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 now in charge of a large pandemic that has led to major economic losses worldwide. Despite the high economic impact of Pseudomonas syringae pv. actinidiae (Psa) on kiwifruit industry, and the many 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 yet elucidated. Our findings shows that plant crude extracts or xylem juices of Actinidia deliciosa and A. chinensis were able to increase Psa virulent 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. Also genes related with biofilm formation, motility and inter cell signaling (e.g. quorum sensing in the broad sense) were differencially expressed according to the type of growing media. Interestringly, many of the studied gene expression responded differentially depending both to plant extracts and Psa cell density. The molecular data were corroborated by phenological studies aiming to test the effect of plant based medium on biofilm formation, motility and symptom development in microcuttings of Actinidia spp. The effect of other plant extracts such as tobacco, tomato, tea, hazelnut and A. arguta were also studied to determine whether the activation of genes and phenotypes observed is related to a non specific plant molecules. To determine which plant molecules are specifically recognized by PSA, the putative bacterial receptors have been cloned, expressed heterologously, purified and immobilized in chromatographic columns in order to capture their ligands present in those plant extracts specifically activating Psa genes. The presented study demonstrates the existence of a not yet identified, host-specific group of molecules able to active the pathogenesis machinery of Psa. Moreover these molecules could led 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.