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Understanding the molecular mechanisms underpinning the ecological success of plant pathogens is critical to develop strategies for controlling diseases and protecting of crops. Recent observations have shown that plant pathogenic bacteria are not just interacting with the plant itself, but with a variety of potential predators present in the soil, such as nematodes, aphids, larvae and amoebae. Bacteria are therefore under strong selective pressure to avoid or survive predation and I hypothesise that bacteria have evolved mechanisms to escape predation. To identify the gene systems that contribute to the ecological success of P. syringae, with a specific focus on anti-predation and pathogenicity mechanisms I have used a Rapid Virulence Annotation (RVA) screening. Three cosmid libraries for P. syringae pv. aesculi, pv. tomato and pv. phaseolicola comprising approximately 2000 cosmids each were screened and a number of potential genes involved in bacterial survival identified. These included also genes encoding for Type 6 secretion system, hemolysins and biofilm formation, motility and adhesion. A comparison of the genes conferring resistance within other P. syringae strains showed differences in the organization of these clusters, indicating evolutionary changes. These data provide an important understanding of how bacteria cope with different biotic pressures and to give insight to the function of the genes and their role in ecological success. Objective of this thesis was also to characterize virulence mechanisms of P. syringae pv. aesculi the agent of the bleeding canker disease of Horse Chestnut. In this thesis I characterized the origin of this pathogen, its evolution and the factors that allow it to cause disease, focussing on the Type 3 Secretion System. hopAB1 gene was identified to be the main determinant in virulence both in N. benthamiana and A. hippocastanum. Analysis and expression in Pae of hopAB1 orthologues allowed the definition of two amino acids responsible successful virulence. |
