1. Plant Responses to Internal & External Signals

2. Signal transduction pathways link signal reception to response
A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation
After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally
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3. (a) Before exposure to light (b)
Figure 39.2
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes.
(a) Before exposure to light
(b)
After a week’s exposure to natural daylight

4. (a) Before exposure to light
Figure 39.2a
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes.
(a) Before exposure to light

5. After a week’s exposure to natural daylight
Figure 39.2b
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes.
(b)
After a week’s exposure to natural daylight

6. A potato’s response to light is an example of cell-signal processing
The stages are reception, transduction, and response
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7. Activation of cellular responses
Figure 39.3
CELL WALL
CYTOPLASM 1
Reception 2
Transduction 3
Response
Relay proteins and
Activation of cellular responses
second messengers
Receptor
Figure 39.3 Review of a general model for signal transduction pathways.
Hormone or environmental stimulus
Plasma membrane

8. Reception
Internal and external signals are detected by receptors, proteins that change in response to specific stimuli
In de-etiolation, the receptor is a phytochrome capable of detecting light
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9. Transduction
Second messengers transfer and amplify signals from receptors to proteins that cause responses
Two types of second messengers play an important role in de-etiolation: Ca2+ ions and cyclic GMP (cGMP)
The phytochrome receptor responds to light by
Opening Ca2+ channels, which increases Ca2+ levels in the cytosol
Activating an enzyme that produces cGMP
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10. 1 Reception CYTOPLASM Plasma membrane Phytochrome Cell wall Light
Figure 1
Reception
CYTOPLASM
Plasma membrane
Phytochrome
Cell wall
Light
Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response.

11. Protein kinase 1 Protein kinase 2
Figure 1
Reception 2
Transduction
CYTOPLASM
Plasma membrane
cGMP
Protein kinase 1
Second messenger
Phytochrome
Cell wall
Protein kinase 2
Light
Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response.
Ca2 channel
Ca2

12. De-etiolation (greening) response proteins
Figure 1
Reception 2
Transduction 3
Response
Transcription factor 1
CYTOPLASM
NUCLEUS
Plasma membrane
cGMP
Protein kinase 1 P
Second messenger
Transcription factor 2
Phytochrome P
Cell wall
Protein kinase 2
Transcription
Light
Translation
Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response.
Ca2 channel
De-etiolation (greening) response proteins
Ca2

13. Response
A signal transduction pathway leads to regulation of one or more cellular activities
In most cases, these responses to stimulation involve increased activity of enzymes
This can occur by transcriptional regulation or post-translational modification
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14. Plant Hormones
Hormones are chemical signals that coordinate the various parts of an organism
A hormone is a compound produced in one part of the body which is then transported to other parts of the body, where it triggers responses in target cells and tissues
Examples of human hormones:
Adrenaline, testosterone, estrogen, epinephrine…

15. Hormones: chemical messengers that coordinate different parts of a multicellular organism
Important plant hormones:
Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide)
Cytokinins – cell division (cytokinesis) & differentiation
Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development
Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy
Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

16. Auxin Stimulates stem elongation Stimulates development of fruit
Involved in phototropism and gravitropism

17. Cytokinins Stimulate cell division and growth
Stimulate cytokinesis
Stimulate germination and flowering

18. Gibberellins Trigger seed and bud germination
Promote stem elongation and leaf growth
Important in the growth of fruit

19. Ethylene Gas: Fruit Ripening
Canister of ethylene gas to ripen bananas in shipping container
Untreated tomatoes vs. Ethylene treatment

20. Abscisic Acid Induces seed dormancy Inhibits cell growth
Anti-gibberellin
Inhibits cell growth
Anti-cytokinin
Inhibits fruit ripening
Anti-ethylene
Closes stomata during water stress, allowing many plants to survive droughts

21. Tropisms
Tropisms are growth responses that result in curvatures of whole plant organs toward or away from a stimuli
There are three major stimuli that induce tropisms
Light (Phototropism)
Gravity (Gravitropism/Geotropism)
Touch (Thigmotropism)

22. Phototropism Phototropism is the growth of a shoot towards light
This is primarily due to the action of auxin
Auxin elongates the cells on the non-light side

23. Positive gravitropism in roots: the statolith hypothesis.

24. Thigmotropism: Toward or away from a stimulus; rapid turgor movements by Mimosa plant  action potentials

25. Biological Clocks/Circadian Rhythms
A physiological cycle with a frequency of about 24 hours is called a circadian rhythm
Even without external, environmental cues, circadian rhythms persist in humans and in all eukaryotes
Example: jet lag in humans

26. Night length is a critical factor!
Photoperiodism: physiological response to the relative length of night & day (i.e. flowering)
Short-day plants: flower when nights are long (mums, poinsettia)
Long-day plant: flower when nights are short (spinach, iris, veggies)
Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)
Night length is a critical factor!

27. Plant Defenses
Plants defend themselves against herbivores in several ways
Physical defenses, such as thorns
Chemical defenses, such as
producing distasteful/toxic
compounds

28. Plant responses to stress

29. Flooding (O2 deprivation):
Drought (H2O deficit):
close stoma
release abscisic acid to keep stoma closed
Inhibit growth
roll leaves  reduce SA & transpiration
deeper roots
Flooding (O2 deprivation):
release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

30. Excess Salt: Heat: Cold: cell membrane – impede salt uptake
produce solutes to ↓ψ - retain H2O
Heat:
evap. cooling via transpiration
heat shock proteins – prevent denaturation
Cold:
alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity)
increase cytoplasmic solutes
antifreeze proteins

31. Herbivores: Pathogens: physical (thorns) chemicals (garlic, mint)
recruit predatory animals (parasitoid wasps)
Pathogens:
1st line of defense = epidermis
2nd line = pathogen recognition, host-specific