1. Plant Responses
AP Biology Chapter 39

2. Plants respond by signal transduction pathways just like we do!
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor
Stimulation of the receptor initiates a specific signal transduction pathway

3. A potato’s response to light is an example of cell-signal processing
Fig. 39-2
A potato’s response to light is an example of cell-signal processing
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. Ca2+ channel opened Ca2+ Fig. 39-4-3 Reception Transduction Response1
Reception 2
Transduction 3
Response
Transcription
factor 1
CYTOPLASM
NUCLEUS
NUCLEUS
Specific
protein
kinase 1
activated
Plasma
membrane
cGMP P
Second messenger
produced
Transcription
factor 2
Phytochrome
activated
by light P
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response
De-etiolation
(greening)
response
proteins
Ca2+ channel
opened
Ca2+

6. Signaling pathways due to Auxin

7. 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 involved in photosynthesis and chlorophyll production
They may also lead to changes in gene expression.

8. The Discovery of Plant Hormones
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
Tropisms are often caused by hormones

9. 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
They postulated that a signal was transmitted from the tip to the elongating region

10. F. Went concluded that the chemical was auxin and that it migrated to the shady side of the plant and caused cell growth in that area. .

11. Boysen-Jensen demonstrated that the substance was mobile and could move through a block of gelatin.

13. But, maybe the light stimulates a GROWTH INHIBITOR on the lighted side!

14. RESULTS Shaded side of coleoptile Control Light Illuminated side of
Fig. 39-5a
RESULTS
Shaded
side of
coleoptile
Control
Light
Illuminated
side of
coleoptile
Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted?

16. A Survey of Plant Hormones
In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells
Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ

17. How does auxin work in stimulating cell elongation in phototropism?

18. AUXIN
The term auxin refers to any chemical that promotes elongation of coleoptiles.
The Role of Auxin in Cell Elongation
According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane
The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric
With the cellulose loosened, the cell can elongate

19. 3 4 2 1 5 Expansins separate microfibrils from cross-
Fig. 39-8
Expansins separate
microfibrils from cross-
linking polysaccharides. 3
Cell wall–loosening
enzymes
Cross-linking
polysaccharides
Expansin
CELL WALL
Cleaving allows
microfibrils to slide. 4
Cellulose
microfibril
H2O
Cell
wall
Cell wall
becomes
more acidic. 2
Plasma
membrane
Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis
Auxin
increases
proton pump
activity. 1
Nucleus
Cytoplasm
Plasma membrane
Vacuole
CYTOPLASM 5
Cell can elongate.

20. Uses of auxin Cell elongation in phototropism and gravitropism
root formation and branching
affects secondary growth by stimulating cambium growth.
An overdose of synthetic auxins can kill eudicots ! weedkillers

21. Plant growth involves interaction between metabolites such as sugars, phytohormones and their action on gene expression. Auxin as a signaling molecule has various effects depending upon the tissue where it acts.

22. CYTOKININS
Cytokinins are so named because they stimulate cytokinesis (cell division).
Cytokinins retard the aging of some plant organs

23. Control of Apical Dominance
Cytokinins, auxin, and other factors 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

24. (a) Apical bud intact (not shown in photo)
Fig. 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

25. Gibberellins
Gibberellins or gibberellic acid (GA) have a variety of effects, such as stem elongation, fruit growth, and seed germination

26. Seed Germination Gibberellins (GA) send signal to aleurone.
Fig
Gibberellins (GA)
send signal to
aleurone. 1
Aleurone secretes
-amylase and other enzymes. 2
Sugars and other
nutrients are consumed. 3
Aleurone
Endosperm
-amylase
Sugar GA GA
Water
Figure Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley
Radicle
Scutellum
(cotyledon)

27. Abscisic Acid Abscisic acid (ABA) slows growth
Two of the many effects of ABA:
Seed dormancy
In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold
Drought tolerance
ABA is the primary internal signal that enables plants to withstand drought

28. Ethylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Also induces leaf fall (abscision) and fruit ripening.

29. The dosage effect of ethylene on impatiens
The dosage effect of ethylene on impatiens. Plants not exposed to ethylene (A). Plants exposed to 2 ppm ethylene for one day (B), two days (C), and three days (D). Initially only open flowers abscised, then buds began to abscise. After three days of exposure, all flowers and buds had been shed

32. Light Cues in Plants
Effects of light on plant morphology are called photomorphogenesis

33. (b) Coleoptile response to light colors
Fig b
Light
Time = 0 min
Effects of light on plant morphology are called photomorphogenesis
Time = 90 min
Figure Action spectrum for blue-light-stimulated phototropism in maize coleoptiles
(b) Coleoptile response to light colors

34. Phytochromes as Photoreceptors
Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life
These responses include seed germination and shade avoidance
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses

35. Fig Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
Figure Phytochrome: a molecular switching mechanism
Absorption of red light causes the Pr to change to Pfr. Far-red light reverses the conversion. Mostly, it is the Pfr that switches on physiological and developmental responses.

36. Fig
How does the order of red and far-red illumination affect seed germination?
RESULTS
red-light ?
Far-red ?
Determing factor?
Are the effects reversible?
Simulates
Inhibits
Final-light exposure
yes
Dark (control)
Red
Dark
Red
Far-red
Dark
Figure How does the order of red and far-red illumination affect seed germination?
Red
Far-red
Red
Dark
Red
Far-red
Red
Far-red

37. 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

38. Fig
Figure Sleep movements of a bean plant (Phaseolus vulgaris)
Noon
Midnight

39. 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
Some processes, including flowering in many species, require a certain photoperiod

40. Critical Night Length
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

41. What does this experiment indicate?
Fig
24 hours
(a) Short-day (long-night)
plant
What does this
experiment indicate?
Light
Flash of
light
Darkness
Critical
dark period
(b) Long-day (short-night)
plant
Figure Photoperiodic control of flowering
Red light (received by phytochromes) can interrupt the nighttime portion of the photoperiod
Flash of
light

42. A flash of far-red can reverse the effect though.
Fig
24 hours
A flash of far-red can reverse the effect though. R
RFR
Figure Reversible effects of red and far-red light on photoperiodic response
RFRR
RFRRFR
Short-day
(long-night)
plant
Long-day
(short-night)
plant
Critical dark period

43. Other Responses: Gravity
Response to gravity is known as gravitropism
Roots show positive gravitropism; shoots show negative gravitropism
Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains

44. (a) Root gravitropic bending (b) Statoliths settling
Fig
Statoliths
20 µm
Figure Positive gravitropism in roots: the statolith hypothesis
(a) Root gravitropic bending
(b) Statoliths settling

45. Mechanical Stimuli
The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance
Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

46. Fig
Figure Altering gene expression by touch in Arabidopsis

47. Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants
Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials

48. Fig
(a) Unstimulated state
(b) Stimulated state
Side of pulvinus with
flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Figure Rapid turgor movements by the sensitive plant (Mimosa pudica)
Pulvinus
(motor
organ)
Vein
0.5 µm
(c) Cross section of a leaflet pair in the stimulated state (LM)

49. How plants react to environmental stresses
Drought: close stomata, slow leaf growth, reduce exposed surface, deep roots
Heat stress – heat shock proteins protect them
Cold – alter lipids in cell membrane
Salt – increased solute conc in cells
Flooding – make air spaces in root cortex

50. How plants resist herbivores and pathogens
Physical and chemical defenses
Recruit predatory animals
Immune system – gene for gene recognition, hypersensitive response, system acquired response, salicylic acid*
*In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments.

51. Beware! Chemical Defenses

52. Physical Defenses

53. Recruiting predatory animals
Ants and acacia tree

54. 4 3 1 1 2 Recruitment of parasitoid wasps that lay their eggs
Fig 4
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars 3
Synthesis and
release of
volatile attractants
Figure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 1
Wounding 1
Chemical
in saliva 2
Signal transduction
pathway

55. Recognizing plant pathogens
Fig
Recognizing plant pathogens
Signal
Hypersensitive
response
Signal
transduction
pathway
Signal transduction
pathway
Acquired
resistance
Figure Defense responses against an avirulent pathogen
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance