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Title: Plant Responses


1
Plant Responses
2
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.

3
  • 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.

4
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
5
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
8
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.

9
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.

10
auxin
  • 3. Enhance apical dominance
  • Produced in the growing tip of a plant and
    transported downward (polar transport)
  • Terminal bud suppresses lateral growth

11
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.

12
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.

13
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

14
  • 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

17
  • 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

18
  • 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.

19
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

20
  • 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.

21
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.

22
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
25
  • 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
26
Phototropism
  • Ability of a plant to respond to light.
  • Auxin moves down stem on dark side causing
    elongation on cells.
  • Unequal distribution of auxin

27
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.

28
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

29
  • 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.

30
Photoperiodism
31
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

32
  • 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.

33
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.

34
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.

35
  • 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
36
  • 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.

37
  • 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.
38
  • 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.

39
  • If there are short periods of dark during the day
  • no change
  • Flashes of red light during the night
  • resets the clock.

40
  • In the 1940s, researchers discovered that it is
    actually night length, not day length, that
    controls flowering and other responses to
    photoperiod.

41
  • 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.

42
  • 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.

43
  • 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
44
  • 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.
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