Molecular signalling inPseudomonas syringaepv.actinidiae

Abstract

Bacterial communication is a complex phenomenon that involves a heterogeneous class of compounds acting as signal molecules and receptors that specifically recognize some of the signal molecules. This communication system occurs between bacteria of the same species, bacteria of different species and also between bacteria and eukaryotes; it affects different physiological features of the bacteria, including biofilm formation, motility and virulence. Pseudomonas syringae pv. actinidiae (Psa), the pathogen responsible of the most severe disease of kiwifruit, lacks a canonical signal molecule but harbours at least three signal receptors. This study aimed to determine i) whether Psa was able to sense its own density population using a yet-to-be characterized cell-density signalling system, ii) whether Psa was sensing the presence of other bacteria that share its ecological niche, and iii) whether any inter-kingdom interaction occurred between Psa and its hosts. Expression of genes related to Psa biofilm production, motility and virulence were analysed by qPCR in Psa samples derived from different culture conditions [high/low density, exogenous addition of synthetic acyl homoserine lactones (AHLs), co-culture with other bacteria and presence of plant-derived medium]. Biofilm formation, motility and growth in planta seemed to be only slightly influenced by Psa cell density. However, the addition of C6-OH-homoserine lactones and C8-OH-homoserine lactones increased both motility and biofilm formation. The presence of AHL-producing bacteria also modified Psa phenotype, suggesting the existence of an interspecies signalling system between kiwifruit epiphytic bacteria. In addition, we showed that crude extracts of Actinidia deliciosa were able to influence biofilm formation, Psa mobility and virulence. This phenomenon was confirmed by qPCR analysis of biofilm, motility and virulence-related gene expression. Non-host plant extracts did not affect Psa behaviour or phenotype.