David Showalter Exit Seminar

Nov 17, 2016 (All day)

David Showalter PhD Exit Seminar


Dept. of Plant Pathology


Thursday November 17 at 9 am in Kottman Hall room 333 with video link to Selby 203.




The Nature and Role of Host Defenses in Forest Pest Invasions:  A Case Study Using Emerald Ash Borer




Emerald ash borer (EAB) is causing massive ecological and economic damage as it devastates North American ash (Fraxinus spp.) populations. Resistance of its coevolved hosts is considered to limit EAB outbreaks and ash mortality in its native Asia, but an understanding of resistance mechanisms is still developing. To begin, an argument will be made that once bark- and wood-boring insects, as well as canker- and wilt-inducing pathogens, are recognized as newly established and damaging, work should begin to transition forests into “defense-constrained space” by developing host resistance. From this background, the current understanding of host defenses in the EAB-ash system, in regards to both their nature (chemical or biological identity and function) and role (contribution to larval performance and tree survival), will be summarized.  

Recent experimental results showing lower rates of larval survival and development in coevolved Manchurian ash (F. mandshurica) compared to evolutionarily naïve white ash (F. americana) indicate that the higher EAB resistance of Manchurian ash results from phloem traits that decrease larval performance, as well as the previously documented lower oviposition preference. Next, the idea that the pro-oxidant potential of ash phloem may be associated with resistance to EAB larval feeding, will be explored by profiling phloem phenolics and pro-oxidant enzyme activities in response to EAB larval feeding in susceptible white and resistant Manchurian ash. The results of this study have informed a revised hypothesis emphasizing the importance of pro-oxidant-associated peroxidase activities over polyphenol oxidase and β-glucosidase activities in ash resistance to EAB.



Finally, patterns of gene expression in response EAB larval feeding in resistant Manchurian (F. mandshurica) and susceptible white (F. americana) ash. The results provide little support for previous hypothesis of impaired recognition and signaling in susceptible ash species, but also identified a rich complement of transcription factors requiring further analysis. Expression results support the association of proteins in the of major allergen Bet v 1 family with ash resistance to EAB. New mechanisms for further investigation relating to EAB resistance suggested by this study include lectin binding, oxidoreductase activity, LRRTK activity, and terpene synthase activities.  Individual markers and their broader pathways and mechanisms may be used to assist in ongoing EAB-resistance breeding efforts in North American and European ash.