The method of action of 2,4,5-T and 2,4-D.


Auxins are plant hormones.

The most important Auxin produced by plants is indole-3-acetic acid (IAA). It plays important roles in a number of plant activities, including:

  1. Phototropism.
  2. Gravitropism.
  3. Apical dominance.
  4. Fruit development.
  5. Abscission.
  6. Root initiation.

A couple of synthetic auxins are 2,4-D and 2,4,5-T.


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

Auxin effects are mediated by two different pathways:

  1. Direct effects on the cell.
  2. "Turning on" of new patterns of gene expression.

1. Direct effects of Auxin.

The arrival of Auxin at the surface of the cell initiates such immediate responses as:

Auxin initiates these events after binding to specific receptors at the cell surface, probably transmembrane proteins such as ABP1 ("Auxin-binding protein 1")

2. Effects of Auxin on gene expression.

Many Auxin effects are mediated by changes in the transcription of genes. The steps appear to be:



This sequence is found in the promoters of Auxin-responsive genes; that is, it is an Auxin response element. The action of Auxin on gene transcription is quite similar to the action of steroid hormones in animals.


Synthetic Auxins as weed killers.

Some of the most common weed killers are synthetic Auxins. Such as: 2,4-D and 2,4,5-T.

2,4-D and its many variants are popular because they are selective herbicides, killing broad-leaved plants but not grasses.


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. It is the resulting accumulation of 2,4-D within the cell that kills it.


More detail can be found at: