Download Chapter 33

Chapter 33
Control Systems in Plants
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What Are the Health Benefits of Soy?
• Soy protein contains all the essential amino
acids
– A healthy meat substitute
• Soybeans contain nonnutritive phytochemicals
– Phytoestrogens are plant hormones
chemically similar to human estrogen
• Isoflavones seem to exert a weak
hormonal effect on humans
– May provide health benefits but also
risks
Copyright © 2005 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
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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.
Video: Phototropism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
33.2 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
33.3 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
• One hypothesis is that auxin stimulates cell
elongation by weakening cell walls
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
33.4 Cytokinins stimulate cell division
• Cytokinins are growth regulators that promote
cell division
– Produced in actively growing tissues
• Cytokinins and auxins interact in the control of
apical dominance
– Act antagonistically in regulating axillary
bud growth
– Cause lower buds to develop into branches
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 33-04
Terminal bud
No terminal bud
33.5 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
33.6 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
33.7 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 33-07b
Leaf
stalk
Stem
(twig)
Protective
layer
Stem
Abscission
layer
Leaf stalk
CONNECTION
33.8 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
GROWTH RESPONSES AND BIOLOGICAL
RHYTHMS IN PLANTS
33.9 Tropisms orient plant growth toward or away
from environmental stimuli
• Tropisms are directed growth responses that
help a plant stay in tune with the environment
– Phototropism: response to light
– Gravitropism: response to gravity
• An adaptation that makes shoots grow
upward into light and roots grow
downward into soil
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Thigmotropism: response to touch
– Responsible for coiling of tendrils and vines
around objects
Video: Gravitropism
Video: Mimosa Leaf
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
33.10 Plants have internal clocks
• Circadian rhythms are innate biological cycles
of about 24 hours
– Occur with or without external stimuli
– Controlled by internal biological clocks
• Light/dark signals of day and night keep
biological clocks precisely synchronized with
the outside world
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The cellular nature of plant biological clocks is
not known
– May depend on synthesis of a protein that
regulates its own production through
positive feedback
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 33-10
Noon
Midnight
33.11 Plants mark the seasons by measuring
photoperiod
• Photoperiod is the relative lengths of night and
day
• The timing of flowering is one of the seasonal
responses to photoperiod
– Actually controlled by night length
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
– Short-day (long-night) plants flower when
nighttime darkness is of a critically long
duration.
• Brief flash of light in the middle of the
period will stop blossoming
– Long-day (short-night) plants flower when
nighttime darkness is of a critically short
duration
• Brief flash of light in the middle of a
longer period of darkness will induce
blossoming
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Critical night length
LE 33-11
24
Time (hr)
Darkness
Flash
of light
Light
0
Short-day (long-night) plants
Long-day (short-night) plants
33.12 Phytochrome is a light detector that may
help set the biological clock
• Phytochrome is a protein with a light-absorbing
component
• Phytochrome interconverts between two forms
differing slightly in structure and wavelength
– Pr absorbs red light; Pfr absorbs far-red light
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The wavelength of the last flash of light affects
a plant's measurement of night length
– Flowering is induced by a last flash of red
light in long-day plants; of far-red light in
short-day plants
– In nature, conversion between the two
forms is related to sunrise and sunset
• Interactions between phytochrome and the
biological clock enable plants to respond to
night, day, and seasons
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 33-12a
Short-day (long-night) plant
20
R
FR
R
FR
R
R
FR
R
FR
R
Time (hr)
16
12
8
4
1
2
3
0
Long-day (short-night) plant
4
Critical night length
24
LE 33-12b
Red
light
Rapid conversion
in daylight
Pfr
Pr
Slow conversion
in darkness
Far-red
light
TALKING ABOUT SCIENCE
33.13 Joanne Chory studies the effects of light
and hormones in the model plant Arabidopsis
• Dr. Chory has used Arabidopsis to investigate
the role of signal-transduction pathways and
how light regulates plant development
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PLANT DEFENSES
33.14 Defenses against herbivores and infectious
microbes have evolved in plants
• Plant defenses against herbivores
– Physical defenses such as thorns
– Chemical defenses such as canavanine
– Recruitment of predatory animals such as
wasps that kill caterpillars
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 33-14a
Recruitment of wasp
Wasp
lays
eggs
Synthesis
and release
of chemical
attractants
Plant cell
Damage to plant
and chemical in
caterpillar saliva
Signal
transduction
pathway
• Plant defenses against pathogens
– Physical barriers: epidermis
– Chemical defenses
• Release of microbe-killing chemicals by
infected plant cells
• Coevolution of resistance to specific
pathogens and pathogen's avirulence in
response
• Systemic acquired resistance: hormones
trigger generalized defense responses in
other organs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 33-14b
Binding of
pathogen’s signal
molecule to
plant’s receptor
molecule
Enhanced
local
response
Avirulent
pathogen
Hormones
Signal
transduction
pathway
Additional
defensive
chemicals
Signal
transduction
pathway
R-Avr recognition leading to a
strong local response
Systemic acquired
resistance
TALKING ABOUT SCIENCE
33.15 Plant biochemist Eloy Rodriguez studies
how animals use defensive chemicals made by
plants
• Dr. Rodriguez is involved in the relatively new
field of zoopharmacognosy
– The study of how animals may medicate
themselves with plants
– May have potential use in human medicine
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings