Everything about Arabidopsis Thaliana totally explained
Arabidopsis thaliana (
A-ra-bi-dóp-sis tha-li-á-na;
thale cress,
mouse-ear cress or
Arabidopsis), is 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 in plant biology and genetics. Its
genome is one of the smallest plant genomes and was the first plant genome to be sequenced. Arabidopsis is a popular tool for understanding the
molecular biology of many plant traits, including
flower development and
light sensing.
Habitat, morphology, and life cycle
Arabidopsis is native to
Europe,
Asia, and northwestern
Africa. It is an
annual (rarely
biennial) plant usually growing to 20–25 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. Leaves are covered with small unicellular hairs (called
trichomes). 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. Roots are simple in structure, with a single primary root that grows vertically downwards, later producing smaller lateral roots. These roots form interactions with
rhizosphere bacteria such as
Bacillus megaterium.
Arabidopsis can complete its entire life cycle in six 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 petri plates or pots, under fluorescent lights or in a greenhouse.
Use as a model organism
Arabidopsis 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 animal biology. Although
Arabidopsis thaliana has little direct significance for agriculture, it has several traits that make it a useful model for understanding the genetic, cellular, and molecular biology of flowering plants.
The small size of its
genome make
Arabidopsis thaliana useful for genetic mapping and
sequencing — with about 157 million
base pairs and five
chromosomes, Arabidopsis has one of the smallest genomes among plants. It was the first plant genome to be sequenced, completed 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 also advantageous for research. Having specialized as a
spring ephemeral, it has been used to found several laboratory strains that take about six weeks from germination to mature seed. The small size of the 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", 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.
History of Arabidopsis research
The first mutant in Arabidopsis was documented in 1873 by
Alexander Braun, describing a
double flower phenotype (the mutated gene was likely
Agamous, cloned and characterized in 1990). However, it wasn't until 1943 that
Friedrich Laibach (who had published the chromosome number in 1907) proposed Arabidopsis as a model organism. 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.
Research
Flower development
Arabidopsis has been extensively studied as a model for flower development. The developing flower has four basic organs:
sepals,
petals,
stamens, and
carpels (which go on to form
pistils). These organs are arranged in a series of whorls: four sepals on the outer whorl, followed by six petals inside this, six stamens, and a central carpel region.
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.
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. Although developed through study of
Arabidopsis flowers, this model is generally applicable to other
flowering plants.
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.
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.
Light response was even found in roots, which were thought not to be particularly sensitive 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.
Non-Mendelian inheritance
In 2005, scientists at
Purdue University proposed that
Arabidopsis possessed 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' genes 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 Since then, alternative models have been proposed which would explain the
phenotype without requiring a new model of inheritance More recently the whole phenomenon is being challenged as a being a simple artifact of pollen contamination. "
When Jacobsen took great pains to isolate the plants, he couldn't reproduce the [reversion] phenomenon", notes
Steven Henikoff. 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, can't be explained by cross pollination.
Multigen
This is an ongoing experiment on the
International Space Station, it's being performed by the
European Space Agency. The goals are to study the growth and reproduction of plants from seed to seed in
microgravity.
Further Information
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