Chemical Communication in Plants: Auxins

This lesson covers: 

1. What plant growth factors are
2. How IAA controls growth by elongation

Plant growth factors

Plant growth factors, sometimes called plant hormones or plant growth regulators, are chemicals that influence plant development mainly by modulating cell elongation and division.

Differences between plant and animal hormones:

• Plant hormones usually act locally at the site of production, while animal hormones usually act on distant organs.
• Plant hormones are produced in a variety of tissues throughout the plant, while animal hormones are produced in specialised cells within glands.

Plant hormones interact with receptors on the surface of cells or in the cytoplasm or nucleus. These receptors usually initiate a series of chemical or ionic signals within the cell.

The role of IAA in shoot elongation

Auxins are plant growth factors that can stimulate, or inhibit, cell elongation. This influences many aspects of growth, including elongation of roots and shoots.

Auxin, specifically indole-3-acetic acid (IAA), is synthesised in the growing tips (meristems) of shoots and roots, where the cells are dividing. Auxins like IAA play a key role in shoot growth via cell elongation.

This occurs as follows:

1. Auxins are synthesised in meristem cells in the shoot tip.
2. Auxins diffuse downwards away from the tip.
3. Auxins bind to receptor proteins on the cell-surface membrane.
4. ATPase proton pumps move hydrogen ions from the cytoplasm into the cell wall, so a low pH develops in the cell walls.
5. This activates expansins, which loosen bonds between cellulose microfibrils so the cells can absorb water by osmosis, increasing the internal pressure and causing the cell walls to expand.
6. Cells elongate and the plant grows.