Associations between phloem microbiota and metabolomes in three North American ash species (Fraxinus spp.) susceptible to emerald ash borer (Agrilus planipennis)

BACKGROUND: Microorganisms play crucial roles in the survival and fitness of their plant and insects hosts, including invasive species. The emerald ash borer (Agrilus planipennis, Fairmaire; EAB) is an invasive insect from Asia. It represents a significant threat to North American forest ecosystems, causing widespread mortality in susceptible native ash (Fraxinus) species. While previous studies have shown differences in specific plant defense metabolites between susceptible North American ash species and their more resistant Asian counterparts, widely targeted metabolite profiles and their interactions with phloem microbiota in response to EAB infestation has thus far received little attention. This study aimed to profile microbial communities associated with ash phloem and EAB larval guts and their relationship to ash phloem metabolites in three native susceptible North American ash species: F. pennsylvanica (green ash), F. nigra (black ash) and F. americana (white ash). RESULTS: Using metabarcoding to characterize the microbial communities associated with the larval gut and host tree phloem and widely targeted metabolomics to establish the first global metabolomic profile of phloem in these ash species, we examined interspecies differences in profiles and associations of ash phloem microbiota and metabolites in relation to EAB infestation. Multivariate analysis revealed that fungal communities were distinct in all ash species, while F. pennsylvanica (green ash) harbored bacterial communities distinct from black ash. Only black ash showed a phloem profile significantly associated with EAB attack symptoms and had the largest number of differentially abundant bacterial taxa. In contrast, larval gut bacterial communities from green ash were distinct from those in other ash species. Green ash displayed a distinct global metabolite profile from the other two species and had the highest number of differentially regulated metabolites, while black ash had the least. Green and white ash shared a strong upregulation of terpenoid compounds, several of which were among compounds significantly associated with microbial communities in green ash phloem or the EAB larval gut. CONCLUSIONS: Our results provide the first comparative analysis of phloem-associated microbial communities and metabolomes across three susceptible North American ash species and their response to EAB. We found that microbiota and metabolites in green ash showed a distinct response to EAB infestation from the other ash species and we identified specific metabolites exhibiting significant correlations with microbial communities in ash phloem or the EAB larval gut. These findings contribute novel insights into interspecies variability in host-associated microbial communities and metabolomes and their response to an invasive insect.