Quorum sensing in Pseudomonas syringae pv. actinidiae (Psa): an interspecific signalling communication

The quorum sensing (QS) is a complex bacterial intra and inter communication system which is regulated by signalling molecules produced by cell density dependent mechanisms. The QS may regulate different phenotypes such as virulence, motility and biofilm production. The aim of the research was to investigate the existence of a QS system in Pseudomonas syringae pv. actinidiae (Psa). Psa lacks of the canonical QS system based on signal-synthase gene. However, at least three signal receptors (PsaR1, PsaR2, PsaR3) were recently identified. The presented study was conducted in order to clarify the effects of different cell densities on the intraspecific communication in Psa cultures. Therefore, interspecific interaction between Psa and other bacteria associated with the plant host (i.e. in the phyllosphere) or known to have an active QS system were also investigated. A bioinformatic research was conducted for the identification of genes putatively involved in Psa QS system. About 30 genes were selected due to their involvement in influencing specific phenotypes such as virulence, density perception, biofilm formation and motility. The regulation of these genes was analysed by QPCR in Psa samples derived from different culture conditions (i.e. density; co-cultures). Therefore, after a nucleotide sequences blast analysis of several bacterial signal synthase promoters that respond to all Psa signal receptors, homology with the promoter region in front of a transcriptional regulator was found. We suppose that this regulator may be involved in the QS signal transduction. Motility and biofilm formation crucial phenotypes in the early stages of host colonization, are under control of QS circuitries. The experiments conducted demonstrated Psa ability of producing biofilms in LB liquid media in a population density manner. Psa motility resulted particularly enhanced on QS-bacteria supernatants (such as Pseudomonas fluorescens, or an overproducing QS-signals strain of Pseudomonas putida). Quantitative expression of QS-related genes was analysed by QPCR. The expression fold change significantly varied in Psa populations depending on the cell density (from 105 up to 108 cfu/ml). Therefore, the gene expression profiles also changed when Psa cultures were grown in bacterial co-cultured media, remarking the importance of interspecific communication on Psa QS-system and potentially on Psa virulence. In conclusion, from our preliminary studies we observed that QS-phenotypes seem to be influenced not only by the cell density, but also by the microbial community that may share with Psa the same environment. For now, these findings are leading to the hypothesis that Psa has evolved to eavesdrop signals coming from the environment to regulate the complexity of its QS mechanisms. Presumably only by considering a multiple model system, where Psa interacts both with the plant host and other bacterial species, the functionality of Psa QS-system will be fully elucidated.