4/24/2014
BI 103: Plant Responses: Tropism
& Hormones
Learning Objectives
How do plants detect light?
What are three main tropisms and
what hormones are involved?
What is involved in signaling the plant
to flower?
Know where hormones are
synthesized and how they function.
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Photoperiodism
How can plants sense the
seasons? How do they know when
to germinate? Break bud? Flower?
Biological response to
length of day and night
Phytochrome –
pigment that detects
light
Circadian cycles (24
hours) are biological
clocks
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The Effects of Night Length on Flowering
short night
interrupted night
day-neutral plant
(rose)
short-day plant
(chrysanthemum)
long-day plant
(iris)
long night
night day
Fig. 45-8
Tropism
from Greek: trope “to turn”
Plants turn toward (positive) or
against (negative) environmental
stimuli
Gravity, Light, Touch, Water
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Plant Hormones
What is a hormone?
> A chemical signal.
These chemicals are produced by cells in one
location that influence other cells
e.g. growth in another region of the organism.
Typically they are proteins or steroids, which are lipid
based.
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Auxins
Made in:
Apical meristem of shoot system.Several Forms:
Move from tip downward.
Affects:
Cell elongation
Indoleacetic acid (IAA)
Phenylacetic acid (PAA)
4-chloroindoleacetic
acid (4-chloroIAA)
Indolebutyric acid
(IBA)
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Auxin and Elongation
Cut tip off, no
growth
Place agar on cut shoot, it grows like
normal!
Cut tip on agar block, auxin
diffuses into agar
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4/24/2014
Auxins
Made in:
Apical meristem of shoot system.
Move from tip downward.
Affects:
Cell elongation
Lateral roots- Adventitious roots
Several Forms:
Indoleacetic acid (IAA)
Phenylacetic acid (PAA)
4-chloroindoleacetic
acid (4-chloroIAA)
Indolebutyric acid
(IBA)
Development of vascular tissue
Fruit development
Retards senescence in leaves & fruits
Tropisms– Photo & Gravotropism
Phototropism
Auxin migrates away from light, and accumulates
in greater amounts on opposite side, promoting
greater elongation of cells on dark side.
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Auxin and Phototropism
shaded
side
opaque shield
light
transparent cap
opaque cap
remove tip
(see pp. 890-891)
illuminated
side
gelatin
mica
Phototropism and Auxin
Continued
cut tip on
agar block
chemical moves into
agar
Asymmetric distribution of
auxin causes cell elongation!
no
tip
just
agar
offset chemical
block
no directional
light!
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4/24/2014
Gravitropism
Gravity may be perceived by statoliths (aka
amyloplasts) in root cap
Amyloplasts: organelles in plant cells which
synthesize starch & store it
Q: Where might amyloplasts be in the root?
Growth Responses - Gravity
Time:
0 hr
gravity
Statoliths- specialized amyloplasts with lots of
dense starch
statoliths
Time:
1 hr
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Auxins
Made in:
Apical meristem of shoot system.
Move from tip downward.
Affects:
Cell elongation
Lateral roots- Adventitious roots
Several Forms:
Indoleacetic acid (IAA)
Phenylacetic acid (PAA)
4-chloroindoleacetic
acid (4-chloroIAA)
Indolebutyric acid
(IBA)
Development of vascular tissue
Fruit development
Retards senescence in leaves & fruits
Tropisms– Photo & Gravotropism
Cytokinins
Made in:
-- root
–moves to shoot
Affects:
Stimulates embryo development
Promotion of sprouting of lateral buds.
Development of chloroplasts
Stimulation of the onset of fruits
Stimulate plant metabolism
Delays the aging of plant parts especially leaves
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Apical Dominance: the inhibition of lateral stem
development by the top of the shoot (apical)
Terminal bud
Terminal (apical) bud: meristem
cells at the tip of main stem and
branches.
Lateral buds: meristem cells that
form new branches or leaves
Lateral shoot
Lateral bud
Apical Dominance: the maintenance of a single top
auxin
(SAM)
auxin
(SAM)
Removal of
SAM
inhibited axillary
buds
cytokinin
(roots)
growing lateral
shoots
cytokinin
(roots)
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Gibberellins
Made in:
• Stem apical meristem
• young leaves
• Embryos
Affects:
Germination
Sprouting of buds
Elongation of stems & leaves
Stimulates flowering
Affects the development of fruit
Gibberellins (commercial uses)
Stem growth enhancer (e.g., rice, cotton)
Fruit growth enhancer (e.g., apples, pears, grapes)
Gibberellin spray for grapes
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Gibberellins and Germination
Water moves into seed
Embryo releases gibberellin
Aleurone cells release amylase
Amylase digest starch into
sugar
Sugar fuels growth
Abscisic Acid
Made in:
throughout plant body, especially when stressed
Affects:
Dormancy of buds & seeds
Dropping of leaves
Growth inhibition – blocks protein synthesis
Closing of stomata
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Ethylene
Made in:
Fruit & plant body
Affects:
• Fruit ripening
Ethylene from apple caused
senescense of holly leaves
• Flower maturation
• Senescense- leaf drop
Ethylene and Thigmotropism
Where plant touches
object, ethylene inhibits
cell elongation (auxin).
Cells not touching
object grow normally,
causing bending around
object.
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Growth Responses - Touch
Thigmotropism – movement
in response to touch
Mystery: Why didn’t the lupines
flower last summer?
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Hormones throughout the year
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