Bio 100 – Guide 27
http://www.offthemarkcartoons.com/cartoons/1994-11-09.gif
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http://pgjennielove.files.wordpress.com/2008/06/apical_meristem.png
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• Experiments on how plants turn toward light led
to the discovery of a plant hormone
– Phototropism is growth of a plant shoot in
response to light
• Adaptive response, directing shoots and
seedlings toward sunlight
– In experiments, cells on the darker side of a
seedling elongated faster than those on the
light side
• Different rates made plant bend toward
light
Figure 33.1a
LE 33-01b
Shaded
side of
shoot
Light
Illuminated
side of
shoot
– Experiments demonstrating the mechanism of
phototropism in grasses
• Darwin demonstrated that the tip of a
seedling is responsible for detecting light
• Boysen-Jensen showed that a mobile
chemical is the signal for phototropism
• Went isolated the specific chemical
messenger (auxin)
• Experiments with grasses opened up
research on plant hormones
LE 33-01c
Light
Control
Tip
removed
Tip
covered by
opaque
cap
Tip
covered
by transparent
cap
Base
covered by
opaque
shield
Darwin and Darwin (1880)
Tip
separated
by gelatin
block
Tip separated
by mica
Boysen-Jensen (1913)
LE 33-01d-4
Shoot tip placed on agar
block. Chemical (later
called auxin) diffuses
from shoot tip into agar.
Agar
Block with
chemical
stimulates
Control growth.
No light
Offset blocks with
chemical stimulate
curved growth.
Other controls:
Blocks with no
chemical have
no effect.
• Five major types of hormones regulate
plant growth and development
– Plant hormones exert control by affecting
cell division, elongation, and
differentiation
• Auxins
• Cytokinins
• Gibberellins
• Abscisic acid
• Ethylene
Table 33.2
• Auxin stimulates the elongation of cells in young
shoots
– Auxins are a class of chemicals that promote
seedling elongation
– The apical meristem of shoot tips is a major
source of IAA synthesis
– At different concentrations, auxin stimulates
or inhibits the elongation of shoots and roots
LE 33-03b
+
Stems
0
Roots
0.9 g/L
−
10− 8
10− 6
10− 4
10− 2
Increasing auxin concentration (g/L)
1
102
LE 33-03c
H2O
Cell wall
Plasma
membrane
Cellulose
molecule
Cell
wall
H+
H+
Cell
elongation
Vacuole
H + pump
(protein)
Enzyme
Cytoplasm
Cellulose loosens; cell can elongate
Cellulose Cross-linking
molecule molecule
–Auxin has a number of effects other
than elongation
• Promotes growth in stem diameter
• Promotes growth of fruit
• Can induce fruit development
without pollination
LE 33-04
Terminal bud
No terminal bud
• Gibberellins affect stem elongation and
have numerous other effects
– Gibberellins stimulate cell elongation in
stems and leaves
• Enhance the action of auxin
– Gibberellin-auxin combinations can also
influence fruit development
– Gibberellins are important in seed
germination in many plants
Unnumbered Figure 33.662
Figure 33.5a
Figure 33.5b
• Abscisic acid inhibits many plant processes
– Abscisic acid (ABA) slows growth
• Inhibits germination during seed
dormancy and maturation
• Some desert plant seeds remain dormant
until rains wash out ABA
• Ratio of ABA to gibberellin can
determine whether a seed will remain
dormant or germinate
– ABA causes stomata to close in wilting
plants
LE 33-07b
Leaf
stalk
Stem
(twig)
Protective
layer
Stem
Abscission
layer
Leaf stalk
http://www.plant-hormones.info/cytokinins.htm
• Ethylene triggers fruit ripening and other aging
processes
– Ethylene production in fruit triggers ripening
• Includes enzymatic breakdown of cell walls
• Spreads from fruit to fruit as a gas
– Ethylene probably also plays a role in the
autumnal changes in deciduous trees
• Leaf abscission is promoted by a change in
the ethylene-auxin balance
–Triggered by shorter days
Figure 33.7a
• Plant hormones have many agricultural uses
– Auxin is used both to delay and to promote
fruit drop
– Gibberellins can be used to produce
seedless fruits or promote seed production
– The weed killer 2,4-D is a synthetic auxin
that disrupts hormones regulating plant
growth
– The use of synthetic versus natural
pesticides and herbicides is a highly
debated question
Figure 33.8
The End