Plant growth factors are chemicals produced in plants that influence their growth and development.

Plant growth factors differ from animal hormones because:

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

Plant growth factors are sometimes called plant growth regulators, or simply plant hormones.

Auxins are plant growth regulators that can stimulate cell elongation, influencing many aspects of growth including elongation of roots and shoots.

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.

Auxin, specifically indole 3-acetic acid (IAA), is synthesised in the growing tips (meristems) of shoots and roots, where the cells are dividing.

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