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Ch. 39: Plant Responses to InternalAnd External Signals - PowerPoint PPT Presentation

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Ch. 39: Plant Responses to Internal And External Signals Signal transduction pathways link internal and environmental signals to cellular responses. Example: Plants response to light Once a shoot reaches the sunlight, greening takes place.

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Ch. 39:

Plant Responses to Internal

And External Signals

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  • Example: Plants response to light

Once a shoot reaches the sunlight, greening

takes place.

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B. The Went Experiment:

Auxin: Hormone

that causes cell

elongation, which

causes a


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  • Auxins: stimulates stem elongation (only

  • low concentrations), root growth, cell

  • differentiation, and branching, regulates

  • development of fruit, enhances apical

  • dominance, functions in phototropism

  • and gravitropism.

Cell elongation: The acid growth hypothesis

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  • Cytokinins:

  • -Source = roots

  • -Affect root growth and differentiation,

  • stimulate cell division and growth (must be

  • with auxin), stimulate germination, delay

  • senescence (anti-aging), signal axillary

  • buds to grow (counteract auxin).

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  • Gibberellins (over 100 kinds):

  • -Source = meristem of apical buds and

  • roots, young leaves, embryo

  • -Promote seed and bud germination, stem

  • elongation, and leaf growth, stimulate

  • flowering (bolting) and developing of fruit,

  • affect root growth and differentiation.

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  • Abscisic Acid:

  • -Source = Leaves, stems, roots, green fruit

  • -Inhibits growth, closes stomata during

  • water stress, counteracts breaking of

  • dormancy

  •  Many seeds will start to germinate once

  • abscisic acid is removed or inactivated.

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  • Ethylene: gaseous hormone

  • -Source = tissues of ripening fruit, nodes of

  • stems, aging leaves and flowers

  • -Promotes fruit ripening, opposes some

  • auxin effects, promotes or inhibits growth

  • and development of roots, leaves, and

  • flowers, depending on species; also, there is

  • a burst of ethylene during apoptosis.

-Responsible for the “triple

response” in seedlings:

a. Horizontal growth of

seedling if there is an

obstacle (rock) on top

of it.

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-Ethylene causes cell to produce enzymes

that digest the cellulose of cells walls

for leaf abscission. When an autumn leaf

falls, the breaking point is at the abscission


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  • Brassinosteroids:

  • -Source = seeds, fruits, shoots, leaves, and

  • floral buds

  • -Inhibits root growth, retards leaf abscission,

  • promotes xylem differentiation

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  • Plant response to light

  • A. Photomorphogenesis: the effects of light

  • on plant morphology.

  • B. Blue-light detection due to 3 pigments:

  • cryptochromes, phototropin, zeaxanthin

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1. Light influences circadian rhythms.

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In 1920, Garner and Allard termed:

-Short day plants only flower if the light

period is less than a critical length

-Long day plants only flower if the light

period is more than a critical period

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Short day plants only flower when

they have continuous dark periods that

exceed the critical period.

Long day plants only flower when they

have continuous dark periods that do not

exceed the critical period.

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Corn roots

  • Statoliths: specialized plastids (starch

  • grains) in root cells.

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Gravity pulls the statoliths groundward. This

redistributes calcium, which triggers an

accumulation of auxin. High conc. of auxin

prevents cell elongation  Curvature

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Touched Untouched

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A signal travels 1 cm per second by an

electrical impulse (action potential); creates

turgid and flaccid cells.

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  • Herbivores:

  • Chemical defense:

  • a.Canavanine: resembles amino acid,

  • arginine. When eaten by an insect,

  • canavanine is incorporated into the

  • insect’s protein in place of arginine.

  •  Kills insect because canavanine is

  • different enough from arginine to

  • allow for incorrect function of protein.

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  • Plant defense against pathogens:

  • 1.Epidermis of plant body

  • 2.Gene-for-gene resistance: avirulent

  • pathogens and plants have a compromise.

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R gene =

codes for receptor that

signals the plant’s

defenses so it can


  • This compromise

    allows for the

    pathogen and plant

    to survive.

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-Sealing off of

infected area

-Self destruction


-Signal (salicylic

acid) received

from infected