Auxin

From Freepedia

Auxins are a group of plant growth substances (often called phytohormones or plant hormones). Auxins play essential role in coordination of many growth and behavioral processes in the plant life.

1 Overview
2 Hormonal activity
3 Locations
4 Effects
5 Molecular mechanisms of auxin action
6 Herbicide manufacture
7 See also
8 Sources

Overview

Auxins have been demonstrated to be the basic coordinative signal of plant development. Their transport throughout plants is complex, and often they also control action of other plant hormones. As a result, a plant can (as a whole) react on external conditions and adjust to them, without requiring a nervous system. They are sometimes referred to as cardinal plant hormones.

The most important member of the auxin family is indole-3-acetic acid (IAA), which is believed to be the most effective native auxin. It generates the majority of auxin effects in intact plants. Unfortunately, molecules 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). Synthetic auxins include 1-naphthaleneacetic acid (NAA - with nearest efects to IAA), 2,4-dichlorophenoxyacetic acid (2,4D), and others.

Commercially, auxins are often used to promote root growth as a main compound of rooting stimulators (beneficial mainly in horticulture for treating of stem cuttings). They are also used 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 growth and may cause leaves to fall and kill the plant. Some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (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

Auxins coordinate development at all levels of plants, from the cellular level to organs and ultimately the whole plant.

Image:Plant cell structure.png

On a cellular level

On the cellular level auxins' 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 activates enzymes known as expansins, which break bonds in the cell wall structure, making the cell wall less rigid. When the cell wall is degraded (not entirely) by the action of auxins, this now less rigid wall is expanded by the pressure coming from within the cell, especially by growing vacuoles.

Organ patterns

Growth and division of plant cells results in growth of tissue and specific tissue growth contributes to the development of plant organs. Growth of cells contributes to the plant's size, but uneven localized growth produces bending, turning and directionalization of organs. For example, stems turning toward light sources (phototropism), growth of roots in response to gravity (gravitropism), and other tropisms.

Organization of the plant

As auxins contribute to organ shaping, they are also fundamentally required for proper development of the plant itself. Without hormonal regulation and organization, plants would be merely proliferating heaps of similar cells. Auxin employment begins in the embryo of the plant, where directional distribution of auxin ushers in subsequent growth and development 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.

An important principle of plant organization based upon auxin distribution is apical dominance, which means that the auxin produced by the apical bud (or growing tip) diffuses downwards and inhibits the development of ulterior lateral bud growth, which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressive hormone allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth. This behavior is used in pruning by horticulturists.

Uneven distribution of auxin. To cause growth in the required domains, it is necessary that auxins should be active preferentially in them. Auxins are not synthesized everywhere, but each cell retains the potential ability to do so, and only under specific conditions will auxin synthesis be activated. For that purpose auxins have to be not only translocated toward those sites where they are needed, but there has to be an established mechanism to detect those sites. Translocation is driven throughout the plant body primarily from peaks of shoots to peaks of roots. For long distances, relocation occurs via the stream of fluid in phloem vessels, but for short distance transport a unique system of coordinated polar transport directly from cell to cell is exploited. This process of polar auxin transport is directional and very strictly regulated. It is based in uneven distribution of auxin efflux carriers on the plasma membrane, which send auxins in the proper direction.

Locations

Synthesized in shoot (and root) meristematic tissue
Synthesized in young leaves
Synthesized in mature leaves in very small amounts
Synthesized in mature root cells
Transported throughout the plant
Released by meristematic cells when they 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
IAA peaks during the day

Effects

Stimulates cell elongation (if gibberellins are also present the effect is stronger)
If cytokinins are also present, they together stimulate cell division
Induces formation and organization of phloem (and xylem)
Involved in phototropism, gravitropism, tropism toward moisture and other developmental changes
Induces new root formation by breaking root apical dominance induced by cytokinins
Induces shoot apical dominance
Directly stimulates ethylene synthesis (stimulation of ethylene in lateral buds cause of inhibition of its growth and it is principle of action of apical dominance)
IAA inhibits ethylene formation and transport of precursor
Inhibits abscission prior to formation of abscission layer (inhibits senescence of leaves)
Induces sugar and mineral accumulation at the site of application
Flower initiation
Sex determination
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

Molecular mechanisms of auxin action

Although auxins and their effects have been known for a long time, mechanisms of action in plants have remained unknown for a long time. In 2005 it was demonstrated that the F-box protein TIR1, which is part of the ubiquitin ligase complex SCFTIR1, is an auxin receptor. Upon auxin binding TIR1 recruit specific transcriptional repressors (the Aux/IAA repressors) for ubiquitination by the SCF complex. This marking process leads to the degradation of the repressors by the proteasome, alleviating repression and leading to specific gene expression in reponse to auxins.

Another protein called ABP1 (Auxin Binding Protein 1) is a putative receptor, but its role is unclear.

Herbicide manufacture

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.