Constitutive phloem proteins, phenolics, and lignin in ash species resistant and susceptible to the emerald ash borer (Agrilus planipennis)

The emerald ash borer has the potential to eliminate ash tree species (Fraxinus spp.) from North American forests and urban environments. Current attempts to quarantine the EAB to already infested regions have failed and the wave of EAB induced destruction continues to spread. The literature contains relatively few studies addressing mechanisms of resistance of deciduous trees to wood-boring beetles. Several studies have hypothesized that both constitutive traits and feeding-induced responses in phloem tissue are key to resistance. Constitutive phloem traits associated with resistance to herbivory fall into two major categories: 1) the presence of low and high-molecular weight secondary metabolites and defense related proteins and 2) physical barriers (i.e. lignin) inhibiting spread of the invader. This study assessed the biochemical and molecular constitutive phloem differences of five susceptible and two resistant ash species. Protein profiles were compared using differential in-gel electrophoresis (DIGE) while phenolics were measured using HPLC. Constitutive lignin was quantified spectrophotometrically. The results of this study may lead to the development of molecular markers in North American ash or North American/Asian ash hybrids that could accelerate selection of resistant individuals for use in ecosystem restoration, urban forestry, and commercial exploitation.