1. Plant Responses
Chapter 39

2. Signal Transduction
Plants have cellular receptors that detect important changes in their environment

3. Grown in Dark 1 week exposure to Light
Greening Response
Etiolation

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.
What kinds of molecules are the receptors?

5. Signal Transduction Pathway
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.

6. Plant Hormones
Chemical signals that coordinate different parts of an organism
only minute amounts are required
produced by 1 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. The Discovery of Plant Hormones
Tropisms : Phototropism

10. Signal is a a mobile chemical

11. Early experiments of phototropism

12. The Went Experiments Went named the chemical “auxin”
Later purified as indolacetic acid
(IAA)

13. How Auxins Cause Cell Elongation
Increase in H+ activates expansins
Weakened cell wall allows more water to enter
H+ pump

14. Auxins Herbicides2-4-D
Affects secondary growth Synthetic auxins sprayed on green house plants induce fruit development

15. Cytokinins Stimulate Cytokinesis
Are produced in actively growing tissues

16. Gibberellins Stimulate cell elongation and division Bolting
Fruit Growth
Germination
Effects
stem elongation
fruit growth
seed germination

17. Figure 39.11 The effect of gibberellin treatment on seedless grapes

18. Abscisic Acid (ABA) Slows growth Maintains seed dormancy
Drought tolerance

19. Ethylene A magic gas? Response to stress Leaf abscission
Fruit ripening
Slowing, thickening, curving

20. ETHYLENE A magic gas? Response to stress Leaf abscission
Ethylene induces the triple response in pea seedlings Slowing, thickening,curving
ETHYLENE
A magic gas?
Response to stress
Leaf abscission
Fruit ripening

21. Abscission of a maple leaf
Apoptosis
Caused by ethylene/auxin shift

22. Responses to gravity
How does a sprouting shoot “know” to grow towards the surface from underground?
environmental cues?
roots = positive gravitropism
shoots = negative gravitropism
settling of statoliths (dense starch grains) may detect gravity

24. Phytochromes

25. Molecular Switch Mechanism of Phytochromes

26. Phytochrome photoreceptors
Molecular switch reaction to red light
conversion of Pr  Pfr in sunlight stimulates germination, flowering, branching…
conversion of Pfr  Pr in dark inhibits response, & stimulates other responses: growth in height
Chromophore
Photorecptor
Light induced
Kinase activity
Phytochrome
Phytochrome
Response:
Vertical growth

27. Figure An example of signal transduction in plants: the role of phytochrome in the greening response

28. Both pathways lead to expression of proteins that function in the greening response

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

30. Figure 39.21 Sleep movements of a bean plant

31. Figure 39.x1 Biological clocks

32. Figure 39.22 Photoperiodic control of flowering

33. Figure 39.23 Reversible effects of red and far-red light on photoperiodic response

34. Is there a flowering hormone?
Plant on left is induced to flower & then grafted onto plant on right
plant on right is triggered to flower
What can you conclude?

35. Thigmomorphogenesis
Change in form resulting from mechanical perturbation
Altering gene expression by touch in Arabidopsis

36. Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)

37. Plant Defenses Defense against herbivores Defense against pathogens
Systematic acquired resistance

38. Figure A corn leaf recruits a parasitoid wasp as a defensive response to an herbivore, an army-worm caterpillar

39. Figure 39.31 Defense responses against an avirulent pathogen