LECTURE PRESENTATIONS

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

© 2011 Pearson Education, Inc.

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 exposureto natural daylight

Figure 39.3

Reception

CELLWALL

2 31 Transduction

CYTOPLASM

Response

Relay proteins and

second messengers

Activationof cellularresponses

Receptor

Hormone orenvironmentalstimulus Plasma membrane

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

processing

Figure 39.4-1

Reception1

CYTOPLASM

Plasmamembrane

Phytochrome

Cellwall

Light

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

• In de-etiolation, the receptor is capable of detecting light

Reception

Figure 39.4-2

Reception 21 Transduction

CYTOPLASM

Plasmamembrane

Phytochrome

Cellwall

Light

cGMP

Secondmessenger

Ca2

Ca2 channel

Proteinkinase 1

Proteinkinase 2

• Second messengers transfer and amplify signals

from receptors to proteins that cause responses

Transduction

Figure 39.4-3

Reception 2 31 Transduction Response

CYTOPLASM

Plasmamembrane

Phytochrome

Cellwall

Light

cGMP

Secondmessenger

Ca2

Ca2 channel

Proteinkinase 1

Proteinkinase 2

Transcriptionfactor 1

Transcriptionfactor 2

NUCLEUS

Transcription

Translation

De-etiolation(greening)

response proteins

P

P

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 changes to the protein

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Concept 39.2: 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

• Tropism: growth of a plant towards or away from a stimulus

• Positive tropism (growing towards stimulus)

• Negative tropism (growing away from stimulus)

• Thigmotropism: touch

• Geotropism or Gravitropism: gravity

• Phototropism: Light

• Photoperiodism: the physiological response to the photoperiod

• Photoperiod: environmental stimulus a plant uses to detect the time of year

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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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Noon Midnight

Photoperiodism and Control of Flowering

• Some processes, including flowering in many species, require a certain photoperiod

• Depends on length of night, not day

– 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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Figure 39.2124 hours

Light Flashoflight

Darkness

Criticaldark period

Flashof light

(b) Long-day(short-night) plant

(a) Short day(long-night) plant

Gravity• Response to gravity is known as gravitropism

• 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 maizebending gravitropically(LMs)

Statoliths settling tothe lowest sides ofroot cap cells (LMs)

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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• 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

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(a) Unstimulated state (b) Stimulated state

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

Leafletsafterstimulation

Pulvinus(motororgan)

Side of pulvinuswith flaccid cells

Side of pulvinuswith turgid cells

Vein

0.5

m

Figure 39.26

A Survey of Plant Hormones

• Plant hormones are produced in very low

concentration, but a minute amount can greatly

affect growth and development of a plant organ

• In general, hormones control plant growth and

development by affecting the division,

elongation, and differentiation of cells

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Complete # 1-9

Auxin

• Effects

– controls cell division

& differentiation

– phototropism

• growth towards light

• asymmetrical distribution of auxin

• cells on darker side elongate faster

than cells on brighter side

– apical dominance

What do Gibberellin and ABA do?

Gibberellins

• Family of hormones

– over 100 different gibberellins identified

• Effects

– stem elongation

– fruit growth

– seed germination

plump grapes in grocery

stores have been treated

with gibberellin hormones

while on the vine

Abscisic acid (ABA)

• Effects

– slows growth

– seed dormancy

• high concentrations of abscisic acid

– germination only after ABA is inactivated or leeched out

• survival value:

seed will germinate only

under optimal conditions

– light, temperature, moisture

Complete #21-26

Ethylene• Hormone gas released by plant cells

• Effects

– fruit ripening

– leaf drop

• like in Autumn

• apoptosis

One bad apple spoils the whole bunch…

Fruit ripening

• Adaptation

– hard, tart fruit protects developing seed from herbivores

– ripe, sweet, soft fruit attracts animals to disperse seed

• Mechanism

– triggers ripening process• breakdown of cell wall

– softening

• conversion of starch to sugar

– sweetening

– positive feedback system• ethylene triggers ripening

• ripening stimulates more ethylene production

Figure 39.UN03

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

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Abiotic Stresses

• 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

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

• Salt: increased salt in the soil can reduce water

uptake

– Plants respond to salt stress by producing solutes

tolerated at high concentrations

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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

• Many plants, as well as other organisms, have

antifreeze proteins that prevent ice crystals from

growing and damaging cells

Figure 39.UN05

Quiz Tomorrow

• Explain water transport

• Explain stomata opening and closing

• Explain sugar transport

• Know the Hormones!

• Tropisms

• Short Day v. Long Day Plants