Jeff Mulhollem, Pennsylvania State University
Genes in green ash trees that may confer some resistance to attacks by the emerald ash borer express themselves only once the tree detects the invasive beetle’s feeding, according to Penn State researchers.
Knowing this, geneticists may be able to selectively breed trees to strengthen them and perhaps move the resistance response earlier to ward off the beetles’ onslaught, explained John Carlson, professor of molecular genetics.
Green ash, an ecologically and economically valuable tree species native to eastern and central North America, is under severe threat from the rapid invasion of emerald ash borer, a wood-boring beetle native to Asia. Penn State scientists and others are trying to save the species.
Prior observations in a green ash provenance trial—an experiment to see how plants adapt—planted at Penn State in 1978 by Kim Steiner, professor of forest biology and director of The Arboretum at Penn State, and colleagues in the U.S. Forest Service, show that a very small percentage of ash trees survive emerald ash borer infestations, seemingly because their tissues do not nourish and perhaps even sicken the beetles.
“Emerald ash borer probably entered the provenance trial unnoticed around 2008 and trees started showing symptoms of attack by 2012,” Carlson said. “All but eight or nine of the approximately 1,800 trees that Kim planted have subsequently been killed by the beetles.”
Ash trees succumb after adult beetles lay eggs on their bark. When the eggs hatch, the larvae bore into the bark and feed on the transportation tissues of the tree. This disrupts the movement of nutrients and water within the tree, girdling it and causing death.
“To better understand the response of green ash trees to emerald ash borer, we compared gene expression data for resistant versus susceptible green ash genotypes exposed to attack by the beetles,” said Carlson, director of Penn State’s Schatz Center for Tree Molecular Genetics. “By comparing RNA-sequence data from stems attacked by emerald ash borer to multiple tree tissues under other stresses, we could identify differences in the gene expression profiles specific to emerald ash borer resistance.”