Plant Responses to Internal and External Signals

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A Survey of Plant Hormones

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.

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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.

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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.

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Cell elongation in response to auxin: acid growth hypothesis

Cross-linking

polysaccharides

Cellulose

microfibril

Cell wall

becomes

more acidic.

2

1

Auxin

increases

proton pump

activity.

Cell wall–loosening

enzymes

Expansin

Expansins separate

microfibrils from cross-

linking polysaccharides.

3

4

5

CELL WALL

Cleaving allows

microfibrils to slide.

CYTOPLASM

Plasma membrane

H2O

Cell

wall

Plasma

membrane

Nucleus

Cytoplasm

Vacuole

Cell can elongate.

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Auxins

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

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Cytokinins

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.

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