Arabidopsis thaliana

Arabidopsis thaliana, or thale cress, is a small flowering plant in the mustard family. Arabidopsis has no inherent agricultural value and is even considered a weed, but it is one of the favored model organisms of plant geneticists and molecular biologists, and it is the most thoroughly studied plant species at the molecular level. Model organisms have traits that make them attractive and convenient for biologists, who anticipate being able to extend their findings to other, less easily studied species. Arabidopsis is small and easy to grow, allowing researchers to cultivate it with minimal investments in effort and laboratory space. It has a short generation time, taking about six weeks for a seed to grow into a mature plant that produces more seeds. This rapid maturation enables biologists to conduct genetic cross experiments in a relatively short period of time. A single mature plant can produce over 5,000 seeds, another property that makes Arabidopsis convenient for use in genetic analysis.

A Small and Simple Genome

Beyond these basic traits, other attributes of Arabidopsis make it particularly well-suited for analysis by modern molecular genetic methods. Its genome (the amount of DNA in each set of chromosomes) is only about 125 million base pairs. This is small compared to many other plants, and makes searching for particular genes easier in Arabidopsis than in plants with larger genomes. For comparison, the genome sizes for rice (Oryza sativa), wheat (Triticum aestivum), and corn (Zea mays) are 420 million, 16 billion, and 2.5 billion base pairs, respectively. Furthermore, the Arabidopsis genome is contained on just five pairs of chromosomes, making it easier for geneticists to locate specific genes.

Geneticists can carry out crosses (interbreeding two different plant strains) with Arabidopsis by introducing the pollen from one plant to the stigma on another. This mode of reproduction, called outcrossing, is useful for combining mutations from different plants. Alternatively, Arabidopsis can reproduce by a process called selfing, in which an individual plant uses its own pollen to fertilize its ovules.

Selfing, which is not possible in many plants, is very useful for geneticists who wish to study mutations. Most mutations are recessive, which means that they physically manifest themselves (display a phenotype) only when they are present on the chromosomes contributed by both the ovule and the fertilizing pollen. In selfing, heterozygous mutations (which are present on only one of the two sets of chromosomes) will become homozygous (present on both) in one quarter of the progeny produced in this manner.

Arabidopsis and Transformation

Another property that endears Arabidopsis to plant molecular biologists is that it is easily transformed. Transformation is a method for introducing foreign DNA into an organism. This technique is invaluable for studying how genes function and interact with other genes. Biologists usually transform plants by infecting them with genetically engineered varieties of a bacterium, Agrobacterium tumefaciens. In nature, when Agrobacterium infects plants, it inserts certain genes directly into the plant cells, causing a disease called crown gall. The genetically engineered Agrobacterium strains have had their disease-causing genes removed. They can still infect a plant and insert their DNA, but do not cause a disease. To transform plants, the molecular biologist inserts the foreign gene to be studied into Agrobacterium, which will then transfer the gene to a plant that it infects. This transformation technique does not work well on many other plant species, limiting the utility of those plants for molecular genetic analysis.

Arabidopsis researchers also use a variation on the Agrobacterium-mediated transformation technique to introduce mutations in the plant. Studying the effect of a mutation in a particular gene often yields critical information about the normal function of that gene. Because Agrobacterium inserts its transforming DNA randomly in the genome, in many cases the DNA gets inserted directly within a gene sequence. This usually destroys the function of the disrupted gene, resulting in a "knockout mutant." Furthermore, the piece of transformed DNA (T-DNA) that is inserted in the disrupted plant gene can serve as a flag for tracking down the gene by molecular biology methods. Large-scale projects using this T-DNA insertion technique are underway to mutate, identify, and characterize every gene in the Arabidopsis genome.

The First Completely Sequenced Plant Genome

At the end of 2000, an international team of researchers announced that Arabidopsis was the first plant to have its complete genome sequence—the exact order of essentially all 125 million DNA base pairs—determined. The project revealed that Arabidopsis contains over 25,500 genes. By identifying and studying these genes, biologists are learning lessons about plant biology that could provide important advances in agriculture, such as improved crop resistance to pathogens, salt, light stress, and drought, and to the production of more healthful edible oils, pharmaceuticals, and biodegradable plastics.

Arabidopsis research has also produced important discoveries in fundamental plant science, such as the identification of a plant hormone receptor, a clearer understanding of how plants sense and respond to light, and more about the processes that induce plants to form flowers. Arabidopsis research may even have direct relevance to human biology. For example, a photoreceptor protein that regulates circadian rhythms in Arabidopsis was found to share sequence similarity to a retinal photoreceptor, which may perform a similar role in mammals.

Paul J. Muhlrad

Bibliography

Meinke, David W., et al. "Arabidopsis thaliana: A Model Plant for Genome Analysis." Science 283, no. 5389 (1998): 662-681.

Internet Resources

The Arabidopsis Information Resource. <http://www.arabidopsis.org/>.

Nature, Vol. 408, December 2000. (Issue devoted to Arabidopsis thaliana; <http://www.nature.com/genomics/papers/a_thaliana.html>).