Plant Responses

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Plant Responses
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Introduction

• At every stage in the life of a plant,
  sensitivity to the environment and coordination
  of responses are evident.
• One part of a plant can send signals to other
  parts.
• Plants can sense gravity and the direction of
  light.
• A plants morphology and physiology are
  constantly tuned to its variable surroundings.

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• Upon receiving a stimulus, a receptor initiates
  a specific series of biochemical steps, a signal
  transduction pathway.

• Ultimately, a signal-transduction pathway leads
  to the regulation of one or more cellular
  activities.

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How do plants control their growth in response to
environmental stimuli?

• Most plants do this by way of chemical messengers
  known as hormones.
• A hormone is a chemical that is produced in one
  part of an organism and transferred to another
  part to affect the activities of that part of the
  plant.

Hormone-producing cells
Movement of hormone
Target cells
Hormone-producing cells
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Research on how plants grow toward light led to
the discovery of plant hormones

• Studies of grass seedlings, particularly oats.
• In the late 19th century, Charles Darwin and his
  son observed that a grass seedling bent toward
  light only if the tip of the coleoptile was
  present.
• This response stopped if the tip was removed or
  covered
• Later, Peter Boysen-Jensen demonstrated that the
  signal was a mobile chemical substance.

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• In 1926, F.W. Went extracted the chemical
  messenger for phototropism, naming it auxin.

Animation
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Auxin

• 1.Auxins are responsible for regulating
  phototropism in a plant by stimulating the
  elongation of cells.
• 2.High concentrations of auxin help promote the
  growth of fruit and minimize the falling off of
  fruit from the plant.
• When the auxin concentrations decrease in the
  autumn, the ripened fruit will fall. The plants
  will begin to lose their leaves.

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Auxin

• Auxin also alters gene expression rapidly,
  causing cells in the region of elongation to
  produce new proteins within minutes.
• Some of these proteins are short-lived
  transcription factors that repress or activate
  the expression of other genes.

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auxin

• 3. Enhance apical dominance
• Produced in the growing tip of a plant and
  transported downward (polar transport)
• Terminal bud suppresses lateral growth

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auxin

• In growing shoots auxin is transported
  unidirectionally, from the apex down to the
  shoot.
• Auxin enters a cell at its apical end as a small
  neutral molecule, travels through the cell as an
  anion, and exits the basal end via specific
  carrier proteins.
• Outside the cell, auxin becomes neutral again,
  diffuses across the wall, and enters the apex of
  the next cell.
• Auxin movement is facilitated by chemiosmotic
  gradients established by proton pumps in the cell
  membrane.

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auxin

• Auxin is transported through the plant body by
  polar transport.
• Requires specific carrier proteins built into the
  cell membrane.
• ATP provides the energy for a proton pump that
  pumps protons out of the cytoplasm, creating a pH
  gradient necessary for the transport of auxin.

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With
Can be used as a rooting powder
5. Treating a detached leaf or stem with rooting
powder containing auxin often causes adventitious
roots to form near the cut surface.
4. Auxin is also involved in the branching of
roots

• Without

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• 6. Auxin also affects secondary growth by
  inducing cell division in the vascular cambium
  and by influencing the growth of secondary xylem.
• 7. Developing seeds synthesize auxin, which
  promotes the growth of fruit.

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Cytokinins

• Stimulate cytokinesis and cell division
• Experiment with plant embryos
• Work with auxin to promote growth and cell
  division
• Work against auxin in relation to apical
  dominance
• Delay senescence (aging) by inhibiting protein
  breakdown
• Florists spray flowers to keep them fresh.
• Produced in roots and travel upward in the plant

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• EXTRAS
• Discovered in 1931
• 1941- coconut milk
• 1961-first isolated cytokinin from corn (Zeatin)
• Now found in almost all higher plants
• Highest in meristematic areas.
• Made in roots

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• Cytokinins interact with auxins to stimulate cell
  division and differentiation.
• In the absence of cytokinins, a piece of
  parenchyma tissue grows large, but the cells do
  not divide.
• In the presence of cytokinins and auxins, the
  cells divide.
• If the ratio of cytokinins and auxins is
  balanced, then the mass of growing cells, called
  a callus, remains undifferentiated.
• If cytokinin levels are raised, shoot buds form
  from the callus.
• If auxin levels are raised, roots form.

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Gibberellins

• Growth hormones that cause plants to grow taller.
  
• They also increase the rate of seed germination
  and bud development.
• There are certain tissues in the seeds that
  release large amounts of gibberellins to signal
  that it is time to sprout.
• Production occurs mainly in the roots
  and young leaves

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• The effects of gibberellins in enhancing stem
  elongation are evident when certain dwarf
  varieties of plants are treated with
  gibberellins.
• if applied to normal plants,
  there is often no
  
  response.

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Abscisic acid

• It inhibits plant growth during times of stress,
  such as cold temperatures or drought.
• Closes stomata during times of stress
• Promotes seed dormancy.
• Overcome by the leaching of ABA in water
• First thought to control abscission.

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Ethylene

• Gas
• Promotes fruit ripening
• Involved in flower production
• Influences leaf abscission a

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Tropisms
Tropisma plants response to their
environment Geotropisma plants response to
gravity Phototropisma plants response to
light Thigmotropisma plants response to touch
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• Geotropism/Gravitropism
• Response of stems and roots to the force of
  gravity.
• It is important when seeds are sprouting.
• Both Auxin and gibberellins are involved.
• If stem is horizontal, auxin concentrates on the
  underside causing elongation of cells.

Gravitropism Clip
Animation
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Phototropism

• Ability of a plant to respond to light.
• Auxin moves down stem on dark side causing
  elongation on cells.
• Unequal distribution of auxin

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Thigmotropism

• The response of a plant to touch.
• Climbing plants, ivy, and vines use thigmotropism
  in order to find their way up or around a solid
  object for support.

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Nastic Movements

• Nastic movements are rapid, reversible responses
  to non-directional stimuli (eg. Temperature,
  Humidity, Light irradiance). Nastic movement is
  caused by turgor pressure change due to movement
  of water in cells as opposed to tropic movement
  which is actual growth and therefore irreversible

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• Mimosa plant
• This occurs when 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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Photoperiodism
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Introduction

• Light is an especially important factor in the
  lives of plants.
• photosynthesis
• light also cues many key events in plant growth
  and development.
• light reception is also important in allowing
  plants to measure the passage of days and
  seasons.
• Photoperiodism is the response of plants to
  changes in the photoperiod.
• Photoperiod- relative length of daylight and night

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• Action spectra reveal that red and blue light are
  the most important colors regulating a plants
  photomorphogenesis.
• These observations led researchers to two major
  classes of light receptors
• a heterogeneous group of blue-light
  photoreceptors
• a family of photoreceptors called phytochromes
  that absorb mostly red light.

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1. Blue-light photoreceptors are a heterogeneous
group of pigments

• The action spectra of many plant processes
  demonstrate that blue light is most effective
  in initiating a diversity of responses.

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2- Phytochrome

• Phytochrome, a protein modified with light
  absorbing chromophore.
• 2 forms
• Pr (p660)and Pfr (p730)
• They are photoresiversible
• When one is exposed to the other wavelength, it
  will convert to the other.
• This conversion helps the plant keep track of
  time.

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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.

Lettuce seeds exposed to flashes of light
p730
p660
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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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• During the day Pr (p660)and Pfr (P730) are in
  equilibrium.
• Pr accumulates at night
• No sunlight to make the conversion
• Pfr breaks down faster than Pr
• At daybreak, Pr begins to be converted to Pfr
• So, night length is responsible for resetting the
  clock.

When there is a short day (long night), a lot of Pfr will be degraded to Pr. 
   When there is a long day (short night), little Pfr will be degraded to Pr.
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• In addition to phytochrome, another chemical
  called cryptochrome has been found to be
  responsible for initiating flowering as a result
  of exposure to blue light.

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• If there are short periods of dark during the day
• no change
• Flashes of red light during the night
• resets the clock.

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• In the 1940s, researchers discovered that it is
  actually night length, not day length, that
  controls flowering and other responses to
  photoperiod.

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• Short-day plant will only flower when the light
  period shorter than a critical length to flower.
• Ex chrysanthemums, poinsettias, and some soybean
  varieties.
• Long-day plants will only flower when the light
  period is longer than a critical number of hours.
• Ex include spinach, iris, and many cereals.
• Day-neutral plants will flower when they reach a
  certain stage of maturity, regardless of day
  length.
• Ex tomatoes, rice, and dandelions.

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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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• 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.

Fig. 39.23
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• A higher proportion of FR light allows plants to
  detect when they are shaded.
• Plants adapted for growth in full sun will
  display greater stem elongation when they are
  transferred to shade.  They also develop smaller
  leaves and less branching.  This change is due to
  greater proportion of Pr to Pfr.