ShareThisBy LEE BOWMAN
The black cottonwood seldom inspires poetry, but it was worthy of being the first tree to have its genetic code mapped, scientists say, largely because it grows so rapidly.
"Poplars are already valued for research because you can see results in them relatively fast, but this gives us a new edge in understanding tree biology," said Stefan Jansson, a researcher at Umea University in Sweden and a member of the team of more than 100 scientists who plotted the approximately 45,000 genes of the black cottonwood.
The team published the results of the project Friday in the journal Science.
Although the tree has about 480 million DNA building blocks, that's about 50 times fewer than the units found in a pine tree. But it has four times as many DNA blocks as arabidopsis, a species of mustard plant widely used in research and the first plant to be genetically mapped.
"Under optimal conditions, poplars can add a dozen feet of growth each year and reach maturity in as few as four years, permitting selective breeding for large-scale plantation forestry," said Sam Foster, a biologist with the U.S. Forest Service.
The gene map was produced from samples taken from a female black cottonwood growing on the banks of the Nisqually River in Washington state.
Gerald Tuskan of the Oak Ridge National Laboratory in Tennessee, who headed the project with Daniel Rokhsar of the Joint Genome Institute in Walnut Creek, Calif., said the research so far has already revealed a significant evolutionary history for the poplars and other flowering trees.
For one thing, compared to the mustard plant and rice, the other plant gene-mapped so far, poplars evolve at a relatively slow pace.
The study also shows that the tree has duplicated its stock of genes at least three times, most recently about 65 million years ago. Having double genes allows the second copy of the gene to evolve into a new function, which, it seems, a lot of poplar genes have done.
While some of the copies have been lost over time, the tree has retained others that fend off diseases and insect pests, synthesize the tough cellulose material that makes up the walls of tree cells, transport nutrients and program cells to die _ all of which help the tree grow as quickly as it does.
For instance, the team has already identified 93 genes that are associated with the building of plant cell walls.
That's important, said Carl Douglas of the University of British Columbia, because "if we want to make trees a more efficient source of biofuel, we need to understand the best way to break those walls down" to release the sugars inside the cells, which could then be fermented into ethanol.
Although corn and other grains are easily convertible to ethanol, scientists and government officials want to diversify the sources of such renewable energy. And because trees cover about a third of Earth's landmass, they're favored candidates.
"Biofuels are not only attractive for their potential to cut reliance on oil imports, but also their reduced environmental impact," said Tuskan.
"Fine-tuning plants for biofuels production is one of the keys to making biofuels economically viable and cost-effective," said Raymond Orbach, undersecretary for science at the Department of Energy, which runs both Oak Ridge and the Joint Genome Institute.
"The genome gives us a set of genes to work with to find combinations that offer us the best possible properties in poplars and other hardwoods to incorporate into breeding programs," Douglas said.
On the Net: www.sciencemag.org
