Document Sample
Photosynthesis Powered By Docstoc
					                           Control of Growth
                           and Responses in    1
Ch. 27 - Plant Responses
                                      Control of Growth
                                      and Responses in    2
     Ch. 27 - Plant Responses
 Plant growth toward or away from a unidirectional
  stimulus is called a tropism
  - Positive is towards stimulus
  - Negative is away from stimulus
  - Due to differential growth - one side of organ
    elongates faster than the other
 Three types of tropisms:
   - Phototropism - movement in response to light
   - Gravitropism - movement in response to gravity
   - Thigmotropism - in response to touch
Phototropism   3
                                      Control of Growth
                                      and Responses in    4

Positive phototropism:
 Studied by Charles & Francis Darwin
  Occurs because cells on the shady side of the
  stem elongate
 A pigment related to riboflavin thought to act as a
  photoreceptor when phototropism occurs
   - Plant hormone called auxin migrates from lighted
     side of stem to shady side of stem
   - Cells on the shady side elongate faster than those
     on the bright side, causing stem to curve toward the
                                       Control of Growth
                                       and Responses in    5
When a plant is placed on its side, the stem
 grows upward, opposite of the pull of gravity.
 This is an example of negative response called
Roots, in contrast, show positive gravitropism, as
 they grow downwards.
 Roots without root caps don’t respond to gravity
 Root cap cells contain sensors called statoliths,
  which are starch grains located within
  amyloplasts, a type of plastid.
  - Amyloplasts settle to lower part of cell & cause
     bending of root.
Gravitropism   6
                  Gravitropism   7

Negative gravitropism of stems

Positive gravitropism of roots

 Sedimentation of statoliths
                                        Control of Growth
                                        and Responses in    8
Auxin is responsible for:
  Positive gravitropism of roots, and
  Negative gravitropism of shoots
How does auxin do this:
  Amyloplasts come in contact with ER which
   releases stored calcium ions.
  This leads to activation of auxin pumps & auxin
   enters the cells
  Roots & stems respond differently to auxin:
   - Auxin inhibits growth of root cells, so cells on upper
      surface elongate so root curves downward
   - Auxin stimulates growth of stem cells, so cells on
      lower surface elongate so stem curves upward
                  Gravitropism   9

Negative gravitropism of stems

Positive gravitropism of roots

 Sedimentation of statoliths
                                     Control of Growth
                                     and Responses in    10

Unusual growth due to contact with solid objects
 is called thigmotropism
  Ex: Coiling of tendrils
The plant grows straight until it touches
  Cells in contact with object grow less while those
   on the opposite side elongate.
  Response can be quite rapid; within 10 minutes
  Sometimes it seems to need light which might be
   a need for ATP for the response.
Coiling Response   11
                                    Control of Growth
                                    and Responses in    12
      Nastic Movements
Nastic movements:
  Do not involve growth and
  Are not dependent on the stimulus direction
Seismonastic movements result from:
  Touch, shaking, or
  Thermal stimulation
Due to loss of turgor pressure within a few cells
 located in a thickening, called a pulvinus, at the
 base of each leaflet. Touch causes K+ to flow
 out of cells & then water follows.
 •Ex: Mimosa leaves & Venus flytrap
           Seismonastic Movement   13

Mimosa pudica
                                   Control of Growth
                                   and Responses in    14
     Nastic Movements

Sleep movements:
 Occur daily in response to light and dark changes
 Ex: Prayer Plant
 Movement due to changes in turgor pressure of
  motor cells in a pulvinus located at the base of
  each leaf.
               Sleep Movement   15

Prayer plant
                                      Control of Growth
                                      and Responses in    16
     Circadian Rhythms

Biological rhythms with a 24-hour cycle
 Tend to be persistent
   - Rhythm is maintained in the absence of
     environmental stimuli
   - Caused by a biological clock
   - Without environmental stimuli, circadian rhythms
     continue but the cycle extends to 25 or 26 hours
   - Believed that the clocks are synchronized by
     external stimuli such as length of daylight
     compared to length of darkness. This is called the
                                  Control of Growth
                                  and Responses in    17
     Plant Hormones

Almost all communication on a plant is done by
 Chemical signals produced in very low
  concentrations in one part of plants and then
  active in another part of the plant
 Hormones travel within phloem, or from cell to
  cell, in response to the appropriate stimulus
 Each hormone has a specific chemical structure
                                   Control of Growth
                                   and Responses in    18
The most common naturally occurring auxin is
  indoleacetic acid (IAA).
It is produced in shoot apical meristem and is
  found in young leaves and in flowers and fruits
Auxins affect many aspects of plant growth &
                                      Control of Growth
                                      and Responses in    19
     Effects of Auxin

Apical Dominance
 Apically produced auxin prevents the growth of
  axillary buds (side buds)
 When a terminal bud is removed, the nearest
  lateral buds begin to grow, and the plant branches
   - Pruning the top of a plant generally achieves a
     fuller look by removing the apical dominance
   - Weak solution of auxin applied to woody cutting
     causes rapid growth of adventitious roots
   - Promotes fruit growth
Apical Dominance   20
                                        Control of Growth
                                        and Responses in    21
     Effects of Auxin

Auxin production by seeds also promotes the
 growth of fruit.
   - As long as auxin is concentrated in leaves or fruits
     rather than in the stem, leaves and fruits do not fall
   - Trees can be sprayed with auxin to keep mature
     fruit from falling to ground
   - Auxin is sprayed on tomatoes to induce
     development of fruit without pollination creating
     seedless tomatoes
                                  Control of Growth
                                  and Responses in    22
     Phototropism Experiments

Darwin & Darwin (1880s)
     1. Used coleoptiles (grass shoots)
     2. Found that shoots bend if:
         a. Tips of shoots are present &
              Normal
              Covered with clear cap
              Opaque base
      3. No bending if:
          a. Tip covered with cap
          b. Tip was removed
      4. Concluded tip senses light
Phototropism Experiments   23
                           Control of Growth
                           and Responses in    24
Peter Boysen-Jensen (1913)
1. Removed tips of shoots
2. Placed gelatin on stump
3. Replaced tip on top of gelatin:
   a. Shoots bent towards light
4. Put piece of impermeable mica
   between shoot and tip:
   a. No phototropic response
5. Concluded that some mobile
   chemical is responsible for the
   phototropic response
Phototropism Experiments   25
                           Control of Growth
                           and Responses in    26
 A. Paal (1918)
1. Removed tips of shoots
2. Put shoots in the dark
3. Replaced tips back on stumps
   but put them off-center on stumps
4. Tip placed on right side:
   a. Shoots bent towards left
5. Tip placed on left side:
   a. Shoots bent towards right
6. Suggested tip produces chemical
   that moves down shoot & causes
   cells below it to grow
   a. Light must alter its amount
Phototropism Experiments   27
                           Control of Growth
                           and Responses in    28
 Fritz Went (1926)
1. Removed tips & placed them on
    blocks of agar for an hour
2. Put blocks of agar only on cut
    ends of stumps
3. If placed in center of stump:
    a. Shoots grew straight upward
4. If placed off-center of stump
    a. Shoots grew & bent to opposite
5. Blank agar blocks didn’t grow
   a. Definitive evidence of a hormone
      He named the hormone auxin.
Demonstrating Phototropism -
    Went’s Experiment
Phototropism   30
                                     Control of Growth
                                     and Responses in    31
      How Auxins Work                     Plants

When a stem is exposed to unidirectional light, auxin
 moves to the shady side.
Auxins bind to plasma membrane receptors which leads
 to a series of reactions & the generation of at least
 three specific second messengers:
 1. Activates a proton, H+, pump
    Acidic conditions cause cell wall to loosen
    Cellulose fibrils are weakened
 2. Activates Golgi apparatus
    Sends out vesicles laden with cell wall materials
                                   Control of Growth
                                   and Responses in    32
      How Auxins Work                   Plants

3. Stimulates DNA-binding protein
   Activates a particular gene
   Leads to production of growth factors
Cell walls become extensible & then fill with water
 by osmosis.
Turgor pressure increases due to the entry of
 water & the cell elongates.
This occurs on side opposite to the light so the
 stem lengthens on shady side causing a
 bending toward the light.
Auxin Mode of Action   33

                                     Control of Growth
                                     and Responses in    35

Growth promoting hormones
 Bring about internode elongation of stem cells
 Gibberellic acid (GA3) = most common
   - Stem elongation
   - Can cause dwarf plants to grow huge
   Sources of gibberellin:
   - Young leaves, roots, embryos, seeds & fruits
   Commercial uses:
   - Break dormancy of buds & seeds, induce flowering,
     increase size of flowers, produce larger seedless
      Effect of Gibberellins             36


No treatment
                                    Control of Growth
                                    and Responses in    37

How GA3 acts as a chemical messenger:
 Embryo produces gibberellins
 Amylase, enzyme that breaks down starch,
  appears in cells just inside seed coat
 GA3 is the first messenger
   - Attaches to receptor in plasma membrane
   Second messenger, calcium ions, combines with a
     DNA-binding protein
   - Believed to activate the gene that codes for
     amylase. This acts on starch to release sugars
     used as source of energy for growing embryo
      Gibberellic Acid:        38
Structure and Mode of Action
                                       Control of Growth
                                       and Responses in    39

 A class of plant hormones that promote cell
  division (cytokinesis)
 First isolated in 1967 from corn = zeatin
 Produced in dividing tissues or roots & in seeds &
   - Promotes cell division
   - Prevents senescence (Aging process. Leaves die
     and fall off)
   - Initiates leaf growth. Lateral buds will grow when
     cytokinin is applied to them.
                                            Control of Growth
                                            and Responses in    40
      Abscisic Acid
Abscisic acid (ABA): (aka stress hormone)
  Initiates and maintains seed and bud dormancy
  Brings about closure of stomata
  Dormancy occurs when a plant readies itself for adverse
   conditions by stopping growth
   - ABA moves from leaves to vegetative buds in fall
   - Buds are converted to winter buds which get covered by
     thick, hardened scales
   - In spring, reduction in level of ABA & increases in
     gibberellins break seed and bud dormancy.
  Produced by:
   - Any “green tissue” with chloroplasts
   - Monocot endosperm, and
   - Roots
              Abscisic Acid:                              41
         Control of Stoma Opening

ABA binding leads to influx of Ca2+ & the opening of K+
 channels. Water exits guard cells & stoma closes.
                                      Control of Growth
                                      and Responses in    42
Ethylene is involved in abscission, the dropping of
 leaves, fruits & flowers from a plant
 Once abscission has begun:
   - Ethylene stimulates certain enzymes like cellulase
   - Causes leaves, fruits, or flowers to drop
 Also ripens fruit by increasing activity of enzymes
  that soften fruit
 Uses in agriculture:
   - To hasten ripening of green fruits
   - To create pleasing colors before sales
 Ethylene is a gas that can induce ripening of
  nearby fruits
Functions of Ethylene   43
                                Control of Growth
                                and Responses in    44
Photoperiodism is any physiological response
 prompted by changes in day or night length

    1. The relative lengths of day and night
    2. This changes with the seasons
    3. Flowering, germination & dormancy all
       occur at specific times of year
    4. Thus, photoperiod is the major
       environmental factor that needs to be
       measured by plants
                                   Control of Growth
                                   and Responses in    45
Three Types of Plants:
   1. Short-Day Plants (Long-night)
      a. Flower when days are short (fall, winter)
      b. Actually controlled by night length: Night
        length must be longer than a critical
      c. Continuity of darkness is what
         matters. A flash of light will disrupt
      d. Examples: Chrysanthemums,
          poinsettias, rice, ragweed
Photoperiodism and Flowering   46
                              Control of Growth
                              and Responses in    47

2. Long-Day Plants (Short-night)
   a. Flower when days are long (late spring,
   b. Night must be shorter than a critical
   c. A flash of light during the night can
      induce flowering during the wrong
   d. Examples: spinach, wheat,
      lettuce, iris, petunia, mustard
Photoperiodism and Flowering   48
                             Control of Growth
                             and Responses in    49

3. Day-Neutral Plants
   a. Day length doesn’t matter
   b. Flower year round
   c. Examples: Roses, carnations,
      dandelions, sunflowers
                                Control of Growth
                                and Responses in    50
How is photoperiod detected?
  1. Involves phytochrome, a light-absorbing
     pigment that exists in 2 inter-changeable
     a. Pr strongly absorbs red light (660-680
     b. Pfr absorbs far-red light (700–730 nm)
 2. When Pr absorbs red light it is converted
    quickly to Pfr
 3. When Pfr absorbs far-red light it is
    converted slowly to Pr
Phytochrome Conversion Cycle   51

      happens quickly

           (happens slowly)
                                Control of Growth
                                and Responses in    52

4. Pfr slowly reverts back to Pr in the dark
5. At sunset, far-red light is common
   a. So Pfr begins to convert to Pr
   b. This marks end of day; start of night.
   c. Pr accumulates slowly all night
6. At sunrise, red-light is common
   a. So Pr converts to Pfr relatively quickly
   b. This marks end of night; start of day.
Phytochrome Conversion Cycle   53

      happens quickly

           (happens slowly)