Plant Defenses in Non-Model Systems: A Multi-Year Analysis of the Chemical Defenses Deployed by Green Ash (Fraxinus pennsylvanica) Against Emerald Ash Borer (Agrilus planipennis) and the Changes in Gene Expression Associated with Successful Defenses

The emerald ash borer (Agrilus planipennis, EAB) invasion in North America threatens most North American Fraxinus species, including green ash (F. pennsylvanica), the mostly widely distributed species. A small number of green ash (0.1-1%) both survive for years after all surrounding green ash have been killed by EAB and kill more EAB larvae when challenged in greenhouse studies. We combined untargeted metabolomics and associative transcriptomics with intensive phenotyping of structured populations of progeny from susceptible or lingering ash parents to detect chemotypes and differentially expressed genes associated with defensive responses to EAB.

The work in this dissertation utilizes the phenotypic distribution (0-100% of larval killed) of progeny from lingering-by-lingering crosses and susceptible controls to provide novel insights into effective and ineffective responses deployed by green ash against EAB as well as to provide resources for future research. This investigation has identified select secoiridoids indicative of a metabolite-based response that appears to be ineffective at killing larvae, alkaloids that may have a role in an effective defense response, and a collection of differentially expressed features in the transcriptome association with response to infestation and high larval kill, providing future avenues to explore the effective defense deployed by select green ash against emerald ash borer. This work integrates intensive phenotyping of structured populations with metabolomics and transcriptomics to explore plant insect interactions with a multiyear analysis of metabolome reproducibility. From this work, we propose a two-part model for the North American Fraxinus response to EAB wherein every individual has the capacity to respond to EAB, but only certain trees mount an effective induced defense response that kills enough EAB larvae to prevent lethal damage to the vascular system.