the production and detection of signal molecules [1,2]
Purification of acyl homoserine lactones (AHL) was performed on spent supernatants of 11 Psa isolates of Italy both from kiwigreen (A. deliciosa) and kiwigold (A. chinensis); it was established that all the Psa strains did not produce detectable AHL molecules in TLC plates using three different AHL bacterial biosensors which can detect a wide range of structurally different AHLs
These results suggest that AHL mediated LuxI/R type QS is either absent in Psa strains or AHLs could not be detected by the analysis performed in this study as these might be produced in very low quantities or have structures which are not detected by the sensors
Summary
Quorum sensing (QS) is an intercellular communication system in bacteria that links bacterial cell density to gene expression via the production and detection of signal molecules [1,2]. In Gramnegative bacteria, N-acyl homoserine lactones (AHL) signal molecules are most commonly used; they are produced by an AHL synthase, which belongs to the LuxI-protein family and a transcriptional regulator belonging to the LuxR family. The LuxR-family protein forms a complex with the cognate AHL at threshold (‘quorum’) concentration and affects the transcription of target genes [3]. QS LuxRs display surprisingly low homologies (18–25%); 95% share 9 highly conserved amino acid residues [6,25]. Six of these are hydrophobic or aromatic and form the cavity of the AHL-binding domain and the remaining three are in the HTH domain [26]
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