1. Plant responses to hormones

2. What the spec says….

3. Types of plants hormone 1. Auxins 2. Gibberellins 3. Cytokinins 4. Abscisic acid 5. Ethene Promotors of growth Inhibitors of growth / Antagonists to growth stimulators Cell divisionCell elongation Differentiation Initiation of organs

4. Growth substances can be…  Synergistic – enhance each others’ effects  Antagonistic – counteract each other

5. Auxin  Indoleacetic acid  Causes elongation of cells –Increases plasticity of cells –Cell walls soften –Cell becomes less turgid and take up more water –Cell expands  Responsible for phototropism

6. Phototropism experiments

7. Other activities of auxins  Apical dominance – apical bud inhibits lateral (axillary buds) –E.g. pruning stimulates axillary buds  Formation of lateral roots – stimulates INITIATION of lateral roots and adventitious roots –E.g. napthalene acetic acid (NAA) –Indole butyric acid (IBA)

8. More auxin functions  Abscission of leaves and fruit  Abscission = falling of fruit and leaves  IAA – delays early stages, promotes later stages –Stimulates ethene production –Ensure fruit stays on trees until harvest

9. Even more auxin functions  Fruit development –Produced in pollen & developing seed –Stimulates fruit development after fertilisation  Used in production of seedless fruits  Weed killers –2,4-D causes abnormal growth of Dicots (e.g. dandelions) monocots are unaffected

10. Giberellins  Stem elongation in dwarf plants –E.g. dwarf peas  Bolting of long day plants when the days are short i.e. cause them to flower  Overcome lack of cold period required for flowering in long day plants

11.  A wild type rice plant (left) compared to a semidwarf transgenic plant (right) containing the gai mutant allele.  A wild type rice plant (left) compared to a semidwarf transgenic plant (right) containing the gai mutant allele.  The gai mutant allele confers giberellin insensitivity upon the transgenic plant, and causes its reduced stature.

12. More giberellins  Fruit development –Stimulates growth of fruit stalk –Delays senescence (aging)  Seed germination – stimulates  Stimulate α – amylase production in seeds, improves malt yields from barley

13. Cytokinins  Produced by meristemic tissue –Apices of roots and shoots  Development of roots, shoots and lateral buds  Cell enlargement  Maturation of chloroplasts  Dependant on auxin for function

14.  Cytokinins are commonly used to stimulate shoot initiation in culture as is shown by shoot/bulblet formation on bulb scales of Bowiea volubilis. Benzyladenine was used here to stimulate shoot growth

15. Abscissic acid  Inhibit / alter growth  Linked to environmental stress –Seeds can become dormant –Abscission of leaves, flowers and fruits –Close stomata in times of water deficiency

17. Ethene (ethylene)  Promotes ripening of fruits  Promotes abscission (auxin prevents it) –Abscission layer  Antagonistic to auxin