Behavior of Plants in Response to Hormones

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Behavior of Plants in Response to Hormones

• Chapter 39

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Plants Respond to Hormones

• Hormone chemical signals that coordinates the
  structure and function of an organism
• Produced in one structure/area
• Transported to a target area/structure
• Binds to a protein receptor at target site
• Triggers a signal transduction response at target
  cells/tissues

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Tropism

• Tropism Growth pattern in response to an
  environmental stimulus
• Phototropism (response to light)
• () towards (-) away
• Gravitropism (response to gravity)
• () towards earth (-) away from
  earth
• 3) Thigmotropism (response to touch)
• - ex. Climbing vines
• () towards contact (-) away from
  contact

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Types of Plant Hormones

• Auxin or (Indoleacetic Acid - IAA)
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic Acid Growth Inhibitor
• Phytochromes
• Florigen

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Auxin (Indoleacetic Acid or IAA)

• Auxin Hormone that promotes elongation in parts
  of cells
• Produced in apical meristem of shoots and
  transported to areas in the plant where cell
  elongation is needed

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Auxin Transport

• Anionic form of auxin is transported across
  membrane through a protein into the cell wall,
  where a a hydrogen ion (proton) is picked up

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Auxin Transport

• In the cytoplasm, the pH of the cell causes the
  auxin to ionize again.
• The H ion is transported by ATPase back into the
  cell wall, maintaining a voltage difference (or
  membrane potential) between the cytoplasm and
  wall

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Auxin Transport

• Voltage difference contributes to the favoring of
  anion transport out of the cytoplasm, so anionic
  auxin leaves the cytoplasm of the cell
• as this cycle continues, auxin can be
  transported throughout the plant

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Phototropism in Plant Stem
Elongation of cells on one side of the stem (due
to auxin) causes bending of the stem
Normal-sized cells on the other side
If apical meristem is removed, no phototropism
can occur because that is where auxin is produced
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The Acid-Growth Hypothesis
ATP
ADP
H
H
Expansin protein
Protons activate Expansin Protein, which (breaks
down Hydrogen bonds in cell wall)
Cell elongation occurs as cell wall stretches in
response to turgor pressure from the vacuole
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Gravitropism in Stem
Auxin accumulates on the bottom side of stem,
causing elongation that turns the plant upwards
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Auxin has opposite effect in roots!

• In roots, instead of expanding and elongating the
  cell, high auxin concentration tends to inhibit
  growth in roots.
• http//www.bio.psu.edu/People/Faculty/gilroy/ali/g
  raviweb/toc.htm

Auxin produced by apical meristem of roots
accumulate at the bottom and inhibits growth on
this side, causing a bend in the roots towards
gravity
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Gibberellins (Gibberellic Acid GA)

• Gibberellins a group of plant hormones (gt100
  types) that promotes cell growth
• 1. Causes bolting rapid elongation
• (evident when dwarf plants are treated with GA,
  they grow to normal size)
• 2. Often works with auxin in the following
• a) fruiting auxin gibberellins are necessary
  for fruit to set
• b) germination auxin gibberellins are
  necessary to cause seeds to break dormancy

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Cytokinins (CK)

• Cytokinins hormones that stimulate cytokinesis
• 1. Effect of Cytokinins depends on relative
  concentration of auxin (IAA)
• IAA CK ? cell dividision w/o
  differenctiation
• IAA lt CK ? shoots form
• IAA gt CK ? roots form

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Cytokinins (CK)

• 2. CK weakens apical dominance and promotes the
  growth of auxillary bud
• 3. Anti-aging properties of plant organs by
  inhibiting breakdown of plant proteins (florists
  often use CKs to keep flowers fresh)

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Ethylene (CH2)

• Ethylene a gas that acts like a hormone and is
  used by plants to cope with stress
• (CH2) produced during times of stress like
  drought, flooding, etc.)
• - Stimulates flowering and fruit ripening
• w/ auxin (IAA), promotes dropping of leaves
  (abscission) during the fall and prevents
  elongation of roots and stems

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Abscisic Acid Growth Inhibitor (ABA)

• Abscisic Acid hormone responsible for
  preventing growth
• Acts as anti-auxin, cytokinins, and gibberrelins
• Keeps seeds dormant during drought
• - once rains come, the rains wash out the
  ABA, allowing seeds to break dormancy with the
  help of gibberrellins and auxins.

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Phototropism

• Phototropism the response of plants to changes
  in season
• Photoperiod relative length of night and day
• Circadian rhythm internal clock that measures
  the length of night and day
• Circadian rhythm is controlled by
• - endogenous (internal) factors and/or
• - exogenous (external) factors
• 4. Phytochrome protein (has a light absorbing
  chromophore) helps maintain the circadian rhythm

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Phytochromes have 2 isomeric forms

• Pr the inactive form that absorbs
  wavelengths of red light (660 nm)
• Pfr the active form that absorbs wavelengths
  of far-red light (730 nm)

Far Red 730 nm Pfr
Red 600nm Pr
Absorbs red
Absorbs far red
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How are phytochromes used by plants to measure
day and night?

• 1. Pr (inactive) is made by plants at night
• 2. Pr is high
• 3. As daybreak approaches and more red light is
  available, Pr ? Pfr
• 4. Since sunlight has both red and far-red
  spectrums, Pr Pfr at mid-day
• 5. Evening decreases the Pfr while increases in
  the Pr helps reset the circadian rhythm

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What triggers flowering?

• Critical Night Length (not day length) triggers
  flowering

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Flowering Responses to Changes in Photoperiod

• Three classifications
• Short-day plants (flower when daylight decreases
  in early fall/late summer)
• Critical night length gt daylight
• 2. Long-day plants (flower when daylight
  increases in spring/early summer)
• Critical night length lt daylight
• 3. Day-neutral plants (other factors trigger
  flowering, like availability of water, etc.)

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Florigen

• Depending on what classification of plant they
  belong in, florigen hormone is produced at
  different periods of the season to trigger
  flowering