The first tree to have its full genetic make-up unravelled is a poplar.

The genome of the black cottonwood (Populus trichocarpa) was sequenced in a four-year international project led by US institutions.

The work, reported in Science journal, shows the poplar tree has far less DNA in its cells than humans or other mammals, but twice the number of genes.

Researchers say the new information will be a boon to the understanding of plant biology and evolution.

The forestry industry also expects the genomic data to help it improve the yield and quality of its products, such as bioethanol.

"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 sustainable plantation forestry," said Dr Sam Foster of the US Forest Service.

"This rapid growth coupled with conversion of the lignocellulosic portion of the plant to ethanol has the potential to provide a renewable energy resource along with a reduction of greenhouse gases."

Doubling up

Trees are a dominant feature on planet Earth. Forests cover some 30% of the world's land area, accommodate two-thirds of life on Earth, and are responsible for 90% of the biomass on solid ground.

Poplars themselves are thought to cover more than 75 million hectares worldwide.


The double-stranded DNA molecule is held together by four chemical components called bases

Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G)

Groupings of these "letters" form the "code of life"; there are about 485m base-pairs in the poplar genome wound into 17 distinct bundles, or chromosomes

Written in the DNA are about 45,000 genes which poplar cells use as starting templates to make proteins; these sophisticated molecules build and maintain the organism

The tree was chosen for sequencing because of the comparatively small size of its genome – it has "just" 485 million base-pairs, the DNA "letters" that make up the genetic code.

This is about 50 times fewer base-pairs than a pine tree. On the other hand, the poplar has four times as much DNA as Arabidopsis thaliana, the tiny cress which in 2000 became the first plant to have its genome cracked.

The research team has identified more than 45,000 possible genes – humans and other mammals such as mice and dogs have about 20-25,000 genes. By comparing the poplar information with that of Arabidopsis, the team has been able to show the two organisms split from a common plant ancestor about 100 to 120 million years ago.

The analysis shows that for about 10% of the poplar genes, there are no comparable genetic units in Arabidopsis.

It also reveals the poplar has increased the size of its genome at least twice in its evolutionary history. This expansion would have freed up many duplicated genes to acquire functions important for trees, such as wood formation.

Next 'revolution'

As well as answering specific questions about trees and plants in general, the new information is expected to have industrial implications.

The research team discovered 93 genes of the poplar associated with the production of cellulose, the building blocks of plant cell walls. Cellulose, the most abundant organic material on Earth, can be broken down into sugar, fermented into alcohol, and distilled to produce fuel-quality ethanol.

"Biofuels are not only attractive for their potential to cut reliance on oil imports but also their reduced environmental impact," said Dr Gerald Tuskan, the lead author of the Science report.

Comparisons with Arabidopsis tell scientists about plant evolution

'Little weed' makes history

"Biofuels emit fewer pollutants than fossil fuels such as gasoline. In addition, poplar and related plants are vital managers of atmospheric carbon. Trees store captured carbon dioxide in their leaves, branches, stems, and roots.

"This natural process provides opportunities to improve carbon removal from the air by producing trees that effectively shuttle and store more carbon below ground in their roots and the soil," the Oak Ridge National Laboratory scientist said.

And Dr Stephen DiFazio, from the Eberly College of Arts and Sciences at West Virginia University, added: "The genome sequence is already having a profound impact on forest biotechnology research.

"This is greatly accelerating the discovery of genes that control many different aspects of forest tree biology, and is paving the way for marked improvements in forest plantation productivity that could rival those of the 'green revolution' in agriculture."

The poplar genome project was led by the US Department of Energy's Joint Genome Institute (DOE JGI) and Oak Ridge National Laboratory (ORNL), uniting the efforts of 34 institutions from around the world.

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