1. Plant Responses to Internal and External Signals31
Plant Responses to Internal and External Signals

2. Outline of topics Phototropism
Role of auxin/plant experiments
Gravitropism, chemotropism, thigmatropism
Plant Hormones
Role of gibberelins/ABA in seed germination
Ethylene
Circadian Rhythms
Photoperiodism
Short day vs long day plants

3. Do now:
Hypothesize the direction of the light source in the picture below.
A tropism is a response towards or away from a stimulus. Identify the type of tropism displayed here and explain the evolutionary benefit of this process.

4. Overview: The Race to Live
Young seedlings must outcompete their neighbors in the race for resources in order to survive
Unlike animals, which respond through movement, plants must respond to environmental challenges by altering their growth and development 4

5. Concept 31.1: Plant hormones help coordinate growth, development, and responses to stimuli
Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant
Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ
Most aspects of plant growth and development are under hormonal control 5

6. 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 6

7. They postulated that a signal was transmitted from the tip to the elongating region
Video: Phototropism 7

8. Results Shaded side Control Light Illuminated side Boysen-Jensen Light
Figure 31.2
Results
Shaded
side
Control
Light
Illuminated
side
Boysen-Jensen
Light
Darwin and Darwin
Gelatin
(permeable)
Mica
(impermeable)
Figure 31.2 Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted?
Light
Opaque
shield
over
curvature
Trans-
parent
cap
Tip
removed
Opaque
cap 8

9. Quick Check:
Why does removing the tip of the coleoptile result in a lack of positive phototropism?
Identify the specific signal and response parts of phototropism in the signal transduction pathway

10. In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance10

11. In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments 11

12. Results Excised tip on agar cube Growth-promoting chemical diffuses
Figure 31.3
Results
Excised tip on
agar cube
Growth-promoting
chemical diffuses
into agar cube
Control
(agar cube
lacking
chemical)
Offset
cubes
Control
Figure 31.3 Inquiry: Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light? 12

13. A Survey of Plant Hormones
The major classes of plant hormones include
Auxin*
Cytokinins
Gibberellins*
Brassinosteroids
Abscisic acid*
Ethylene*
* know these hormones 13

14. Table 31.1
Table 31.1 Overview of plant hormones 14

15. 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 15

16. Results Cell 1 100 m Cell 2 Epidermis Cortex Phloem 25 m Xylem
Figure 31.4
Results
Cell 1
100 m
Cell 2
Epidermis
Cortex
Phloem
Figure 31.4 Inquiry: What causes polar movement of auxin from shoot tip to base?
Xylem
25 m
Basal end
of cell
Pith 16

17. Gibberellins
Gibberellins (GAs) have a variety of effects, such as stem elongation, fruit growth, and seed germination 17

18. Gibberellins are produced in young roots and leaves
Stem elongation: Gibberellins stimulate stem and leaf growth by enhancing cell elongation and cell division
Gibberellins are produced in young roots and leaves
They can induce bolting, rabid growth of the floral stalk 18

19. Figure 31.6
Figure 31.6 Effects of gibberellins on stem elongation and fruit growth 19

20. Germination: After water is imbibed, release of gibberellins from the embryo signals seeds to germinate 20

21. Aleurone Endosperm Water Radicle Scutellum (cotyledon) GA GA -amylase
Figure 31.7
Aleurone
Endosperm
-amylase
Sugar GA GA
Water
Figure 31.7 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley
Radicle
Scutellum
(cotyledon) 21

22. Abscisic Acid Abscisic acid (ABA) slows growth
Two of the many effects of ABA include
Seed dormancy
Drought tolerance 22

23. 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 23

24. ABA accumulation causes stomata to close rapidly
Drought tolerance: ABA is the primary internal signal that enables plants to withstand drought
ABA accumulation causes stomata to close rapidly 24

25. Quick check: What 3 effects do gibberellins have on a plant?
ABA is water soluble while gibberellins are not. Why is it necessary to water a plant once you have planted seeds?

26. Ethylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening 26

27. Explain the phrase “one rotten apple spoils the bunch”
Fruit ripening: A burst of ______ production in a fruit triggers the ripening process
Ethylene triggers ripening, and ripening triggers release of more ethylene
Which kind of feedback is this? Explain.
Explain the phrase “one rotten apple spoils the bunch” 27

28. Quick Check:
Grocery stores need ripe fruit for customers to buy, but it is easier to transport in an unripened state. How could ethylene solve this dilemma?
An enzyme PG softens fruit by enzymatic action of the cell wall to break it down. What would occur if PG was released but the enzyme was not in ideal conditions? Explain.
Why might putting fruits in the refrigerator delay fruit ripening?

29. Do now:
Explain why the following hormones are evolutionarily favorable
Auxin
gibberellins/ABA
ethylene

30. Do now: Identify the stimuli for the following tropisms: Gravitropism
Thigmotropism
Phototropism

31. Identify the following pictures as positive (towards) or negative (away) and the tropism

32. Identify the following pictures as positive (towards) or negative (away) and the tropism

33. Identify the following pictures as positive (towards) or negative (away) and the tropism

34. Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”
Which plant is at
Noon and which is at
Night?
Do humans have a
circadian rhythm? 34

35. Plants respond to light
There are two major classes of light receptors:
blue-light photoreceptors
phytochromes*, photoreceptors that absorb mostly red light
How does it function ? (refer to bozeman video questions) 35

36. The Effect of Light on the Biological Clock
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues 36

37. 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 37

38. 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 ________
Plants that flower when a light period is longer than a certain number of hours are called __________
Flowering in day-neutral plants is controlled by plant maturity, not photoperiod 38

39. If this is a short day plant, describe
for each scenario what the status
of the plant would be (flowered vs
Non flowered)
Definition:
Plants that flower when a light period is shorter than a critical length

41. If this is a long day plant, describe
For each scenario what the status
Of the plant would be (flowered vs
Non flowered)
Definition:
plants that flower when a light period is longer than a certain number of hours

43. Critical night length: In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length 43

44. 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 44

45. Quick check:
Why is it an error to call plants “short day” and “long day” plants? Use evidence from the observations shown

46. 24 hours (a) Short-day (long-night) plant Light Flash of light
Figure 31.16
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
Flash
of light 46

47. 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 47

50. Short-day (long-night) plant Long-day (short-night) plant
Figure 31.17
24 hours R R F R
Figure Reversible effects of red and far-red light on photoperiodic response R F R R R F R R F R
Short-day
(long-night)
plant
Long-day
(short-night)
plant
Critical dark period 50

51. Summary Quick Check
If you were a florist, and you had short day plants, in which seasons would they flower?
What can you do to induce flowering in the other seasons? (so you don’t go out of business?)

52. Exit Slip
A student makes the claim that plants behave according to environmental cues. Support or refute this claim with some examples.

53. Other plants need several successive days of the required photoperiod
Some plants flower after only a single exposure to the required photoperiod
Other plants need several successive days of the required photoperiod
Still others need an environmental stimulus in addition to the required photoperiod
For example, vernalization is a pretreatment with cold to induce flowering 53

54. Concept 31.3: Plants respond to a wide variety of stimuli other than light
Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms 54

55. Gravity Response to gravity is known as gravitropism
Roots show positive gravitropism by growing downward; shoots show negative gravitropism by growing upward
Plants may detect gravity by the settling of statoliths, dense cytoplasmic components
Video: Gravitropism 55

56. (a) Primary root of maize bending gravitropically (LMs)
Figure 31.19
Statoliths
20 m
Figure Positive gravitropism in roots: the statolith hypothesis
(a) Primary root of maize
bending gravitropically
(LMs)
(b) Statoliths settling to
the lowest sides of
root cap cells (LMs) 56

57. 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 57

58. Figure 31.20
Figure Altering gene expression by touch in Arabidopsis 58

59. Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants
Another example of a touch specialist is the sensitive plant, Mimosa pudica, which folds its leaflets and collapses in response to touch
Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials
Video: Mimosa Leaf 59

60. (a) Unstimulated state (leaflets spread apart)
Figure 31.21
Figure Rapid turgor movements by the sensitive plant (Mimosa pudica)
(a) Unstimulated state (leaflets
spread apart)
(b) Stimulated state (leaflets folded) 60