Researchers discover fungus in beetle degrades lignin

January 1, 1970

BY Erin Krueger

Web exclusive posted August 22, 2008 at 9:35 a.m. CST

Penn State researchers have discovered a fungus in the gut of Asian longhorned beetles that may prove useful in breaking down biomass for generating biofuels. It has already been known that microbes in insect guts can break down cellulose, but little has been known about how, or if, insects can degrade lignin, which is a natural polymer that helps plants stay upright and protects them from most microbial attacks.

Before the Penn State researchers released their report on Aug. 18, it was thought that insects are unable to extensively break down lignin and so they ate wood that has already degraded or lived close to fungi that could degrade wood for them. However, this theory failed to explain the ability of insects to feed and grow within healthy living trees.

Kelli Hoover, a Penn State associate professor of entomology, along with Scott Geib, a doctoral student in entomology and lead author of the report, and Ming Tien, a Penn State professor of biochemistry and molecular biology completed the research and co-authored the report. The researchers studied two insects, the Asian longhorned beetle and the Pacific dampwood termite. To measure the degree of change in the lignin, they fed pin oak wood to the Asian longhorned beetle and ponderosa pine wood to the termite, which typically eats only dead wood.

The researchers compared the chemical structure of non-degraded wood before and after it had passed through the gut of the insects. Chemical analysis of feces from the two bugs indicated that they are able to alter the chemical structure of lignin by selectively adding or removing certain groups of molecules from the polymer. According to Geib, such alternations make it easier for the insect to break down wood.

"This fungus has genes that then make enzymes," Hoover said. While the researchers have identified the fungus in the gut of the Asian longhorned beetle, they have yet to find one in the gut of the termite.

Chemical changes caused by the fungus are similar to the chemical changes seen with white-rot fungus. "This type of fungus [in the Asian longhorned beetle] is known to cause disease in plants," said Hoover. "But this particular strain appears to be unique. It looks like these insects somehow acquired the fungus to live in their gut and help them break down wood." She explained that the fungi found inside the beetles are more efficient than their free-ranging counterparts. While those fungi can take months or years to break down wood, the gut-borne fungi seem to do it much faster.

However, Geib cautions that while the gut-borne fungus is certainly a key player in degrading wood, it may just be part of a bigger picture. "It is likely that there is an interaction among enzymes produced by the fungus, hundreds of bacteria within the insect gut, and the insect itself," he said. "It is a consortium that is doing the job."

"Getting rid of the lignin barrier and making the cellulose more accessible is the most expensive and environmentally unfriendly part of making ethanol from biomass," Geib said, adding that the team's discovery could lead to the potential development of cheaper and more efficient enzymes for converting wood into ethanol.

The research has been ongoing for approximately five years. According to Tein, the next step in the research will involve taking a closer look at the enzymes produced within the Asian longhorned beetle. The team's research findings appeared in the Aug. 18 issue of Proceedings of the National Academy of Sciences.

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