1. Plant Responses to Internal and External Signals
Chapter 39
Plant Responses to Internal and External Signals

2. Overview: Stimuli and a Stationary Life
Linnaeus noted that flowers of different species opened at different times of day and could be used as a horologium florae, or floral clock
Plants, being rooted to the ground, must respond to environmental changes that come their way
For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light
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3. Figure 39.1
Figure 39.1 Can flowers tell you the time of day?

4. (1) CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response
Figure 39.3
(1)
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

5. Glucose 1-phosphate (108 molecules)
Figure 11.16
Reception
Binding of epinephrine to G protein-coupled receptor (1 molecule)
Transduction
(41)
Inactive G protein
Active G protein (102 molecules)
Inactive adenylyl cyclase
Active adenylyl cyclase (102)
ATP
Cyclic AMP (104)
Inactive protein kinase A
Active protein kinase A (104)
Figure Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine.
Inactive phosphorylase kinase
Active phosphorylase kinase (105)
Inactive glycogen phosphorylase
Active glycogen phosphorylase (106)
Response
Glycogen
Glucose 1-phosphate (108 molecules)

6. Concept 39.1: 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 (2-3)
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7. (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

8. Activation of cellular responses second messengers
Figure 39.3
A potato’s response to light is an example of cell-signal processing
The stages are reception, transduction, and response
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

9. 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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10. 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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11. De-etiolation (greening) response proteins
Figure
(4-6) 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

12. 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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13. Post-Translational Modification of Preexisting Proteins
Post-translational modification involves modification of existing proteins in the signal response
Modification often involves the phosphorylation of specific amino acids
The second messengers cGMP and Ca2+ activate protein kinases directly
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14. Activated relay molecule
Figure 11.10
Signaling molecule
Receptor
Activated relay molecule
Inactive protein kinase 1
Active protein kinase 1
Inactive protein kinase 2
ATP
Phosphorylation cascade
ADP
Active protein kinase 2 P PP
P i
Figure A phosphorylation cascade.
Inactive protein kinase 3
ATP
ADP P
Active protein kinase 3 PP
P i
Inactive protein
ATP
ADP P
Active protein
Cellular response PP
P i

15. Transcriptional Regulation
Specific transcription factors bind directly to specific regions of DNA and control transcription of genes
Some transcription factors are activators that increase the transcription of specific genes
Other transcription factors are repressors that decrease the transcription of specific genes
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16. De-Etiolation (“Greening”) Proteins
De-etiolation activates enzymes that
Function in photosynthesis directly
Supply the chemical precursors for chlorophyll production
Affect the levels of plant hormones that regulate growth
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17. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
Hormones were first discovered and animals and defined with the following criteria: 1. signal produced that acts elsewhere in the body 2. Binds to a specific receptor and triggers responses 3. Travels via circulatory system
Plant hormones (aka plant growth regulators) are chemical signals that modify or control one or more specific physiological processes within a plant (7-8)
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18. The Discovery of Plant Hormones
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light
They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present (9)
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19. (10-11) RESULTS Shaded side Control Light Illuminated side
Figure 39.5
RESULTS
(10-11)
Shaded side
Control
Light
Illuminated side
Boysen-Jensen
Light
Darwin and Darwin
Figure 39.5 Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted?
Light
Gelatin (permeable)
Mica (impermeable)
Tip removed
Opaque cap
Trans- parent cap
Opaque shield over curvature

20. Figure 39.6
Control
RESULTS
Excised tip on agar cube
Growth-promoting chemical diffuses into agar cube
Control (agar cube lacking chemical)
Offset cubes
Went Experiment
He gave this substance the name auxin (greek for increase). Its structure was later found to be IAA (indoleacetic acid) (12-13)
Figure 39.6 Inquiry: Does asymmetrical distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?

21. Auxin
The term auxin refers to any chemical that promotes elongation of coleoptiles
Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA
Auxin is produced in shoot tips and is transported down the stem
Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell
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22. Auxin Roles Stimulates stem elongation (low concentrations)
Promotes formation of lateral and adventitius roots
Regulates development of fruit
Enhances apical dominance
Fuctions in phototropism and gravitropism
Promotes vascular differentiation
Retards leaf abscission (14)
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23. Practical Uses for Auxins
The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings
Monocots have inactivating enzymes, dicots do not
An overdose of synthetic auxins can kill plants
For example 2,4-D is used as an herbicide on eudicots (15)
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24. Cytokinins
Cytokinins are so named because they stimulate cytokinesis (cell division)
Control of Cell Division and Differentiation
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin to control cell division and differentiation in shoots and roots. (16-17partial next two on following slides)
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25. Control of Apical Dominance
Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds
If the terminal bud is removed, plants become bushier
Cytokinin promotes lateral bud growth.
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26. “Stump” after removal of apical bud
Figure 39.9
Lateral branches
“Stump” after removal of apical bud
(b) Apical bud removed
Figure 39.9 Apical dominance.
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem

27. Anti-Aging Effects
Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
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28. Gibberellins
Gibberellins have a variety of effects, such as 1) stem elongation, 2) fruit growth, 3) pollen development and 4)seed germination
(18)
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29. Gibberellins are produced in young roots and leaves
Stem Elongation
Gibberellins are produced in young roots and leaves
Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division
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30. Grapes from control vine (left) and gibberellin-treated vine (right)
Figure 39.10
(b)
Grapes from control vine (left) and gibberellin-treated vine (right)
Figure Effects of gibberellins on stem elongation and fruit growth.
(a)
Rosette form (left) and gibberellin-induced bolting (right)

31. Abscisic Acid Abscisic acid (ABA) slows growth
Unlike previous discussed hormones it inhibits growth
When first discovered it was thought to play a primary role in leaf abscission (no longer thought)
Two of the many effects of ABA
Seed dormancy
Drought tolerance (Closes stomata)
Inhibited growth (19-20)
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32. Seed Dormancy
Seed dormancy ensures that the seed will germinate only in optimal conditions
In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold
Precocious (early) germination can be caused by inactive or low levels of ABA
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33. Ethylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Auxin can also stimulate ethylene production.
The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening (21-22)
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34. The Triple Response to Mechanical Stress
Ethylene induces the triple response, which allows a growing shoot to avoid obstacles
The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth
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35. Ethylene concentration (parts per million)
Figure 39.13
Figure The ethylene-induced triple response.
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)

36. Senescence Senescence is the programmed death of cells or organs
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
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37. Leaf Abscission
A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls
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38. Table 39.1
Table 39.1 Overview of Plant Hormones

39. Fruit Ripening
A burst of ethylene production in a fruit triggers the ripening process
Ethylene triggers ripening, and ripening triggers release of more ethylene
Fruit producers can control ripening by picking green fruit and controlling ethylene levels
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40. Concept 39.3: Responses to light are critical for plant success
Light cues many key events in plant growth and development
Effects of light on plant morphology are called photomorphogenesis
Plants detect not only presence of light but also its direction, intensity, and wavelength (color)
A graph called an action spectrum depicts relative response of a process to different wavelengths
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41. Phytochromes absorb red wavelengths of light.
Different plant responses can be mediated by the same or different photoreceptors
There are two major classes of light receptors: blue-light photoreceptors and phytochromes
Phytochromes absorb red wavelengths of light.
Red light (660nm) increased germination, while far-red light (730nm) inhibited germination
The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome (24-25 & 27)
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42. RESULTS Red Dark Red Far-red Dark Dark (control) Red Far-red Red Dark
Figure 39.17
RESULTS
Red
Dark
Red
Far-red
Dark
Dark (control)
Figure Inquiry: How does the order of red and far-red illumination affect seed germination?
Red
Far-red
Red
Dark
Red
Far-red
Red
Far-red

43. Blue-Light Photoreceptors
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism (26)
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44. (28-30) Cross out 31 Pr Pfr Red light
Figure 39.19 Pr
Pfr
Red light
Responses: seed germination, control of flowering, etc.
Synthesis
Far-red light
Figure Phytochrome: a molecular switching mechanism.
Slow conversion in darkness (some plants)
Enzymatic destruction
(28-30) Cross out 31

45. Biological Clocks and Circadian Rhythms
Many plant processes oscillate during the day
Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions
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46. Figure 39.20
Figure Sleep movements of a bean plant (Phaseolus vulgaris).
Noon
Midnight

47. Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”
Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle
The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes (32 Skip examples)
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48. Photoperiodism and Responses to Seasons
Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year
Photoperiodism is a physiological response to photoperiod (33)
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49. Photoperiodism and Control of Flowering
Some processes, including flowering in many species, require a certain photoperiod
Plants that flower when a light period is shorter than a critical length are called short-day plants
Plants that flower when a light period is longer than a certain number of hours are called long-day plants
Flowering in day-neutral plants is controlled by plant maturity, not photoperiod (34)
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50. Critical Night Length
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness
Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness
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51. (35) 24 hours (a) Short day (long-night) plant Light Flash of light
Figure 39.21
24 hours
(a)
Short day (long-night) plant
Light
Flash of light
Darkness
Critical dark period
(b)
Long-day (short-night) plant
Figure Photoperiodic control of flowering.
(35)
Flash of light

52. Red light can interrupt the nighttime portion of the photoperiod
A flash of red light followed by a flash of far-red light does not disrupt night length
Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light
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53. Short-day (long-night) plant Long-day (short-night) plant
Figure 39.22
24 hours R R FR
Figure Reversible effects of red and far-red light on photoperiodic response. R FR R R FR R FR
Short-day (long-night) plant
Long-day (short-night) plant
Critical dark period

54. Long-day plant grafted to short-day plant
Figure 39.23
24 hours
24 hours
24 hours
Graft
Figure Experimental evidence for a flowering hormone.
Short-day plant
Long-day plant grafted to short-day plant
Long-day plant

55. A Flowering Hormone?
Photoperiod is detected by leaves, which cue buds to develop as flowers
The flowering signal is called florigen
Florigen may be a macromolecule governed by the FLOWERING LOCUS T (FT) gene (36)
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56. Figure 39.UN03
Figure 39.UN03 Summary figure, Concept 39.2