During his Ph.D. work at Ohio State University, Justin Whitehill witnessed the devastation caused by the emerald ash borer and helped to identify the first mechanism of resistance in ash trees against the pest.
“What this really highlighted to me was that there was a deep need for developing genetic solutions to combat some of the challenges that trees face,” he said.
Whitehill is now an assistant professor and Extension specialist in Christmas tree genetics at North Carolina State University. He discussed his research in a presentation sponsored by the Real Christmas Tree Board (RCTB) and Michigan State University.
Whitehill is now working to advance the genetic improvement of Fraser fir primarily but also other species of Christmas trees.
According to Whitehill, Fraser fir accounts for 40% of all Christmas trees sold annually in America. The species is considered the Cadillac of Christmas trees because of its soft needles, good needle retention, strong pliable branches and its long-lasting sweet aroma. Taking into account the other fir species, 70% of Christmas trees are in the fir (Abies) genus.
Most fir species are adapted to cool, moist environments. “When we plant them as Christmas trees, we’re taking them out of that environment, which puts a little bit more stress on them,” Whitehill explained.
Additionally, the impacts of a changing climate – increased temperatures, heavy rainfall events and drought – are creating more challenges for fir trees. Without the necessary cool temperatures which trigger dormancy pre-harvest, the needles of fir can rapidly fall off. Stressed trees are more prone to insect and disease damage. Particularly devastating to Fraser fir is Phytophthora root rot.
Whitehill believes the answer to the challenges facing Christmas tree growers lies in part in genetic improvements. One route is to take trees from a wild population and attempt to domesticate them over time and eventually identify traits in individual trees that have elite performance in specific traits that will vary by grower needs. This process is called gradual population improvement.
While effective, it is a slow, tedious process. First, you need to identify parent trees and grow a genetically identical tree. This can be done by grafting branch tips or scions of each selection onto seedling rootstock to establish a breeding orchard. Control-pollinations are then performed among the selections to produce progeny (children).
The problem is that by the time you grow out an identical parent tree and then its progeny about 16 years have gone by. Another eight years passes before you can identify traits – for example, needle retention – of the progenies’ progeny (grandchildren).
Whitehill and other researchers believe there is a better way. This is to take our knowledge of biology, genetics and computer science (genomics) and employ it to speed up genetic improvements in Christmas trees.
“Instead of having to plant a tree in the field and see it if it grows fast or if it dies, or if it smells good, or if the needles are good or the needles are bad, we can use what we call genomic selection or any of these genomic tools where we basically go in and we look at the DNA of the plant,” Whitehill said.
He is in the process of publishing data from a grant titled the “Christmas Tree Genome Project to Advance Genetic Improvement.” The grant was funded by the RCTB.
The Whitehill Lab Christmas Tree Genetics Program visits western North Carolina Christmas tree country during harvest in November 2023. The Whitehill Lab met with Ashe County Extension agents and NC Department of Ag team members to observe the harvest season. (L – R) Extension Agent Brad Edwards, NCDA Upper Mountain Research Station Superintendent Tracy Taylor, Extension Agent Blake Williams and CTG program Lab Manager Angela Chiang. Photo courtesy of Justin Whitehill
Currently, Whitehill and his colleagues have developed a fragmented blueprint of the Fraser fir genome. According to Whitehill, the Fraser fir genome is 5.5 times larger than the human genome, so while they don’t have a completed genome, it’s a considerable accomplishment.
The goal of mapping the genome is to help learn about specific gene variants that might predispose a tree to specific traits. Whitehill said, “There’s a lot of traits we can start breeding into our trees – things like fast growth. We can identify trees that have form that can reduce labor. We can use this as a barometer to identify trees that are not going to have great needle retention.”
Using the genome, Whitehill hopes to read the DNA of young trees (one to two years old) and select the best group of 100 to 200 trees from 50,000 trees or more. This speeds up the process of genetic improvement because then they can take seeds from those young desirable trees instead of waiting for them to grow to maturity and analyzing their physical traits.
Once a desirable seed has been identified, genetic improvement can then be sped up through a mass propagation procedure called somatic embryogenesis. With somatic embryogenesis, a researcher can take just one seed and produce thousands of genetically identical trees. Somatic embryogenesis, along with other molecular biology tools, could eventually be used to develop superior genetically engineered trees.
(An example of a genetically engineered tree involves inserting genes from wheat into the American chestnut, whose populations have been decimated by American chestnut blight, a fungus. The wheat genes produce a protein that breaks down an acid created by the fungus and allows the chestnuts to survive.)
Rather than taking traits from an unrelated species and inserting that genetic information into the Fraser fir, Whitehill is looking at the potential of using CRISPR-Cas9 technology to improve the Fraser fir. CRISPR-Cas9 (“genetic scissors”) involves modifying precise regions of a genome.
Whitehill’s team is looking at a Japanese fir species (A. firma), which is closely related to Fraser fir and is 100% resistant to Phytophthora root rot disease. Ideally, the team is hoping there is a single gene that they can identify and insert directly into Fraser fir to make resistant trees.
Whitehill said, “If there is a similar gene in Fraser, it would be great if we could simply edit it to be like the Japanese tree. In reality, it’s unlikely that resistance will fall to one single gene, and instead it will be multiple genes. However, we don’t know yet.”
He is seeking additional funding from the USDA Specialty Crop Research Initiative to continue his genetic work. Whitehill said, “The goal of this ultimately is to try to develop genetic and genomic tools that we can implement to help provide more guidance to growers across the United States to improve their economic returns on investment.
“If this does get implemented, then we’re going to see some really big returns for the industry overall and hopefully make Christmas tree growers’ lives a little easier. Genetic improvement holds significant benefits for Christmas tree growers.”
by Sonja Heyck-Merlin
