Plant Responses to Internal and External Signals

1
Chapter 39
Plant Responses to Internal and External Signals
2
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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Figure 39.1
4
Figure 39.3
(1)
CYTOPLASM
CELLWALL
Response
Reception
Transduction
Relay proteins and
Activationof cellularresponses
second messengers
Receptor
Hormone orenvironmentalstimulus
Plasma membrane
5
Figure 11.16
Reception
Binding of epinephrine to G protein-coupled
receptor (1 molecule)
Transduction
Inactive G protein
(41)
Active G protein (102 molecules)
Inactive adenylyl cyclase
Active adenylyl cyclase (102)
ATP
Cyclic AMP (104)
Inactive protein kinase A
Active protein kinase A (104)
Inactive phosphorylase kinase
Active phosphorylase kinase (105)
Inactive glycogen phosphorylase
Active glycogen phosphorylase (106)
Response
Glycogen
Glucose 1-phosphate (108 molecules)
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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 (2-3)

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Figure 39.2
(a) Before exposure to light
8
Figure 39.3

• A potatos response to light is an example of
  cell-signal processing
• The stages are reception, transduction, and
  response

CELLWALL
CYTOPLASM
Response
Reception
Transduction
Relay proteins and
Activationof cellularresponses
second messengers
Receptor
Hormone orenvironmentalstimulus
Plasma membrane
9
Reception

• Internal and external signals are detected by
  receptors, proteins that change in response to
  specific stimuli
• In de-etiolation, the receptor is a phytochrome
  capable of detecting light

10
Transduction

• Second messengers transfer and amplify signals
  from receptors to proteins that cause responses
• Two types of second messengers play an important
  role in de-etiolation Ca2 ions and cyclic GMP
  (cGMP)
• The phytochrome receptor responds to light by
• Opening Ca2 channels, which increases Ca2
  levels in the cytosol
• Activating an enzyme that produces cGMP

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Figure 39.4-3
(4-6)
Reception
Transduction
Response
Transcriptionfactor 1
CYTOPLASM
NUCLEUS
Plasmamembrane
cGMP
P
Proteinkinase 1
Secondmessenger
Transcriptionfactor 2
Phytochrome
P
Cellwall
Proteinkinase 2
Transcription
Light
Translation
De-etiolation(greening)response proteins
Ca2? channel
Ca2?
12
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

13
Post-Translational Modification of Preexisting
Proteins

• Post-translational modification involves
  modification of existing proteins in the signal
  response
• Modification often involves the phosphorylation
  of specific amino acids
• The second messengers cGMP and Ca2 activate
  protein kinases directly

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Figure 11.10
Signaling molecule
Receptor
Activated relaymolecule
Inactiveprotein kinase1
Activeprotein kinase1
Inactiveprotein kinase2
ATP
Phosphorylation cascade
ADP
P
Activeprotein kinase2
PP
P i
Inactiveprotein kinase3
ATP
ADP
P
Activeprotein kinase3
PP
P i
Inactiveprotein
ATP
P
ADP
Activeprotein
Cellularresponse
PP
P i
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Transcriptional Regulation

• Specific transcription factors bind directly to
  specific regions of DNA and control transcription
  of genes
• Some transcription factors are activators that
  increase the transcription of specific genes
• Other transcription factors are repressors that
  decrease the transcription of specific genes

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De-Etiolation (Greening) Proteins

• De-etiolation activates enzymes that
• Function in photosynthesis directly
• Supply the chemical precursors for chlorophyll
  production
• Affect the levels of plant hormones that regulate
  growth

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

• Hormones were first discovered and animals and
  defined with the following criteria 1. signal
  produced that acts elsewhere in the body 2.
  Binds to a specific receptor and triggers
  responses 3. Travels via circulatory system
• Plant hormones (aka plant growth regulators) are
  chemical signals that modify or control one or
  more specific physiological processes within a
  plant (7-8)

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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
• In the late 1800s, Charles Darwin and his son
  Francis conducted experiments on phototropism, a
  plants response to light
• They observed that a grass seedling could bend
  toward light only if the tip of the coleoptile
  was present (9)

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Figure 39.5
RESULTS
(10-11)
Shaded side
Control
Light
Illuminatedside
Boysen-Jensen
Light
Darwin and Darwin
Light
Gelatin(permeable)
Mica(impermeable)
Trans-parentcap
Tipremoved
Opaquecap
Opaqueshield overcurvature
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Figure 39.6
Went Experiment
He gave this substance the name auxin (greek for
increase). Its structure was later found to be
IAA (indoleacetic acid) (12-13)
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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 is produced in shoot tips and is
  transported down the stem
• 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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Auxin Roles

• Stimulates stem elongation (low concentrations)
• Promotes formation of lateral and adventitius
  roots
• Regulates development of fruit
• Enhances apical dominance
• Fuctions in phototropism and gravitropism
• Promotes vascular differentiation
• Retards leaf abscission (14)

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• Practical Uses for Auxins
• The auxin indolbutyric acid (IBA) stimulates
  adventitious roots and is used in vegetative
  propagation of plants by cuttings
• Monocots have inactivating enzymes, dicots do not
• An overdose of synthetic auxins can kill plants
• For example 2,4-D is used as an herbicide on
  eudicots (15)

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Cytokinins

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

• 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 in shoots and
  roots. (16-17partial next two on following slides)

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• Control of Apical Dominance
• Cytokinins, auxin, and strigolactone interact in
  the control of apical dominance, a terminal buds
  ability to suppress development of axillary buds
• If the terminal bud is removed, plants become
  bushier
• Cytokinin promotes lateral bud growth.

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Figure 39.9
Lateral branches
Stump afterremoval ofapical bud
(b) Apical bud removed
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem
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• Anti-Aging Effects
• Cytokinins slow 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
  1) stem elongation, 2) fruit growth, 3) pollen
  development and 4)seed germination
• (18)

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• Stem Elongation
• Gibberellins are produced in young roots and
  leaves
• Gibberellins stimulate growth of leaves and stems
• In stems, they stimulate cell elongation and cell
  division

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Figure 39.10
31
Abscisic Acid

• Abscisic acid (ABA) slows growth
• Unlike previous discussed hormones it inhibits
  growth
• When first discovered it was thought to play a
  primary role in leaf abscission (no longer
  thought)
• Two of the many effects of ABA
• Seed dormancy
• Drought tolerance (Closes stomata)
• Inhibited growth (19-20)

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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 (early) germination can be caused by
  inactive or low levels of ABA

33
Ethylene

• Plants produce ethylene in response to stresses
  such as drought, flooding, mechanical pressure,
  injury, and infection
• Auxin can also stimulate ethylene production.
• The effects of ethylene include response to
  mechanical stress, senescence, leaf abscission,
  and fruit ripening (21-22)

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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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Figure 39.13
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
36

• Senescence
• Senescence is the programmed death of 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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Table 39.1
39

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

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• Different plant responses can be mediated by the
  same or different photoreceptors
• There are two major classes of light receptors
  blue-light photoreceptors and phytochromes

• Phytochromes absorb red wavelengths of light.
• Red light (660nm) increased germination, while
  far-red light (730nm) inhibited germination
• The photoreceptor responsible for the opposing
  effects of red and far-red light is a phytochrome
  (24-25 27)

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Figure 39.17
RESULTS
Dark
Red
Red
Far-red
Dark
Dark (control)
Red
Far-red
Dark
Red
Red
Red
Far-red
Far-red
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Blue-Light Photoreceptors

• Various blue-light photoreceptors control
  hypocotyl elongation, stomatal opening, and
  phototropism (26)

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Figure 39.19
Pr
Pfr
Red light
Responsesseedgermination,control
offlowering, etc.
Synthesis
Far-red light
Enzymaticdestruction
Slow conversionin darkness(some plants)
(28-30) Cross out 31
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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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Figure 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 (32 Skip examples)

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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 (33)

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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 (34)

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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

• 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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Figure 39.21
24 hours
Light
Darkness
Flashoflight
Criticaldark period
(35)
Flashof light
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• Red light can interrupt the nighttime portion of
  the photoperiod
• A flash of red light followed by a flash of
  far-red light does not disrupt night length
• Action spectra and photoreversibility experiments
  show that phytochrome is the pigment that
  receives red light

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Figure 39.22
24 hours
R
R
FR
R
R
FR
R
R
FR
FR
Short-day(long-night)plant
Long-day(short-night)plant
Critical dark period
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Figure 39.23
24 hours
24 hours
24 hours
Graft
Short-dayplant
Long-day plantgrafted toshort-day plant
Long-dayplant
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A Flowering Hormone?

• Photoperiod is detected by leaves, which cue buds
  to develop as flowers
• The flowering signal is called florigen
• Florigen may be a macromolecule governed by the
  FLOWERING LOCUS T (FT) gene (36)

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Figure 39.UN03