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

2. Overview: Stimuli and a Stationary Life
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

4. Concept 39.1: Signal transduction pathways link signal reception to response
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor

5. A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation

6. LE 39-2
Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots.
After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.

7. After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

8. A potato’s response to light is an example of cell-signal processing
The stages are reception, transduction, and response

9. CELL WALL CYTOPLASM Reception Transduction Response Activation
LE 39-3
CELL
WALL
CYTOPLASM
Reception
Transduction
Response
Activation
of cellular
responses
Relay molecules
Receptor
Hormone or
environmental
stimulus
Plasma membrane

10. Reception
Internal and external signals are detected by receptors, proteins that change in response to specific stimuli

11. Transduction
Second messengers transfer and amplify signals from receptors to proteins that cause responses

12. LE 39-4_3 Reception Transduction Response Transcription factor 1
CYTOPLASM
NUCLEUS
Specific
protein
kinase 1
activated
Plasma
membrane
cGMP
Second messenger
produced
Transcription
factor 2
Phytochrome
activated
by light
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
De-etiolation
(greening)
response
proteins
Ca2+ channel
opened
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

14. Transcriptional Regulation
Transcription factors bind directly to specific regions of DNA and control transcription of genes

15. Post-Translational Modification of Proteins
Post-translational modification involves activation of existing proteins in the signal response

16. De-Etioloation (“Greening”) Proteins
Many enzymes that function in certain signal responses are directly involved in photosynthesis
Other enzymes are involved in supplying chemical precursors for chlorophyll production

17. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
Hormones are chemical signals that coordinate different parts of an organism

18. 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

19. In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light
They observed that a 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
Video: Phototropism

20. Shaded side of Control coleoptile Light Illuminated side of coleoptile
LE 39-5a
Shaded
side of
coleoptile
Control
Light
Illuminated
side of
coleoptile

21. Darwin and Darwin (1880) Light Tip removed Tip covered by opaque cap
LE 39-5b
Darwin and Darwin (1880)
Light
Tip
removed
Tip
covered
by opaque
cap
Tip
covered
by trans-
parent
cap
Base covered
by opaque
shield

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

23. Boysen-Jensen (1913) Light Tip separated by gelatin block
LE 39-5c
Boysen-Jensen (1913)
Light
Tip
separated by
gelatin block
Tip separated
by mica

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

25. LE 39-6 Excised tip placed on agar block Growth-promoting
chemical diffuses
into agar block
Agar block
with chemical
stimulates growth
Control
(agar block
lacking
chemical)
has no
effect
Offset blocks
cause curvature
Control

26. 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

28. Auxin
The term auxin refers to any chemical that promotes cell elongation in target tissues
Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell

29. Cell 1 100 µm Cell 2 Epidermis Cortex Phloem Xylem 25 µm Basal end
of cell
Pith

30. 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

31. Cell wall enzymes Cross-linking cell wall polysaccharides Expansin
LE 39-8a
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Expansin
CELL WALL
Microfibril
ATP
Plasma membrane
CYTOPLASM

32. LE 39-8b
H2O
Cell
wall
Plasma
membrane
Nucleus
Cytoplasm
Vacuole

33. Lateral and Adventitious Root Formation
Auxin is involved in root formation and branching

34. Auxins as Herbicides
An overdose of auxins can kill eudicots

35. Other Effects of Auxin
Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

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

37. Control of Cell Division and Differentiation
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin

38. 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

39. Plant with apical bud removed
LE 39-9
Axillary buds
“Stump” after
removal of
apical bud
Lateral branches
Intact plant
Plant with apical bud removed

40. If the terminal bud is removed, plants become bushier

41. Anti-Aging Effects
Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

42. Gibberellins
Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination

43. Stem Elongation Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division

44. Fruit Growth
In many plants, both auxin and gibberellins must be present for fruit to set
Gibberellins are used in spraying of Thompson seedless grapes

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

47. Aleurone Endosperm a-amylase Sugar GA GA Water Radicle Scutellum
(cotyledon)

48. Brassinosteroids
Brassinosteroids are similar to the sex hormones of animals
They induce cell elongation and division

49. Abscisic Acid Two of the many effects of abscisic acid (ABA):
Seed dormancy
Drought tolerance

50. Seed Dormancy
Seed dormancy ensures that the seed will germinate only in optimal conditions
Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes

51. LE 39-12
Coleoptile

52. Drought Tolerance
ABA is the primary internal signal that enables plants to withstand drought

53. Ethylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection

54. 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

55. Ethylene concentration (parts per million)
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)

56. Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene

57. ein mutant ctr mutant LE 39-14
ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene.
ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene.

58. Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis

59. LE 39-15 Ethylene synthesis Ethylene inhibitor added Control Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)

60. Apoptosis: Programmed Cell Death
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

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

62. LE 39-16
0.5 mm
Protective layer
Abscission layer
Stem
Petiole

63. Fruit Ripening
A burst of ethylene production in a fruit triggers the ripening process

64. Systems Biology and Hormone Interactions
Interactions between hormones and signal transduction pathways make it hard to predict how genetic manipulation will affect a plant
Systems biology seeks a comprehensive understanding that permits modeling of plant functions

65. 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

66. 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

67. Phototropic effectiveness relative to 436 nm
LE 39-17
1.0
0.8
0.6
Phototropic effectiveness relative to 436 nm
0.4
0.2
400
450
500
550
600
650
700
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.

68. There are two major classes of light receptors: blue-light photoreceptors and phytochromes

69. Blue-Light Photoreceptors
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

70. Phytochromes as Photoreceptors
Phytochromes regulate many of a plant’s responses to light throughout its life

71. Phytochromes and Seed Germination
Studies of seed germination led to the discovery of phytochromes

72. In the 1930s, scientists at the U. S
In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds
Dark (control)
Dark
Dark

73. LE 39-18 Dark (control) Red Dark Red Dark Red Red Dark Red Red Far-red

74. The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome

75. LE 39-19
A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains.
Chromophore
Photoreceptor activity
Kinase activity

76. Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses

77. Pr Pfr Red light Responses: seed germination, control of Synthesis
LE 39-20 Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction

78. Phytochromes and Shade Avoidance
The phytochrome system also provides the plant with information about the quality of light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded

79. 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

80. LE 39-21
Noon
Midnight

81. Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long
Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle

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

83. 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

84. 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
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

85. Darkness Flash of light 24 hours Critical dark period Light
LE 39-22
Darkness
Flash of
light
24 hours
Critical
dark
period
Light
“Short-day” plants
“Long-day” plants

86. Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light
Red light can interrupt the nighttime portion of the photoperiod

87. LE 39-23 24 FR R 20 R FR FR FR Critical dark period R R R R 16 Hours12 8 4
Short-day (long-night) plant
Long-day (short-night) plant

88. A Flowering Hormone?
The flowering signal, not yet chemically identified, is called florigen
Florigen may be a hormone or a change in relative concentrations of multiple hormones

89. LE 39-24
Graft
Time
(several
weeks)

90. Meristem Transition and Flowering
For a bud to form a flower instead of a vegetative shoot, meristem identity genes must first be switched on
Researchers seek to identify the signal transduction pathways that link cues such as photoperiod and hormonal changes to the gene expression required for flowering

91. Concept 39.4: 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

92. Gravity Response to gravity is known as gravitropism
Roots show positive gravitropism
Stems show negative gravitropism

93. Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains
Video: Gravitropism

94. LE 39-25
Statoliths
20 µm

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

97. Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants

98. Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials
Video: Mimosa Leaf

99. LE 39-27 Unstimulated Stimulated Side of pulvinus with flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Pulvinus
(motor
organ)
Vein
Motor organs
0.5 mm

100. Environmental Stresses
Environmental stresses have a potentially adverse effect on survival, growth, and reproduction
They can have a devastating impact on crop yields in agriculture

101. Drought
During drought, plants respond to water deficit by reducing transpiration
Deeper roots continue to grow

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

103. Control root (aerated) Experimental root (nonaerated)
LE 39-28
Vascular
cylinder
Air tubes
Epidermis
100 µm
100 µm
Control root (aerated)
Experimental root (nonaerated)

104. Salt Stress
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

105. Heat Stress
Heat-shock proteins help plants survive heat stress

106. Cold Stress
Altering lipid composition of membranes is a response to cold stress

107. Concept 39.5: Plants defend themselves against herbivores and pathogens
Plants counter external threats with defense systems that deter herbivory and prevent infection or combat pathogens

108. 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 chemical defenses such as distasteful or toxic compounds

109. Some plants even “recruit” predatory animals that help defend against specific herbivores

110. Recruitment of parasitoid wasps that lay their eggs
LE 39-29
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
Synthesis and
release of
volatile attractants
Wounding
Chemical
in saliva
Signal transduction
pathway

111. Defenses Against Pathogens
A plant’s first line of defense against infection is its “skin,” the epidermis or 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 inherited ability to recognize certain pathogens

112. Gene-for-Gene Recognition
A virulent pathogen is one that a plant has little specific defense against
An avirulent pathogen is one that may harm but not kill the host plant
Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes

113. A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant

114. LE 39-30a
Signal molecule (ligand)
from Avr gene product
Receptor coded by R allele R
Avr allele
Avirulent pathogen
Plant cell is resistant
If an Avr allele in the pathogen corresponds to an R allele in the host plant, the host plant will have resistance, making the pathogen avirulent.

115. If the plant host lacks the R gene that counteracts the pathogen’s Avr gene, then the pathogen can invade and kill the plant

116. LE 39-30b R No Avr allele; virulent pathogen R allele; plant cell
becomes diseased
Avr allele
Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
No Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen will be virulent, causing disease to develop.

117. Plant Responses to Pathogen Invasions
A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection

118. LE 39-31 Signal Signal transduction pathway Signal transduction
Hypersensitive
response
Signal transduction
pathway
Acquired
resistance
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance

119. Systemic Acquired Resistance
Systemic acquired resistance (SAR) is a set of generalized defense responses in organs distant from the original site of infection
Salicylic acid is a good candidate for one of the hormones that activates SAR