Gene expression profiles of early regulators in ash (Fraxinus spp.)

Ash (Fraxinus spp.) are common trees in North America (NA) in both forest and urban landscapes. Currently, the survival of NA ash is being threatened by the emerald ash borer (EAB; Agrilus planipennis), an exotic, invasive wood-borer from Asia. Since its accidental introduction, millions of white (F. americana), green (F. pennsylvanica), and black (F. nigra), and to a lesser degree, blue (F. quadrangulata) ash trees have been killed. However, in eastern Asia, the incidence of damage to native ash species (e.g. Manchurian ash; F. mandshurica) is limited and appears restricted to stressed trees, suggesting Asian hosts are generally resistant due to their co-evolutionary history with EAB. Despite efforts to contain EAB in NA, it continues to spread at an alarming rate. Given the lack of molecular knowledge of the host species, and to better understand the resistance mechanism(s) deployed by Manchurian ash, we conducted a genomic analysis of non-induced (constitutive) ash phloem using 454 pyrosequencing. We evaluated the constitutive gene expression patterns of early regulators (ERs) in susceptible (green and black) and resistant (Manchurian) ash using quantitative real time PCR. The ERs, including protein kinases (CDPKs), transcription factors (MYB, WRKYs), and a lipoxygenase (LOX3), were previously reported to be involved in plant defense and were differentially expressed in resistant compared to susceptible ash. This study of early regulators in ash trees provides foundational information not only on specific EAB resistance mechanisms but also, more generally, on the basis of interactions between exotic insect pests and their naïve and co-evolved hosts.