Mimics a Natural Plant Hormone

This class of herbicides is termed the ’auxinics’ because:

  1. They resemble the structure of the natural plant hormone auxin (indole-3-acetic acid, also known as IAA) (Figure 2) and
  2. They cause physiological effects in sensitive plants similar to those caused by high doses of IAA.

The term auxin is from the Greek word, auxein, meaning ’to increase’ and was first used by Frits Went in 1926 to describe the natural plant hormone, indole-3-acetic acid (IAA), which is the compound that caused curvature of oat coleoptiles.


Fig. 2: Structural similarities between the natural auxin, IAA, and an auxinic herbicide, 2,4-D

Synthetic auxinic herbicides possess several similarities to the natural auxin, IAA. These similarities include:

  1. Dose-response patterns whereby at low doses they act as plant-growth regulators and have a stimulatory effect on plant-cell growth, while at high doses they display phytotoxic effects;
  2. Some (e.g., 2,4-D, picloram, dicamba) can replace IAA as the hormone supplement used for proper cell development in plant-cell culture medium;
  3. Differential sensitivity has been observed among different tissues as well as between tissues at different physiological stages of growth; and
  4. Auxin and auxinic herbicides induce growth by cell elongation as opposed to cell division.