Scientists with Ohio State University's Ohio Agricultural Research and Development Center (OARDC) have received a three-year, $1.4 million grant to continue their groundbreaking work toward the development of a tree that can be used for preservation of ash in natural and urban forests. The U.S. Department of Agriculture's Animal and Plant Health Inspection Service provided the funds.
An accidental import from Asia, EAB is an invasive insect that has killed millions of ash trees in the eastern U.S., the Midwest and Canada. It is so devastating that virtually all ash trees within 31 miles of the initial EAB infestation in southeastern Michigan are now dead. And the tiny beetle is predicted to cause an unprecedented $10-$20 billion in losses to urban forests over the next decade.
The EAB-resistant tree project -- which began in 2003, one year after the first U.S. EAB infestation was confirmed near Detroit -- is a collaboration between Ohio State, Wright State University, Michigan State University and the U.S. Forest Service. It involves an interdisciplinary team of entomologists, plant pathologists, biologists and ecologists.
"This grant will allow us to build upon the powerful framework that we have established with our previous research," says Dan Herms, project leader and a professor of entomology with OARDC and OSU Extension. "We have made important discoveries to help us elucidate the resistance mechanisms at work in trees that are not susceptible to EAB, have identified candidate genes that could confer that resistance, and have made progress overcoming barriers to hybridizing Asian and North America ash tree species."
Indeed, researchers in Herms' team have confirmed that Asian ash tree species (such as Manchurian ash) are naturally resistant to EAB because of their evolutionary history with the insect. In separate experiments, they also found that this resistance trait is shared by saplings propagated from parent trees.
Later on, team scientists working in Pierluigi Bonello's lab at Ohio State and Don Cipollini's lab at Wright State identified certain compounds (secondary metabolites and defense proteins) in the phloem tissue of resistant Asian trees that may be responsible for such resistance. Phloem is the living tissue just under the bark that carries water and nutrients throughout the tree. It is also the part of the tree in which EAB larvae live and feed -- ultimately choking the tree to death.
Using functional genomics, researchers have compared resistant Asian species with susceptible North American species at the genetic level -- identifying in the process several candidate genes that could be used to breed a hybrid tree that still looks like a native North American ash but carries resistance to EAB like the Asian types. They have also identified genes in EAB larvae that potentially allow them to detoxify, or get rid of, defenses or stressors generated by ash trees in response to EAB attacks.
"With this grant, we seek to further explore the functionality of these candidate genes for their specific roles (resistance and detoxification) in ash and EAB," says OARDC entomologist Omprakash Mittapalli, who is using cutting-edge molecular tools, such as 454 pyrosequencing and RNA-Seq, to advance EAB resistance studies. "We will obtain full-length sequences for our candidate resistant genes and develop a gene knock-out strategy that will allow us to decipher their specific functions."
Another success story in the project has been the development of genetic markers to screen small ash trees for resistance traits, which will help speed up the hybridization process. "With these tools, we don't have to wait until the trees are big enough to be attacked by EAB (which takes several years) to know if they are resistant or not," Herms pointed out. "Right away we can decide which trees to keep for our selection process and which ones to discard."
Moreover, through extensive surveys conducted in Michigan and Ohio, members of the team have discovered a small number of healthy native ash trees in heavily EAB-infested areas where more than 99 percent of all ashes have died. These so-called "lingering" ash trees will be studied as part of the new grant to find out what's responsible for their survival.
"Our hope is that we will find signs of resistance to EAB in the progeny of lingering ash, providing a source of rare resistance genes in native trees," says Jennifer Koch, a research biologist with the U.S. Forest Service's Northern Research Station in Delaware, Ohio. Such potential discovery would add to the genetic material already identified in resistant Asian species for hybridization purposes, as well as become another piece of the puzzle in understanding mechanisms of resistance to EAB.
The team began an experimental planting this fall on OARDC's Wooster campus that includes hundreds of Asian and hybrid ash trees. These trees, Herms said, will be evaluated both for their resistance traits and their silvicultural features (adequate growth and aesthetics among them).
The project has also managed to produce its first experimental Asian-North American hybrids. In small containers right now, these trees will be planted next fall and subsequently evaluated for their resistance and silvicultural attributes as well.
"By the end of this three-year grant, we anticipate being at the phase where we have selected and bred resistant genotypes and we are in a position to evaluate them for silvicultural traits," Herms explains. "Then we will pick the best for further breeding -- those trees with the highest resistance to EAB and the most desirable growth features."
In other words, trees that would allow ash to continue to be a key component of North American urban plantings and forests for generations to come.