Auxin

Auxins are plant hormones. The most important auxin produced by plants is indole-3-acetic acid (IAA).

• phototropism
• gravitropism
• apical dominance
• fruit development
• abscission
• root initiation

1. Phototropism

Mechanism

• The direction of light is detected at the tip of the shoot.
  • Blue light is most effective.
  • It is absorbed by flavoproteins. These are proteins, such as cryptochromes, that contain flavin as a prosthetic group.
• Auxin is synthesized at the tip and translocated down along the shady side of the shoot.
• Auxin stimulates elongation of the cells on the shady side causing the shoot to bend toward the light.

2. Gravitropism

• Plant shoots display negative gravitropism: when placed on its side, a plant shoot will grow up
• Roots display positive gravitropism: they grow down.

Mechanism of gravitropism in roots

• Amyloplasts (organelles containing starch grains) settle to the bottom of cells in the root tip.
• Auxin sent down from the shoot arrives in the central tissues of the root tip and is then translocated back along the under side of the root.
• This INHIBITS root cell elongation. (Roots and shoots differ in their sensitivity to auxin: concentrations of auxin that stimulate shoot growth inhibit root growth.)
• So the cells at the top surface of the root elongate, causing the root to grow down.

3. Apical dominance

Growth of the shoot apex (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. This phenomenon is called apical dominance.

If the terminal shoot of a plant is removed, the inhibition is lifted, and lateral buds begin growth. Gardeners exploit this principle by pruning the terminal shoot of ornamental shrubs, etc. The release of apical dominance enables lateral branches to develop and the plant becomes bushier. The process usually must be repeated because one or two laterals will eventually outstrip the others and reimpose apical dominance.

Apical dominance seems to result from the downward transport of auxin produced in the apical meristem. In fact, if the apical meristem is removed and IAA applied to the stump, inhibition of the lateral buds is maintained.

4. Fruit development

Pollination of the flowers of angiosperms initiates the formation of seeds. As the seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit that covers the seeds.

Some commercial growers deliberately initiate fruit development by applying auxin to the flowers. Not only does this ensure that all the flowers will "set" fruit, but it also maximizes the likelihood that all the fruits will be ready for harvest at the same time.

5. Abscission

Auxin also plays a role in the abscission of leaves and fruits. Young leaves and fruits produce auxin and so long as they do so, they remain attached to the stem. When the level of auxin declines, a special layer of cells - the abscission layer -forms at the base of the petiole or fruit stalk. Soon the petiole or fruit stalk breaks free at this point and the leaf or fruit falls to the ground.

Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping.

5. Root initiation

Auxins stimulate the formation of adventitious roots in many species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant.

Horticulturalists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting. The process can often be hastened by treating the cuttings with a solution or powder containing a synthetic auxin.

How does auxin achieve its many different effects in the plant?

• immediate, direct effects on the cell
• turning on of new patterns of gene expression

1. Direct effects of auxin

• changes in movement of ions in and out of the cell through the plasma membrane
• extension of the cell wall causing elongation of the cell

• Auxin binds to specific receptors at the cell surface.
  • These are transmembrane proteins, e.g., ABP1 ("Auxin-binding protein 1")
• This interaction activates G proteins that, in turn,
• activate adenylyl cyclase.
• This produces the "second messenger", cyclic AMP (cAMP) which turns on a variety of cell activities in the cytosol.

2. Effects of auxin on gene expression

• auxin enters the cell by active transport through special auxin transporter molecules in the plasma membrane
• Auxin binds to molecules in the cytosol such as ARF1 ("auxin response factor 1")
• ARF1 is a transcription factor
• it enters the nucleus and binds to the DNA sequence

  TGTCTC
  ACAGAG
  

• This sequence is found in the promoters of auxin-responsive genes; that is, it is an auxin response element

Synthetic auxins as weed killers

Some of the most widely-used weed killers are synthetic auxins. These include 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T).

As the formulas show, 2,4,5-T is 2,4-D with a third chlorine atom, instead of a hydrogen atom, at the #5 position in the benzene ring (blue circles).

2,4-D and its many variants are popular because they are selective herbicides, killing broad-leaved plants but not grasses (no one knows the basis of this selectivity).

Why should a synthetic auxin kill the plant? Auxin (IAA) is actively transported into cells by a transmembrane transporter and leaves the cells by facilitated diffusion through a different transporter. It turns out that the importer works fine for 2,4-D but that 2,4-D cannot leave the cell through the exporter. Perhaps it is the resulting accumulation of 2,4-D within the cell that kills it.

A mixture of 2,4,-D and 2,4,5-T was the "agent orange" used by the U.S. military to defoliate the forest in parts of South Vietnam.