Plant Responses to Internal and External...

LECTURE PRESENTATIONS

For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

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Lectures by

Erin Barley

Kathleen Fitzpatrick

Plant Responses to Internal and

External Signals

Chapter 39

Concept 39.1: Signal

transduction pathways link

signal reception to response • 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

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(a) Before exposure to light

(b) After a week’s exposure to natural daylight

• A potato’s response to light is an example of cell-signal processing

• The stages are reception, transduction, and response

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Reception

CELL WALL

2 3 1 Transduction

CYTOPLASM

Response

Relay proteins and

second messengers

Activation of cellular responses

Receptor

Hormone or environmental stimulus Plasma membrane

Figure 39.3

Reception

CELL WALL

2 3 1 Transduction

CYTOPLASM

Response

Relay proteins and

second messengers

Activation of cellular responses

Receptor

Hormone or environmental stimulus

Plasma membrane

Figure 39.4-1

Reception 1

CYTOPLASM

Plasma membrane

Phytochrome

Cell wall

Light

- capable of detecting light

- responds to light by: Opens Ca2+

channels, which increases Ca2+ levels in

the cytosol.

- Activates an enzyme that produces cGMP

Figure 39.4-2

Reception 2 1 Transduction

CYTOPLASM Plasma membrane

Phytochrome

Cell wall

Light

cGMP

Second messenger

Ca2

Ca2 channel

Protein kinase 1

Protein kinase 2

- Opens Ca2+ channels,

which increases Ca2+

levels in the cytosol.

Responds to light by:

Activating an enzyme that

produces cGMP

Figure 39.4-3

Reception 2 3 1 Transduction Response

CYTOPLASM

Plasma membrane

Phytochrome

Cell wall

Light

cGMP

Second messenger

Ca2

Ca2 channel

Protein kinase 1

Protein kinase 2

Transcription factor 1

Transcription factor 2

NUCLEUS

Transcription

Translation

De-etiolation (greening)

response proteins

P

P Stimulation involve

increased activity of enzymes.

This can occur by

transcriptional regulation

Concept 39.2: Plant hormones help

coordinate growth, development, and

responses to stimuli

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Tropism -

• Any response resulting in curvature of organs toward or away from a stimulus.

Phototropism-

• A plant bending toward light only if the tip of the coleoptile was present.

• That means a signal must transmitted from

the tip to the elongating region of the plant.

Figure 39.5

Control

Light

Shaded side

Illuminated side

Boysen-Jensen

Light

Light

Darwin and Darwin

Gelatin (permeable)

Mica (impermeable)

Tip removed

Opaque cap

Trans- parent cap

Opaque shield over curvature

RESULTS

• In 1913, Peter Boysen-

Jensen demonstrated

that the signal was a

mobile chemical

substance.

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• In 1926, Frits Went

extracted the chemical

messenger for

phototropism, auxin, by

modifying earlier

experiments

Control

RESULTS

Excised tip on agar cube

Growth-promoting chemical diffuses into agar cube

Control (agar cube lacking chemical)

Offset cubes

Auxin

The Role of Auxin in Cell Elongation

• - Acid Growth Hypothesis - 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

Plasma membrane

Cell wall

Nucleus Cytoplasm

Vacuole

H2O

• Refers to any chemical that promotes elongation of coleoptiles. Indoleacetic acid (IAA) is a common auxin in plants;

• Auxin is produced in shoot tips

and is transported down the stem

Auxin’s Role in Plant Development

• Pattern formation of the developing plant

• Reduced auxin flow from the shoot of a branch stimulates growth in lower branches

• Plays a role in phyllotaxy, the arrangement of leaves on the stem

• Directs leaf venation pattern

• Activity of the vascular cambium is under control of auxin transport

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An overdose of synthetic auxins can kill plants For example 2,4-D is used as an herbicide on eudicots

Cytokinins • stimulate cytokinesis (cell division)

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• Cytokinins work together with auxin to

control cell division and differentiation

Control of Apical Dominance

• Terminal bud suppresses development of axillary

buds

• If the terminal bud is removed, plants become

bushier

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

(b) Apical bud removed

(c) Auxin added to decapitated stem

Axillary buds

Lateral branches

“Stump” after removal of apical bud

Figure 39.9

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

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•Produced in young roots and leaves

•Stimulate growth of leaves and stems

•Stimulate cell elongation and

cell division

(a) Rosette form (left) and gibberellin-induced Bolting (right)

(b) Grapes from control vine (left) and gibberellin-

treated vine (right)

Fruit Growth Gibberellins are used in

spraying of Thompson

seedless grapes

Germination

• After water is imbibed, release of gibberellins

from the embryo signals seeds to germinate

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Aleurone Endosperm

Water

Scutellum (cotyledon)

Radicle

-amylase Sugar GA

GA

1 2 3

Abscisic Acid

Abscisic acid (ABA) slows growth

Two of the many effects of ABA

• Seed dormancy

• Drought tolerance

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Seed dormancy

• Ensures that the seed

will germinate only in

optimal conditions

• Dormancy is broken

when ABA is removed by

heavy rain, light, or

prolonged cold

Drought Tolerance

• ABA accumulation

causes stomata to close

rapidly

• Primary internal signal

that enables plants to

withstand drought

Ethylene

Plants produce ethylene in response to stresses such

as drought, flooding, mechanical pressure, injury,

and infection

Effects of ethylene include

• response to mechanical stress

• Senescence

• leaf abscission

• fruit ripening

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• Senescence is the

programmed death of cells

(apoptosis) or organs

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0.5 mm

Stem Petiole

Protective layer Abscission layer

• Leaf Abscission

A change in the balance

of auxin and ethylene

controls- abcission layer

grows and cuts off

nutrients to leaf.

Fruit Ripening

• Ethylene triggers ripening, and ripening triggers

release of more ethylene

• Fruit producers can control ripening by picking

green fruit and controlling ethylene levels

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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 light’s direction, intensity, and wavelength (color)

• An action spectrum depicts relative response of a process to different wavelengths

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(a) Phototropism action spectrum

(b) Coleoptiles before and after light exposures

1.0

0.8

0.6

0.4

0.2

0

436 nm

400 450 500 550 600 650 700

Wavelength (nm)

Ph

oto

tro

pic

eff

ecti

ven

ess

Light

Time 0 min

Time 90 min

• There are two major classes of light receptors:

• blue-light photoreceptors - control

hypocotyl elongation, stomatal opening, and

phototropism

• phytochromes - responses include seed

germination and shade avoidance

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Figure 39.17

RESULTS

Red Red

Red Red Red Red

Far-red

Far-red Far-red Far-red

Dark (control)

Dark Dark

Dark

• 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

germination

Biological Clocks and Circadian Rhythms

• Many plant processes oscillate during the day

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Noon Midnight

• Circadian rhythms

cycles that are about 24 hours

long and are governed by an

internal “clock”

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

• the relative lengths of night and day

• the environmental stimulus plants use most often to

detect the time of year

Photoperiodism and Control of Flowering

• Short-day plants

flower when a light period is shorter than a critical length

• Long-day plants

flower when a light period is longer than a certain number of hours

• Day-neutral plants

Flowering controlled by plant maturity, not photoperiod

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Needs a minimum number

of hours of darkness

responses to photoperiod are actually

controlled by night length

Critical Night Length

Needs a maximum number

of hours of darkness

Figure 39.21

24 hours

Light Flash of light

Darkness

Critical dark period

Flash of light

(b) Long-day (short-night) plant – spring time bloomers

(a) Short day (long-night) plant – Cool season bloomers

A Flowering Hormone? • Photoperiod is detected by leaves, which cue buds

to develop as flowers

• The flowering signal is called florigen

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24 hours

Graft

Short-day plant

Long-day plant grafted to

short-day plant

Long-day plant

24 hours 24 hours

Gravity

• Gravitropism - response to gravity

• Roots show positive gravitropism;

• Shoots show negative gravitropism

• Plants may detect gravity by the

settling of statoliths, dense

cytoplasmic components

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Primary root of maize bending gravitropically (LMs)

Mechanical Stimuli

• Thigmomorphogenesis -changes in growth that result from “touch”- results in “clingy/wrapping” growth

• occurs in vines and other climbing plants (Cudzu, Ivy)

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How do you grow Cudzu! It’s easy. You throw the seeds to the left and you run to the right!

• Plants counter excessive herbivory with:

• physical defenses - thorns and trichomes

• chemical defenses - distasteful or toxic compounds

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

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Defenses Against Herbivores

Wounding

Signal transduction pathway

Chemical in saliva

Synthesis and release of volatile attractants

Recruitment of parasitoid wasps that lay their eggs within caterpillars

2

1 1

3

4

Figure 39.28

Wounding

Signal transduction pathway

Chemical in saliva

Synthesis and release of volatile attractants

Recruitment of parasitoid wasps that lay their eggs within caterpillars

2

1 1

3

4

Figure 39.UN03

Figure 39.UN05