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Nature's defenses: How the Sitka spruce fends off weevil invasion

In the lush green forests of North America’s west coast, a towering tree is silently waging a biological war against a notorious pest. A fascinating study from North Carolina State University reveals how the Sitka spruce tree defends itself against the voracious spruce weevil.

The findings will provide insights for breeding more resistant species of this tree, which could ultimately safeguard its survival against the voracious spruce weevil, Pissodes strobi.

The primary objective of the study was to investigate the genetic mechanisms of natural pest resistance. “We wanted to learn about the genetic basis for natural pest resistance that certain Sitka spruce trees have evolved to prevent insects from feeding on the plant,” explained study first author Professor Justin Whitehill.

Formation of stone cells

The research, which Whitehill began during his time as a postdoctoral researcher at the University of British Columbia, sheds light on an intriguing defense feature of the Sitka spruce – the formation of stone cells.

“The trait we studied in Sitka spruce is a physical defense known as stone cells, which are found in almost all plant species,” Whitehill explained. Stone cells are the culprits behind the gritty texture one feels when eating a pear, revealing their existence in more familiar contexts.

Professor Whitehill emphasized the complexity of stone cell development, noting that thousands of genes contribute to this process. The research team, however, has managed to identify certain key genetics involved in the initial stages of these cells’ development.

Sitka spruce vulnerability 

Distributed along the West Coast from California to Alaska, the Sitka spruce is a large conifer tree. Despite its replacement in North America with other timber species due to susceptibility to the weevil, it maintains a strong presence as a timber species in Europe. 

Professor Whitehill highlighted an interesting historical cause of the tree’s vulnerability. He noted that the primary population cultivated for forestry products on the West Coast originated from an island, where they had never been exposed to the weevil, leading to a heightened susceptibility in these trees.

Some trees built resistance 

But there was a silver lining in this tale of botanical hardship. A group of resistant Sitka spruce trees that developed stone cells was discovered in Canada. These rigid cells uniquely proliferate in less than an inch of the top of budding branches, coincidentally the very region favored by the weevil for feeding.

The stone cells play a critical role in the tree’s defense. “The stone cells slow down the progression of the insect and give time for the resin found in the trees’ bark to coat the insect and make it too sticky to feed more,” explained Professor Whitehill. Their effect is to obstruct the feeding insects and hinder their progress sufficiently to avoid significant damage to the tree.

What the researchers discovered 

The comprehensive analysis led to the identification of almost 1,300 genes that were expressed at higher levels in stone cells. 

The experts also pinpointed a key gene functioning as a “master switch,” activating thousands of other genes known to control the development of thick-walled cells in other plants.

Professor Whitehill succinctly summarized the import of their findings: “This paper lays out a roadmap of the genes involved in stone-cell development. We’re showing it’s strongly controlled by genetics involved in secondary cell walls.”

The execution of this study hinged on the use of a microdissection tool that employs a laser to meticulously slice tissue into thin sections. This technology enabled the researchers to study genes expressed specifically in stone cells during their formation.

Future research plans

With an updated version of this tool now funded for use at NC State, the research team plan to delve into the genetic makeup of the Fraser fir tree, a leading Christmas tree in the United States grown in western North Carolina. 

The size of the Fraser fir’s genome, which is five times bigger than the human genome, has raised hopes of unlocking insights into the tree’s viability, fragrance, and pest resilience.

The application of the laser microdissection technique is viewed by Whitehill as a promising path forward: “We’re using this approach now to look for genes involved in resistance to pathogens and pests, and to understand complex ecological interactions at the genetic level.” 

More about the Sitka spruce tree 

The Sitka spruce (Picea sitchensis) is a species of spruce tree native to the west coast of North America. Its range extends from Alaska, down through British Columbia, Washington, Oregon, and as far south as northern California. It’s named after Sitka, a city in southeast Alaska.

This tree species is notable for its size. It is one of the largest spruce species and among the tallest trees in the world. Some mature Sitka spruce trees can reach heights of over 300 feet (90 meters), although they typically average around 125 to 180 feet (38 to 55 meters) tall. The trunk diameter can be up to 16 feet (5 meters).

The Sitka spruce has a conical shape, with dense branches that droop slightly and are covered with sharp, stiff, needle-like leaves. Its bark is thin and scaly, and its cones are cylindrical and hang down from the branches.

Sitka spruces are adapted to cool, wet climates and grow well in both sun and shade. They’re commonly found in coastal rainforests and near bodies of water, as they require a lot of moisture to thrive.

The wood of the Sitka spruce is light, strong, and flexible, making it highly valued in various industries. It’s often used in the construction of musical instruments, especially soundboards for pianos and tops for guitars, due to its excellent resonant properties. The wood is also used in aircraft construction, particularly in small light aircraft, because of its strength-to-weight ratio.


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