Lab plant makes history

A member of the mustard family: Arabidopsis

By BBC News Online Science Editor Dr David Whitehouse

Researchers have decoded the complete DNA sequence on two chromosomes in a tiny plant that that has become hugely important to science.

Arabidopsis thaliana (thale cress or mouse-eared cress) is used as a laboratory model to test ideas about plant biology in the same way that mice are used to help us learn about the workings of the human body.

A. thaliana, which is in the mustard family, has a total of five chromosomes. These structures are used by cells to bundle up the long DNA molecules that carry all the genetic information required to build and maintain the plant.

The international project which has sequenced the chromosomes has involved over 200 scientists working in 35 laboratories. Their work is reported in the journal Nature.

Convenient model

"Arabidopsis was chosen as the subject of international efforts to sequence an entire plant genome in 1996," said Professor Mike Bevan, of the John Innes Research Centre, Norwich, UK, and co-ordinator of the chromosome four sequencing work.

"This apparently unlikely choice was based on the fact that Arabidopsis has a relatively simple genome, which is, for example, only 5% of the size of the maize genome.

"It has also been the subject of much research because its small size and rapid growth make it a convenient model plant for laboratory work.

"This report describes the first phase of the sequencing. We expect to complete the whole genome by the end of 2000."

Wheat and rice

Many plants have large and complex genomes that would have required more time and resources to determine the precise order of their genetic code.

Arabidopsis has a relatively simple genome

Instead, the scientists decided to begin with the small and relatively simple genome of Arabidopsis and to use the information gained about its genes, and their functions, as a starting point in studying a wide range of other plants.

"This strategy has been a great success," said Professor Bevan. "Many Arabidopsis genes, when transferred into distantly related plants such as wheat and rice are found to perform the same functions."

A variety of genes controlling disease resistance, cold tolerance and sensing of the seasons, plant shape and structure, and the production of important food materials such as vitamins, fats and starch, have been identified in Arabidopsis. These are providing new and fundamental knowledge about important plant processes.

Different conditions

Ultimately, this information may lead to crop plants with enhanced nutritional content, disease resistance and environmental adaptability.

Different strains of Arabidopsis grow successfully in a wide range of climates from northern Sweden to the tropical Cape Verde Islands. It will now be possible to identify and understand the genetic variations underlying the ability of Arabidopsis to adapt to these differing conditions.

The sequencing project has so far completed nearly 70% of the 130-million DNA-base genome of Arabidopsis.

The sequenced regions are thought to have 7,781 genes, approximately 30% of the predicted 26,000 genes that scientists expect to find in the completed genome.

Common ancestor

In contrast, a similar-sized region of the human genome, chromosome 22, has only 545 genes. This demonstrates that sequencing the small genome of Arabidopsis will provide a rich harvest of information.

It will also help scientists to understand the fundamental differences between plants and animals. The two are thought to have diverged from a common ancestor 1,500 million years ago and have since followed different evolutionary paths.

"As for how many genes plants and animals have in common, we don't have a number yet, as no plant genome is completely sequenced," Elliot Meyerowitz, of the California Institute of technology, told BBC News Online.

"But from what we have now, it would seem that half of the genes found in Arabidopsis have some recognisable relative in either bacteria, fungi, or animals. How many relate only to animals has not, as far as I know, been calculated yet."