Plant Responses

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Plant Responses

• AP Biology Chapter 39

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Respond to the environment

• Plants have cellular receptors that detect
  changes in the environment

Potato grown in the dark w/ long shoots
Potato after exposure to light
Etiolationadaptions for growing in darkness
De-etiolationadaptions for growing in light
(greening)
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Signal Transduction Pathways (involved w/
de-etiolation)

• Reception
• Extracellular chemicals change the shape of
  membrane proteins
• Phytochrome in cytoplasm
• Transduction
• Second messengers (small molecules) amplify the
  signal and transfer it from receptor to other
  proteins to carry out the response
• Cyclic GMP Ca2 activate kinases, cytoskeletal
  proteins, or specific ion channels (phosphotases
  turn things off)
• Response
• The product of the transduction regulates
  the gene expression and
  usually increased enzyme
  activity or activated transcription
  factors

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LE 39-3
CELL WALL
CYTOPLASM
Reception
Transduction
Response
Signal Transduction Pathways
Activation of cellular responses
Relay molecules
Receptor
Hormone or environmental stimulus
Plasma membrane
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Phototropism

• Plant response towards light (positive) or away
  from light (negative)
• Tropism is plant growth towards or away from
  stimuli

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LE 39-5b
Darwin and Darwin (1880)
Phototropism occurs only when the tip is
illuminated!
Light
Base covered by opaque shield
Tip covered by trans- parent cap
Tip covered by opaque cap
Tip removed
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LE 39-5c
Phototropism occurs when the tip is separated by
a permeable barrier but not an impermeable barrier
Boysen-Jensen (1913)
Light
Tip separated by mica
Tip separated by gelatin block
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Frits Went Experiment
Agar block gathered chemicals from tip over time.
When place directly on top, the grass grew
straight. When place to one side, the grass bent
on the side opposite the chemical. The chemical
was named AUXIN!
Dark
Light
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Gravitropism

• Plant response to gravity
• Roots undergo positive whereas shoots undergo
  negative

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Thigmotropism

• Plant response to touch
• Sensitive plant (loss of turgor pressure in plant
  motor organs causes leaflets to fold together
  quickly)
• Tendrils of many vines climbing plants coil
  around objects

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Hormones

• Control growth development of plants
• Signaling molecule produced in small amounts in
  one part of an organism and sent to other parts
  where it binds to a receptor and triggers
  responses

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Auxin

• a hormone that promotes cell elongation
• Responsible for phototropisms (due to unequal
  distributions)
• Enhances apical dominance (suppresses the
  development of axial buds)
• Indoleacetic acids (IAA) is a naturally occurring
  auxin.
• In rooting powder it causes faster root
  development
• In strong doses can be a weed killer
• Synthetic auxins can induce a tomato plant to
  make fruit without pollination (seedless)

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Cytokinins

• Interact with auxins to stimulate cell division
  (cytokinesis) and cell differentiations
• Also delay senescence (aging) by inhibiting
  protein breakdown
• Produced in roots and travels up with xylem sap

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Gibberellins

• Act in concert with auxins to promote stem
  elongation
• Induces bolting
• Rapid growth of a tall flower stalk ensuring
  pollination seed dispersal
• Also causes seeds to break dormancy after seed is
  imbibed with water

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Abscisic Acid (ABA)

• Inhibits growth
• Promotes seed dormancy until conditions are
  favorable (enough water to wash away or prolonged
  exposure to light or cold weather deactivate it)
• Ratio of ABA to gibberellins determines seed
  dormancy or germination
• Drought tolerance
• Stomates closes

Insufficient ABA in tomato seeds
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Ethylene

• Gaseous hormone responsible for effects caused by
  stress, senescence, leaf abscission fruit
  ripening
• Triple Response growing tip runs into rock,
  ethylene gets released, stem elongation slows,
  stem thickens, stem begins to grow horizontally
  until amount of ethylene lessens (no more hard
  surface).
• Senescence programmed death of certain cells or
  organs or the entire plant, but will salvage
  materials if possible (Apoptosis)
• Leaf abscission leaves falling off and a scar
  forms to protect the tree
• Fruit ripening fruits will mature in the
  presence of ethylene (Positive Feedbackmore
  ripening causes more ethylene production)
• Fruit sellers pick perishable fruit early and
  then spray it with ethylene when they are needed
  to ripen.

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Response to Light

• Blue light photoreceptors
• Initiate phototropism light-induced opening of
  stomata
• Phytochromes (red light)
• Regulate, De-etoiliation, seed germination,
  shade avoidance
• In seed germination the last flash of light
  determines the responseredgerminate
  far-redinhibition
• In shade avoidance the ratio of red to far-red
  can tell the plant to grow taller or to grow more
  laterally to maximize photosynthesis

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Plants Biological Clocks

• Plants have circadian rhythms too!
• Whether in constant light or dark
• The ratio of phytochrome red to far-red helps the
  plant reset its biological clock (like us
  adjusting to jet lag)
• Photoperiodism is a physiological response to
  photoperiod (lengths of night day)

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Response to Dangers

• Against herbivores
• Can have thorns
• Can produce distasteful or toxic compounds
• Some can even recruit predatory animals
• Against pathogens
• Primary defense is epidermis/periderm
• Secondary defense is recognition of pathogen
  derived molecules called elicitors by R (disease
  Resistant) proteins
• Hypersensitive response causes cell tissue
  death near the infection site
• Systemic Acquired Resistance substance is
  produced around infection site and sent by phloem
  throughout the plant that activates a signal
  transduction pathway that induces resistance to
  pathogens

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