Plant Responses to Internal and External Signals

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Chapter 39

• Plant Responses to Internal and External Signals

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Response to Stimuli

• Plants are sensitive to a wide range of stimuli.
• They elicit a response.
• They use a signal transduction pathway.

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Response to Stimuli

• Consider a forgotten potato in the cupboard.
• The eyes of the potato (axillary buds) sprout
  shoots that are suited to their function. They
  are pale and lack broad green leaves. They lack
  elongated roots.

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Etiolation

• These adaptations for growing in the dark are
  called etiolation.
• The stimulus for growth is complete darkness.
• The plant uses all energy to elongation of the
  stem so the leaves can open after reaching the
  surface.

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Etiolation

• During etiolation, there is no evaporative loss
  of water.
• Leaves would be a hindrance to the shoot passing
  through the soil.
• Theres no need for chlorophyll--theres no light.

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De-etiolation

• When the shoot hits the light, de-etiolation
  occurs.
• Leaves now expand.
• Elongation of the stem slows.
• Roots elongate.

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Signal Reception

• Signals are detected by receptors.
• Proteins change in response to the stimulus.

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Signal Transduction

• Second messengers are small, internally produced
  chemicals.
• They transfer and amplify signals from the
  receptor to the other proteins causing a
  response.
• One signal receptor protein can give rise to
  hundreds of specific enzymes.
• In this way, 2nd messenger signal transduction
  leads to rapid amplification of the signal

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Response

• Signal transduction leads to one or more cellular
  pathways being regulated.
• Usually, this leads to an increase in the
  activity of certain enzymes.

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Response 2 Main Mechanisms

• 1. Stimulating transcription of mRNA.
• 2. Activating existing enzyme molecules.

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1. Stimulating Transcription of mRNA

• This is called transcriptional regulation.
• These transcription factors bind directly to DNA
  molecules and control the transcription of
  specific genes.

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The De-etiolation Response and Phytochrome

• The receptor involved in de-etiolation is a
  phytochrome.
• The mutant tomato studied has lower levels of
  phytochrome.
• They green less when exposed to light than normal
  tomatoes.

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The De-etiolation Response and Phytochrome

• When the mutants were injected with phytochrome
  from other plants they exhibited a normal
  de-etiolation response when exposed to light.

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The De-etiolation Response and Phytochrome

• Small amounts of light can trigger the
  de-etiolation response.
• In the phytochrome example, small amounts of
  light give rise to activated phytochrome.
• This gives rise to hundreds of second messenger
  molecules which leads to hundreds of activated
  enzymes.

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The Change in Phytochrome

• Light causes the conformation of phytochrome to
  change.
• This leads to an increase in cGMP (2nd messenger)
  and Ca2 influx.
• cGMP activates protein kinases.

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• If we inject the mutant tomatoes with cGMP, we
  get a partial de-etiolation response--even
  without the addition of phytochrome.

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• Protein kinases are activated by cGMP and Ca2,
  and can act to phosphorylate and activate other
  enzymes.
• These can be used to stimulate or shut down
  transcription.
• When transcription is affected, the enzymes can
  now synthesize proteins for chlorophyll
  production and other de-greening proteins.

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• The mechanism by which a signal promotes a new
  developmental course depends on the activation of
  positive or negative control factors.

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• Post-translational modification involves
  activating existing enzyme molecules.
• This is where existing proteins are
  modified--usually via phosphorylation.

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Kinases

• Often, kinases become activated by
  phosphorylation which activates more kinases, and
  so on.
• Eventually, the cascades link initial stimuli to
  responses at the gene level where they are
  expressed.

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Signal Pathways

• Signal pathways lead to both a turning on and off
  of genes.
• For example, putting a potato back into the
  cupboard activates many phosphatases which
  dephosphorylate specific proteins and switch off
  certain pathways.

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The Idea of Signal Pathways

• Classic experiments studying grass uncovered the
  notion of chemical messengers.
• The movement of a plant shoot toward or away from
  a stimulus is called tropism.
• The hormone pathway.

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Signal Pathways and Grass Seedlings

• Phototropism is the process that directs plants
  toward sunlight for photosynthesis.
• Grass shoots kept in the dark will grow straight
  up.
• So will those illuminated equally on all sides.

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Signal Pathways and Grass Seedlings

• If illuminated from only one side, the plant will
  grow toward the stimulus.
• This results in differential growth on the
  opposite side of the stimulus.

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The Darwins Experiments

• Observations
• Plants will only bend toward the light source if
  the coleoptile is present--no tip, no curve.
• Covering the tip with an opaque cap prevents
  curving.
• Covering the tip with a transparent cap or
  placing a cover below the tip--curving.

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The Darwins Experiments

• Their conclusions
• The tip of the coleoptile is responsible for
  curvature.
• Also, the curvature of the plant actually was the
  result of differential growth some distance below
  the coleoptile.
• Some signal must be responsible for elongation of
  the coleoptile.

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Peter Boysen-Jensen

• A few decades later
• He separated the tip of the coleoptile with a
  block of gelatin.
• The cells still showed the normal growth response.

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Peter Boysen-Jensen

• Using mica, there was no response.

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Frits Went

• A Dutchman that modified Boysen-Jensens
  experiment to extract the chemical messenger.
• He removed the coleoptile tip and placed it on an
  agar block.
• If the messenger could diffuse into the block,
  then it could be substituted for the tip and
  placed on the decapitated coleoptile resulting in
  normal growth.

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Frits Went

• Results
• Decapitated coleoptile--no growth.
• Decapitated coleoptile agar only--no growth.
• Decapitated coleoptile agar with
  hormones--growth.

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Frits Went

• Also, placing the block on one side of the
  coleoptile or the other caused unequal growth on
  the side containing the block causing curvature
  in the opposite direction.

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Frits Went

• Conclusions
• The plants curved due to the higher concentration
  of growth promoting chemical on the dark side of
  the plant.
• Went named this hormone auxin.

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Wents Model

• This model doesnt necessarily occur in all
  plants.
• There is still an unequal distribution of auxin
  in a plant causing curvature.
• Some plants show an increase in growth inhibitors
  on the light side of the plant.

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Plant Hormones

• There are many different classes of plant
  hormones.
• They all have different effects on plants.
• Most are produced in very small amounts and often
  have profound effects on the plant.
• The hormones are often amplified and acts to
  alter gene expression.

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3 Common Plant Hormones

• Auxin-stimulate growth. Produced in the embryo,
  growth tissue, and meristematic tissue.
• Gibberillins--produced in the apical meristems of
  buds and roots, young leaves and embryos.
• Ethylene--promotes ripening. Opposes auxin.

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Light

• Light is an important environmental factor in the
  growth and development of plants.
• Photomorphogenesis is the effect of light on
  plant morphology.
• The ability of a plant to perceive light allows
  plants to measure the passage of days and seasons.

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Circadian Rhythms

• Common to all eukaryotic life, and is not
  governed by an known environmental factor.

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Photoperiodism

• Is the physiological response of plant due to a
  change in the lengths of night and day--a
  photoperiod.

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Different Types of Plants

• There are 3 general varieties of plants
  classified according to their light requirements
  for flowering
• 1. Short-day plants
• 2. Long-day plants
• 3. Day-neutral plants.

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Short-Day Plants

• Respond the long nights.
• A.k.a. long-night plants.
• They usually flower in the late summer, fall, or
  winter as the light period is shorter than 14
  hours.

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Long-Day Plants

• Respond to short nights.
• A.k.a. short-night plants.
• They flower when the light period is longer than
  14 hours.

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Day-Neutral Plants

• These are unaffected by the light period, and
  flower when they reach maturity.
• Tomatoes, rice, and dandelions.

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Classic Experiments

• In the 1940s scientists began experimenting with
  photoperiods.
• They looked at the length of the night and day.
• They found that short-day plants flower when days
  are 16 hours or shorter (nights are 8 hours or
  longer).

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Classic Experiments

• In the short-day plants, they looked at
  flowering
• They found that if the daytime portion of
  photoperiod is broken by a brief period of
  darkness, there is no effect.
• However, if the nighttime portion of the
  photoperiod is interrupted by a short period of
  dim light, the plant doesnt flower.

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Classic Experiments

• The opposite is true for long-day plants
• When long day plants are grown in a photoperiod
  of a long night, flower doesnt occur.
• However, if the long night portion of the
  experiment is interrupted by a brief period of
  dim light, flowering will occur.

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From These Experiments

• Red light is most effective at interrupting the
  nighttime portion of the photoperiod.
• Scientists have demonstrated that phytochrome is
  the pigment that measures the photoperiod.

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Extending the Experiments

• Scientists at the USDA conducted these
  experiments.
• Phytochrome was demonstrated to be the pigment
  responsible for seed germination.
• From this, they were able to elucidate the
  flowering cycle.

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USDA Flowering Experiments

• Seeds were subjected to a variety of
  monochromatic light.
• Red and far-red light opposed each other in their
  germinating ability.
• One induced germination, the other inhibited it.

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USDA Flowering Experiments

• It was determined that the two different forms of
  light switched the phytochrome back and forth
  between two isomeric forms.

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USDA Flowering Experiments

• One form caused seed germination, the other
  inhibited the germination response.

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USDA Flowering Experiments

• The question How do plants in nature illicit a
  response to light and begin germination?

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USDA Flowering Experiments

• If seeds are kept in the dark, they synthesize
  Pr.
• When seeds are illuminated with sunlight, they
  begin to be converted to Pfr.
• The appearance of Pfr is one of the ways plants
  detect sunlight.
• Adequate sunlight converts Pr to Pfr and triggers
  germination.

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USDA Flowering Experiments

• In the flowering response, scientists were able
  to show the effects of the red and far red light
  on the flowering ability in plants.
• Again, the 2 forms of light canceled each other.

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Other Stimuli

• There are also a wide variety of stimuli other
  than light that effects plant growth.
• Gravity, mechanical stimuli, and environmental
  stress also play a role in plant growth and
  development.