1. Chapter 39.
Plant Response

2. Plant Reactions Stimuli & a Stationary Life
animals respond to stimuli by changing behavior
move toward positive stimuli
move away from negative stimuli
plants respond to stimuli by adjusting growth & development

3. What mechanism causes this response?
grown in dark
1 week exposure to light

4. Signal Transduction Pathway model
signal triggers receptor
receptor triggers internal cellular messengers & then cellular response
receptor
signal pathway (2° messengers)
response
General model for signal transduction pathways:
A hormone or another signal binding to a specific receptor stimulates the cell to produce relay molecules, such as second messengers. These relay molecules trigger the cell’s various responses to the original signal. In this diagram, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside.

5. Signal Transduction Pathway example
1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
Signal Transduction Pathway controlling greening (de-etiolation)
of plant cells, like in a sprouting potato tuber.
Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
One pathway uses cGMP as a 2nd messenger to activate a protein kinase.The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.
Both pathways lead to expression of genes for proteins that function in greening response of plant.
2. One pathway uses cGMP as a 2nd messenger to activate a protein kinase.
The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.
3. Both pathways lead to expression of genes for proteins that function in greening response of plant.

6. Plant hormones
Chemical signals that coordinate different parts of an organism
minute amounts required
produced by one part of body
transported to another part
binds to specific receptor
triggers response in target cells & tissues

7. Plant hormones auxins cytokinins gibberellins brassinosteroids
abscisic acid
ethylene

8. Auxin Indolacetic acid (IAA) stimulates cell elongation
enhances apical dominance
classical explanation of phototropism

9. Phototropism
Growth towards light

10. Cytokinins Family of hormones Effects
produced in roots, fruits & embryos
Effects
control of cell division & differentiation
Enhance axillary growth
interaction of auxin & cytokinins

11. Gibberellins Family of hormones Effects
over 100 different gibberellins
Effects
stem elongation
fruit growth
seed germination

12. Brassinosteroids Steroids Effects Slow leaf loss (abscission)
cell elongation & division in shoots & seedlings

13. Abscisic acid (ABA) Effects slows growth seed dormancy
survival value: seed will germinate only under optimal conditions
light, temperature, moisture
drought tolerance
rapid stomate closing

14. Ethylene Ethylene is a gas released by plant cells Multiple effects
response to mechanical stress
triple response
slow stem elongation
thickening of stem
curvature to horizontal growth
apoptosis
leaf abscission
fruit ripening

15. Fruit ripening
Hard, tart fruit protects developing seed from herbivores
Ripe, sweet, soft fruit attracts animals to disperse seed
burst of ethylene triggers ripening process
conversion of starch to sugar = sweetening
positive feedback system
ethylene triggers ripening
ripening stimulates more ethylene production

16. Responses to light Photomorphogenesis Light detection
effect of light on plant growth
Light detection
intensity
direction
wavelength

17. Flowering Response Triggered by photoperiod
relative lengths of day & night
night length—“critical period”— is trigger
Long-day plants
flower if dark shorter than critical period
Short-day plants
flower if dark longer than critical period
Short-day plants
Long-day plants

18. Circadian rhythms Internal (endogenous) 24-hour cycles 4 O’clock
Noon
Midnight
Morning glory

19. Responses to gravity roots = positive gravitropism
shoots = negative gravitropism
settling of statoliths detects gravity

20. Responses to touch
Thigmotropism

21. Plant defenses
Defenses against herbivores

22. Plant defenses Defenses against herbivores
Parasitoid wasp larvae emerging from a caterpillar