Proteomic differences between ash species resistant and susceptible to the emerald ash borer

Larvae of the emerald ash borer (EAB; Agrilus planipennis) feed in the phloem of ash (Fraxinus spp.) trees. Since its accidental introduction from Asia, EAB has killed millions of ash trees in the United States and Canada. All NA ash species are susceptible to EAB while an Asian species, Manchurian ash (MA; F. mandshurica), is resistant. Phylogenetic analyses based on ITS and chloroplast gene sequences of all 43 Fraxinus species place NA black ash (BA; F. nigra) and MA in the same clade and section (Fraxinus section Fraxinus), yet BA is highly susceptible to EAB. The contrast between phylogenetic similarity and phenotypic divergence provides an opportunity to investigate the genetic differences between the two species that may have a role in defense/resistance. In order to identify proteins that may be responsible for EAB resistance in MA, we used Differential Gel Electrophoresis (DIGE) to compare the phloem proteomes of resistant MA with susceptible BA. Differentially expressed proteins were sequenced and putative functions were assigned. Hierarchical clustering of 355 proteins with concentrations differing significantly (> 1.5-fold, P < 0.05) between BA and MA phloem revealed the two ash species had almost opposite protein expression patterns. Proteins that may be associated with resistance include enzymes involved in the synthesis of isoflavones and coumarins, oxidative enzymes, a PR-10 protein, and an aspartic protease. Future functional studies are needed to identify what roles these proteins may play in EAB defense. Discovery of resistance related proteins in NA or NA/Asian ash hybrids could accelerate selection of resistant trees for ecosystem restoration/urban forestry.