Constitutive Metabolite Profiling of European and Asian Fraxinus with Varying Susceptibility to Ash Dieback

Hymenoscyphus fraxineus is an invasive pathogen native to East Asia, responsible for the widespread mortality of European ash (Fraxinus excelsior) throughout Europe. Asian ash species, which co-evolved with H. fraxineus, are considered more tolerant than European ash. However, within European ash populations, a small proportion of genotypes show low susceptibility to the pathogen. This study sought to characterize the underlying defence mechanisms to H. fraxineus by performing untargeted constitutive metabolomics profiling of phloem and leaf tissue of from thirteen F. excelsior genotypes (nine tolerant and four susceptible) and five genotypes representing three Asian ash species (F. mandshurica, F. platypoda, and F. chinensis). Here we report 57 and 36 compounds associated with lower or higher disease susceptibility, from phloem and leaf tissue, respectively. Flavonoids and coumarins were the main classes of detected compounds. In particular, quercitrin and fraxetin exhibited greater variation among the groups. In phloem tissue, quercitrin and fraxetin were more abundant in tolerant than in susceptible European ash and, lowest in Asian ash species. In leaves, however, quercitrin was highest in Asian ash, followed by tolerant and then susceptible European ash. Other flavonoids, coumarins, and iridoid glycosides also showed variation among groups, with stronger differences in phloem than in leaf tissue. Overall, this study advances our understanding of metabolite composition in Fraxinus species with different co-evolutionary histories and susceptibility to H. fraxineus and demonstrates the potential of untargeted metabolomics for investigating defence-related mechanisms in plant-pathogen interactions.