Exploring the Exclusive Isolation of <scp> <i>Pseudomonas syringae</i> </scp> in <i>Peltigera</i> Lichens via Metabolite Analysis and Growth Assays

The specific association of the potentially plant-pathogenic Pseudomonas syringae with Peltigera lichens raises questions about the factors driving this host specificity. To explore this, the metabolic profile of seven lichen species belonging to three genera (Cladonia, Peltigera and Stereocaulon) was analysed using LC-MSMS. In addition, we assessed the growth of P. syringae strains in media supplemented with extracts from each lichen species. This revealed that Peltigera exhibits lower metabolite richness compared to other genera, but shows a higher chemical investment in specific compounds. Growth kinetics showed comparable P. syringae growth across lichen-supplemented media, except for Cladonia arbuscula and Cladonia sp., where the former exhibited lower growth rates. Inhibition assays with lichen extracts showed no inhibition of P. syringae. The lichen metabolome is predominantly composed of lipids and organic acids. Furthermore, specific compounds, such as aminoglycosides, may facilitate P. syringae presence in Peltigera by inhibiting Bacillus subtilis and other antagonists. In addition, compounds absent in Peltigera, like anthracene, might serve as a carbon source inhibitor like Bacillus velezensis.