Charles and Francis Darwin (1880)There is an ‘influence’ which moves from the tip to the cells below.

Plant Hormones

Went purified the ‘influence’and identified Indole-3-aceticacid (IAA)

Very small molecules – can move across cell walls

Hormone = compound produced by one part of an organism that is transported to other parts where it triggers a response in target cells

PLANT RESPONSES

CELLWALL

CYTOPLASM

  1 Reception 2 Transduction 3 Response

Receptor

Relay molecules

Activationof cellularresponses

Hormone orenvironmentalstimulus

Plasma membrane

Functions of Hormones• Coordinate growth and development by

affecting division, elongation and differentiation of cells

• Regulate responses to environmental stimuli

• Varying concentrations affect target cells differently; and different target cells are affected differently

Plant Hormones (growth regulators)

Auxin or IAA

Gibberellins

Cytokinins

Ethene

Abscisic acid

Auxin

Meristem

Growing point – part of plant which grows most

Auxins are produced in meristematic tissue

Auxin will migrate to the dark side of an illuminated stem

Geotropism

Movement of dense molecules in sensitive

cells.

AMYLOPLASTS (statoliths - starch granules)

Cause redistribution of auxin to lower parts

Auxins act as inhibitors in roots

Cytokinins• Often have opposite effect to auxin• Stimulate lateral buds bushiness

Giberellins• Produced at growing tips longer internodes• promote seed germination• stem thickening

Other Hormones• Ethene ripens fruit• Abscisic acid (ABA) inhibits auxins,

cytokinins and giberellin