Arabidopsis Thaliana


Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, Arabidopsis is popular as a model organism for understanding the molecular biology of plants. Its genome is one of the smallest plant genomes that was completely sequenced. Its common name is mouse-ear cress.

Kingdom: Plantae

Angiosperms

Eudicots

Rosids

Order: Brassicales

Family: Brassicaceae

Genus: Arabidopsis

Species: A. thaliana

BIOLOGY

Arabidopsis is native to Europe, Asia, and northwestern Africa. It is an annual plant, 20–25 cm tall, with leaves forming a rosette at the base and few leaves also on the flowering stem. The basal leaves are green to slightly purplish in colour, with coarsely serrated margin; the stem leaves are smaller, unstalked, with entire margin. Leaves are covered with small unicellular hairs called trichomes. The flowers are 3 mm in diameter.. The fruit is a siliqua 5–20 mm long, containing 20–30 seeds.

Arabidopsis can complete its entire life cycle in 6 weeks. The central stem that produces flowers grows after about three weeks, and the flowers naturally self-pollinate. In the lab Arabidopsis may be grown in petridishes or pots, under fluorescent lights or in a greenhouse.

SPECIAL FEATURES

Arabidopsis is widely used as one of the model organisms for studying plant sciences, including genetics and plant development. As a photosynthetic organism, Arabidopsis requires only light, air, water and a few minerals to complete its life cycle. It has a fast life cycle that produces numerous self progeny in limited space requirements and is easily grown in a greenhouse or indoor growth chamber.

The small size of its genome (about 157 million base pairs and five chromosomes) makes Arabidopsisthaliana useful for genetic mapping and sequencing. Arabidopsis has one of the smallest genomes among plants and is the first plant genome to be sequenced in 2000 by the Arabidopsis Genome Initiative. Much work has been done to assign functions to its 27,000 genes and the 35,000 proteins they encode.

The plant’s small size and rapid life cycle are useful for cultivation in a small space and it produces several thousand seeds. It takes about six weeks from germination to mature seed.

Homeotic mutations in Arabidopsis result in the change of one organ to another — in the case of the Agamous mutation. For example, stamens become petals and carpels are replaced with a new flower, resulting in a recursively repeated sepal-petal-petal pattern.

Finally, plant transformation in Arabidopsis is routine, using Agrobacterium tumefaciens to transfer DNA to the plant genome. The current protocol, termed “floral-dip”, involves simply dipping a flower into a solution containing Agrobacterium, the DNA of interest, and a detergent. This method avoids the need for tissue culture or plant regeneration.

The availability of a broad base of knowledge about Arabidopsis and the previously developed research toolkit invites scientists to establish new techniques, develop new approaches, and test new concepts in Arabidopsis prior to their application in other species.

Some of its advantages as a model organism are as follows:

  • It has one of the smallest genomes in the plant kingdom: 115,409,949 base pairs of DNA distributed in 5 chromosomes (2n = 10).
  • Very little of this is “junk” DNA, so most of the DNA encodes its 25,498 genes.
  • Transgenic plants can be made easily using Agrobacterium tumefaciens as the vector to introduce foreign genes.
  • The plant is small — a flat rosette of leaves from which grows a flower stalk 6–12 inches high.
  • It can be easily grown in the lab in a relatively small space.
  • Development is rapid. It only takes 5– 6 weeks from seed germination to the production of a new crop of seeds.
  • It is a prolific producer of seeds (up to 10,000 per plant) making genetics studies easier.
  • Mutations can be easily generated by irradiating the seeds or treating them with mutagenic chemicals.
  • It is normally self-pollinated so recessive mutations quickly become homozygous and thus expressed.
  • However, Arabidopsis can easily be cross-pollinated to do genetic mapping and to produce strains with multiple mutations.

CONTRIBUTION TO SCIENCE

The first mutant in Arabidopsis was documented in 1873 by Alexander Braun, describing a double flower phenotype. However, it was not until 1943 that Friedrich Laibach proposed Arabidopsis as a model organism. His student Erna Reinholz published her thesis on Arabidopsis in 1945, describing the x-ray mutagenesis

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 by writing several scholarly reviews which were instrumental in introducing the model to the scientific community. The breakthrough year for Arabidopsis as the preferred model plant came in 1986 when T-DNA mediated transformation was first published, which coincided with the first gene to be cloned and published.

Observations of homeotic mutations led to the formulation of the ABC model of flower development by E. Coen and E. Meyerowitz. According to this model floral organ identity genes are divided into three classes: class A genes, which affect sepals and petals, class B genes, which affect petals and stamens and class C genes, which affect stamens and carpels. These genes code for transcription factors that combine to cause tissue specification in their respective regions during development.

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. Arabidopsis has also been important in understanding the functions of another blue light receptor, cryptochrome, which is especially important for light entrainment to control the plants circadian rhythms.

In 2005, scientists at Purdue University proposed that Arabidopsis possessed an alternative form of DNA repair, which they called “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.




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