Auxin

Auxins are a group of plant growth substances (often called phytohormones or plant hormones). Auxins play essential role in coordination of many growth and behavorial processes in the plant life. In fact auxins and theirs, strikingly very ingenious and complex, transport througtout the plant have been demonstrated to be the basic coordinative signal of plant developement. They also often control action of other plant hormones. As a result, plant can as a whole react on external conditions and adjust to them, while they have no nervous system. So they are sometimes referred to as cardinal plant hormones.

The most important member of auxin family is indole-3-acetic acid (IAA), which is believed to be the most efective native auxin, generating majority of auxin efects in intact plant. Unfortunately moleculas of IAA are chemically unstable, so they can´t be used commercially. Native auxins further include 4-chloro-indoleacetic acid, phenylacetic acid(PPA) and indole-3-butyric acid (IBA). Syntetic auxins include 1-naphtaleneacetic acid (NAA - with nearest efects to IAA),2,4-dichlorophenoxyacetic acid (2,4D) and others.

Commercially are auxins often used to promote root growth as a main compound of rooting stimulators (beneficial mainly in horticulture for treating of stem cuttings). They are used also to promote uniform flowering, and to set fruit and prevent premature fruit drop. Used in high doses, it stimulates the production of ethylene which stops the growth and may cause leaves to fall and can kill the plant. Some synthetic auxins such as 2, 4-D and 2, 4, 5-T can be used as herbicides, broad-leaved weeds like dandelions are much more susceptible to auxins than narrow-leaved plants like grass and cereal crops.

Hormonal activity

Auxin is required either on a cellular level, organ formation level or on the level of whole plant developement.

On a cell level auxin presence is essential both for cell division and respective cell growth resulting usually in its axial elongation. According to the "acid growth theory", auxins may stimulate cell elongation, for example, by causing responsive cells to actively tranport hydrogen ions out of the cell, thus lowering the pH around cells. This acidification of the cell wall region surrounding the cell activates enzymes known as expansins, which break bonds in the cell wall structure, making the cell wall less rigid and allowing elongation.

Shaping the plant organs. Growth and division of plant cells results in growth of tissue and specific tissue growth contribute to the developement of plant organs. Such developement profiting from growth of cells contributes not only to plant size, but throught uneven localized growth produces bending, turning and directionalization of organs. E.g. stems toward light sources phototropism, growth of roots in response to gravity gravitropism or another tropism.

Plant As it contributes to organ shaping, it is also fundamentaly required for proper development of plant itself. Without hormonal regulation and organization, plants would be merely proliferating heaps of similar cells. Its employment begins in the embryo of the plant, where directional distibrution of auxin ushers in subseqent growth and developement of primary growth poles, then forms buds of future organs. Throughout the plant's life, auxin helps the plant maintain the polarity of growth and recognize where it has its branches (or any organ) connected.

uneven distribution For addressing the growth in required domains is necessary that auxins should be preferentially active in them. Auxin is not synthesized everywhere, but each cell retains the potential ability to do so, and in very specific conditions it activates. For that purpose they have to be not only translocated toward that sites, but able to detect that sites. Translocation is driven throughout the plant body primarily from peaks of shoots to peaks of roots. For long distances it relocates in stream of fluid in vessels, but for short distance transport it exploits a very unique system of coordinated polar transport from cell to cell (that mean acros the tissue, cell by cell) This process is directional and very strictly regulated. Polar Auxin Transport is based in uneven distribution of auxin efflux carriers on the Plasma membrane, which then sends auxins in the proper direction.

The defoliant Agent Orange was a mix of 2, 4-D and 2, 4, 5-T. 2, 4-D is still in use and is thought to be safe, but 2, 4, 5-T was more or less banned by the EPA in 1979. The dioxin TCDD is an unavoidable contaminant produced in the manufacture of 2, 4, 5-T, as a result of the integral dioxin contamination, 2, 4, 5-T has been implicated in leukaemia, miscarriages, birth defects, liver damage, and other diseases.

Location, Characteristics and Occasions for Synthesis Induction

• Synthesized in shoot and root meristematic tissue
• Synthesized in young leaves
• Synthesized in mature leaves in very small amounts
• Transported throughtout the plant
• IAA peaks during the day
• Synthesized in mature root cells
• Released by meristematic cells when they have enough sugar and oxygen to support both themselves and any dependent cells and are in good growing conditions
• Released by all cells when they are experiencing conditions which would normally cause a shoot meristematic cell to produce auxin
• Directly or indirectly induced by high levels of ethylene

Effects

• Stimulates cell elongation
• Stimulates cell division with CK
• Induces xylem and phloem
• Directly stimulates ethylene synthesis
• IAA inhibits ethylene formation and transport of precursor
• Induces shoot apical dominance
• Inhibits abscission prior to formation of abscission layer (inhibits senescence of leaves)
• Involved in phototropism, gravitropism, tropism toward moisture
• Induces sugar and mineral accumulation at the site of application
• Flower initiation
• Sex determination
• Induces new root formation by breaking root apical dominance induced by CK
• Inhibits root hair growth and causes them to die back
• Stimulates the rate of metabolism of cells in the root (who are not at their peak metabolism rates) in response to an increase in the levels sugar and essential gases

fr:Auxine ja:オーキシン