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Chapter 39

Plant Responses to Internal and External Signals

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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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Fig. 39-1

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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
Stimulation of the receptor initiates a specific signal transduction pathway

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

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Fig. 39-2

(a) Before exposure to light

(b) After a week’s exposure to

natural daylight

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

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Fig. 39-3

CELL

WALL

CYTOPLASM

Reception

Transduction

Response

Relay proteins and

second messengers

Activation

of cellular

responses

Hormone or

environmental

stimulus

Receptor

Plasma membrane

1

2

3

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Reception

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

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Transduction

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

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Fig. 39-4-1

CYTOPLASM

Reception

Plasma

membrane

Cell

wall

Phytochrome

activated

by light

Light

Transduction

Second messenger

produced

cGMP

NUCLEUS

1

2

Specific

protein

kinase 1

activated

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Fig. 39-4-2

CYTOPLASM

Reception

Plasma

membrane

Cell

wall

Phytochrome

activated

by light

Light

Transduction

Second messenger

produced

cGMP

Specific

protein

kinase 1

activated

NUCLEUS

1

2

Specific

protein

kinase 2

activated

Ca²⁺ channel

opened

Ca²⁺

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Fig. 39-4-3

CYTOPLASM

Reception

Plasma

membrane

Cell

wall

Phytochrome

activated

by light

Light

Transduction

Second messenger

produced

cGMP

Specific

protein

kinase 1

activated

NUCLEUS

1

2

Specific

protein

kinase 2

activated

Ca²⁺ channel

opened

Ca²⁺

Response

3

Transcription

factor 1

Transcription

factor 2

NUCLEUS

Transcription

Translation

De-etiolation

(greening)

response

proteins

P

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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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Transcriptional Regulation

Specific transcription factors bind directly to specific regions of DNA and control transcription of genes
Positive transcription factors are proteins that increase the transcription of specific genes, while negative transcription factors are proteins that decrease the transcription of specific genes

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Post-Translational Modification of Proteins

Post-translational modification involves modification of existing proteins in the signal response
Modification often involves the phosphorylation of specific amino acids

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De-Etiolation (“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

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Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli

Hormones are chemical signals that coordinate different parts of an organism

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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

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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

Video: Phototropism

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Fig. 39-5

RESULTS

Control

Light

Darwin and Darwin: phototropic response

only when tip is illuminated

Illuminated

side of

coleoptile

Shaded

side of

coleoptile

Tip

removed

Light

Tip covered

by opaque

cap

Tip

covered

by trans-

parent

cap

Site of

curvature

covered by

opaque

shield

Boysen-Jensen: phototropic response when tip separated

by permeable barrier, but not with impermeable barrier

Tip separated

by gelatin

(permeable)

Tip separated

by mica

(impermeable)

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Fig. 39-5a

RESULTS

Control

Light

Illuminated

side of

coleoptile

Shaded

side of

coleoptile

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Fig. 39-5b

RESULTS

Light

Tip

removed

Darwin and Darwin: phototropic response

only when tip is illuminated

Tip covered

by opaque

cap

Tip

covered

by trans-

parent

cap

Site of

curvature

covered by

opaque

shield

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

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Fig. 39-5c

RESULTS

Light

Boysen-Jensen: phototropic response when tip is separated

by permeable barrier, but not with impermeable barrier

Tip separated

by gelatin

(permeable)

Tip separated

by mica

(impermeable)

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

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Fig. 39-6

Excised tip placed

on agar cube

RESULTS

Growth-promoting

chemical diffuses

into agar cube

Agar cube

with chemical

stimulates growth

Offset cubes

cause curvature

Control

(agar cube

lacking

chemical)

has no

effect

Control

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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

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Table 39-1

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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 transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell

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Fig. 39-7

100 µm

RESULTS

Cell 1

Cell 2

Epidermis

Cortex

Phloem

Xylem

Pith

Basal end

of cell

25 µm

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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

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Fig. 39-8

Cross-linking

polysaccharides

Cellulose

microfibril

Cell wall

becomes

more acidic.

2

1

Auxin

increases

proton pump

activity.

Cell wall–loosening

enzymes

Expansin

Expansins separate

microfibrils from cross-

linking polysaccharides.

3

4

5

CELL WALL

Cleaving allows

microfibrils to slide.

CYTOPLASM

Plasma membrane

H₂O

Cell

wall

Plasma

membrane

Nucleus

Cytoplasm

Vacuole

Cell can elongate.

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Lateral and Adventitious Root Formation

Auxin is involved in root formation and branching

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Auxins as Herbicides

An overdose of synthetic auxins can kill eudicots

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Other Effects of Auxin

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

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Cytokinins

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

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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

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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

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Fig. 39-9

(a) Apical bud intact (not shown in photo)

(c) Auxin added to decapitated stem

(b) Apical bud removed

Axillary buds

Lateral branches

“Stump” after

removal of

apical bud

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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

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Gibberellins

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

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Stem Elongation

Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division

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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

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Fig. 39-10

Gibberellin-induced stem

growth

(b) Gibberellin-induced fruit

growth

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Germination

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

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Fig. 39-11

Gibberellins (GA)

send signal to

aleurone.

Aleurone secretes

α-amylase and other enzymes.

Sugars and other

nutrients are consumed.

Aleurone

Endosperm

Water

Scutellum

(cotyledon)

Radicle

1

2

3

GA

α-amylase

Sugar

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Brassinosteroids

Brassinosteroids are chemically similar to the sex hormones of animals
They induce cell elongation and division in stem segments

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Abscisic Acid

Abscisic acid (ABA) slows growth
Two of the many effects of ABA:

Seed dormancy
Drought tolerance

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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 germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes

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Fig. 39-12

Early germination

in red mangrove

Early germination

in maize mutant

Coleoptile

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Drought Tolerance

ABA is the primary internal signal that enables plants to withstand drought

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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

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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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Fig. 39-13

Ethylene concentration (parts per million)

0.10

0.00

0.20

0.40

0.80

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Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene
Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis

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Fig. 39-14

ein mutant

ctr mutant

(a) ein mutant

(b) ctr mutant

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Senescence

Senescence is the programmed death of plant cells or organs
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

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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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Fig. 39-15

0.5 mm

Protective layer

Stem

Abscission layer

Petiole

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Fruit Ripening

A burst of ethylene production in a fruit triggers the ripening process

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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

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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

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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

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Phototropic effectiveness

Fig. 39-16

436 nm

1.0

0.8

0.6

0.4

0.2

0

400

450

500

550

600

650

700

Wavelength (nm)

(a) Action spectrum for blue-light phototropism

Light

Time = 0 min

Time = 90 min

(b) Coleoptile response to light colors

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Phototropic effectiveness

Fig. 39-16a

436 nm

1.0

0.8

0.6

0.4

0.2

400

450

500

550

600

650

700

Wavelength (nm)

0

(a) Action spectrum for blue-light phototropism

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Fig. 39-16b

Light

Time = 0 min

(b) Coleoptile response to light colors

Time = 90 min

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

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Blue-Light Photoreceptors

Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

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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

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Phytochromes and Seed Germination

Many seeds remain dormant until light conditions change
In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds

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Fig. 39-17

Dark (control)

RESULTS

Dark

Red

Far-red

Red

Dark

Red

Far-red

Red

Far-red

Red

Far-red

Dark

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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

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Fig. 39-18

Two identical subunits

Chromophore

Photoreceptor activity

Kinase activity

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Fig. 39-UN1

Red light

Far-red light

P_r

P_fr

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Phytochromes exist in two photoreversible states, with conversion of P_r to P_fr triggering many developmental responses

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Fig. 39-19

Synthesis

P_r

Far-red

light

Slow conversion

in darkness

(some plants)

Enzymatic

destruction

Responses:

seed germination,

control of

flowering, etc.

P_fr

Red light

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Phytochromes and Shade Avoidance

The phytochrome system also provides the plant with information about the quality of light
Shaded plants receive more far-red than red light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of P_r when a tree is shaded

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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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Fig. 39-20

Noon

Midnight

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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

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The Effect of Light on the Biological Clock

Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues

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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

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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

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Critical Night Length

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

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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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Fig. 39-21

24 hours

Light

Critical

dark period

Flash

of

light

Darkness

(a) Short-day (long-night)

plant

Flash

of

light

(b) Long-day (short-night)

plant

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

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Fig. 39-22

24 hours

R

RFR

RFRR

RFRRFR

Critical dark period

Short-day

(long-night)

plant

Long-day

(short-night)

plant

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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

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A Flowering Hormone?

The flowering signal, not yet chemically identified, is called florigen
Florigen may be a macromolecule governed by the CONSTANS gene

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Fig. 39-23

24 hours

Graft

Short-day

plant

24 hours

Long-day plant

grafted to

short-day plant

Long-day

plant

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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

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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

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Gravity

Response to gravity is known as gravitropism
Roots show positive gravitropism; shoots show negative gravitropism

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Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains

Video: Gravitropism

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Fig. 39-24

Statoliths

20 µm

(b) Statoliths settling

(a) Root gravitropic bending

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Some mutants that lack statoliths are still capable of gravitropism
Dense organelles, in addition to starch granules, may contribute to gravity detection

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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

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Fig. 39-25

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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

Video: Mimosa Leaf

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Fig. 39-26

(a) Unstimulated state

Leaflets

after

stimulation

Pulvinus

(motor

organ)

(c) Cross section of a leaflet pair in the stimulated state (LM)

(b) Stimulated state

Side of pulvinus with

flaccid cells

Side of pulvinus with

turgid cells

Vein

0.5 µm

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Fig. 39-26ab

(a) Unstimulated state

(b) Stimulated state

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Fig. 39-26c

Leaflets

after

stimulation

(c) Cross section of a leaflet pair in the stimulated state (LM)

Side of pulvinus with

flaccid cells

Side of pulvinus with

turgid cells

Vein

Pulvinus

(motor

organ)

0.5 µm

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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

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Drought

During drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface area
Growth of shallow roots is inhibited, while deeper roots continue to grow

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Flooding

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

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Fig. 39-27

(a) Control root (aerated)

Vascular

cylinder

Air tubes

Epidermis

(b) Experimental root (nonaerated)

100 µm

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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

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Heat Stress

Excessive heat can denature a plant’s enzymes
Heat-shock proteins help protect other proteins from heat stress

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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

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Concept 39.5: Plants respond to attacks by herbivores and pathogens

Plants use defense systems to deter herbivory, prevent infection, and combat pathogens

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

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Fig. 39-28

Recruitment of

parasitoid wasps

that lay their eggs

within caterpillars

Synthesis and

release of

volatile attractants

Chemical

in saliva

Wounding

Signal transduction

pathway

1

2

3

4

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Plants damaged by insects can release volatile chemicals to warn other plants of the same species
Methyljasmonic acid can activate the expression of genes involved in plant defenses

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Defenses Against Pathogens

A plant’s first line of defense against infection is 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 inherited ability to recognize certain pathogens

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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

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Gene-for-gene recognition involves recognition of pathogen-derived molecules by 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

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The Hypersensitive Response

The hypersensitive response

Causes cell and tissue death near the infection site
Induces production of phytoalexins and PR proteins, which attack the pathogen
Stimulates changes in the cell wall that confine the pathogen

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Fig. 39-29

Signal

Hypersensitive

response

Signal transduction

pathway

Avirulent

pathogen

Signal

transduction

pathway

Acquired

resistance

R-Avr recognition and

hypersensitive response

Systemic acquired

resistance

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Systemic Acquired Resistance

Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response
Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance

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Fig. 39-UN2

Plasma membrane

Reception

Response

CELL

WALL

CYTOPLASM

Transduction

Receptor

Hormone or

environmental

stimulus

Relay proteins and

second messengers

Activation

of cellular

responses

1

2

3

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Fig. 39-UN3

Photoreversible states of phytochrome

P_r

P_fr

Red light

Far-red

light

Responses

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Fig. 39-UN4

Control

Ethylene

added

Ethylene

synthesis

inhibitor

Wild-type

Ethylene insensitive

(ein)

Ethylene

overproducing (eto)

Constitutive triple

response (ctr)

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Fig. 39-UN5

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You should now be able to:

Compare the growth of a plant in darkness (etiolation) to the characteristics of greening (de-etiolation) 5-7

*8-17 cell communication "AP Bio core concept"

List six classes of plant hormones and describe their major functions 27

Describe the phenomenon of phytochrome photoreversibility and explain its role in light-induced germination of lettuce seeds 67-72

Explain how light entrains biological clocks 79-81

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Distinguish between short-day, long-day, and day-neutral plants; explain why the names are misleading 82-86

Describe how plants tell up from down 92-98

Distinguish between thigmotropism and thigmomorphogenesis 97-101

Describe the challenges posed by, and the responses of plants to, drought, flooding, salt stress, heat stress, and cold stress 107-111

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Describe how the hypersensitive response helps a plant limit damage from a pathogen attack 117-122