One hundred years after Hugo De Vries rediscovered Gregor Mendel¿s famed rules of genetic inheritance, researchers have described the first plant genome in full detail. A team published the complete gene sequence of a modest weed known as Arabidopsis thaliana, or the thale cress, last week in Nature. Though the plant itself is of little direct use to humans, the new data will provide an unprecedented means for evaluating the genes that control developmental and physiological processes in many flowering plant species¿including commercially important crops.

Researchers sequenced the first two Arabidopsis chromosomes last year. Now, four years after an international consortium known as the Arabidopsis Genome Initiative was formed, they have completed the other three. On these five chromosomes scientists have identified some 26,000 genes made up of about 125 million base pairs. This is small for a genome. Many crop plants contain genomes that are hundreds or tens of thousands times larger: wheat, for example, has an estimated 15 billion base pairs. But geneticists chose Arabidopsis as their model organism based on its relative genetic simplicity, as well as its short generation time.

With an entire plant genome to work from, biologists can now probe the genes responsible for such basic plant activities as budding, blooming, sleeping and seeding. Knowing where those genes are in Arabidopsis will help researchers to locate their counterparts in plants with much larger genomes. As a result, researchers will be able to build better plants. In fact, genes first identified in Arabidopsis have already offered lessons on how to ripen tomatoes, protect wheat from disease and increase rape seed oil (canola) yields, among other things.

Although the Arabidopsis sequence can itself serve as a guide to other plant genomes, plans are underway to sequence the rice genome and researchers are debating the merits of sequencing maize and other crops. But the money and technology required to unravel these genomes tower over those needed for the simple thale cress. At 400-million base pairs long, the rice genome is one of the more manageable prospects as far as cereal crops are concerned. Yet even that is four times longer than the Arabidopsis genome. Still, Japan¿s Ministry of Agriculture, Forestry and Fisheries (MAFF) is hoping to finish rice by 2004.

The significance of the Arabidopsis sequencing goes beyond agricultural implications. This wealth of new data should also shed light on the evolutionary history of flowering plants. Moreover, considering that many of Arabidopsis¿s genes have human counterparts, knowing the locations and functions of the Arabidopsis genes will enable geneticists to locate the human genes and learn more about various disorders. "Gaining a better understanding of the functions genes perform in cells, whether plant or animal, is going to help us understand how to diagnose and treat diseases in humans," says Richard K. Wilson of Washington University. Indeed, as geneticist Michael Bevan of the John Innes Centre in Norwich, England told Science, the genome sequence from the humble thale cress, "will have as much impact as the human genome."