1. Plant Hormones

2. Overview
Plant cells, like animal cells, respond to the actions of a variety of chemical signals, called hormones
Just like animal hormones they are:
often triggered by external environmental factors (stimuli)
very effective at low concentrations

3. Overview The best understood plant hormones are:
Auxins
Gibberellins
Cytokinins
Abscisic acid
Ethylene gas
The same hormone can produce different responses in different parts of the plant

4. Auxins
Auxins are water-soluble chemicals produced in growing tips of roots and shoots
Auxin stimulates the growth of cells in shoots, but inhibits the growth of cells in roots!

5. Auxins
Auxins produced in the growing tips of plants promote the elongation of the cells below

6. this selective pruning is how gardeners produce bushy plants
Auxins
When the bud at the apex is removed, the lateral buds lower down the stem begin to develop
Indoleacetic acid (IAA), is responsible for apical dominance as it inhibits the growth of lateral buds near the growing tip at the apex of the plant
this selective pruning is how gardeners produce bushy plants

7. increased concentration of auxins
Tropism is when plant growth is in response to a stimulus like light, water or gravity
Auxin moves away from light, causing plant stems to bend towards light!
increased concentration of auxins
cells in this region grow faster and longer, resulting in the plant bending towards the light
phototropism

8. increased concentration of auxins
Geotropsim is when plant growth is in response to gravity
Gravity causes auxin to accumulate on lower side of horizontal shoots or roots
This causes shoots to bend upwards and roots to bend downwards (as auxins inhibit growth in roots)
increased concentration of auxins

9. Gibberellins
Growth hormones that stimulate cell elongation and cell reproduction in stems and leaves
They also instigate seed germination and bud development
Best known gibberellin is gibberellic acid, which initiates a transduction pathway in seed germination
induces production of
gibberellic acid
amylase
starch
glucose
activated by water

10. Cytokinins
Growth hormones that, when present in high concentrations, promote rapid cell reproduction in growing shoots, roots and young fruits
Produced in roots and travel to stem via phloem
Also produced in young fruits to stimulate their own growth

11. Abscisic Acid Growth-inhibiting hormone found in dormant buds
This hormone also influences stomal closure to stop water loss
Leaves produce, and accumulate, abscisic acid if they begin to wilt
Abscisic acid somehow influences ion concentrations in guard cells, causing the stomata to close
Therefore abscisic acid is important for water retention in plants

12. Abscisic Acid
Also found in high levels in fruit and leaves that are about to fall from a plant – a process called abscission
It was once though that abscisic acid caused abscission (hence the similarity in names); but it doesn’t
Ethylene is now known to promote abscission in aging leaves

13. Ethylene Gas Ethylene is produced by some fruits as they ripen
e.g. apples, bananas and tomatoes
NOT cherries, grapes and citrus fruits
Ethylene increases the rate of respiration, and thus the rate of ripening
If one fruit produces ethylene, it can promote ripening in nearby fruits, due to the diffusion of the gas
Ethylene gas is used to control the ripening of fruit so that they are ready to eat when they reach the shops

14. SUMMARY
Plant hormones, like animal hormones, are chemical signalling molecules required for growth and development
Different plant hormones affect different parts of a plant differently
e.g. auxins in stems and roots
Plant hormones, like animal hormones, result in signal transduction pathways in plant cells