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When Trees Attack, Fungus Can Parry

24 January 2011 4:42 pm
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DiGustini et al, PNAS Early Edition (2011)

Beetle henchman. The mountain pine beetle (first frame) burrows through the trunks (second frame) of lodgepole pine trees. Aided by the blue stain fungus (third frame), it evades the trees' natural defenses and cuts off their nutrients (last frame).

All across British Columbia, from the Pacific Ocean past the Rocky Mountains, more than 40 million acres of coniferous forest stand brown and desiccated, ravaged by the mountain pine beetle. Over the past decade, the pest has spread virtually unchecked, rupturing ecosystems and maiming British Columbia's timber industry. A new genetic analysis reveals how the beetle's partner in crime—the fungus Grosmannia clavigera—helps the insect elude pine trees' natural defenses, providing it safe passage to the tree's core.

Although the pine beetle gets most of the blame for destroying forests, many researchers think that G. clavigera is the more deadly of the duo. Commonly known as blue stain fungus for the color it leaves on the wood of trees, G. clavigera travels from tree to tree in the beetle's mouth. The fungus, beetle, and pine tree are three competitors in a "never-ending arms race," says molecular biologist Joerg Bohlmann of the University of British Columbia, Vancouver, in Canada.

To learn more about how G. clavigera helps the beetle do its dirty work, Bohlmann and colleagues took the fungus back to the lab. When a beetle burrows through the bark of a pine tree, the tree secretes a toxic resin into the tunnel, trapping—and often killing—the beetle. (This pine resin is an ingredient in Pine-Sol antimicrobial cleaner, which kills most fungi.) But when the researchers treated cultures of G. clavigera with substances from the resin or with extracts from the bark of a tree in the process of repelling a beetle, the fungus continued growing happily. Under attack by these toxins, the fungus switched on a different set of genes. Some of these genes allow it to break the toxins down and even use them as food. The researchers suspect that this ability helps protect the beetle from being poisoned as it burrows through the tree.

"[The fungus] has taken a step further in evolution: It can not only tolerate the resin but can use it as a carbon source for its own benefit," says Bohlmann, whose team reports its findings online today in the Proceedings of the National Academy of Sciences. His interdisciplinary group of researchers, known as the Tria Project, plans to determine what other genomic mechanisms may help the fungus kill trees and interact with the beetle.

So far, the pine beetle epidemic has mostly killed lodgepole pines, which make up the majority of British Columbia's forests. But if the fungus-beetle combo can adjust its counterdefense system to overcome the weaponry of other pines, it could spread much farther. "The potential to spread into Alberta's jack pines would provide the beetle a conduit to the east coast [of Canada]," says forest entomologist Brian Aukema of the University of Minnesota, Twin Cities, who was not involved in the research. He adds that from there, it could easily move south into the United States. "The genome analysis gives us an insight into what's going on under the hood."