Plant responses to hormones

What the spec says….

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

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

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

Phototropism experiments

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)

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

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

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

 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.

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

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

 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

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

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