| The emerald ash borer (EAB; Agrilus planipennis Fairmaire) is an invasive phloem-feeding, wood-boring insect that was introduced to southeastern Michigan from Asia and first discovered in the U.S. in 2002. EAB larvae kill ash (Fraxinus spp.) trees by disrupting the flow of water and nutrients throughout the main stem of host plants. While all North American species of ash are susceptible to attack by EAB, an Asian species, Manchurian ash, is resistant. It is hypothesized that Manchurian ash resists EAB colonization by deploying targeted defenses that developed over time by virtue of its co-evolutionary history with the insect. These defenses can be broken down into constitutive and induced, physical and chemical defenses. We investigated constitutive and induced mechanisms of resistance in ash using proteomic and metabolomics approaches to study physiologically altered trees belonging to resistant and susceptible species from diverse phylogenetic backgrounds.;We found significantly increased EAB attack rates below the girdle in susceptible ash species but not in resistant Manchurian ash, while exogenous treatment with a regulatory defense phytohormone, methyl jasmonate (MeJA) provided protection from attack by EAB at the same rate as an insecticide formulated against phloem feeding insects (Chapter 2). Application of MeJA to the outer bark of Manchurian and white ash also had significant effects on phloem phenolic profiles (Chapter 5). Furthemore, four proteins were constitutively expressed at significantly higher levels in the phloem tissues of Manchurian ash than in susceptible black, green, and white ashes and may therefore represent important components of constitutive resistance of Manchurian ash (Chapter 3). Comparisons of constitutive phloem phenolic profiles of six ash species revealed differences in secondary chemistry that were reflective of their phylogenetic relationships and may be associated with constitutive resistance (Chapter 4). Finally, proteomic and metabolomic characterization of oral secretions of 4th instar EAB larvae revealed a highly active molecular environment reflective of the dynamic interaction between ash and EAB (Chapter 6). |
