Transcriptome analysis of Fraxinus spp. through 454 pyrosequencing

Since the accidental introduction of the exotic insect pest, emerald ash borer (EAB; Agrilus planipennis), millions of white, green, blue and black ash trees (Fraxinus spp.) have been killed in North America. However, in eastern Asia, where EAB is indigenous, the incidence of damage to ash (Chinese and Manchurian) is isolated and restricted to stressed trees, suggesting Asian hosts are resistant by virtue of their coevolutionary history. To date, no transcriptomic information is available for any of the Fraxinus spp. This information would aid in understanding EAB interactions with different ash species. Here, a cDNA library constructed from the phloem of different Fraxinus species, including susceptible (North American) and resistant (Asian) ash, was sequenced using the new-generation 454 Titanium platform, yielding ~66 million bp from which 55,000 unigenes were generated. Gene Ontology (GO) analysis for the entire set of unigenes revealed several groups of genes encoding for proteases, pathogenesis related (PR) proteins,and inhibitors, which are thought to play important roles in plant defense. The derived expressed sequence tag (EST) database of Fraxinus spp. not only represents a powerful tool for mining ash genes putatively involved in resistance to EAB but also lays the foundation for future functional studies. Such information will be of great importance in breeding programs and in studies aimed to better understand interactions between forest trees and their exotic and indigenous herbivores.