Nerve activates contraction

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CHAPTER 39
PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS
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Responses to Stimuli

• Plants respond to a wide array of stimuli
  throughout its lifecycle
• Hormonal signals
• Gravity
• Direction of light
• Plant interactions between environmental stimuli
  and internal signals.

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Responses to Stimuli

• Animals and Plants differ in how they respond to
  stimuli
• Animals
• mobility
• behavioral
• Plants
• environmental cues
• Patterns of growth development

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Responses to Stimuli

• The ability to receive specific environmental and
  internal signals and respond to them in ways that
  enhance survival and reproductive success.
• Cellular receptors detect environmental changes
• Hormonal changes
• Injury repair
• Seasonal changes

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Signal-transduction pathways link internal and
environmental signals to cellular responses.

• Plant growth patterns vary dramatically in the
  presence versus the absence of light.
• Potato grown in dark Potato grown in light

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• Morphological adaptations in seedling growth
• The shoot does not need a thick stem.
• Leaves would be damaged as the shoot pushes
  upward.
• Dont need an extensive root system
• No chlorophyll produced
• Energy allocated to stem growth

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• The effect of sunlight on shoots (greening)
• The elongation rate of the stems slow.
• The leaves expand and the roots start to
  elongate.
• The entire shoot begins to produce chlorophyll.

(b) After a weeks exposure to natural
daylight
(a) Before exposure to light
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• Signal transduced pathways greening response.

• Three stages
• Reception
• Signal transduction
• Response

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• Reception for Greening
• The receptor is called a phytochrome a
  light-absorbing pigment attached to a specific
  protein.
• Located in the cytoplasm.
• Sensitive to very weak environmental and chemical
  signals
• Signal is then amplified by a second messenger

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• Transduction
• Second messenger produced by the interaction
  between phytochrome and G-protein
• G-protein activates enzyme with produces Cyclic
  GMP (2nd messenger)
• Ca2-calmodulin is also a 2nd messenger

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• Response
• Cyclic GMP and Ca2-calmodulin pathways lead to
  gene expression for protein that activates
  greening response
• Response ends when switch-off is activated
  (protein phosphatases)

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Signal Transduction in Plants Greening
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Hormone

• Hormones- are chemical signals that travel to
  target organs
• Only small amts are needed
• Often the response of a plant is governed by the
  interaction of two or more hormones.
• Phototropism and Negative phototropism

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Early Experiments of Phototropism
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• Went Experiment (1926) of Phototropism
• auxin

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• Some major classes of plant hormones
• Auxin- phototropism
• Cytokinins- root growth
• Gibberellins- growth
• Abscisic acid- inhibits growth
• Ethylene- promote fruit ripening
• Brassinosteroids- inhibits root growth
• Many function in plant defense against pathogens

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Polar Auxin Transport A Chemiosmotic Model
Fig. 39-8
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Expansins separate microfibrils from
cross- linking polysaccharides.
Cell wallloosening enzymes
Cross-linking polysaccharides
Expansin
CELL WALL
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Cleaving allows microfibrils to slide.
Cellulose microfibril
H2O
Cell wall
Cell wall becomes more acidic.
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Plasma membrane
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Auxin increases proton pump
activity.
Nucleus
Cytoplasm
Plasma membrane
Vacuole
CYTOPLASM
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Cell can elongate.
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Auxin

• Stimulates the elongation of cells in young
  shoots.
• Auxins are used commercially in the vegetative
  propagation of plants by cuttings.
• Synthetic auxins are used as herbicides

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Cell elongation in response to auxin the acid
growth hypothesis
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Cytokines

• Cytokines stimulate cytokinesis, or cell
  division.
• The active ingredient is a modified form of
  adenine
• They are produced in actively growing tissues,
  particularly in roots, embryos, and fruits.
• Cytokinins interact with auxins to stimulate cell
  division and differentiation.
• A balanced level of cytokinins and auxins results
  in the mass of growing cells, called a callus,
  that remains undifferentiated.
• High cytokinin levels ? shoot buds form from the
  callus.
• High auxin levels ? roots form.

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• Cytokinins, auxin, and other factors interact in
  the control of apical dominance, the ability of
  the terminal bud to suppress the development of
  axillary buds.
• The direct inhibition hypothesis - proposed that
  auxin and cytokinin act antagonistically in
  regulating axillary bud growth.
• Auxin levels would inhibit axillary bud growth,
  while cytokinins would stimulate growth.

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• Many observations are consistent with the direct
  inhibition hypothesis.
• If the terminal bud, the primary source of auxin,
  is removed, the inhibition of axillary buds is
  removed and the plant becomes bushier.
• This can be inhibited by adding auxins to the cut
  surface.

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• The direct inhibition hypothesis predicts that
  removing the primary source of auxin should lead
  to a decrease in auxin levels in the axillary
  buds.
• However, experimental removal of the terminal
  shoot (decapitation) has not demonstrated this.
• In fact, auxin levels actually increase in the
  axillary buds of decapitated plants.

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• Cytokinins retard the aging of some plant organs.
• They inhibit protein breakdown by stimulating RNA
  and protein synthesis, and by mobilizing
  nutrients from surrounding tissues.
• Leaves removed from a plant and dipped in a
  cytokinin solution stay green much longer than
  otherwise.
• Cytokinins also slow deterioration of leaves on
  intact plants.
• Florists use cytokinin sprays to keep cut flowers
  fresh.

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Gibberellins

• Gibberellin
• Stem elongation
• Fruit growth
• Germination

Roots and leaves are major sites of gibberellin
production
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• Stem Elongation
• Dwarf pea plants treated with gibberellins.
• After treatment dwarf pea plant grew to normal
  height.

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• Fruit Growth
• In many plants, both auxin and gibberellins must
  be present for fruit to set.
• Individual grapes grow larger the internodes of
  the grape bunch elongate.

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• Germination
• Seeds treated with gibberellins will break
  dormancy.

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Abscisic Acid

• Abscisic acid (ABA)
• ABA generally slows down growth.
• Often ABA antagonizes the actions of the growth
  hormones - auxins, cytokinins, and gibberellins.
• It is the ratio of ABA to one or more growth
  hormones that determines the final physiological
  outcome.
• Functions in seed dormancy

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Ethylene

• Ethylene causes leaves to drop from trees.
• Its produced in response to stresses such as
  drought, flooding, mechanical pressure, injury,
  and infection.
• Ethylene production also occurs during fruit
  ripening and during programmed cell death.
• Ethylene is also produced in response to high
  concentrations of externally applied auxins.
• Ethylene produced during apoptosis (programmed
  cell death)

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• Ethylene triple response in seedlings that
  enables a seedling to circumvent an obstacle.
• Ethylene production is induced by mechanical
  stress on the stem tip.
• In the triple response, stem elongation slows,
  the stem thickens, and curvature causes the
  stem to start growing horizontally.

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• Arabidopsis mutants fail to undergo the triple
  response after exposure to ethylene.
• Some lack a functional ethylene receptor.

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• Other mutants undergo the triple response in the
  absence of physical obstacles.

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• The various ethylene signal-transduction mutants
  can be distinguished by their different responses
  to experimental treatments.

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Leaf Abscission

• In deciduous trees, its an adaptation to prevent
  desiccation during winter when roots cannot
  absorb water from the frozen ground.
• Essential elements are salvaged prior to leaf
  abscission and stored in stem parenchyma cells.
• These nutrients are recycled back to developing
  leaves the following spring.

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Hormones responsible for leaf abscission

• A change in the balance of ethylene and auxin
  controls abscission.
• Aged leaves produce less auxin
• Cells become more sensitive to ethylene
• The cells in the abscission layer produce enzymes
  that digest the cellulose and other components of
  cell walls.

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Fruit Ripening

• The consumption of ripe fruits by animals helps
  disperse the seeds of flowering plants.
• Ethylene production helps ripen fruit
• The production of new scents and colors helps
  advertise fruits ripeness to animals, who eat
  the fruits and disperse the seeds.
• Fruit ripens quickly in closed paper bag
• Prevent ripening in produce by spraying CO2

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Brassinosteroids

• Brassinosteroids are steroids chemically similar
  to cholesterol and the sex hormones of animals.
• Brassinosteroids induce cell elongation and
  division in stem segments and seedlings.
• They also retard leaf abscission and promote
  xylem differentiation.
• Brassinosteroids are nonauxin hormones.

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The effect of light on plants

• Light is an especially important factor on the
  lives of plants.
• Photosynthesis
• Cue many key events in plant growth and
  development.
• Photomorphogenesis- the effects of light on plant
  morphology.
• Light reception circadian rhythms.

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Action Spectrum

• Plants detect the direction, intensity, and
  wavelengths of light.
• For example, the measure of physiological
  response to light wavelength, the action
  spectrum, of photosynthesis has two peaks, one in
  the red and one in the blue.
• These match the absorption peaks of chlorophyll.

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Blue-light photoreceptors are a heterogeneous
group of pigments

• Blue light is most effective in initiating a
  diversity of responses.

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• The biochemical identity of blue-light
  photoreceptors was so elusive that they were
  called cryptochromes.
• Analysis Arabidopsis mutants found three
  completely different types of pigments that
  detect blue light.
• cryptochromes (for the inhibition of hypocotyl
  elongation)
• phototropin (for phototropism)
• zeaxanthin (for stomatal opening) a
  carotenoid-based photoreceptor called

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Phytochromes function as photoreceptors in many
plant responses to light

• Phytochromes were discovered from studies of seed
  germination.
• Seed germination needs optimal environmental
  conditions, especially good light.
• Such seeds often remain dormant for many years
  until a change in light conditions.
• For example, the death of a shading tree or the
  plowing of a field may create a favorable light
  environment.

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• Action spectrum for light-induced germination of
  lettuce seeds.
• Seeds were exposed to a few minutes of
  monochromatic light of various wavelengths and
  stored them in the dark for two days and recorded
  the number of seeds that had germinated under
  each light regimen.
• While red light increased germination, far red
  light inhibited it and the response depended on
  the last flash.

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• The photoreceptor responsible for these opposing
  effects of red and far-red light is a phytochrome.

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• This interconversion between isomers acts as a
  switching mechanism that controls various
  light-induced events in the life of the plant.
• The Pfr form triggers many of the plants
  developmental responses to light.
• Exposure to far-red light inhibits the
  germination response.

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• Plants synthesize phytochrome as Pr and if seeds
  are kept in the dark the pigment remains almost
  entirely in the Pr form.
• If the seeds are illuminated with sunlight, the
  phytochrome is exposed to red light (along with
  other wavelengths) and much of the Pr is
  converted to (Pfr), triggering germination.

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• The phytochrome system also provides plants with
  information about the quality of light.
• During the day, with the mix of both red and
  far-red radiation, the Pr ltgtPfr photoreversion
  reaches a dynamic equilibrium.
• Plants can use the ratio of these two forms to
  monitor and adapt to changes in light conditions.

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Biological clocks control circadian rhythms in
plants and other eukaryotes

• Many plant processes oscillate during the day
• transpiration
• synthesis of certain enzymes
• opening and closing stomata
• Response to changes in environmental conditions
• Light levels
• Temperature
• Relative humidity

24 hr day/night cycle
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• Many legumes lower their leaves in the evening
  and raise them in the morning.
• These movements will be continued even if plants
  are kept in constant light or constant darkness.
• circadian rhythms- internal clock no
  environmental cues

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Light entrains the biological clock

• Many circadian rhythms are greater than or less
  than the 24 hour daily cycle
• Desynchronization can occur when denied
  environmental cues.
• Humans experience jetlag.
• Eventually, our circadian rhythms become
  resynchronized with the external environment.
• Plants are also capable of re-establishing
  (entraining) their circadian synchronization.

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Photoperiodism synchronizes many plant responses
to changes of season

• The appropriate appearance of seasonal events are
  of critical importance in the life cycles of most
  plants.
• Seed germination, flowering, and the onset and
  breaking of bud dormancy.
• The environmental stimulus that plants use most
  often to detect the time of year is the
  photoperiod, the relative lengths of night and
  day.
• A physiological response to photoperiod, such as
  flowering, is called photoperiodism.

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• Photoperiodism and the Control of Flowering
• Long-day plants will only flower when the light
  period is longer than a critical number of hours.
• Examples include spinach, iris, and many cereals.
• Day-neutral plants will flower when they reach a
  certain stage of maturity, regardless of day
  length.
• Examples include tomatoes, rice, and dandelions.
• Night length, not day length, controls flowering
  and other responses to photoperiod

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• Short-day plants are actually long-night plants,
  requiring a minimum length of uninterrupted
  darkness.
• Cocklebur is actually unresponsive to day length,
  but it requires at least 8 hours of continuous
  darkness to flower.

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• Similarly, long-day plans are actually
  short-night plants.
• A long-day plant grown on photoperiods of long
  nights that would not normally induce flowering
  will flower if the period of continuous darkness
  are interrupted by a few minutes of light.

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• Red light is the most effective color in
  interrupting the nighttime portion of the
  photoperiod.
• Action spectra and photoreversibility experiments
  show that phytochrome is the active pigment.
• If a flash of red light during the dark period
  is followed immediately by a flash of far-red
  light, then the plant detects no interruption
  of night length, demonstrating red/far-red
  photoreversibility.

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Bleeding hearts flower in May for a brief time

• While buds produce flowers, it is leaves that
  detect photoperiod and trigger flowering.

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• Plants lacking leaves will not flower, even if
  exposed to the appropriate photoperiod.
• The flowering signal may be hormonal

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Introduction

• Because of their immobility, plants must adjust
  to a wide range of environmental circumstances
  through developmental and physiological
  mechanisms.
• While light is one important environmental cue,
  other environmental stimuli also influence plant
  development and physiology.

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Plants respond to environmental stimuli through a
combination of developmental and physiological
mechanisms

• Both the roots and shoots of plants respond to
  gravity, or gravitropism, although in
  diametrically different ways.
• Roots demonstrate positive gravitropism
• Shoots exhibit negative gravitropism
• Auxin plays a major role in gravitropic responses
  
• Statoliths- specialized plastids containing dense
  starch grains, play a role in gravitropism

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Statolith hypothesis for root gravitropism
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• Thigmomorphogenesis- plants can change form in
  response to mechanical stress
• Differences seen in members of the same species
  grown in different environments
• Windy mtn ridge
• stocky tree
• Sheltered location
• taller, slenderer tree

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• Rubbing the stems of young plants a few times
  results in plants that are shorter than controls.

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• Some plant species have become, over the course
  of their evolution, touch specialists.
• For example, most vines and other climbing plants
  have tendrils that grow straight until they touch
  something.
• Contact stimulates a coiling response,
  thigmotropism, caused by differential growth of
  cells on opposite sides of the tendril.
• This allows a vine to take advantage of whatever
  mechanical support it comes across as it climbs
  upward toward a forest canopy.

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• Some touch specialists undergo rapid leaf
  movements in response to mechanical stimulation.
• Mimosas leaflets fold together when touched.
• This occurs when pulvini, motor organs at the
  joints of leaves, become flaccid from a loss of
  potassium and subsequent loss of water by
  osmosis.
• It takes about ten minutes for the cells to
  regain their turgor and restore the
  unstimulated form of the leaf.

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Response to Stress

• Environmental factors that can harm plants
• Flooding
• Drought
• Salt
• Excessive Heat
• Freezing

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Plant Interactions

• Plants do not exist in isolation, but interact
  with many other species in their communities.
• Beneficial interactions
• fungi in mycorrhizae
• insect pollinators
• Negative interactions
• Attack by herbivores
• Attacks by pathogenic viruses, bacteria, and
  fungi.

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Defenses to deter predation

• Physical defenses
• Chemical defenses

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A corn leaf recruits a parasitoid wasp as a
defensive response to a herbivore
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Australian Pine Chemical defense
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Plants defense against pathogens

• Epidermal barrier (1o)
• Periderm (2o)
• viruses, bacteria, and the spores and hyphae of
  fungi can through injuries or through natural
  openings in the epidermis, such as stomata.
• Once a pathogen invades, the plant mounts a
  chemical attack as a second line of defense that
  kills the pathogens and prevents their spread
  from the site of infection.

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Plants defense against pathogens

• Invasion by pathogens
• Viruses
• Bacteria
• Spores and hyphae of fungi
• Invasion can occur through injuries or through
  natural openings in the epidermis, such as
  stomata.
• Plant mounts a chemical defense against pathogen