Genetic study on giant conifers reveals clues about pest resistance

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Recent research on a cluster of giant conifers is helping scientists better understand why some trees are able to survive pests, and may help foresters raise trees with the resilience needed to survive in the face of new and emerging challenges to forest life. forest health.

The problem is a cluster of Sitka spruce trees – coastal giants that can live hundreds of years and reach 300 feet tall on the west coast from California to Alaska. Most Sitka trees are susceptible to a hungry pest called the spruce weevil which can stunt tree growth, cause warping, and ultimately kill trees.

However, foresters have discovered a small cluster of Sitka trees with natural resistance to the pest. In a new analysis, a researcher at North Carolina State University conducted a study to identify patterns of gene expression in resistant trees that could allow them to control the spruce weevil. The results, published in the journal Plant-environment interactions, could help researchers develop resistance.

The summary spoke with lead author of the study, Justin Whitehill, an assistant professor in the North Carolina State Department of Forestry and Environmental Resources, about the findings. Whitehill also heads the North Carolina State Christmas Tree Genetics Program, which works to identify genes and traits that can be used to create genetically improved Christmas trees in North Carolina.

AT: How common is natural resistance to this pest in Sitka spruce?

Whitehill: Sitka Spruce can live hundreds of years on the West Coast to grow into giant trees. Some of the largest spruce trees are over 300 feet tall and are nearly 1,000 years old. Because they have such long generation times, it is difficult for them to quickly develop resistance to pests and diseases.

In terms of resistance, forest geneticists working in British Columbia, Canada noticed a small population of Sitka spruce growing quite well, even though it was in areas heavily infested with weevils. Geneticists have worked to integrate these trees into their breeding program in order to develop resistant varieties. This work has been going on for decades. It is only recently that we have started to understand how trees are able to resist the attacks of this pest.

AT: What do we already know about the natural resistance of these trees?

Whitehill: In the early 2010s, a specialized cell type called stone cells was first recognized as a contributing factor to resistance against the spruce weevil. If you’ve ever eaten a pear, the grainy texture is due to the presence of stone cells. In Sitka spruce, they are very lignified, which means they are very similar to the cells of wood, which makes them stiff or hard. One of the first clues some people noticed when trying to propagate trees was that these trees were much harder to cut. As a result, their knives wore out faster than on sensitive trees. The areas people were cutting happened to be the same place the weevil tends to hang out and end its life cycle. Ultimately, this area was filled with stone cells.

The abstract: What did genetic analysis show about resistance?

Whitehill: The big story is that the resistant tree is already well defended against the weevil long before an attack even takes place. It seems that the genes are already put in the “on” position, so the defenses are still there, and the main defense is the presence of many calculus cells.

Another interesting finding is that we identified a set of genes belonging to fungi that were constantly present in resistant trees and susceptible trees actively attacked by the weevil. We believe this means that resistant trees and susceptible trees attacked by the weevil harbor a natural fungal community. This hitherto unexplored interaction could have negative impacts on the survival of weevils.

There may be other complex ecological interactions that need to be further explored. Through the use of new molecular tools, we are now beginning to paint a more complete picture of this complex system.

AT: Was there something in the Sitka spruce genome that made these trees difficult to study?

Whitehill: One of the challenges of studying the genomes of the spruce, or any conifer, is that their genome is about seven to eight times the size of the human genome, on average, with about 21 to 22 billion pairs of genetic code bases. Not only do conifers have huge genomes, but they also have these extremely long pieces of genetic code that repeat themselves multiple times throughout the genome. This makes it difficult to sequence the gene segments.

Gene space is also not well assessed and defined in conifers. If you think of the human genome, the Human Genome Project was started in 1990 and cost billions of dollars before a finalized Human Genome Project was published in 2003. In the case of Sitka Spruce, we had to base our findings on the next best-studied organism to understand what the genes potentially did. Most of the plant genes that have been functionally validated are found in a small mustard plant called Arabidopsis. We base the function of genes in conifers on Arabidopsis Genoa.

AT: Why is it important to study this particular spruce?

Whitehill: Sitka spruce is a coastal species found primarily in the Pacific Northwest from Canada to northern California and throughout the coast of Alaska. Economically, it has been planted as a source of wood all over the coast of British Columbia, but because of this pest, its use in planting has been reduced for the past 30 years or more.

The tree I used in my study comes from a population of trees that are very susceptible to the weevil. However, these trees grow very fast and have a very good quality of wood. Due to their rapid growth and good quality timber, these trees were brought back to England in the 1800s and now form the basis of the English forest industry.

The ultimate goal of this project, and my plan for the NC State Christmas Tree Genetics program, is to apply advanced genomic techniques and tools to understand how trees tolerate stress, identify the underlying mechanisms that support resistance. and, finally, to use new genomic tools. approaches to rapidly develop pest-resistant trees and ensure their long-term survival in the face of increasing forest health challenges.


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