PowerPoint Presentation · Concept 39.2: Plant hormones help coordinate growth, development, and...

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