Auxins are a group of plant growth substances (often called phytohormones or plant hormones), the most common example being indoleacetic acid (IAA). According to the "acid growth theory", auxins may stimulate cell elongation in young stems, 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.

Auxins are also responsible for phototropism which is the bending toward light sources. 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.

Auxins include indoleacetic acid, phenylacetic acid , and 4-chloro-indoleacetic acid . Commercially, auxins are used to promote root growth, to promote uniform flowering, and to set fruit and prevent premature fruit drop . Synthetic auxins such as 2, 4-D and 2, 4, 5-T have been used as herbicides, broad-leaved weeds like dandelions are much more susceptible to auxins than narrow-leaved plants like grass and cereal crops.

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
• 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
• (From Theory II) 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