Model plant mapped

June 23, 2000

The gene sequence of a cress will have a huge impact on agriculture. Clare Sansom reports.

Any day now the draft sequence of the human genome will be completely published. It will be seen as the most significant scientific achievement of the year 2000. But amid the fanfares, another important achievement of genetics will go almost unnoticed.

This summer will also see the completion of the genome sequence of an insignificant looking weed, Arabidopsis thaliana. This achievement may have almost as much significance for agriculture as the human genome will for medicine. Arabidopsis thaliana is the Latin name of thale cress, or mouse-ear cress. It is a tiny member of the cabbage family, although it is not a crop in its own right. It was discovered in Germany in the 16th century by Johannes Thal, whose name it bears. Since the late 19th century it has been studied by botanists and, later, by geneticists.

Mutants were reported as early as 1873; the Arabidopsis Stock Centre in Nottingham now holds over 50,000 different strains. Sean May, director of the stock centre, explains its importance: "Arabidopsis does everything a plant does - it has roots, shoots, leaves and flowers. It is a miniature brassica." Arabidopsis's short life cycle and ease of cultivation make it easy for geneticists to work with.

The sizes of plant genomes are extremely variable. The Arabidopsis genome is only about 5 per cent the size of the human genome and 1 per cent that of some crop plants, such as wheat. The genes are contained within just five chromosomes to our 23. But even so, the biochemistry of Arabidopsis is not much less complex than that of wheat. Most of the extra genetic material in wheat is made up of duplicate genes and "junk DNA". This is one factor that makes Arabidopsis valuable as a "model organism": one of a set of representative species chosen for detailed genetic analysis.

The closest edible relation of Arabidopsis is cress and its more distant relatives include species that are important in the diets of many cultures: cabbages, cauliflower and broccoli. Therefore, geneticists argue, it should be possible to use it to model these crop plants.

Sean May says: "Anyone looking at a gene in a brassica should be able to find a similar gene in Arabidopsis and use it to test a hypothesis."

However, in strictly agronomic terms, brassicas and other "dicots" (dicotyledons, or plants with two seed leaves) pale into insignificance before a few monocotyledonous species: barley, wheat, maize and rice. The only other plant genome anywhere near completion is that of rice.

The biotechnology company Monsanto is hoping to regain public goodwill by making its draft sequence of one variant of rice available to the non-profit International Rice Genome Sequencing Project. A comparison of the genomes of these two model plants, one monocot and one dicot, will yield unambiguous information about the overall genetic similarity of all higher plants.

The sequencing of Arabidopsis took about nine years from the establishment of the Multinational Coordinated Arabidopsis thalania Genome Research Project, and again Monsanto is one of the key players. The company has identified many parts of the genome, but other profit-making and public-sector organisations have worked on other parts so there is no single owner of the intellectual property. The work started slowly, as with the human genome, and most of the publication has taken place in the past two years.

A plant is a complex organism and it is impossible to predict how even a simple modification to one gene will affect the whole plant. It will soon be possible to test hypotheses by laying out all the approximately 25,000 genes of the Arabidopsis genome on a single "DNA chip". These chips are used to investigate patterns of gene expression. Sean May explains: "By comparing expression patterns in normal and transgenic plants, it will be possible to see how a change in any one gene will affect all the plant's biochemical pathways."

However, the benefits of plant molecular genetics are not apparent to everyone. After all, the British public have shown little enthusiasm for genetically modified food. Sir Bob May, the government's chief scientific adviser, is not surprised by the British public's rejection of the GM foods currently available.

During the Bernal Lecture at Birkbeck College, London, recently, he praised their common sense: "There is a possible risk, and people do not want this risk. This is eminently sensible. I do not see a change in attitudes until there are tangible benefits."

Scientifically assisted plant breeding did not start with genetic modification. It is about a century old. Even in 1906, William Bateson, who coined the word "genetics", was able to write: "The study of hybridisation and plant-breeding, from being a speculative pastime to be pursued in the hope that something would turn up has become a developed science."

Plants have been deliberately exposed to radiation in the hope of creating useful mutants for more than 30 years. This procedure changes genes at random; with the newer techniques it is possible to introduce such changes at will.

Most of the first traits to be introduced in plants by genetic modification have been resistance related. Sean May agrees that, as each company introduced seeds that were resistant to its own pesticides, it is hardly surprising that the public viewed the whole enterprise as "tainted by an early rush for profits".

Knowing the precise details of plant genes, their functions, and how they interact, will make it easier for scientists to engineer plants with "tangible benefits". These might be variants of staple foods that can introduce essential micronutrients into Third World diets, like the vitamin A rich "golden rice", or peanuts that are safe for those afflicted by the most serious type of nut allergy.

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