Arabidopsis thaliana

Arabidopsis thaliana (A-ra-bi-dóp-sis tha-li-á-na; thale cress, mouse-ear cress or Arabidopsis), is a species of Arabidopsis, native to Europe, Asia, and northwestern Africa, from the British Isles south to the Azores and Morocco, east to Japan, and southeast to northern India.^[1]

It is an annual (rarely biennial) plant growing to 5–30 cm (rarely to 50 cm) tall. The leaves form a rosette at the base of the plant, with a few leaves also on the flowering stem. The basal leaves are green to slightly purplish in colour, 1.5–5 cm long and 2–10 mm broad, with an entire to coarsely serrated margin; the stem leaves are smaller, unstalked, usually with an entire margin. Both leaves and stems are thinly hairy, with 1 mm stellate hairs. The flowers are 3 mm in diameter, arranged in a corymb; their structure is that of the typical Brassicacaea. The fruit is a siliqua 5–20 mm long, containing 20–30 seeds. It has a chromosome number of 2n = 10.^[2][3][4][5]

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Model organism

It is widely used as one of the model organisms for studying plant sciences, including genetics and plant development. It plays the role for agricultural sciences that mice and fruit flies (Drosophila) play in human biology.

Although Arabidopsis thaliana has little direct significance for agriculture, it has several advantages that made it the model for understanding the genetic, cellular, and molecular biology of flowering plants.

The small size of its genome made it useful for genetic mapping and sequencing. At about 157 million base pairs^[6] and five chromosomes, it is a small genome for a plant species. It was the first sequenced plant genome, in 2000. Much work has been done to assign functions to its 27,000 genes and the 35,000 proteins they encode. ^[7]

The plant's small size and rapid life cycle are also advantages. For many commonly used laboratory strains, it takes about six weeks from germination to mature seed. The small size of this plant is convenient for cultivation in a small space and it produces many seeds. Further, the selfing nature of this plant assists genetic experiments. Also, as an individual plant can produce several thousand seeds, each of the above criteria leads to Arabidopsis thaliana being valued as a genetic model organism.

Finally, plant transformation in arabidopsis is routine, using Agrobacterium tumefaciens to transfer DNA to the plant genome. The current protocol, termed "floral-dip", does not involve tissue culture or plant regeneration.

History of Arabidopsis as a model organism

The first mutant in arabidopsis was documented by Alexander Braun in 1873. Yet the potential of arabidopsis as a model organism was not documented until 1943. This mutant is now known as AGAMOUS, and the mutated gene was isolated by cloning in 1990.

Friedrich Laibach, known as the "father of Arabidopsis", published the chromosome number of arabidopsis in 1907 and proposed it as a model organism in 1943. His student Erna Reinholz published her thesis on arabidopsis in 1945, describing the first collection of arabidopsis mutants that they generated using x-ray mutagenesis. Laibach continued his important contributions to arabidopsis research by collecting a large number of ecotypes. With the help of Albert Kranz, these were organised into the current ecotype collection of 750 natural accessions of Arabidopsis thaliana from around the world.

In the 1950s and 1960s John Langridge and George Rédei played an important role in establishing arabidopsis as a useful organism for biological laboratory experiments. Rédei wrote several scholarly reviews instrumental in introducing the model to the scientific community.

The start of the arabidopsis research community dates to a newsletter called Arabidopsis Information Service (AIS), established in 1964. The first International Arabidopsis Conference was held in 1965, in Göttingen, Germany.

In the 1980s arabidopsis started to become widely used in plant research laboratories around the world. It was one of several candidates that included maize, petunia and tobacco. The latter two were attractive since they were easily transformable with the then current technologies, while maize was a well established genetic model for plant biology. The breakthrough year for arabidopsis as the preferred model plant came in 1986 when T-DNA mediated transformation was first published and this coincided with the first gene to be cloned and published.

Research

Non-Mendelian inheritance

In 2005, scientists at Purdue University discovered in arabidopsis an alternative to previously known mechanisms of DNA repair, which one scientist called a "parallel path of inheritance". It was observed in mutations of the HOTHEAD gene. Plants mutant in this gene exhibit organ fusion, and pollen can germinate on all plant surfaces, not just the stigma. After spending over a year eliminating simpler explanations, it was indicated that the plants "cached" versions of their ancestors' genetic code going back at least four generations, and used these records as templates to correct the HOTHEAD mutation and other single nucleotide polymorphisms. The initial hypothesis proposed that the record may be RNA-based^[8] Since then, alternative models have been proposed which would explain the phenotype without requiring a new model of inheritance^[9][10] More recently the whole phenomenon is being challenged as a being a simple artifact of pollen contamination.^[11] "When Jacobsen took great pains to isolate the plants, he couldn't reproduce the [reversion] phenomenon", notes Steven Henikoff.^[12] In response to the new finding, Lolle and Pruitt agree that Peng et al. did observe cross-pollination but note that some of their own data, such as double reversions of both mutant genes to the regular form, cannot be explained by cross pollination.^[13]

Light sensing

The photoreceptors phytochrome A, B, C, D and E mediate red light based phototropic response. Understanding the function of these receptors has helped plant biologists understand the signalling cascades that regulate photoperiodism, germination, de-etiolation and shade avoidance in plants.

Arabidopsis was used extensively in the study of the genetic basis of phototropism, chloroplast alignment, and stomatal aperture and other blue light-influenced processes. These traits respond to blue light, which is perceived by the phototropin light receptors. Another blue light receptor, cryptochrome, is also know to function in arabidopsis and is especially important for light entrainment to control the plants circadian rhythms.

Light response was even found in roots, which were thought not to respond to light. While gravitropic response of arabidopsis root organs is their predominant tropic response, specimens treated with mutagens and selected for the absence of gravitropic action showed negative phototropic response to blue or white light, and positive response to red light.

Multigen

This is an ongoing experiment on the International Space Station, it is being preformed by the European Space Agency. The goals are to study the growth and reproduction of plants from seed to seed in microgravity.

See also

• Botany
• Molecular biology
• Nottingham Arabidopsis Stock Centre

References