Chapter 28
Lecture Outline
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When touched, the leaves of the sensitive plant
(Mimosa pudica) quickly fold, and only slowly unfold
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Chapter 28
Flowering Plants: Behavior
Chapter Outline:

Overview of Plant Behavioral Responses

Plant Hormones

Plant Responses to Light

Plant Responses to Gravity and Touch

Plant Responses to Attack
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Overview of Plant
Behavioral Responses

Time-lapse photography shows that most plants
are constantly in motion
 Bending,

twisting, or rotating – known as nutation
Other examples of plant behavior:
 Shoots
grow toward light and against gravity
 Roots
grow toward water and toward gravity
 Seeds
germinate when they detect light and moisture
 Flowers,
 Plants
fruit, and seeds grow only at the right season
respond to attack by microbes or animals
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Responses to internal and external
stimuli

Internal stimuli

Chemical signals – hormones, phytohormones or plant growth
substances


Often interact with each other and external signals to
maintain homeostasis and progress through life stages
Environmental stimuli

Light, atmospheric gases (CO2 and water vapor), temperature,
touch, wind, gravity, water, rocks, and soil stimuli

Herbivores, pathogens, organic chemicals from neighboring
plants, and beneficial or harmful soil organisms

Agricultural chemicals including hormones
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Physical stimuli
Biological stimuli
Internal:
circadian
rhythms
hormones
Environmental:
Light
Atmospheric
gases including
CO2
Environmental:
Herbivores
Agricultural
hormone
applications
Humidity
Temperature
Pathogens
Touch, wind
Gravity
Soil water
Rocks and
Other barriers
Soil
minerals
Organic
chemicals
emitted by
other plants
Soil
microorganisms
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Plant responses

Receptor molecules located in plant cells
sense various kinds of stimuli and lead to
appropriate responses

ex: Phototropism – involves both a cellular
perception of light and a growth response of
stem tissue to an internal chemical signal (auxin)
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Plant signal transduction

Process in which a cell perceives a signal,
switching on an intercellular pathway that leads
to cellular response

Three stages
1.
Receptor activation
2.
Transduction of the signal via second messengers
3.
Cellular response via effector molecules
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
Receptors or sensors



Proteins that become activated when they receive a specific type
of signal
Messengers or second messengers

Transmit messages from many types of activated receptors

Cyclic AMP, IP3, and calcium ions
Effectors

Molecules that directly influence cellular responses

Calcium-dependent protein kinases (CDPKs) are important

Signal transduction ends when an effector causes a cellular
response
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BIOLOGY PRINCIPLE
Cells are the simplest units of life
Internal and environmental stimuli are received
and elicit responses at the cellular level.
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Plant Hormones

Chemical signals transported within the plant body that
bind to cellular receptors, thereby causing responses

About a dozen small molecules synthesized in metabolic
pathways

Auxins, cytokinins, gibberellins, ethylene, abscisic
acid and brassinosteroids

One hormone often has multiple effects

Different concentrations or combinations can produce
distinct responses
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Auxins

“Master” plant hormone

Indoleacetic acid (IAA) is one plant auxin

Promotes expression of diverse genes known as
auxin-response genes
 Under
low auxin conditions Aux/IAA repressors
prevent gene expression
 High
enough auxin conditions cause breakdown of
repressors allowing gene expression
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Auxin transport

Produced in apical shoot tips and young leaves

Directionally transported

May enter cells by diffusion

AUX1 plasma membrane protein (auxin influx carrier)
needed to transport IAA

Apical end of cell
PIN proteins transport auxin out of cells
(they are auxin efflux carriers)

Basal end of cell
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
Polar transport – auxin flows down in shoots into roots
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Auxin effects

Establishes the apical-basal polarity of seed
embryos

Induces vascular tissue to differentiate

Mediates phototropism

Promotes formation of adventitious roots

Stimulates fruit development

Used for cloning in plant tissue culture
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
Many effects of practical importance to humans
 Seedless

fruit production
Stimulates flower ovaries to mature into fruits
 Retards
 Root
premature fruit drop
development on stem cuttings
 Pinching
topmost shoots produces bushy plants
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Cytokinins

Increase rate of cytokinesis
or cell division

Root tips major production site

Also produced in shoots and seeds

At shoot and root tips, cytokinins influence meristem size,
stem cell activity, and vascular tissue development

Also involved in root and shoot growth and branching, the
production of flowers and seeds, and leaf senescence
(aging)
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Gibberellins

Also known as gibberellic acids or GA

Produced in apical buds, roots, young leaves, and seed
embryos

Foster seed germination

Enhance stem elongation and flowering

Retard leaf and fruit aging

Arise from stimulatory effects on cell division and
elongation
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EVOLUTIONARY CONNECTIONS
Plant gibberellin responses evolved
in a step-wise manner

In flowering plants, gibberellin works by helping to liberate
repressed transcription factors

In the absence of gibberellin, DELLA proteins prevent
expression of gibberellin-responsive genes

DELLAs function as brakes to restrain cell division and expansion

Gibberellin binds to GID1, leads to destruction of DELLAs

Compared flowering plant proteins to homologous proteins
in bryophyte and lycophyte

Necessary components (DELLAs and GID1 proteins) were
present earlier, but did not assemble into a growth regulation
system until later in plant evolutionary history
EVOLUTIONARY CONNECTIONS
Ethylene

Important in coordinating developmental and stress
responses

Produced during seedling growth, flower development
and fruit ripening

Important roles in leaf and petal aging and drop

Defense against osmotic stress and pathogen attack

Influences cell expansion, often with auxin

Cells tend to expand in all directions rather than
elongating
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Triple Reponse of seedlings
1. Ethylene prevents the seedling
stem and root from elongating
2. Hormone induces the stem
and root to swell radially,
thereby increasing in thickness
3. Seedling stem bends so that
a hook pushes up through
the soil

Result from imbalance
of auxin

This response protects
the delicate meristem
from crusty soil
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Stress hormones

Help plants respond to environmental stresses such as
flooding, drought, high salinity, cold, heat, and attack by
microorganisms and herbivores

Examples:
 Abscisic
acid (ABA)
 Brassinosteroids
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Abscisic acid (ABA)

Slows or stops plant
metabolism when growing
conditions are poor

May induce bud and seed dormancy

Stimulates formation of protective scales around buds
of perennial plants in preparation for winter
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Brassinosteroids

Found in seeds, fruits,
shoots, leaves, and flower
buds of all types of plants

Induce vacuole water intake
and influence enzymes that alter cell-wall carbohydrates,
thereby fostering cell expansion

Impede leaf drop

Stimulate xylem development

Can be applied to crops to help protect plants from heat,
cold, high salinity, and herbicide injury
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Plant Responses to Light

Based on the presence of light receptors within
cells

Photoreceptors respond to light absorption by
switching on signal transduction

Results in

Sun tracking

Phototropism

Flowering

Seed germination
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Phytochrome
A plant “light-switch”
 Red- and far-red-light receptor
 Flips back and forth between 2 conformations

– conformation that only absorbs far-red light and
activates cellular responses
 Pfr
 When
left in the dark, Pfr transforms to red light
absorbing Pr


Pr can only absorb red light and cannot activate cellular
responses
Plays a critical role in photoperiodism and
plant responses to shading
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DARKNESS
In darkness, seeds do not
germinate because phytochrome
remains in the inactive Pr
conformation.
Red
Far
red
Red
Exposure to red light after far-red
light switches phytochrome back to
the active Pfr conformation, so
seeds germinate.
Red
Even a brief exposure to red
light generates the active Pfr
conformation of phytochrome,
allowing seeds to germinate.
Red
Red
Far
red
Red
Far
red
The most recent light exposure
determines whether phytochrome
occurs in the active Pfr or in the
inactive Pr conformation. If in the
latter, most seeds do not germinate.
Far
red
Exposure to far-red light after
red-light exposure converts active
Pfr to inactive Pr, so seeds do not
germinate.
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(1–5): © Prof. and Mrs. M. B.Wilkins/University of Glasgow
Photoperiodism

Phytochromes play a critical role

Influences the timing of dormancy and flowering

Flowering plants can be classified as long-day,
short-day, or day-neutral according to the way
their flowering responds to night length

Plants measure night length – not day length
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
Long-day plants – flower in spring or early
summer, when the night period is shorter (thus
day length is longer) than a defined period

Short-day plants – flower only when the night
length is longer than a defined period such as in
late summer, fall, or winter, when days are short

Day-neutral plants – flower regardless of the
night length, as long as day length meets the
minimal requirements for plant growth
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Shading responses

Also mediated by phytochrome

Responses include
 Extension
of leaves from shady portions of a dense
tree canopy into the light
 Growth
that allows plants to avoid being shaded by
neighboring plants

Occur by the elongation of branch internodes

Leaves detect shade as an increased proportion
of far-red light to red light
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Plant Responses to
Gravity and Touch
Why do plant stems grow up and roots grow down?
Responses to gravity and touch

Gravitropism
 Growth
in response to the force of gravity
 Shoots
are said to be negatively gravitropic
 Most
roots are positively gravitropic
 Statocytes
contain starch-heavy plastids called
statoliths
 Heavy
statoliths sink, causing changes in calcium ion
messengers, inducing lateral auxin transport
 Changes
direction of root or shoot growth
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Gravity and touch response are related

Roots encounter rocks as they grow down
 Touch
response temporarily supersedes
the response to gravity
 Roots
grow horizontally until they get around
the barrier, then downward growth in response to
gravity resumes

More rapid responses, such as sensitive plant,
based on changes in water content of cells

Cells in pulvinus become limp when touched

Additional folding from electrical impulse
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Plant Responses to Attack

Structural barriers (cuticles, epidermal
trichomes, and outer bark) help to reduce
infection and herbivore attack
Herbivore attack
 Wide
variety of chemical defenses
 Make
plants taste bad
 Some
chemicals attract enemies of their attackers or
cause neighbor plants to produce defensive
compounds
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Pathogen attack

Elicitors are molecules produced by bacterial and
fungal pathogens that promote infection

Plants have several defense strategies
 Plasma
membrane receptors that bind microbial
molecules, such as lipopolysaccharides or chitin


Encoded by R genes (resistance genes)
MicroRNAs in the cytosol destroy the nucleic acids
of invading viruses
 Receptors
in the cytosol recognize injected elictors,
triggering the production of chemical defenses or
programmed cell death
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
Hypersensitive response (HR)
 Occurs
when a plant recognizes a pathogen by
chemical means and responds in such a way that the
disease symptoms are limited
 Several
components including increased production
of hydrogen peroxide
 Nitric
oxide also produced
 Synthesis
of hydrolytic enzymes, defensive
secondary metabolites, the hormone salicylic acid,
and tough lignin in cell walls of nearby tissues
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
Systemic acquired resistance (SAR)
 Localized
hypersensitive response can result in the
production of alarm signals that travel to noninfected
regions of a plant and induce widespread resistance to
diverse pathogens
 Jasmonic
acid
 May
produce defensive enzymes or tannins (toxic to
microorganisms)
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Defensive
responses
Defensive
responses
Methyl
salicylate
Jasmonic
acid
Salicylic
acid
Defensive
responses
Systemin
Sites of
pathogen
attack
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