Cellulose Breakdown

THE DECODED genetic sequence of the soft-rot fungus Trichoderma reesei reveals important clues about how the organism breaks down plant fibers into simple sugars, according to a study carried out by a large team of government, academic, and industrial scientists (Nat. Biotechnol., DOI: 10.1038/nbt1403). The findings should help researchers customize the fungus to produce enzymes that more easily and cost-effectively deconstruct recalcitrant cellulose in plant cell walls, the first step in converting biomass into transportation fuels and chemical building blocks.

T. reesei was originally discovered as it degraded military uniforms and canvas tents in the South Pacific during World War II. Scientists quickly found that the fungus and its variants are prolific producers of cellulose-degrading enzymes, which are now widely used to make textiles and other industrial goods.

To better understand how the dark green fungus produces these enzymes, the team, led by researchers at Los Alamos National Laboratory (LANL) and the Department of Energy's Joint Genome Institute, sequenced the genome of T. reesei and compared it with the genetic sequences of 13 other cellulose-degrading fungi.

What the researchers found was unexpected, notes lead author Diego Martinez, a biologist affiliated with LANL and the University of New Mexico. Compared with other cellulose-degrading fungi, T. reesei has fewer genes dedicated to producing cellulases, hemicellulases, and other degrading enzymes. These genes are clustered together in the genome rather than randomly distributed, as in other fungi genomes.

"We were aware of T. reesei's reputation as a producer of massive quantities of degrading enzymes; however, we were surprised by how few enzyme types it produces," Martinez says. He suggests that the fungus has devised an exceptionally efficient way of secreting the few enzymes it does produce.

"Sequencing the T. reesei genome is a major step toward using renewable feedstocks for the production of fuels and chemicals," notes Joel Cherry, senior director of biofuels biotechnology at enzymes producer Novozymes, which collaborated on the study. Information from the genome will allow scientists to better understand how the fungus operates, making it possible to reduce the current high cost of converting cellulosic biomass to fermentable sugars, he says.