Deciphering the Cross-Talk between Actinidia spp. and Pseudomonas Syringae pv. Actinidiae (Psa).

Pseudomonas syringae pv. actinidiae (Psa) is the causal agent of bacterial canker disease of kiwifruit plants worldwide. The steps of the PhD project followed a logical approach, starting the study of the effect of the interactions first in pure cultures of Pseudomonas syringae pv. actinidiae at different densities, then in more complex systems in which Psa was made interact with synthetic molecules, microbial biocoenosis and the host. Intraspecific communication systems in many bacteria rely on signals synthesis and perception as function of cell density and it is often referred to as “quorum-sensing” (QS). Psa displays three QS-signal receptors, but not the signal synthase gene of N-acyl-homoserine lactones (AHLs). Gene expression by qPCR was analysed at different culture densities in order to evaluate potential effects of the intraspecific communication on pathogenicity. It was established that Psa exploited swarming, swimming and twitching motilities and that the addition of AHLs influenced motility but not the biofilm formation nor virulence in vivo. Analysis of gene expression by qPCR supported in vitro results and revealed that very little resulted density-dependent. It was also evaluated the effect of the bacterial cross-talk considering several microbial species. Those bacteria that share with Psa the same environment on kiwifruit plants primed the gene regulation and the phenotypes such as biofilm production and motility, thus indicating that interspecific signalling may occur and play a crucial role during host colonization. Moreover, Psa phenotypic bioassays and relative gene expression quantitation were characterized in Actinidia spp. plant extracts and xylem saps. In the tested conditions, plant material stimulated biofilm formation, motility and virulence, leading also to high levels of gene expression and disease when inocula grown in plant material was used for infections. Four mutants (psaR1-, psaR3-, algD-, Tr-) were also used to investigate the processes occurring during intraspecific, interspecific and interkingdom communications.