Plant Responses to Internal and External Signals

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Plant Responses to Internal and External Signals

• Chapter 39

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Plant Hormones

• What is a Plant hormone?
• Compound produced by one part of an organism that
  is translocated to other parts where it triggers
  a response in target cells and tissues.

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Experiments on PhototropismDiscovery of Hormones

• 1. Darwin and Darwin
• Removed the tip of the coleoptile of a grass
  seedling, and it failed to grow toward light.
• 2. Boysen-Jensen
• Put block of gelatin on coleoptile tip to allow
  chemical diffusion
• Auxin purified later by Thimann
• 3. Went
• Modified Boysen-Jensen experiments
• Extracted the chemical messenger responsible

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Functions of Plant Hormones

• Control plant growth and development by affecting
  division, elongation, and cell differentiation
• Effect depends on size of action, stage of plant
  growth and hormone concentration
• Hormonal signal is amplified by gene expression,
  enzyme activity, or membrane properties

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Five Classes of Plant Hormones

• 1. Auxins
• 2. Cytokinins
• 3. Gibberellins
• 4. Abscisic acid
• 5. Ethylene

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Which hormones cause the following.

• Apical dominance from apical bud
• Abscission
• Stimulates growth of axillary buds
• Root growth
• Stimulates closing of stomata
• Causes fruit ripening
• Stimulates seeds to break dormancy and germinate
• Growth inhibitor
• Cell division and differentiation
• Cell elongation
• Seedless fruit

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Answers

• Auxin
• Ethylene
• Cytokinins
• Gibberellins
• Abscisic acid
• Ethylene
• Gibberellins
• Abscisic acid
• Cytokinins
• Auxin
• Gibberellins

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Which hormone is made at each location?

• Made in roots and transported upwards
• cytokinins
• Found in meristems of apical buds and seed
  embryos
• auxin
• Found in tissues of ripening fruit
• ethylene
• Leaves stems, roots and green fruit
• gibberellins

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Opposing hormones

• Which two hormones act in opposition to one
  another regarding apical dominance, cell division
  and differentiation?
• Cytokinins and auxins
• Which two hormones work in opposition regarding
  seed dormancy?
• Abscisic acid and gibberellins

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Phototropism
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Acid Growth Hypothesis
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Plant Movement

• A. Tropisms
• growth response toward
• or away from stimuli
• 1. Phototropism
• cells on darker side of shoot
• elongate faster than cells on bright
• side due to auxin distribution
• auxin move laterally across the tip
• from the bright to dark side by an
• unknown mechanism.
• Cells on the dark side grow

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Plant Movement

• 2. Gravitropism (geotropism)
• gravity
• roots --gt positive geotropism
• stems---gt negative geotropism
• Statoliths
• starch grains in root cap cells, they trigger
  calcium redistribution which results in auxin
  movement in root
• auxin inhibits cell elongation
• upper side of root elongates faster than bottom

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Plant Movement

• 3. Thigmotropism
• growth in response to touch
• tendrils contacts solid and coils
• increased production of ethylene
• 4. Hydrotropism
• growth toward water
• willow tree

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Circadian Rhythms and the Biological Clock

• Circadian Rhythm- a physiological cycle with a
  frequency of about 24 hours that persists even
  when an organism is sheltered from environmental
  cues.
• Photoperiodism
• a physiological response to DAY length
• seasonal events

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• Photoperiods Control of Flowering
• the amount of night length controls flowering
• 1. Short day plants
• late summer, fall and winter
• 2. Long day plants
• late spring and summer
• 3. Day- Neutral plants
• unaffected by photoperiods

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Critical Night Length

• Night (dark) actually causes flowering not light
• Leaves detect the photoperiod while buds produce
  flowers
• Florigen-
• scientists believe this unidentified hormone is
  produced in the leaves and moves to buds.

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Phytochrome

• Protein containing chromophore (light-absorbing
  component) responsible for a plants response to
  photoperiod
• Pr - red absorbing
• Pfr- Far red absorbing
• Plants synthesize Pr in dark
• if phytochrome illuminated then Pr---gtPfr
• Pfr triggers many plant responses to light
• In darkness Pfr goes back to Pr

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Response to Stress

• Water deficit
• Flooding
• Salt Stress
• Heat Stress
• Cold Stress
• Herbivores

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Defense Against Pathogens

• Gene for Gene relation between plant and pathogen
• Coevolution between plant and pathogen

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Short Day Plant
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Short Day Plant
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• Spinach Long Day Plant

Spinach Long Day Plant
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AuxinsIAA indoleacetic acid natural auxin

• Promotes elongation secondary growth
• Apical meristem is the major site of auxin
  production
• Inhibits lateral growth
• Induces female floral parts fruit

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Cytokinins

• Move from the roots to tissues by moving up xylem
• Stimulates protein synthesis
• Made in roots
• Function
• 1. Cell division and differentiation
• 2. Apical dominance
• 3. Anti-aging hormones
• slow protein deterioration

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Gibberellins

• Stimulate elongation of cells
• Inhibits root growth
• Stimulate flower part development- bolting
• Signals seeds to break dormancy and germinate

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Abscisic Acid (ABA)

• Growth inhibitor
• returns seeds to dormancy
• inhibits cell division in vascular cambium
• causes rapid closing of stoma during dry periods
• promotes positive geotropism

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Ethylene

• Gaseous hormone
• high auxin induce release of ethylene
• Senescence (aging)
• fruit ripening
• Abscission
• leaves falling