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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 33
Control Systems in Plants
Modules 33.1 – 33.5
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Benefits of Soy
• Soy offers many dietary benefits
– It is one of the few plant proteins that contains
all the essential amino acids, making it a healthy
substitute for meat
– It may reduce the
risk of heart disease
• Examples of soy
products include
soy milk, tofu,
soy flour, and miso
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Soy also contains non-nutritive phytochemicals
– These may have significant metabolic effects on
the human body
• Plants, like humans, use hormones as chemical
signals that control growth and development
• When we consume plants, we consume plant
hormones
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Phytoestrogen is a plant hormone found in soy
– Its chemical structure is similar to that of the
human sex hormone estrogen
Estrogen (Estradiol)
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Phytoestrogen (Genistein)
• Isoflavones are one type of phytoestrogen found
in soy
– Menopausal women
often choose dietary
supplements with
isoflavones over
hormone replacement
therapy
– But scientists have not
established the benefits
and risks of isolated
isoflavones
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
PLANT HORMONES
33.1 Experiments on how plants turn toward light
led to the discovery of a plant hormone
• Hormones coordinate the
activities of plant cells
and tissues
• The study of plant
hormones began with
observations of plants
bending toward light
– This phenomenon is
called phototropism
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 33.1A
• Phototropism results from faster cell growth on
the shaded side of the shoot than on the
illuminated side
Shaded
side of
shoot
Light
Illuminated
side of
shoot
Figure 33.1B
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• Experiments carried out by Darwin and others
showed that the tip of a grass seedling detects
light and transmits a signal down to the
growing region of the shoot
Light
Control
Figure 33.1C
Tip
removed
Tip covered
by opaque
cap
Tip
covered
by transparent cap
DARWIN AND DARWIN (1880)
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Base
covered
by opaque
shield
Tip
separated
by gelatin
block
Tip
separated
by mica
BOYSEN-JENSEN (1913)
• It was discovered in the 1920s that a hormone
was responsible for the signaling Darwin
observed
– This hormone was dubbed auxin
– Auxin plays an important role in
phototropism
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Shoot tip placed on agar block.
Chemical (later called auxin)
diffuses from shoot tip into agar.
Agar
Control
Block with
chemical
stimulates
growth.
Offset blocks with
chemical stimulate
curved growth.
Other controls:
Blocks with no
chemical have
no effect.
NO LIGHT
Figure 33.1D
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33.2 Five major types of hormones regulate plant
growth and development
• Hormones regulate plant growth and
development by affecting
– cell division
– cell elongation
– cell differentiation
• Only small amounts of hormones are
necessary to trigger the signal-transduction
pathways that regulate plant growth and
development
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Table 33.2
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• Plants produce auxin (IAA) in the apical
meristems at the tips of shoots
– At different concentrations, auxin stimulates or
inhibits the elongation of shoots and roots
STEMS
ROOTS
0.9 g/L
Figure 33.3B
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• The effect of auxin on pea plants
Figure 33.3A
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• Auxin may initiate elongation by weakening
cell walls
Auxin
stimulates
Plasma
membrane
Cell
wall
CELL WALL
H+
H+
Activates
Vacuole
H2O
CELL
ELONGATION
H+ pump
(protein)
Enzyme
CYTOPLASM
Cellulose
molecule
Cellulose loosens; cell can elongate
Figure 33.3C
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• Auxin stimulates cell division and the
development of vascular tissues in vascular
cambium
– This promotes growth in stem diameter
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
33.4 Cytokinins stimulate cell division
• Cytokinins are hormones that promote cell
division
– They are produced in actively growing roots,
embryos, and fruits
• The antagonistic interaction of auxin and
cytokinin may be one way a plant coordinates
the growth of its root and shoot systems
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Cytokinins from roots may balance the effects
of auxin from apical meristems, causing lower
buds to develop into branches
– The basil plant on the right has had its terminal
bud removed
– The inhibitory effect of
auxin on axillary buds
was thus eliminated
– Cytokinins from the
roots activated the
axillary buds, making
the plant grow more
branches
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Terminal bud
No terminal bud
Figure 33.4
33.5 Gibberellins affect stem elongation and have
numerous other effects
• Gibberellins stimulate cell elongation and cell
division in stems and leaves
– Foolish seedling
disease occurs
when rice
plants infected
with the
Gibberella
fungus get an
overdose of
gibberellin
Figure 33.5A
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• Gibberellins, in combination with auxin, can
influence fruit development
– Gibberellins can make grapes grow larger and
farther apart in a cluster
– The grapes
at right were
treated with
gibberellin,
while those
at left were
not
Figure 33.5B
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• Gibberellin-auxin sprays can make apples,
currants, and eggplants develop without
fertilization
• Gibberellins released from embryos function
in some of the early events of seed
germination
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings