Plant Responses to Internal and External Signals

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Chapter 39

• Plant Responses to Internal and External Signals

Chapter 39
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Response to stimuli

• Plants, being rooted to the ground must respond
  to whatever environmental change comes their way
• For example, the bending of a grass seedling
  toward light begins with the plant sensing the
  direction, quantity, and color of the light

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Signal Transduction stimulus ? ? ? response

• Signal transduction pathways link signal
  reception to response
• Plants have cellular receptors to detect
  important changes in their environment
• For a stimulus to elicit a response the cell must
  have an appropriate receptor
• Upon receipt of the stimulus the receptor starts
  a series of biochemical steps that lead to a
  response

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Potato Example

• A potato left growing in darkness will produce
  shoots that do not appear healthy, and lack
  elongated roots
• These are morphological adaptations for growing
  in darkness are referred to as etiolation
• After the potato is exposed to light, the plant
  undergoes changes called de-etiolation,
  (greening) in which shoots and roots grow normally

Before exposure to light. Adark-grown potato
has tall,spindly stems and nonexpandedleavesmor
phologicaladaptations that enable theshoots to
penetrate the soil. Theroots are short, but
there is littleneed for water absorptionbecause
little water is lost by theshoots.
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Reception Transduction Response
Reception Internal and external signals are
detected by receptors (proteins that change in
response to specific stimuli) Transduction
Second messengers transfer and amplify signals
from receptors to proteins that cause specific
responses Response Results in regulation of one
or more cellular activities. In many cases this
involves the increased activity of certain
enzymes
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Greeningan example of signal transduction
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Tropisms
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Plant Hormones and Tropisms

• Hormones Chemical signals that coordinate
  growth, development, and responses to stimuli
• The discovery of plant hormones came from work
  with tropisms
• Any growth response that results in curvatures of
  whole plant organs toward or away from a stimulus
  is called a tropism
• Tropisms are often caused by hormones

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Phototropism
Movie
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Darwins experiments with Phototropisms
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Wents experiment

• In 1926, Frits Went

• Extracted the chemical messenger for
  phototropism, auxin, by modifying earlier
  experiments

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Plant Hormones
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Plant hormones

• In general, hormones control plant growth and
  development
• By affecting the division, elongation, and
  differentiation of cells
• Plant hormones are produced in very low
  concentrations
• But a minute amount can have a profound effect on
  the growth and development of a plant organ

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A Survey of Plant Hormones
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Auxin

• The term auxin is used for any chemical substance
  that promotes cell elongation in different target
  tissues
• Auxin is involved in the formation and branching
  of roots (Lateral and Adventitious Root
  Formation)
• Auxin affects secondary growth by inducing cell
  division in the vascular cambium and influencing
  differentiation of secondary xylem
• Auxins as herbicidesan overdose of auxins can
  kill eudicots (2,4-D is a synthetic auxin)

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Cell elongation in response to auxin

• A model called the acid growth hypothesis
  suggests proton pumps play a major role in the
  growth response of cells to auxin

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Cytokinins

• Cytokinins
• Stimulate cell division
• Are produced in actively growing tissues such as
  roots, embryos, and fruits
• Work together with auxin
• Retard the aging of some plant organs (anti-aging
  effects)

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Control of Apical Dominance

• Cytokinins, auxin, and other factors interact in
  the control of apical dominance (The ability of a
  terminal bud to suppress development of axillary
  buds)

If the terminal bud is removed plants become
bushier
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Gibberellins

• Gibberellins have a variety of effects
• stem elongation
• fruit growth
• seed germination

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Fruit Growth

• In many plants both auxin and gibberellins must
  be present for fruit to set
• Gibberellins are used commercially in the
  spraying of Thompson seedless grapes

Untreated
Treated
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Germination

• After water is imbibed, the release of
  gibberellins from the embryo signals the seeds to
  break dormancy and germinate

2
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Brassinosteroids

• Brassinosteroids
• Are similar to the sex hormones of animals
• Induce cell elongation and division

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Abscisic Acid effects

• Seed dormancy
• Seed dormancy has great survival value because it
  ensures that the seed will germinate only when
  there are optimal conditions
• Drought tolerance
• Through a variety of mechanisms (For example, an
  increasing amt of ABA in leaves will cause the
  stomata to close to reduce water loss)
• Inhibits growth

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Ethylene

• Produced in response to stresses such as drought,
  flooding, mechanical pressure, injury, and
  infection
• The Triple Response to Mechanical Stress
• allows a growing shoot to avoid obstacles during
  soil penetration
• Stems elongate less rapidly
• Stems thicken
• Stems grow horizontally

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Other Ethylene effects

• Apoptosis (programmed cell death) a burst of
  ethylene is associated with the programmed
  destruction of cells, organs, or whole plants
• Fruit Ripening a burst of production triggers
  the ripening process
• 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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Plant Responses to Light
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Plant Responses to Light

• Light cues many key events in plant growth and
  development.
• Light reception is important for measuring the
  passage of days and seasons
• Effects of light on plant morphology is called
  photomorphogenesis
• Plants not only detect the presence of light but
  also its direction, intensity, and wavelength
  (color)

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Action Spectra
Researchers exposed maize (Zea mays) coleoptiles
to violet, blue, green, yellow, orange, and red
light to test which wavelengths stimulate the
phototropic bending toward light.
EXPERIMENT
RESULTS
The graph below shows phototropic effectiveness
(curvature per photon) relativeto effectiveness
of light with a wavelength of 436 nm. The photo
collages show coleoptiles before and after
90-minute exposure to side lighting of the
indicated colors. Pronounced curvature occurred
only with wavelengths below 500 nm and was
greatest with blue light.
CONCLUSION
The phototropic bending toward light is caused
by a photoreceptor that is sensitive to blue and
violet light, particularly blue light.
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Light Receptors (two major classes)

• Blue-light photoreceptors
• Control hypocotyl elongation, stomatal opening,
  and phototropism
• Phytochromes
• Regulate many of a plants responses to light
  throughout its life. (such as seed germination)

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Seed Germination Experiment
During the 1930s, USDA scientists briefly
exposed batches of lettuce seeds to red light or
far-red light to test the effects on germination.
After the light exposure, the seeds were placed
in the dark, and the results were compared with
control seeds that were not exposed to light.
EXPERIMENT
The bar below each photo indicates the sequence
of red-light exposure, far-red light exposure,
and darkness. The germination rate increased
greatly in groups of seeds that were last
exposedto red light (left). Germination was
inhibited in groups of seeds that were last
exposed to far-red light (right).
RESULTS
Red light stimulated germination, and far-red
light inhibited germination.The final exposure
was the determining factor. The effects of red
and far-red light were reversible.
CONCLUSION
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Phytochrome switch

• Phytochromes exist in two photoreversible states
  (isomers) with conversion of Pr (red absorbing)
  to Pfr (far-red absorbing) triggering many
  developmental responses
• When seeds are exposed to adequate sunlight for
  the first time, it is the appearance of Pfr that
  triggers germination

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Phytochromes and Shade Avoidance

• The phytochrome system also provides the plant
  with information about the quality of light
• In the shade avoidance response of a tree
• The phytochrome ratio shifts in favor of Pr when
  a tree is shaded. (amount of Pr greater than
  amount of Pfr)
• This causes the tree to allocate more resources
  to growing taller (vertical growth) and less to
  branching
• Lateral branching occurs in plentiful direct
  sunlight because the phytochrome ratio favors Pfr
  (Pfr gtPr)

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Biological Clocks and Circadian Rhythms

• Many plant processes oscillate during the day
• For example, many legumes lower their leaves in
  the evening and raise them in the morning (these
  are called sleep movements)

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Sleep movements
Movie
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Circadian rhythms

• cyclical responses to environmental stimuli
• approximately 24 hours long
• can be entrained (set) to exactly 24 hours by the
  day/night cycle by daily signals from the
  environment
• Human examples include blood pressure, body
  temperature, alertness, sex drive, metabolic
  rate, etc. etc.

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The Effect of Light on the Biological Clock

• Phytochrome conversion marks sunrise and sunset
  providing the biological clock with environmental
  cues
• An increase of red light during the day causes
  Pfr to accumulate, while the amount of Pr
  accumulates in dim light
• 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

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Photoperiodism and Control of Flowering

• Flowering in many species requires a certain
  photoperiod
• Short-day plants (generally flower in late
  summer, fall, or winter) (mums poinsettias)
• Long-day plants (flower in late spring or early
  summer) (lettuceiris)
• Day-neutral plants are unaffected by photoperiod
  and flower at a certain stage of maturity
  regardless of day length at the time
  (tomatodandelion)

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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

During the 1940s, researchers conducted
experiments in which periods of darkness were
interrupted with brief exposure to light to test
how the light and dark portions of a photoperiod
affected flowering in short-day and long-day
plants.
EXPERIMENT
RESULTS
Darkness
Flash oflight
24 hours
Criticaldarkperiod
Light
(a) Short-day plantsflowered only if a period
ofcontinuous darkness waslonger than a critical
darkperiod for that particularspecies (13 hours
in thisexample). A period ofdarkness can be
ended by abrief exposure to light.
(b) Long-day plantsflowered only if aperiod
of continuousdarkness was shorterthan a
critical darkperiod for thatparticular species
(13hours in this example).
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Test for presence of a flowering hormone
Does a flowering hormone exist (florigen)?
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Meristem Transition and Flowering

• Whatever combination of environmental cues and
  internal signals is necessary for flowering to
  occur the outcome is the transition of a buds
  meristem from a vegetative to a flowering state

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Plant response toNon-Light stimuli
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Gravity

• Response to gravity is gravitropism
• Roots show positive gravitropism
• Stems show negative gravitropism

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Statoliths

• Plants may detect gravity by the settling of
  statoliths (specialized plastids containing dense
  starch grains) to lower portions of cells.
• How does it work?...maybe because of their
  density they enhance gravitational sensing in
  some way?

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Gravitropism
Movie
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Response to Mechanical Stimuli

• Thigmomorphogenesis refers to the changes in form
  that result from mechanical perturbation
• Rubbing the stems of young plants a couple of
  times daily results in plants that are shorter
  than controls

Rubbed
Un-rubbed
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Thigmotropism

• Growth in response to touch occurs in vines and
  other climbing plants.

Movie
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Rapid leaf movement in response to mechanical
stimulation-1
Movie
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Rapid leaf movement in response to mechanical
stimulation-2
Movie
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Response to Environmental Stresses

• Environmental stresses
• Have a potentially adverse effect on a plants
  survival, growth, and reproduction
• Can have a devastating impact on crop yields in
  agriculture
• Drought
• During drought plants respond to water deficit by
  reducing transpiration
• Deeper roots continue to grow

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Flooding

• Waterlogged soil lacks air spaces to provide
  oxygen for cellular respiration in roots.
• Oxygen deprivation stimulates ethylene production
  which then leads tooEnzymatic destruction of
  cells and creation of air tubes snorkels that
  provide oxygen to submerged roots

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Other stresses

• Salt StressPlants respond to salt stress by
  producing compatible solutes (solutes tolerated
  at high concentrations) which keeps the water
  potential of cells more negative than that of the
  soil solution
• Heat Stress Heat-shock proteins help plants
  survive heat stress by protecting important
  molecules from denaturation
• Cold StressAltering lipid composition of
  membranes to maintain fluidity of membranes is
  one response to cold. Increasing levels of
  solutes (like sugar) in the cells helps some
  frost-tolerant plants to avoid freezing

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Plant Defenses
Movie
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Defenses Against Herbivores

• Plants counter excessive herbivory
• With physical defenses such as thorns
• With chemical defenses such as distasteful or
  toxic compounds
• Recruitment of predatory animals

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Recruitment of Predatory animals
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Defenses Against Pathogens

• A plants first line of defense against infection
• Is the physical barrier of the plants skin,
  the epidermis and the periderm
• Once a pathogen invades a plant
• The plant mounts a chemical attack as a second
  line of defense that kills the pathogen and
  prevents its spread
• The second defense system is enhanced by the
  plants inherited ability to recognize certain
  pathogens

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Pathogens

• A virulent pathogen
• Is one that a plant has little specific defense
  against
• An avirulent pathogen
• Is one that may harm but not kill the host plant

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Gene-for-gene recognition

• Involves recognition of pathogen-produced
  molecules by the protein products (receptors) of
  specific plant disease resistance (R) genes

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Avirulent pathogen

• A pathogen is avirulent if it has a specific Avr
  gene corresponding to a particular R allele in
  the host plant

Signal molecule (ligand) from Avr gene product
R
Avr allele
Avirulent pathogen
Plant cell is resistant
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Virulent pathogen

• If the plant host lacks the R gene that
  counteracts the pathogens Avr gene
• Then the pathogen can invade and kill the plant

R
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Plant Responses to Pathogen Invasions

• A hypersensitive response against an avirulent
  pathogen seals off the infection and kills both
  pathogen and host cells in the region of the
  infection

4 Before they die,infected cellsrelease a
chemicalsignal, probablysalicylic acid.
3 In a hypersensitiveresponse (HR), plantcells
produce anti-microbial molecules,seal off
infectedareas by modifyingtheir walls, andthen
destroythemselves. Thislocalized
responseproduces lesionsand protects
otherparts of an infectedleaf.
5 The signal is distributed to the
rest of the plant.
Signal
5
4
Signaltransductionpathway
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Hypersensitiveresponse
3
6 In cells remote fromthe infection site,the
chemicalinitiates a signaltransductionpathway.
Signal transductionpathway
Acquiredresistance
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2 This identification step triggers a
signal transduction pathway.
7 Systemic acquired resistance isactivated
theproduction ofmolecules that helpprotect the
cellagainst a diversityof pathogens forseveral
days.
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Avirulentpathogen
1 Specific resistance is based on the
binding of ligands from the pathogen to
receptors in plant cells.
R-Avr recognition and hypersensitive response
Systemic acquired resistance
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Systemic Acquired Resistance

• Systemic acquired resistance (SAR)
• Is a set of generalized defense responses in
  organs distant from the original site of
  infection
• Is triggered by the signal molecule salicylic
  acid (which activates plant defenses throughout
  the plant before infection spreads)