Thale cress

Appearance

''Arabidopsis thaliana'' is an annual 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 color, 1.5–5 cm long and 2–10 mm broad, with an entire to coarsely serrated margin; the stem leaves are smaller and unstalked, usually with an entire margin. Leaves are covered with small, unicellular hairs . The flowers are 3 mm in diameter, arranged in a corymb; their structure is that of the typical Brassicaceae. 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 downward, later producing smaller lateral roots. These roots form interactions with rhizosphere bacteria such as ''Bacillus megaterium''.



''A. thaliana'' can complete its entire lifecycle in six weeks. The central stem that produces flowers grows after about three weeks, and the flowers naturally self-pollinate. In the lab, ''A. thaliana'' may be grown in Petri plates, pots, or hydroponics, under fluorescent lights or in a greenhouse.

Distribution

''A. thaliana'' is native to Europe, Asia, Africa, and human observations indicate its geographic distribution is rather continuous from the Mediterranean to Scandinavia and Spain to Greece. It also appears to be native in tropical alpine ecosystems in Africa and perhaps South Africa. It has been introduced and naturalized worldwide, including in North America ca. the 17th century.

''A. thaliana'' readily grows and often pioneers rocky, sandy and calcareous soils. It is generally considered a weed, due to its widespread distribution in agricultural fields, roadside, railway lines, waste ground and other disturbed habitat, but due to its limited competitive ability and small size it is not categorized as a noxious weed. Like most Brassicaceae species, ''A. thaliana'' is edible by humans as a salad or cooked, but it does not enjoy a widespread use as a spring vegetable.

Behavior

The plant's small size and rapid lifecycle 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 ''A. thaliana'' being valued as a genetic model organism.The photoreceptors phytochromes 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.

The UVR8 protein detects UV-B light and mediates response to this DNA damaging wavelength.

''A. thaliana'' 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. When the onset of darkness is unusually early, ''A. thaliana'' reduces its metabolism of starch by an amount that effectively requires division.

Light responses were even found in roots, previously thought to be largely insensitive to light. While the gravitropic response of ''A. thaliana'' 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, indicating that the roots also show positive phototropism.

In 2000, Dr. Janet Braam of Rice University genetically engineered ''A. thaliana'' to glow in the dark when touched. The effect was visible to ultrasensitive cameras.

Multiple efforts, including the Glowing Plant project, have sought to use ''A. thaliana'' to increase plant luminescence intensity towards commercially viable levels.

Habitat

''A. thaliana'' is native to Europe, Asia, Africa, and human observations indicate its geographic distribution is rather continuous from the Mediterranean to Scandinavia and Spain to Greece. It also appears to be native in tropical alpine ecosystems in Africa and perhaps South Africa. It has been introduced and naturalized worldwide, including in North America ca. the 17th century.

''A. thaliana'' readily grows and often pioneers rocky, sandy and calcareous soils. It is generally considered a weed, due to its widespread distribution in agricultural fields, roadside, railway lines, waste ground and other disturbed habitat, but due to its limited competitive ability and small size it is not categorized as a noxious weed. Like most Brassicaceae species, ''A. thaliana'' is edible by humans as a salad or cooked, but it does not enjoy a widespread use as a spring vegetable.

Evolution

Plants are affected by multiple pathogens throughout their lifetime. In response to the presence of pathogens, plants have evolved receptors on the cell surface to detect and respond to pathogens. ''Arabidopsis Thaliana'' is a model organism used to determine specific defense mechanisms of plant-pathogen resistance. These plants have special receptors on their cell surfaces that allow for detection of pathogens and initiate mechanisms to inhibit pathogen growth. They contain two receptors, FLS2 and EF-Tu , which use signal transduction pathways to initiate the disease response pathway. The pathway leads to the recognition of the pathogen causing the infected cells to undergo cell death to stop the spread of the pathogen. Plants with FLS2 and EF-Tu receptors have shown to have increased fitness in the population. This has led to the belief that plant-pathogen resistance is an evolutionary mechanism that has built up over generations to respond to dynamic environments, such as increased predation and extreme temperatures.

''A. thaliana'' has also been used to study systemic acquired resistance .
This pathway utilizes Benzothiadiazol, a chemical inducer, to induce transcription factors, mRNA, of SAR genes. This accumulation of transcription factors leads to inhibition of pathogen-related genes.

Plant-pathogen interactions are important for understanding of how plants have evolved to combat different types of pathogens that may affect them. Variation in resistance of plants across populations is due to variation in environmental factors. Plants that have evolved resistance, whether it be the general variation or the SAR variation, have been able to live longer and hold off necrosis of their tissue , which leads to better adaptation and fitness for populations that are in rapidly changing environments.