Plant Responses to Internal and External Signals

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Chapter 39

• Plant Responses to Internal and External Signals

Shawn Koshy Peter Jandovitz Jason Lee Cody
Pickel Edwin Mathieu
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Concept 39.1 Signal transduction pathways link
signal reception to response

• All organisms receive specific environmental
  signals and respond to them in ways tat enhance
  survival and reproductive success.
• Responses caused by stimuli can nly happen
  through certain receptors on cells.
• Etiolation morphological plant adapttions for
  growig in the darkness.
• De-etiolation the changes a plant shoot
  undergoes in response to sunlight also known
  informally as greening.

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Reception

• Receptors, or proteins that undergo
  conformational changes in response to a specific
  stimulus, are the first to detect signals.
• Phytochrome the receptor involved in
  de-etiolation (photoreceptor).

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Transduction

• Second messengers small internally produced
  chemicals that transfer and amplify the signal
  from the receptor to other proteins that cause
  the response.
• For example, signal transduction in plants begin
  by the detection of the light signal by the
  phytochrome receptor, which activates at least 2
  signal transduction pathways.
• One pathway uses cGMP as a second messenger that
  activates a specific protein kinase.
• The next pathway causes an increase in
  cytoplasmic Ca2 levels, ultimately activating
  another protein kinase.
• Lastly, both pathways will result in an
  expression of genes for proteins that function in
  the de-etiolation response (greening).

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Response

• A signal transduction pathway leads to regulation
  of 1 or more cellular activities.
• There are 2 main mechanisms by which an enzyme
  can be activated by a signal pathway
• Transcription Regulation stimulates
  transcription of mRNA for the enzyme.
• Post-Translational Modification activates
  existing enzyme molecules.

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Transcription Regulation

• Transcription factors bind directly to particular
  regions of DNA and control the transcription of
  certain genes.
• The activation of positive or negative or both
  types of transcription factors has an affect on
  the mechanism by which a signal promotes a new
  developmental course.

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Post-Translational Modification of Proteins

• The post-translational modification of existing
  proteins is as important as the syntheses of new
  proteins by transduction and translation.
• Chains of phosphorylated protein kinases can
  result in signal pathways ultimately regulating
  the synthesis of new proteins (turning genes on
  and off).
• Protein phosphatases enzymes that
  dephosphorylate specific proteins (switch-off
  processes).

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De-Etiolation (Greening) Proteins

• Enzymes that are involved in photosynthesis
  directly, and enzymes involved in supplying the
  chemical precursors for chlorophyll production,
  and many more also effect hormones that regulate
  plant growth.
• These are either newly transcribed or activated
  by phosphorylation during the de-etiolation
  process.

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

• Tropism- response that causes changes in growth
  away from or toward stimuli
• ex) phototropism- growth towards light
• Tropisms are caused by hormones- chemical
  signals in an organisms

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Discovery of Plant Hormones

• In the late 19th century, Charles and Francis
  Darwin discovered that a phototropic response
  could only be triggered when light could reach
  the tip of the coleoptile.
• Boysen-Jensen observed that a phototrophic
  response was triggered by a light- activated
  mobile chemical
• Later modified experiments by Frits Went led to
  the discovery of auxin.

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Survey of Plant Hormones
Hormone Major Functions
Auxins At lower concentrations, stimulates cell elongation by increasing the activity of proton pumps at higher concentrations, it inhibits cell elongation. Lateral and adventitious root formation and branching. Induces xylem differentiation in developing plants. Promotes the growth of fruits. Can be used as herbicides for broadleaf plants.
Cytokinins Produced in the roots and fruits, and spread throughout the plant, working with auxin to stimulate cell division and differentiation. Works against auxin in controlling apical dominance.
Giberellins Signals a young embryo to break dormancy and begin germination. Stimulate growth of both leaves and stems, especially in bolting, rapid growth of the floral stalk. Stimulates cell elongation by inducing enzymes that facilitate the expansins that loosen cell walls. Used commercially to enhance development and growth of fruits.
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Plant Hormones (contd)
Hormone Major Functions
Brassinosteroid Induce cell elongation and division in stem segments and seedlings at low concentrations. Retard leaf abscission and promote xylem differentiation.
Abscisic Acid Increases in levels to promote seed dormancy . Internal signal that enables plants to withstand drought. Under excessive drought, causes stomata to close rapidly, reducing transpiration.
Ethylene Produced in response to mechanical stresses such as drought, flooding, mechanical pressure, and infection. Instigates the triple response when seed growth reaches an obstacle. Increased levels associated with apoptosis, the programmed destruction of organs or tissues in the plant. Leaf abscission controlled by a balance of auxin and ethylene (higher ethylene levels promote leaf abscission). Ethylene triggers fruit ripening, which in turn triggers more ethylene through positive feedback.
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Figure 1 Triple Response Caused by Ethylene

• The growing shoot on the left undergoes the
  triple response, resulting in a slowing of stem
  elongation, thickening of the stem, and a
  curvature of the stem that causes it to grow
  horizontally. The growing shoot on the right is
  under control conditions, and continues to grow
  vertically.

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Figure 2 Commercial Use of Gibberellins for
Fruit Production

• The picture on the right shows how gibberellins
  enhance the development and growth of fruits. The
  grapes on the right were grown with daily
  spraying of gibberellins. Thompson grapes are an
  example of hoe gibberellins are used in industry
  to increase the size, taste and overall worth of
  fruits to the consumer.

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System Biology and Hormone Interactions

• Interactions between hormones and their signal
  transduction pathways makes it difficult to
  predict the effect of genetic engineering on a
  plant.
• Systems biology strives for better, in-depth
  knowledge of plants that will grant better view
  of these interactions, making genetic engineering
  more effective.

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Concept 39.3 Responses to light are critical for
plant success

• Effects of light on plant morphology are what
  plant biologists call photomorphogenesis
• Light causes many key events in plant growth and
  development
• There are two major classes of light receptors
  blue-light photoreceptors and phytochromes, which
  absorb mostly red light

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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
The graph below shows phototropic effectiveness
(curvature per photon) relative to 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.
RESULTS
The phototropic bending toward light is caused
by a photoreceptor that is sensitive to blue and
violet light, particularly blue light.
CONCLUSION

• An action spectrum depicts the relative
  effectiveness of different wavelengths of
  radiation in driving a particular process

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Blue-Light Photoreceptors

• Blue-light receptors initiate diverse responses
  in plants including
• The light induced opening of stomata
• The light-induced slowing of hypocotyl elongation
  that occurs when a seedling breaks ground
• Phototropism

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Phytochromes as Photoreceptors

• Phytochromes are responsible for many of a
  plants responses to light throughout its
  lifetime
• De-etiolation is regulated by phytochromes

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Phytochromes and Seed Germination

• Phytochromes were discovered during studies of
  seed germination
• In the 1930s, scientists at the U.S. Department
  of Agriculture determined the action spectrum for
  light-induced germination of lettuce seeds

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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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• Figure 3 Phytochromes exist in two
  photoreversible states - Pr and Pfr

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Phytochromes and Shade Avoidance

• The phytochrome system also provides the plant
  with information about the quality of light
• In a shade avoidanc response of a tree, the
  phytochrome ratio shifts in favor of Pr

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Biological Clocks and Circadian Rhythms

• Many plant processes oscillate during the day
• Ex Transpiration
• Synthesis of certain enzymes

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• Figure 4 Some plants lower leaves in the evening
  and raise them in the morning

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• Cyclical responses with a frequency of about 24
  hours and not directly paced by environmental
  variables are called circadian rhythms
• approximately 24 hours long
• can be made to be exactly 24 hours by the
  day/night cycle

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The Effect of Light on the Biological Clock

• Phytochrome conversion marks sunrise and sunset
• This provides the biological clock with
  environmental cues

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Photoperiodism and Responses to Seasons

• Photoperiod - the relative lengths of night and
  day
• Many plants use the photoperiod to detect the
  time of year
• Photoperiodism - a physiological response to
  photoperiod

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Photoperiodism and Control of Flowering

• Some developmental processes require a certain
  photoperiod
• Short-day plants flower in fall or winter due to
  the shorter day lengths and longer nights
• Long-day plants flower in late spring or early
  summer do to the long hours of daylight.
• Day-neutral plants are unaffected by photoperiod
  and flower regardless of daylength

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Critical Night Length

• In the 1940s, researchers discovered that
  responses to photoperiod are 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
Darkness
RESULTS
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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A Flowering Hormone?

• The flowering signal, not yet chemically
  identified is called florigen
• It may be a ormone or change in relative
  concentrations of multiple hormones

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Meristem Transition and Flowering

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

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Section 39. 4 Plants respond to a wide variety
of stimuli other than light
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Gravity

• Gravitropism is a response to gravity.
• Gravitropism functions as soon as the seed
  germinates ensuring that the root grows into the
  soil and the shoot reaches sunlight regardless of
  how the seed happens to be oriented in the soil
• Gravitropism may be either positive (toward) or
  negative (away from).
• In their responses to gravity, roots display
  positive gravitropism and shoots exhibit negative
  gravitropism
• The curvature that occurs in reaction to gravity
  is due to differences in cell elongation on the
  opposite sides of a root or shoot.
• The molecule called auxin promotes cell
  elongation in shoot and inhibits it in roots.

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Gravity (cont.)

• Plants may detect gravity by the settling of
  statoliths, specialized plastids containing dense
  starch grains, to the lower portions of cells
• According to one hypothesis, the settling of
  statoliths in cells of the root cap triggers
  movement of calcium, which causes the lateral
  transport of auxin.
• The calcium and auxin accumulate on the lower
  side of the growing root, where the high
  concentration of auxin inhibits cell elongation,
  causing the root to curve downward.
• The settling of the protoplast and large
  organelles may distort the cytoskeleton and also
  signal gravitation direction.

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• Figure 5 Positive gravitropism in roots the
  statolith hypothesis

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Mechanical Stimuli

• Thigmomorphogenesis refers to the morphological
  changes in its form that result from mechanical
  stress
• Plants are very sensitive to mechanical stress
• Mechanical stimulation activates a signal
  transduction pathway that increases the cytosolic
  Ca2 , which in turn mediates the activation of
  specific genes, some of which encode for proteins
  that affect cell wall properties.
• Rubbing the stems of a young plant a couple of
  times daily results in plants that are shorter
  than controls (see Figure 2)

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• Figure 6
• Thigmomorphogenesis

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Mechanical Stimuli (Cont.)

• Thigmotropism is the directional growth as a
  response to contact with a solid object
• For example, when the compound leaf of the
  sensitive plant Mimosa pudica is touched, it
  collapses and its leaflets fold together (see
  Figure 3)
• This response is due to the rapid loss of turgor
  by cells in specialized motor organs called
  pulvini, located at the joints of the leaf
• These cells lose potassium when stimulated,
  resulting in osmatic water loss.
• The message travels through the plant from the
  point of stimulation, perhaps as the result of
  electrical impulses, called Action potentials

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• Figure 7
• Thigmotropism

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Environmental Stresses

• Drought
• Water deficit in a leaf causes guard cells to
  lose turgor, a simple control mechanism that
  slows transpiration by closing stomata
• stimulates increased synthesis and release of
  abscisic acid in the leaf, and this hormone helps
  keep stomata closed by acting on guard cell
  membranes.
• inhibits the growth of young leaves, minimizing
  the transpirational loss of water by slowing the
  increase in leaf surface
• Inhibits the growth of shallow roots while deeper
  roots in moist soil continue to grow

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Environmental Stresses (Cont.)

• Flood
• The air spaces of flooded soil lack the oxygen
  needed for the cellular respiration of the roots
• Oxygen deprivation stimulates the production of
  the hormone ethylene, which causes some of the
  cells in the root cortex to undergo apoptosis
  (programmed cell death).
• Enzymatic destruction of cells creates air tubes
  that function as snorkels, providing oxygen to
  the submerged roots

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Environmental Stresses (Cont.)

• Salt Stress
• Lowers the water potential of the soil solution
  below that of roots, causing the roots to lose
  water
• sodium and certain other ions are toxic to plants
  when their concentrations are relatively high
• The selectively permeable membranes of root cells
  prevent the uptake of most harmful ions, but this
  only aggravates the problem of acquiring water
  from hypertonic soil.
• Plants may respond to moderate soil salinity by
  producing compatible solutes that lower the water
  potential of root cells.
• Halophytes- salt tolerant plants that have salt
  glands that pump salts out across the leaf
  epidermis

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Environmental Stresses (Cont.)

• Heat Stress
• Excessive heat can harm and eventually kill a
  plant by denaturing its enzymes and damaging its
  metabolism in other ways
• Transpiration creates evaporative cooling for a
  plant, but this effect may be lost on hot, dry
  days when stomata close to reduce water loss
• In high temperatures, plant cells produce
  heat-shock proteins that may provide temporary
  support to reduce protein denaturation.

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Environmental Stresses (Cont.)

• Cold Stress
• Plants respond to cold stress by increasing the
  proportion of unsaturated fatty acids in membrane
  lipids in order to maintain the fluidity of cell
  membranes.
• At subfreezing temperatures, ice forms in the
  cell walls and intercellular spaces of most
  plants, lowering the extracellular water
  potential and causing cells to dehydrate
• Plants adapted to cold winters have special
  adaptations that enable them to cope with
  freezing stress, such as changing the solute
  composition of the cytosol

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39.5 Plants defend themselves against herbivores
and pathogens

• Plants do not exist in isolation but interact
  with many species.
• While some of these interactions can be
  beneficial, most are harmful and dangerous to the
  plant.
• As a producer plants are the base of most food
  webs and subject to attack by a wide range of
  animals, as well as infection by pathogenic
  viruses, bacteria, etc.

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Defenses Against Herbivores

• Many plants have physical defenses, such as
  thorns, and chemical defenses, such as toxic
  compounds
• Some plants recruit predatory animals that prey
  on specific herbivore by releasing volatile
  chemicals which attract the predator.
• Volatile chemicals also serve as an alert for
  nearby plants, which allow them to activate genes
  for plant defense
• ex) jasmonic acid

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Defenses Against Pathogens

• Virulent host plant has little defense against
  pathogen.
• Avirulent- pathogen able to harm, but not kill,
  host plant.
• 1) Gene-for-gene recognition- recognition of
  pathogen derived molecules by the protein
  products of specific disease resistant (R) genes.
  
• R proteins recognize pathogen molecules encoded
  from avirulence (Avr) genes, which play a role in
  the infection of pathogen.

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Defenses (contd)

• 2) Plant Responses
• - Elicitors- induce broader type of host
  defense stimulate phytoalexins, antimicrobial
  compounds
• - PR proteins- spread signals to nearby
  cells, as well as aid in attacking pathogens
• - Cross linking of cell walls and release of
  lignin, which produces a barricade to prevent
  further infection
• Hypersensitive Response (HR)- enhance
  production of elicitors and PR proteins

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Systemic Acquired Resistance (SAR)

• Chemical signals sent throughout whole plant,
  stimulating production of phytoalexins and PR
  proteins coupled with HR.
• Salicylic acid- main hormone attributed to SAR