Effect of plant extracts on Pseudomonas syringae pv. actinidiae gene expression, motility and virulence

Pseudomonas syringae pv. actinidiae (Psa), the causative agent of bacterial canker of kiwi, was firstly isolated in 1992, but the second outbreak dating back to 2008 is responsible for the current pandemics that has led to major economic losses worldwide. Despite the high economic impact of Pseudomonas syringae pv. actinidiae (Psa) on kiwifruit industry, and the intense research efforts, the molecular mechanisms involved in pathogenesis are still not fully characterized. In particular, the precise mechanism of recognition/communication between Psa and its host plant has not been elucidated yet. Our findings show that plant crude extracts or xylem juices of Actinidia deliciosa and A. chinensis were able to increase Psa virulence in comparison with artificial growing media such as Luria Broth or Nutrient agar. The study of gene expression in response to plant-based or artificial growing medium was performed by qPCR and showed a differential expression of many virulence-related genes. In addition, genes related to biofilm formation, motility and inter-cell signaling (e.g. quorum sensing in the broad sense) were differentially expressed according to the type of growing media. Interestingly, the expression of many of the studied genes responded differentially both to plant extracts and Psa cell density. The molecular data were corroborated by phenological studies testing the effect of plant-based medium on biofilm formation, motility and symptom development in micro-cuttings of Actinidia spp. The present study demonstrates the existence of a not yet identified, host-specific group of molecules able to activate the pathogenesis machinery of Psa. Moreover, these molecules could lead to different physiological and molecular behavior depending on Psa population density. Finally, it was also demonstrated that these molecules were differentially present in leaves and xylem sap collected by different Actinidia species.