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

2. Went purified the ‘influence’ and identified Indole-3-acetic acid (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

4. PLANT RESPONSES Figure 39.3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus Plasma membrane

5. 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

6. Plant Hormones (growth regulators) Auxin or IAA Gibberellins Cytokinins Ethene Abscisic acid

7. Auxin

8. Meristem Growing point – part of plant which grows most Auxins are produced in meristematic tissue

9. Auxin will migrate to the dark side of an illuminated stem

10. 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

11. Cytokinins Often have opposite effect to auxin Stimulate lateral buds  bushiness

12. Giberellins Produced at growing tips  longer internodes promote seed germination stem thickening

13. Other Hormones Ethene ripens fruit Abscisic acid (ABA) inhibits auxins, cytokinins and giberellin