In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells.
Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ.
Auxin - Polar Movement of Auxins
The term auxin refers to any chemical that promotes elongation of coleoptiles.
Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA.
Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell.
The Role of Auxin in Cell Elongation
According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane.
The proton pumps H+ lower the pH in the cell wall, activating expansins, enzymes that loosen the cell wall’s fabric.
With the cellulose loosened, the cell can elongate.
Cell elongation in response to auxin: acid growth hypothesis
microfibrils from cross-
microfibrils to slide.
Cell can elongate.
Lateral and Adventitious Root Formation
Auxin is involved in root formation and branching
Auxins as Herbicides
An overdose of synthetic auxins can kill eudicots
Other Effects of Auxin
Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem
Cytokinins are so named because they stimulate cytokinesis (cell division).
Control of Cell Division and Differentiation
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits.
Cytokinins work together with auxin to control cell division and differentiation.