Control Systems in Plants—Chapter 39
Plants respond to environmental stimuli by:
 Sending signals between different parts of
the plant
 Tracking the time of day and the time of
year
 Sensing and responding to gravity and
direction of light
 Adjusting their growth pattern and
development
I.
Plant Hormones
A. Research on how plants grow toward light
led to the discovery of plant hormones
 Phototropism—growth toward or away
from light
 Toward light is positive phototropism,
away negative phototropism
 Results from differential growth of cells
on opposite sides of a shoot
 Cells on the darker side elongate faster
than those on the light side
EXPERIMENTS LEADING TO THE DISCOVERY
OF A PLANT HORMONE
 Charles and Francis Darwin—removed tip of
the coleoptile from a grass seedling and it
failed to grow toward light
-tip was responsible for sensing light
-since the curvature occurs some distance
below the tip, the tip sends a signal to the
elongating region.
 Peter Boysen-Jensen separated the tip
from the remainder of the coleoptile by a
block of gelatin, preventing cellular
contact, but allowing chemical diffusion
-seedlings behaved normally
-impenetrable barrier was substituted, no
phototropic response occurred
-these experiments demonstrated that the
signal was a mobile substance
 1926—F.W. Went removed the coleoptile
tip, placed it on an agar block, and then put
the agar (without the tip) on decapitated
coleoptiles kept in the dark—Fig. 39.3
-a block centered on the coleoptile caused
the stem to grow straight up
-if the block was placed off center, the
plant curved away from the side with the
block
-Went concluded the agar block contained
a chemical that diffused into it from the
coleoptile tip, and that this chemical
stimulated growth.
-went called this chemical auxin
B.
Plant hormones help coordinate growth,
development, and responses to
environmental stimuli.
 Effects depend on site of action, stage of
plant growth and hormone concentration
 The hormonal signal is amplified, perhaps
by affecting gene expression, enzyme
activity, or membrane properties
 Reaction to hormones depends on
hormonal balance
Major classes of plant hormones:
1. Auxin
2. Cytokinins
3. Gibberellins
4. Abscisic acid
5. Ethylene
1.
Auxin—a hormone that promotes
elongation of young developing shoots or
coleoptiles
a. apical meristem is a major site of
auxin production
b. moves from the apex down to the
zone of elongation at a rate of about
10 mm per hour
c.
d.
1.
2.
3.
4.
The acid-growth hypothesis states
that cell elongation is due to
stimulation of a proton pump that
acidifies the cell wall—Fig. 39.5
 Causes the crosslinks between the
cellulose myofibrils of the cell
walls to break
 This loosens the wall, allowing
water uptake, which results in
elongation of the cell
Other effects of auxin
affects secondary growth
promotes formation of adventitious
roots
promotes fruit growth in many plants
used as herbicides—affects dicots
selectively allowing removal of
broadleaf weeds
2.
Cytokinins—modified forms of adenine
that stimulate cytokinesis
Function in several areas of plant growth:
 Cell division and differentiation
 Apical dominance
 Anti-aging hormones
3.
Gibberellins—more than 80 natural
occurring gibberellins have been
identified
Function in aspects of:
 Stem elongation
 Fruit growth
 Germination
4.
Abscisic acid (ABA)—produced in the
terminal bud and helps prepare plants for
winter by suspending both primary and
secondary growth.
-Also acts as a stress hormone, closing
stomata in times of water-stress thus
reducing transpirational water loss
5.
Ethylene—a gaseous hormone that
diffuses through air spaces between
plant cells
-can also move in the cytosol
-high auxin concentrations induce release
of ethylene, which acts as a growth
inhibitor
Responsible for aspects of:
 Aging (senescence) in plants
 Fruit ripening
 Leaf abscission—loosing of leaves
Analysis of mutant plants is extending the list
of hormones and their functions
C.
Signal-transduction pathways link cellular
responses to hormonal signals and
environmental stimuli
-Plants cells respond to hormones and
environmental stimuli which are mediated
by intercellular signals—signaltransduction pathway:
1. reception—detection of a hormone
2. transduction—amplification of stimulus
and conversion into a chemical form that
can activate the cell’s response
3. induction—the pathway step in which
the amplified signal induces the cell’s
specific response to the stimulus
II. Plant Movements As Models for Studying
Control Systems
A. Tropisms orient the growth of plant organs
toward or away from stimuli—result in
curvatures of whole plant organs toward or
away from stimuli
1. Phototropism—growth either toward or
away from light—shoot tip is the site of
the photoreception that triggers the
growth response
2. Gravitropism—orientation of a plant in
response to gravity
 Roots display positive gravitropism—
curve down
B.
 Shoots display negative gravitropism—
curve upward
3. Thigmotropism—directional growth in
response to touch
 Thigmomorphogenesis—developmental
response to mechanical stimulation
Turgor movements are relatively rapid,
reversible plant responses
1. Rapid leave movements—travel from the
leaf that was stimulated to adjacent
leaves along the stem
 May be a response to reduce water loss
or protect against herbivores
 Travel wavelike through plant at about
1 cm/sec
 Correlated with action potentials in
animals, but thousands of times slower
 May be widely used as a form of
internal communication since found in
many algae and plants
2. Sleep movements—lowering of leaves to
a vertical position in evening and raising
of leaves to a horizontal position in the
morning—due to daily changes in turgor
pressure
III. Control of Daily and Seasonal Responses
A. Biological clocks control circadian rhythms
in plants and other eukaryotes
 Circadian rhythm is a physiological cycle
with a frequency of about 24 hours
 Probably set due to environmental signals
 A free running period is when an organism
is sheltered from environmental cues
causing the rhythm to deviate, can vary
from 21 to 27 hours
 Most clocks cued by light-dark cycle
resulting from the earth’s rotation, may
take days to reset once the cues change—
jet lag
B. Photoperiodism synchronizes many plant
responses to changes of season—a
physiological response to day length
 Plants detect the time of year by the
photoperiod
1. Photoperiodism and the control of
flowering
2. Critical night length
 If daytime is broken by brief periods of
darkness—plants will flower
 If nighttime is interrupted by short
exposure to light—responses are
disrupted and plants do not flower
3. Types of plants
a. Short day plants—need long nights
b. Long day plants—need only short
nights
c. Day neutral plants—unaffected by
photoperiod
IV. Phytochromes
A. Phytochromes function as photoreceptors
in many plant responses to light and
photoperiod
-phytochromes help plants measure the
length of darkness in a photoperiod
 Phytochromes alternate between two
photoreversible forms: Pr (red absorbing)
and Pfr (far-red absorbing)—the
interconversion between the two forms is a
switching mechanism that controls various
plant events
1. The ecological significance of
phytochrome as a photoreceptor
a. phytochrome functions as a
photodetector that tells the plant if
light is present
b. Photoreception by phytochrome has
a large effect on the whole plant
even though very little pigment is
present in plant cells
c. Complementing phytochromes effect,
other photoreceptors help coordinate
B.
V.
a plant’s growth and development
with its environment
Phytochromes may help entrain the
biological clock
1. Pfr gradually reverts to Pr
a. occurs everyday after sunset
b. Degradative enzymes destroy more Pfr
than Pr
c. At sunrise, the Pfr level increases due to
photoconversion of Pr
2. Plants do not use the disappearance of
Pfr to measure night length since:
 The conversion is complete within a
few hours after sunset
 Temperature affects the conversion
rate, thus, it would not be reliable
3. Night length is measured by the
biological clock, not by phytochrome
Plant Responses to Environmental stress
A. Plants cope with environmental stress
through a combination of developmental
and physiological responses.
B. Some plants have evolutionary adaptations
that enable them to live in environments
that are stressful to other plants.
1. Response to water deficit
a. control systems in both the leaves and
roots that help them cope with water
deficits
b. leaves—help the plant conserve water
by reducing transpirational water loss
c. roots—reduce growth
2. Response to oxygen deprivation
a. waterlogged soil lacks the air spaces
that provide oxygen for cellular
respiration in roots
b. some plants form air tubes that extend
from roots to the surface, therefore O2
can reach the roots
3. Response to salt stress
Excess salts in the soil may:
a. lower the water potential of the soil
solution causing a water deficit even
though sufficient water is present
b. Have toxic effect on the plant at
relatively high concentrations
-Many plants produce compatible
solutes which are an organic compound
that keeps the water potential of cells
more negative than the soil solution
without admitting toxic quantities of
salt
4. Responses to heat stress
a. Transpiration is one mechanism that
helps plants respond to excessive heat
and prevent the denaturing of enzymes
and damage to metabolism
b. Most plants will begin producing heatshock proteins when exposed to
excessive temperatures—back up
system to transpiration
5. Responses to cold stress
a. Chilling of a plant causes a change in
the fluidity of cell membranes
b. Plants respond to the cold stress of
chilling by altering the lipid
composition of their membranes
c. Subfreezing temperatures are the
most severe form of cold stress
because ice crystals begin to form in
the plant.
6. Response to herbivores
a. Plants counter excessive grazing by
herbivores with both physical and
chemical defense measures
-physical—thorns and spines
-chemical—distasteful or toxic
compounds
-recruit predatory animals to help
defend
VI. Defense against Pathogens
A. Resistance to disease depends on a gene
for gene recognition between plant and
pathogen
1. most pathogen-plant interactions are
nonvirulent
2. resistance is achieved with a precise
match-up of an allele in the plant and
the allele in the pathogen
B. The hypersensitive response contains the
infection
1. Plants infected by a virulent pathogen
are capable of resisting infection
through a localized chemical signaling
system. The response involves the
following factors:
a. phytoalexins—antimicrobial
compounds released from wounded
cells
b. activation of genes encoding PR
proteins, some of which are
antimicrobial and others which act as
signals to adjacent cells
c. Lignin synthesis and cross-linking of
cell wall components, actions aimed
at isolating the infection
2. If the pathogen is a virulent, the local
response is more vigorous and is
referred to as a hypersensitive response
(HR)
C.
Systemic acquired resistance (SAR) helps
prevent infection throughout the plant
1. HR response is local and specific, signals
produced from an HR are conveyed
throughout the plant.
2. These “alarm hormones” initiate a nonspecific systemic acquired resistance
response to help protect uninfected
tissue from a pathogen that might
spread from its point of invasion