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

2. 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 2

3. Figure 31.1
Figure 31.1 How do plants detect light? 3

4. 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 4

5. 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 region7

8. Video: Phototropism

9. 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 9

10. Shaded side Control Light Illuminated side Figure 31.2a
Figure 31.2a Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? (part 1: control) 10

11. Darwin and Darwin: Phototropism occurs only
Figure 31.2b
Darwin and Darwin: Phototropism occurs only
when the tip is illuminated.
Light
Opaque
shield
over
curvature
Trans-
parent
cap
Figure 31.2b Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? (part 2: Darwin and Darwin)
Tip
removed
Opaque
cap 11

12. Boysen-Jensen: Phototropism occurs
Figure 31.2c
Boysen-Jensen: Phototropism occurs
when the tip is separated by a permeable
barrier but not an impermeable barrier.
Light
Figure 31.2c Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? (part 3: Boysen-Jensen)
Gelatin
(permeable)
Mica
(impermeable) 12

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

14. In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments 14

15. 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? 15

16. A Survey of Plant Hormones
The major classes of plant hormones include
Auxin
Cytokinins
Gibberellins
Brassinosteroids
Abscisic acid
Ethylene 16

17. Table 31.1
Table 31.1 Overview of plant hormones 17

18. 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 18

19. 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 19

20. 100 m Epidermis Cortex Phloem Xylem Pith Figure 31.4a
Figure 31.4a Inquiry: What causes polar movement of auxin from shoot tip to base? (part 1: LM, lower magnification)
Pith 20

21. Cell 1 Cell 2 25 m Basal end of cell Figure 31.4b
Figure 31.4b Inquiry: What causes polar movement of auxin from shoot tip to base? (part 2: LM, higher magnification)
25 m
Basal end
of cell 21

22. With the cellulose loosened, the cell can elongate
The role of auxin in cell elongation: Polar transport of auxin stimulates proton pumps in the plasma membrane
According to the acid growth hypothesis, 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 22

23. 4 3 H 2 H H H H H H H 1 ATP H 5
Figure 31.5
CELL WALL 4
Cell wall-loosening
enzymes cleave
cross-linking
polysaccharides. 3
Low pH
activates
expansins.
H2O
Cell
wall H
Plasma
membrane 2
Acidity
increases. H H H H H H H
Figure 31.5 Cell elongation in response to auxin: the acid growth hypothesis 1
Proton
pump
activity
increases.
Nucleus
Cytoplasm
Plasma membrane
Vacuole
ATP H 5
Sliding cellulose microfibrils
allow cell to elongate.
CYTOPLASM 23

24. 4 3 H 2 H H H H H H H 1 Plasma membrane H
Figure 31.5a 4
Cell wall-loosening enzymes cleave cross-linking
polysaccharides.
CELL WALL 3
Low pH
activates
expansins. H 2
Acidity
increases. H H H H H H
Figure 31.5a Cell elongation in response to auxin: the acid growth hypothesis (part 1: mechanism) H 1
Proton
pump
activity
increases.
Plasma membrane
ATP H
CYTOPLASM 24

25. Sliding cellulose microfibrils allow cell to elongate.
Figure 31.5b
H2O
Cell
wall
Plasma
membrane
Figure 31.5b Cell elongation in response to auxin: the acid growth hypothesis (part 2: cell elongation)
Nucleus
Cytoplasm
Vacuole 5
Sliding cellulose microfibrils
allow cell to elongate. 25

26. Auxin also alters gene expression and stimulates a sustained growth response26

27. Auxin’s role in plant development: Polar transport of auxin controls the spatial organization of the developing plant
Reduced auxin flow from the shoot of a branch stimulates growth in lower branches
Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem 27

28. Practical uses for auxins
The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings
An overdose of synthetic auxins can kill plants
For example 2,4-D is used as an herbicide on eudicots
Tomato growers spray their plants with synthetic auxins to stimulate fruit growth 28

29. Cytokinins
Cytokinins are so named because they stimulate cytokinesis (cell division) 29

30. Control of cell division and differentiation: Cytokinins work together with auxin to control cell division and differentiation
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits 30

31. 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 31

32. Gibberellins
Gibberellins (GAs) have a variety of effects, such as stem elongation, fruit growth, and seed germination 32

33. 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 33

34. vine (left) and gibberellin- treated vine (right)
Figure 31.6
Grapes from control
vine (left) and gibberellin-
treated vine (right)
Figure 31.6 Effects of gibberellins on stem elongation and fruit growth
(a) Rosette form (left) and
gibberellin-induced bolting
(right) 34

35. (a) Rosette form (left) and gibberellin-induced bolting (right)
Figure 31.6a
Figure 31.6a Effects of gibberellins on stem elongation and fruit growth (part 1: bolting)
(a) Rosette form (left) and
gibberellin-induced bolting
(right) 35

36. vine (left) and gibberellin- treated vine (right)
Figure 31.6b
Grapes from control
vine (left) and gibberellin-
treated vine (right)
Figure 31.6b Effects of gibberellins on stem elongation and fruit growth (part 2: fruits) 36

37. Gibberellins are used in spraying of Thompson seedless grapes
Fruit growth: In many plants, both auxin and gibberellins must be present for fruit to develop
Gibberellins are used in spraying of Thompson seedless grapes 37

38. Germination: After water is imbibed, release of gibberellins from the embryo signals seeds to germinate 38

39. 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) 39

40. Brassinosteroids
Brassinosteroids are chemically similar to cholesterol and the sex hormones of animals
They induce cell elongation and division in stem segments and seedlings
They slow leaf abscission (leaf drop) and promote xylem differentiation 40

41. Abscisic Acid Abscisic acid (ABA) slows growth
Two of the many effects of ABA include
Seed dormancy
Drought tolerance 41

42. 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 42

43. Red mangrove (Rhizophora mangle) seeds Coleoptile Maize mutant
Figure 31.8
Red mangrove
(Rhizophora
mangle) seeds
Coleoptile
Figure 31.8 Precocious germination of wild-type mangrove and mutant maize seeds
Maize mutant 43

44. Red mangrove (Rhizophora mangle) seeds Figure 31.8a
Figure 31.8a Precocious germination of wild-type mangrove and mutant maize seeds (part 1: wild-type mangrove seeds) 44

45. Coleoptile Maize mutant Figure 31.8b
Figure 31.8b Precocious germination of wild-type mangrove and mutant maize seeds (part 2: mutant maize seeds)
Maize mutant 45

46. 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 46

47. 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 47

48. 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 48

49. Other mutants are not responsive to inhibitors of ethylene synthesis
Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene
Some ethylene-overproducing mutants undergo the triple response even in air but are returned to normal growth when treated with ethylene synthesis inhibitors
Other mutants are not responsive to inhibitors of ethylene synthesis 49

50. ein mutant ctr mutant (a) ein mutant (b) ctr mutant Figure 31.9
Figure 31.9 Ethylene triple-response Arabidopsis mutants
(a) ein mutant
(b) ctr mutant 50

51. ein mutant (a) ein mutant Figure 31.9a
Figure 31.9a Ethylene triple-response Arabidopsis mutants (part 1: ein mutant)
(a) ein mutant 51

52. ctr mutant (b) ctr mutant Figure 31.9b
Figure 31.9b Ethylene triple-response Arabidopsis mutants (part 2: ctr mutant)
(b) ctr mutant 52

53. 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 53

54. Leaf abscission: A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls 54

55. 0.5 mm Protective layer Abscission layer Stem Petiole
Figure 31.10
0.5 mm
Figure Abscission of a maple leaf
Protective layer
Abscission layer
Stem
Petiole 55

56. 0.5 mm Protective layer Abscission layer Stem Petiole
Figure 31.10a
0.5 mm
Figure 31.10a Abscission of a maple leaf (part 1: LM)
Protective layer
Abscission layer
Stem
Petiole 56

57. 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 57

58. Concept 31.2: Responses to light are critical for plant success
Light triggers many key events in plant growth and development, collectively known as photomorphogenesis 58

59. Photomorphogenesis
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 59

60. (a) Before exposure to light (b) After a week’s exposure
Figure 31.11
Figure Light-induced de-etiolation (greening) of dark-grown potatoes
(a) Before exposure to light
(b) After a week’s exposure
to natural daylight 60

61. (a) Before exposure to light
Figure 31.11a
Figure 31.11a Light-induced de-etiolation (greening) of dark-grown potatoes (part 1: before)
(a) Before exposure to light 61

62. (b) After a week’s exposure to natural daylight
Figure 31.11b
Figure 31.11b Light-induced de-etiolation (greening) of dark-grown potatoes (part 2: after)
(b) After a week’s exposure
to natural daylight 62

63. 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
Action spectra are useful in studying any process that depends on light 63

64. Phototropic effectiveness
Figure 31.12
1.0
436 nm
0.8
Refracting prism
0.6
Phototropic effectiveness
0.4
White
light
0.2
400
450
500
550
600
650
700
Figure Action spectrum for blue-light-stimulated phototropism in maize coleoptiles
Wavelength (nm)
Light wavelengths below 500nm induce
curvature.
Blue light induces the most curvature
of coleoptiles. 64

65. Phototropic effectiveness
Figure 31.12a
1.0
436 nm
0.8
0.6
Phototropic effectiveness
0.4
0.2
Figure 31.12a Action spectrum for blue-light-stimulated phototropism in maize coleoptiles (part 1: action spectrum)
400
450
500
550
600
650
700
Wavelength (nm)
Light wavelengths below 500nm induce
curvature. 65

66. Blue light induces the most curvature of coleoptiles.
Figure 31.12b
Refracting prism
White
light
Figure 31.12b Action spectrum for blue-light-stimulated phototropism in maize coleoptiles (part 2: prism)
Blue light induces the most curvature
of coleoptiles. 66

67. 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, photoreceptors that absorb mostly red light 67

68. Blue-Light Photoreceptors
Various blue-light photoreceptors control phototropism (movement in response to light), stomatal opening, and hypocotyl elongation 68

69. Phytochrome 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 69

70. Phytochromes and seed germination: Many seeds remain dormant until light conditions are optimal
In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds 70

71. Red light increased germination, while far-red light inhibited germination
The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome 71

72. Results Red Dark Red Far-red Dark Dark (control) Red Far-red Red Dark
Figure 31.13
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 72

73. Dark (control) Figure 31.13a
Figure 31.13a Inquiry: How does the order of red and far-red illumination affect seed germination? (part 1: control) 73

74. Figure 31.13b
Red
Dark
Figure 31.13b Inquiry: How does the order of red and far-red illumination affect seed germination? (part 2: red, dark) 74

75. Red Far-red Dark Figure 31.13c
Figure 31.13c Inquiry: How does the order of red and far-red illumination affect seed germination? (part 3: red, far-red, dark) 75

76. Red Far-red Red Dark Figure 31.13d
Figure 31.13d Inquiry: How does the order of red and far-red illumination affect seed germination? (part 4: red, far-red, red, dark) 76

77. Red Far-red Red Far-red Figure 31.13e
Figure 31.13e Inquiry: How does the order of red and far-red illumination affect seed germination? (part 5: red, far-red, red, far-red, dark) 77

78. Red light triggers the conversion of Pr to Pfr
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses
Red light triggers the conversion of Pr to Pfr
Far-red light triggers the conversion of Pfr to Pr
The conversion to Pfr is faster than the conversion to Pr
Sunlight increases the ratio of Pfr to Pr and triggers germination 78

79. • Inhibition of vertical growth and stimu- lation of branching
Figure 31.14
Responses to Pfr:
• Seed germination
• Inhibition of vertical
growth and stimu-
lation of branching
• Setting internal clocks
• Control of flowering
Red light
Synthesis Pr
Pfr
Far-red
light
Slow conversion
in darkness
(some species)
Enzymatic
destruction
Figure Phytochrome: a molecular switching mechanism 79

80. Leaves in the canopy absorb red light
Phytochromes and shade avoidance: The phytochrome system also provides the plant with information about the quality of light
Leaves in the canopy absorb red light
Shaded plants receive more far-red than red light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded
This shift induces the vertical growth of the plant 80

81. 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 81

82. Figure 31.15
Figure Sleep movements of a bean plant (Phaseolus vulgaris)
Noon
10:00 PM 82

83. Figure 31.15a
Figure 31.15a Sleep movements of a bean plant (Phaseolus vulgaris) (part 1: noon)
Noon 83

84. Figure 31.15b
Figure 31.15b Sleep movements of a bean plant (Phaseolus vulgaris) (part 2: 10 PM)
10:00 PM 84

85. 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 85

86. The Effect of Light on the Biological Clock
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues 86

87. 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 87

88. 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 88

89. Critical night length: In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length 89

90. 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 90

91. 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 91

92. 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 92

93. 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 93

94. 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 94

95. 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 protein governed by the FLOWERING LOCUS T (FT) gene 95

96. 24 hours 24 hours 24 hours Graft Short-day plant Long-day plant
Figure 31.18
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 96

97. 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 97

98. 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 98

99. Video: Gravitropism

100. (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)
100

101. Figure 31.19a
Figure 31.19a Positive gravitropism in roots: the statolith hypothesis (part 1: horizontal root)
101

102. Figure 31.19b
Figure 31.19b Positive gravitropism in roots: the statolith hypothesis (part 2: bent root)
102

103. Figure 31.19c
Statoliths
Figure 31.19c Positive gravitropism in roots: the statolith hypothesis (part 3: statoliths at start)
20 m
103

104. Figure 31.19d
Statoliths
20 m
Figure 31.19d Positive gravitropism in roots: the statolith hypothesis (part 4: settled statoliths)
104

105. Some mutants that lack statoliths are still capable of gravitropism
Mechanical pulling on proteins that connect the protoplast to the cell wall may aid in gravity detection
Dense organelles and starch granules may also contribute to gravity detection
105

106. 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
106

107. Figure 31.20
Figure Altering gene expression by touch in Arabidopsis
107

108. 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
108

109. Video: Mimosa Leaf

110. (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)
110

111. (a) Unstimulated state (leaflets spread apart)
Figure 31.21a
Figure 31.21a Rapid turgor movements by the sensitive plant (Mimosa pudica) (part 1: unstimulated)
(a) Unstimulated state (leaflets spread apart)
111

112. (b) Stimulated state (leaflets folded)
Figure 31.21b
Figure 31.21b Rapid turgor movements by the sensitive plant (Mimosa pudica) (part 2: stimulated)
(b) Stimulated state (leaflets folded)
112

113. Environmental Stresses
Environmental stresses have a potentially adverse effect on survival, growth, and reproduction
Stresses can be abiotic (nonliving) or biotic (living)
Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress
Biotic stresses include herbivores and pathogens
113

114. Drought
During drought, plants reduce transpiration by closing stomata, reducing exposed surface area, or even shedding their leaves
114

115. Flooding
Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding
115

116. (a) Control root (aerated) (b) Experimental root (nonaerated)
Figure 31.22
Vascular
cylinder
Air tubes
Epidermis
Figure A developmental response of maize roots to flooding and oxygen deprivation
100 m
100 m
(a) Control root (aerated)
(b) Experimental root
(nonaerated)
116

117. (a) Control root (aerated)
Figure 31.22a
Vascular
cylinder
Figure 31.22a A developmental response of maize roots to flooding and oxygen deprivation (part 1: aerated)
Epidermis
100 m
(a) Control root (aerated)
117

118. Vascular cylinder Air tubes Epidermis 100 m (b) Experimental root
Figure 31.22b
Vascular
cylinder
Air tubes
Epidermis
Figure 31.22b A developmental response of maize roots to flooding and oxygen deprivation (part 2: nonaerated)
100 m
(b) Experimental root
(nonaerated)
118

119. Salt Stress
Salt can lower the water potential of the soil solution and reduce water uptake
Plants respond to salt stress by producing solutes tolerated at high concentrations
This process keeps the water potential of cells more negative than that of the soil solution
119

120. Heat Stress Excessive heat can denature a plant’s enzymes
Heat-shock proteins, which help protect other proteins from heat stress, are produced at high temperatures
120

121. Cold Stress Cold temperatures decrease membrane fluidity
Altering lipid composition of membranes is a response to cold stress
Freezing causes ice to form in a plant’s cell walls and intercellular spaces
Water leaves the cell in response to freezing, leading to toxic solute concentrations in the cytoplasm
121

122. Many plants, as well as other organisms, have antifreeze proteins that prevent ice crystals from growing and damaging cells
The five classes of antifreeze proteins have markedly different amino acid sequences but similar structure, indicating they arose through convergent evolution
122

123. Concept 31.4: Plants respond to attacks by herbivores and pathogens
Through natural selection, plants have evolved defense systems to deter herbivory, prevent infection, and combat pathogens
123

124. Defenses Against Herbivores
Herbivory, animals eating plants, is a stress that plants face in any ecosystem
Plants counter excessive herbivory with physical defenses, such as thorns and trichomes, and chemical defenses, such as distasteful or toxic compounds
Some plants even “recruit” predatory animals that help defend against specific herbivores
124

125. 4 Recruitment of parasitoid wasps that lay their eggs
Figure 31.23 4
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars 3
Synthesis
and release
of volatile
attractants 1
Wounding 1
Chemical
in saliva
Figure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 2
Signal transduction
pathway
125

126. Plants damaged by insects can release volatile chemicals to warn other plants of the same species
Arabidopsis can be genetically engineered to produce volatile components that attract predatory mites
126

127. Defenses Against Pathogens
A plant’s first line of defense against infection is the barrier presented by the epidermis and periderm
If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread
This second defense system is enhanced by the plant’s ability to recognize certain pathogens
127

128. Host-Pathogen Coevolution
A virulent pathogen is one that a plant has little specific defense against
An avirulent pathogen is one that may harm but does not kill the host plant
128

129. Gene-for-gene recognition involves recognition of effector molecules by the protein products of specific plant disease resistance (R) genes
An R protein recognizes a corresponding molecule made by the pathogen’s Avr gene
R proteins activate plant defenses by triggering signal transduction pathways
These defenses include the hypersensitive response and systemic acquired resistance
129

130. The Hypersensitive Response
Causes localized cell and tissue death near the infection site
Induces production of phytoalexins and PR proteins, which attack the specific pathogen
Stimulates changes in the cell wall that confine the pathogen
130

131. Infected tobacco leaf with lesions
Figure 31.24
Infected tobacco leaf with lesions
Signal 4 5 6
Hypersensitive
response
Signal
transduction
pathway 3 2
Signal transduction pathway
Figure Defense responses against an avirulent pathogen 7
Acquired
resistance 1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
131

132. Signal transduction pathway
Figure 31.24a
Signal 4 5 6
Hypersensitive
response
Signal
transduction
pathway 3 2
Signal transduction pathway 7
Acquired
resistance 1
Figure 31.24a Defense responses against an avirulent pathogen (part 1: art)
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
132

133. Infected tobacco leaf with lesions
Figure 31.24b
Figure 31.24b Defense responses against an avirulent pathogen (part 2: photo)
Infected tobacco leaf with lesions
133

134. Systemic Acquired Resistance
Causes plant-wide expression of defense genes
Protects against a diversity of pathogens
Provides a long-lasting response
Methylsalicylic acid travels from an infection site to remote areas of the plant where it is converted to salicylic acid, which initiates pathogen resistance
134

135. Roots not connected Roots connected Osmotic shock Figure 31.UN01a
Figure 31.UN01a Skills exercise: interpreting experimental results from a bar graph (part 1)
135

136. 15 minutes after osmotic shock 13
Figure 31.UN01b 14
Control
15 minutes after osmotic shock 13 12 11
Stomatal opening (M) 10 9 8
Figure 31.UN01b Skills exercise: interpreting experimental results from a bar graph (part 2) 7 6 1 2 3 4 5 6 7 8 9 10 11
Plant number
136

137. Figure 31.UN02
Figure 31.UN02 Summary of key concepts: plant hormones
137

138. Figure 31.UN04
Figure 31.UN04 Summary of key concepts: environmental stress
138

139. Pea plant (Pisum sativum)
Figure 31.UN01c
Figure 31.UN01c Skills exercise: interpreting experimental results from a bar graph (part 3)
Pea plant (Pisum sativum)
139

140. Red light Pr Pfr Responses Far-red light Figure 31.UN03
Figure 31.UN03 Summary of key concepts: responses to light
140

141. Ethylene synthesis inhibitor Ethylene added
Figure 31.UN05
Ethylene
synthesis
inhibitor
Ethylene
added
Control
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Figure 31.UN05 Test your understanding, question 7 (ethylene mutants)
Constitutive triple
response (ctr)
141