Battle Amid the Ashes: Phloem Defense Expression of Resistant and Susceptible Ash Species and Associated Physiological Responses of Emerald Ash Borer Larvae
The alien, invasive wood boring beetle known as emerald ash borer (EAB, Agrilus planipennis Fairmaire) is devastating native ash (Fraxinus spp.) populations in North American natural and urban forest settings, with annual damage estimates approaching $3.5 billion. The Asian beetle has also become established near Moscow, Russia and further threatens European ash resources already facing ash dieback. A critical part of landscape-scale, long-term EAB management will be deployment of host resistance traits, which are present in coevolved Asian ash species, but have not yet been characterized. We hypothesize that Asian species are resistant because they are able to recognize and respond to an attack more rapidly and/or deploy specialized, inducible defenses that are not available to non-coevolved species. To test these hypotheses, we have conducted a field experiment in which we are profiling gene transcription, hormone signaling, and phenolic metabolism in phloem and cambial tissue from resistant Asian Manchurian ash (F. mandshurica) and susceptible North American white ash (F. americana), both before and immediately after EAB larval attack.Additionally, we have attempted to identify major categories of ash defenses by examining the developmental and physiological responses of EAB larvae feeding on the two contrasting ash species. We have found that larvae feeding on resistant Manchurian ash exhibited slower development and higher activities of various quinone-protective and anti-oxidant enzymes compared to larvae feeding on susceptible white ash. Additionally, Manchurian ash-fed larvae had lower β- glucosidase activity than white ash-fed larvae. These results suggest that pro-oxidant compounds and phenolic glycosides (e.g. verbascoside) are important components of an effective ash defense response against EAB. Ongoing ash phloem transcription profiling is identifying orthologous gene transcripts that are differentially expressed between the two ash species, either constitutively or following EAB attack. Discovery of ash genes of interest, with support from larval enzymatic studies, and future ash hormone signaling and defense metabolite network analyses, will allow us to identify targets for ash resistance breeding programs. More broadly, this research will significantly improve understanding of the genetic basis of angiosperm tree defenses against wood boring insects.
Showalter, D. N.,
Rigsby, C. M.,
Herms, D. A.,
& Bonello, P.
(2015). Battle Amid the Ashes: Phloem Defense Expression of Resistant and Susceptible Ash Species and Associated Physiological Responses of Emerald Ash Borer Larvae. 5th International Workshop on Genetics of Tree-Parasite Interactions, 32.