04 March 2009

International Team Sequences Genome of Brown-Rot Fungus

Findings have implications for biofuel production, wood preservation

A microscopic view of the brown-rot fungus Postia placenta among wood cells.

By Daniel Gorelick
Staff Writer

Washington — In the quest to extract fuel from plants, the sturdy and complex chemical lignin stands in scientists’ way. Now researchers have sequenced the genome of a fungus that extracts energy from plants but leaves lignin intact, uncovering a potential key to making fuel production from plants more cost effective and energy efficient.

Producing ethanol, a biofuel, from plant cellulose is technically challenging and energy intensive. By sequencing the complete genome of Postia placenta, scientists learned the fungus uses a unique strategy, different from that of all known microbes, to efficiently break down cellulose from the protective plant cell wall.

“The microbial world represents a little explored yet bountiful resource for enzymes that can play a central role in the deconstruction of plant biomass — an early step in biofuel production,” said Edward Rubin, director of the Joint Genome Institute at the U.S. Department of Energy, where the genome sequencing was conducted. “The brown-rot Postia placenta’s genome offers us a detailed inventory of the biomass-degrading enzymes that this and other fungi possess.”

“This type of information may empower industrial biotechnologists to devise new strategies to enhance efficiencies and reduce costs associated with biomass conversion for renewable fuels and chemical intermediates,” according to Randy Berka, a scientist at Novozymes Inc. who participated in the study.

“Analysis of the Postia genome also offers unparalleled opportunities for the development of more effective and environmentally preferable wood-preservation techniques,” said Chris Risbrudt, director of the U.S. Forest Service’s Forest Products Laboratory in Wisconsin.

P. placenta attacks wood, leading to rot and producing economic damage. Current treatments for wood rot are not very effective or are environmentally damaging.

The results were reported by a team of scientists working in Austria, Canada, Chile, the Czech Republic, France, Germany, Spain and the United States and published in the February 10 issue of Proceedings of the National Academy of Sciences.

DEGRADING THE CELL WALL

Plants evolved a protective cell wall that resists degradation. The wall, composed of cellulose and lignin (lignocellulose), is a source of energy.

Cellulose can be broken down into simple sugars like glucose, which can then be fermented to produce alcohol, such as ethanol, and used as fuel. Lignin blocks access to cellulose. Current methods of destroying the cell wall to access cellulose use too much energy to be cost effective.

“Fungi like P. placenta, nevertheless, are capable of eating the cellulose without damaging the lignin,” said Antonio Pisabarro, professor of microbiology at the Public University of Navarre in Spain and an author of the study. “Our goal is to find out the modus operandi of these fungi — how they manage to get the cellulose and, in this way, recover the greatest quantity of glucose from the wood in order to use it to produce alcohol.”

“The first step in this study was to elucidate the mechanism Postia uses to degrade cellulose,” said study leader Dan Cullen, a microbiologist at the Forest Products Laboratory.

“Postia has, over its evolution, shed the conventional enzymatic machinery for attacking plant material,” Cullen said. “Instead, the evidence suggests that it utilizes an arsenal of small oxidizing agents that blast through plant cell walls to depolymerize the cellulose. This biological process opens a door to more effective, less-energy intensive and more environmentally sound strategies for lignocellulose deconstruction.”

“For the first time we have been able to compare the genetic blueprints of brown-rot, white-rot and soft-rot fungi, which play a major role in the carbon cycle of our planet,” Berka said. “Such comparisons will increase our understanding of the diverse mechanisms and chemistries involved in lignocellulose degradation.”

Risbrudt cautions that the mechanisms discovered in this study are “a long way from commercialization” but could be a piece that helps solve the puzzle of cost-effective biofuel production.

Apart from the possible applications to biofuel production, Pisabarro highlighted the importance of basic genomics research. “There are aspects of gene regulation that we can study in simple systems such as fungi and obtain answers applicable to ourselves.”

For more information on the P. placenta genome, see the paper in the Proceedings of the National Academy of Sciences.

More information on the Forest Products Laboratory is available on its Web site.