| Summary: | Pseudomonas syringae, traditionally recognized as a plant pathogen, has a broader ecological role beyond agriculture. In diverse non-agricultural environments, researchers have isolated the bacterium revealing non-pathogenic behavior and significant roles, often associated with its ice nucleation property. The primary objective of this project was to achieve the first isolation of P. syringae from lichens. In Paper I, we detail the isolation process from a specific lichen genus, Peltigera, after screening 10 different lichen genera. The analysis extends to studying the P. syringae population within the same sampling points alongside Peltigera, moss, and tracheophytes when feasible. Following the isolation of this potential plant pathogen, the logical progression led to Paper II, where we explored the fitness and pathology of selected P. syringae strains, predominantly from more aggressive phylogroups, across ten different plant species, primarily crops. The results showed similar pathogenicity in some lichen strains compared with epidemic ones and a similar fitness in 8 out of 10 plant species tested. The final project phase aimed to understand the exclusive isolation of P. syringae from a single genus. Paper III, adopting a metabolomics approach, analyzed differences in the profiles of Peltigera and non-Peltigera lichen genera that may influence P. syringae presence. Peltigera's overall profile shows a higher chemical investment, focusing on the production of certain compounds. Kinetics and inhibition analyses suggest a slightly increased growth of P. syringae in Peltigera media, though not considered decisive. The study also outlines specific compounds present or absent in Peltigera, including the absence of ubiquinones, recognized for their resistance role against P. syringae in various plant species. Furthermore, experiments with P. syringae isolated in Iceland gain significance through collaboration with INRAE (Paper IV). Comparisons with a global database reveal that this Icelandic P. syringae population ...
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