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Plant Responses

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Title: Communication Author: Katie Hanson Last modified by: Katie Created Date: 8/30/2009 6:53:30 PM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Plant Responses


1
Plant Responses
  • F215 control, genomes and environment
  • Module 4 responding to the environment

2
Learning Outcomes
  • Explain why plants need to respond to their
    environment in terms of the need to avoid
    predation and abiotic stress.

3
Plant Responses
  • Plants have evolved a wide range of responses to
    a large variety of stimuli, this helps them to
  • Survive long enough to reproduce
  • Avoid stress
  • Avoid being eaten

4
Sensitivity in plants
  • A plants responses to the external environment
    are mainly growth responses
  • Plants must respond to
  • Light
  • Gravity
  • Water
  • Chemicals
  • Touch
  • Plants communicate by plant growth regulators.

5
Learning Outcomes
  • Define the term tropism.
  • Explain how plant responses to environmental
    changes are coordinated by hormones, with
    reference to responding to changes in light
    direction.

6
Plant movements
  • Nastic Movements
  • Usually brought about by changes in turgidity in
    cells
  • Rapid responses
  • examples
  • Venus fly trap shutting
  • Leaves closing
  • Petals closing

7
Nastic Movements
  • Can you think of a nastic movement made by marram
    grass?
  • Describe the response and its adaptive value to
    the plant.

8
Tropisms
  • Slower responses resulting in directional growth
  • is a directional growth response in which the
    direction of the response is determined by the
    direction of the external stimulus

9
Phototropism
  • Phototropism is the response of plant organs to
    the direction of light.
  • A shoot shows Positive phototropism

10
Phototropism
  • This is a growth response towards or away from
    light
  • Look at the worksheet detailing some early
    experiments on phototropisms using oat, barley
    and wheat coleoptiles.
  • Try to draw a conclusion to each experiment.

11
Darwins experiment
12
Darwins conclusions
  • A growth stimulus is produced in the tip of the
    coleoptile
  • Growth stimulus is transmitted to the zone of
    elongation
  • Cells on the shaded side of the coleoptile
    elongate more than the cells on the other side.

13
Boysen-Jensens experiment
14
Boysen-Jensens experiment
15
Boysen-Jensens conclusions
  • Materials which are not permeable to water can
    stop the curvature response in some circumstances
  • Materials which are permeable to water do not
    interfere with the curvature response

16
Wents experiment
17
Wents conclusions
18
Wents conclusions
  • Angle of curvature is related to the number of
    tips used
  • Number of tips used relates to the concentration
    of auxin in the agar block
  • Curvature response is due to a chemical which
    moves from the tip and affects cell elongation

19
Phototropin, auxin and phototropism
20
Phototropin, auxin and phototropism
  • Phototropins
  • Proteins that act as receptors for blue light
  • In plasma membrane of certain cells in plant
    shoots
  • Become phosphorylated when hit by blue light
  • If light is directional, then the phototropin on
    the side receiving the light becomes
    phosphorylated.

21
Phototropin, auxin and phototropism
  • Phosphorylation of phototropin brings about a
    sideways movement of auxin
  • More auxin ends up on the shady side of the shoot
    than on the light side
  • Involves transporter proteins in the plasma
    membranes of some cells in the shoot, these
    actively move auxin out of the cell
  • The presence of auxin stimulates cells to grow
    longer
  • Where there is more auxin there is more growth

22
Auxin action
  • Auxin binds to receptors in plasma membranes of
    cells in the shoot.
  • This affects the transport of ions through the
    cell membrane
  • Build up of hydrogen ions in the cell walls
  • The Low pH activates enzymes that break
    cross-linkages between molecules in walls
  • Cell takes up water by osmosis, cell swell and
    become longer
  • Permanent effect

23
Plant growth
  • Plant growth occurs at meristems
  • Apical meristem
  • Lateral bud meristems
  • Lateral meristems
  • Intercalary meristems

24
Learning outcomes
  • Evaluate the experimental evidence for the role
    of auxins in the control of apical dominance and
    gibberellin in the control of stem elongation.

25
Why plant growth regulators?
  • Exert influence by affecting growth
  • Produced in a region of plant structure by
    unspecialised cells
  • Some are active at the site of production
  • Not specific can have different effects on
    different tissues

26
The Plant growth regulators
  • There are five main groups
  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic acid
  • Ethene

27
Plant growth regulators
  • Produced in small quantities
  • Are active at site of production, or move by
    diffusion, active transport or mass flow.
  • Effects are different depending on concentration,
    tissues they act on and whether there is another
    substance present as well.

28
Interaction of plant growth regulators
  • Synergism
  • 2 or more act together to reinforce an effect
  • Antagonism
  • Have opposing actions and inhibit (diminish) each
    others effects.

29
Auxins
  • Synthesised in shoot or root tips.
  • Most common form is IAA (indole-3-acetic acid
    a.k.a. indoleacetic acid)
  • Main effects of auxins include
  • Promote stem elongation
  • Stimulate cell division
  • Prevent leaf fall
  • Maintain apical dominance.

30
Auxins and Apical Dominance
  • Auxins produced by the apical meristem
  • Auxin travels down the stem by diffusion or
    active transport
  • Inhibits the sideways growth from the lateral buds

31
Apical Dominance
32
Apical Dominance
33
Mechanism for apical dominance
  • Auxin made by cells in the shoot tip
  • Auxin transported downwards cell to cell
  • Auxin accumulates in the nodes beside the lateral
    buds
  • Presence inhibits their activity

34
Evidence for mechanism (1)
  • If the tip is cut off of two shoots
  • Indole-3-acetic-acid (IAA) is applied to one of
    them, it continues to show apical dominance
  • The untreated shoot will branch out sideways

35
Evidence for mechanism (2)
  • If a growing shoot is tipped upside down
  • Apical dominance is prevented
  • Lateral buds start to grow out sideways
  • This supports the theory
  • Auxins are transported downwards, and can not be
    transported upwards against gravity

36
Question and reading
  • Suggest how apical dominance could be an
    advantage to a plant!
  • Read through Page 224 in your textbook apical
    dominance

37
Suggest!!
38
Gibberellins and stem elongation
  • Gibberellin (GA) increases stem length
  • Increases the lengths of the internodes
  • Stimulating cell division
  • Stimulating cell elongation

39
Evidence for GA and stem elongation
  • Dwarf beans are dwarf because they lack the gene
    of producing GA
  • Mendels short pea plants lacked the dominant
    allele that encodes for GA
  • Plants with higher GA concentrations are taller

40
Action of GA
  • Affects gene expression
  • Moves through plasma membrane into cell
  • Binds to a receptor protein, which binds to other
    receptor proteins eventually breaking down DELLA
    protein.
  • DELLA proteins bind to transcription factors
  • If DELLA protein is broken down, transcription
    factor is released and transcription of the gene
    can begin

41
Gibberellins and germination of seeds
  • Monocotyledonous plants e.g. barley and wheat
  • Seeds can lay dormant until conditions are
    suitable for germination.
  • Structure of a seed
  • Pericarp and testa
  • Aleurone layer protein rich
  • Endosperm starch store
  • Scutellum seed leaf
  • Embryo

42
Gibberellins in the germination of barley seeds
  • Germination need suitable conditions, this
    requires presence of water, oxygen and an ideal
    temperature
  • Water enters seed
  • GA secreted by the embryo diffuses across
    endosperm to aleurone layer.
  • GA activates gene coding for amylase
    (transcription)
  • Amylase produced in aleurone and diffuses into
    the endosperm
  • Amylase hydrolyses starch into maltose
  • Maltose is hydrolysed into glucose, which
    diffuses into the embryo.

43
Learning Outcomes
  • Outline the role of hormones in leaf loss in
    deciduous plants.

44
Leaf Abscission
  • Trees in temperate countries shed their leaves in
    autumn.
  • Survival advantage
  • Reduces water loss through leaf surfaces
  • Avoids frost damage
  • Avoid fungal infections through damp, cold leaf
    surfaces
  • Plants have limited photosynthesis in winter

45
Abscission and hormones
  • Three different plant hormones control abscission
  • Auxin
  • Inhibits abscission
  • Ethene (gas)
  • Increase in ethene production inhibits auxin
    production
  • Abscisic Acid

46
Abscisic acid
  • Inhibits growth (antagonistic to GA and IAA)
  • stress hormone
  • Control stomatal closure
  • Plays a role in leaf abcission
  • Abscission falling of leaves or fruit from
    plants.

47
Stages in leaf abscission
  • As leaves age, rate of auxin production declines
  • Leaf is more sensitive to ethene production
  • More ethene produced, inhibits auxin production
  • Abscission layer begins to grow at the base of
    the leaf stalk.

48
Leaf Abscission
49
Abscission Layer
  • The abscission layer is made of thin-walled cells
  • Weakened by enzymes that hydrolyse
    polysaccharides in their walls
  • Layer is so weak that the petiole breaks
  • Leaf falls off
  • Tree grows a protective layer where the leaf will
    break off
  • Cell walls contain suberin
  • Leaves a scar which prevents the entry of
    pathogens

50
Learning Outcomes
  • Describe how plant hormones are used
    commercially.

51
Commercial use of Auxins
  • Sprayed onto developing fruits to prevent
    abscission
  • Sprayed onto flowers to initiate fruit growth
    without fertilisation
  • Parthenocarpy promotes the growth of seedless
    fruits
  • Applied to the cut end of a shoot to stimulate
    root production
  • Synthetic auxins are used as selective herbicides

52
Commercial use of Ethene
  • Fruits harvested before they are ripe allows them
    to be transported without deteriorating, these
    are sprayed with ethene to promote ripening at
    the sale point.
  • E.g. bananas from the Caribbean

53
Commercial use of Gibberellin
  • Sprayed onto fruit crops to promote growth
  • Sprayed onto citrus trees to allow fruit to stay
    on the trees longer
  • Sprayed onto sugar cane to increase the yield of
    sucrose
  • Used in brewing, where GA is sprayed onto barley
    seeds to make them germinate, amylase is
    produced, starch is broken down into maltose, the
    action of yeast on the maltose produces alcohol.

54
Commercial use of cytokinins
  • Delay leaf senescence can be sprayed on lettuce
    leaves to prevent them from yellowing
  • Can be used in tissue culture to mass produce
    plants
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