Plant Responses
Introduction
• At every stage in the life of a plant,
sensitivity to the environment and
coordination of responses are evident.
– One part of a plant can send signals to other
parts.
– Plants can sense gravity and the direction of
light.
– A plant’s morphology and physiology are
constantly tuned to its variable surroundings.
Upon receiving a stimulus, a receptor initiates
a specific series of biochemical steps, a signal
transduction pathway.
•Ultimately, a
signaltransduction
pathway leads
to the
regulation of
one or more
cellular
activities.
How do plants control their growth in response to environmental stimuli?
Most plants do this by
way of chemical
messengers known as
hormones.
A hormone is a chemical
that is produced in one
part of an organism and
transferred to another
part to affect the
activities of that part of
the plant.
Hormone-producing cells
Movement
of
hormone
Target
cells
Hormone-producing cells
Research on how plants grow toward
light led to the discovery of plant
hormones
• Studies of grass seedlings, particularly oats.
• In the late 19th century, Charles Darwin and his
son observed that a grass seedling bent toward
light only if the tip of the coleoptile was
present.
– This response stopped if the tip was removed or
covered
• Later, Peter Boysen-Jensen demonstrated that
the signal was a mobile chemical substance.
In 1926, F.W. Went extracted the chemical
messenger for phototropism, naming it auxin.
Animation
Auxin
1.Auxins are responsible for regulating
phototropism in a plant by stimulating the
elongation of cells.
2.High concentrations of auxin help promote the
growth of fruit and minimize the falling off of
fruit from the plant.
When the auxin concentrations decrease in
the autumn, the ripened fruit will fall. The
plants will begin to lose their leaves.
Auxin
Auxin also alters gene expression rapidly, causing
cells in the region of elongation to produce new
proteins within minutes.
•Some of these proteins are short-lived transcription
factors that repress or activate the expression of
other genes.
auxin
3. Enhance apical dominance
Produced in the growing tip of a plant and
transported downward (polar transport)
Terminal bud suppresses lateral growth
auxin
• In growing shoots auxin is transported
unidirectionally, from the apex down to the
shoot.
– Auxin enters a cell at its apical end as a small
neutral molecule, travels through the cell as an
anion, and exits the basal end via specific carrier
proteins.
– Outside the cell, auxin becomes neutral again,
diffuses across the wall, and enters the apex of
the next cell.
– Auxin movement is facilitated by chemiosmotic
gradients established by proton pumps in the cell
membrane.
auxin
Auxin is transported
through the plant body
by polar transport.
 Requires specific carrier
proteins built into the cell
membrane.
ATP provides the
energy for a proton
pump that pumps
protons out of the
cytoplasm, creating a
pH gradient necessary
for the transport of
auxin.
Can be used as a rooting powder
With
5. Treating a detached leaf or stem
with rooting powder containing auxin
often causes adventitious roots to
form near the cut surface.
4. Auxin is also involved in the
branching of roots
Without
6. Auxin also affects secondary growth by
inducing cell division in the vascular cambium
and by influencing the growth of secondary
xylem.
7. Developing seeds synthesize auxin, which
promotes the growth of fruit.
Cytokinins
• Stimulate cytokinesis and cell division
– Experiment with plant embryos
• Work with auxin to promote growth and cell
division
• Work against auxin in relation to apical dominance
• Delay senescence (aging) by inhibiting protein
breakdown
– Florists spray flowers to keep them fresh.
• Produced in roots and travel upward in the plant
EXTRAS:
• Discovered in 1931
• 1941- coconut milk
• 1961-first isolated
cytokinin from corn
(Zeatin)
• Now found in almost all
higher plants
• Highest in meristematic
areas.
• Made in roots
• Cytokinins interact with auxins to stimulate cell
division and differentiation.
– In the absence of cytokinins, a piece of parenchyma
tissue grows large, but the cells do not divide.
– In the presence of cytokinins and auxins, the cells
divide.
– If the ratio of cytokinins and auxins is balanced,
then the mass of growing cells, called a callus,
remains undifferentiated.
– If cytokinin levels are raised, shoot buds form from
the callus.
– If auxin levels are raised, roots form.
Gibberellins
Growth hormones that cause plants
to grow taller.
They also increase the rate of
seed germination and bud
development.
 There are certain tissues in the
seeds that release large amounts of
gibberellins to signal that it is time
to sprout.
Production occurs mainly
in the roots and young leaves
• The effects of gibberellins in enhancing
stem elongation are evident when certain
dwarf varieties of plants are treated with
gibberellins.
– if applied to
normal plants,
there is
often no
response.
Abscisic acid
•It inhibits plant growth during times of stress,
such as cold temperatures or drought.
•Closes stomata during times of stress
•Promotes seed dormancy.
•Overcome by the leaching of ABA in water
•First thought to control abscission.
Ethylene
•
•
•
•
Gas
Promotes fruit ripening
Involved in flower production
Influences leaf abscission a
Tropisms
Tropism—a plant’s response to their environment
Geotropism—a plant’s response to gravity
Phototropism—a plant’s response to light
Thigmotropism—a plant’s response to touch
Geotropism/Gravitropism
• Response of stems and roots
to the force of gravity.
•It is important when seeds are
sprouting.
•Both Auxin and gibberellins are
involved.
• If stem is horizontal, auxin
concentrates on the underside
causing elongation of cells.
Gravitropism Clip
Animation
Phototropism
Ability of a plant to respond to light.
Auxin moves down stem on dark side causing
elongation on cells.
Unequal distribution of auxin
Thigmotropism
The response of
a plant to touch.
Climbing plants,
ivy, and vines use
thigmotropism in
order to find their
way up or around a
solid object for
support.
Nastic Movements
• Nastic movements are rapid, reversible
responses to non-directional stimuli (eg.
Temperature, Humidity, Light irradiance).
Nastic movement is caused by turgor
pressure change due to movement of water
in cells as opposed to tropic movement which
is actual growth and therefore irreversible
Mimosa plant
• This occurs
when motor
organs at the
joints of
leaves, become
flaccid from a
loss of
potassium and
subsequent
loss of water
by osmosis.
• It takes about
ten minutes for
the cells to
regain their
turgor and
restore the
“unstimulated”
form of the
leaf.
Photoperiodism
Introduction
• Light is an especially important factor in the
lives of plants.
– photosynthesis
– light also cues many key events in plant growth
and development.
– light reception is also important in allowing plants
to measure the passage of days and seasons.
• Photoperiodism is the response of plants to
changes in the photoperiod.
– Photoperiod- relative length of daylight and
night
• Action spectra reveal
that red and blue
light are the most
important colors
regulating a plant’s
photomorphogenesis.
– These observations
led researchers to
two major classes of
light receptors:
• a heterogeneous group
of blue-light
photoreceptors
• a family of
photoreceptors called
phytochromes that
absorb mostly red light.
1. Blue-light photoreceptors are a
heterogeneous group of pigments
• The action spectra
of many plant
processes
demonstrate that
blue light is most
effective in
initiating a
diversity of
responses.
2- Phytochrome
• Phytochrome, a protein
modified with light
absorbing chromophore.
2 forms:
• Pr (p660)and Pfr (p730)
• They are photoresiversible
– When one is exposed to the
other wavelength, it will
convert to the other.
– This conversion helps the
plant keep track of time.
• This interconversion between isomers acts as a
switching mechanism that controls various lightinduced events in the life of the plant.
Lettuce
seeds
exposed to
flashes of
light
p660
p730
• This interconversion between isomers acts as a
switching mechanism that controls various lightinduced events in the life of the plant.
• The Pfr form triggers many of the plant’s
developmental responses to light.
• Exposure to far-red light inhibits the germination
response.
• During the day Pr
(p660)and Pfr (P730)
are in equilibrium.
• Pr accumulates at
night
• No sunlight to make
the conversion
• Pfr breaks down
faster than Pr
• At daybreak, Pr
begins to be
converted to Pfr
• So, night length is
responsible for
resetting the clock.
When there is a
short day (long
night), a lot of Pfr
will be degraded
to Pr.
When there is a
long day (short
night), little Pfr
will be degraded
to Pr.
• In addition to phytochrome, another chemical
called cryptochrome has been found to be
responsible for initiating flowering as a result of
exposure to blue light.
• If there are short periods of dark during
the day
• no change
• Flashes of red light during the night
• resets the clock.
• In the 1940s, researchers discovered that it is
actually night length, not day length, that controls
flowering and other responses to photoperiod.
• Short-day plant will only flower when the light
period shorter than a critical length to flower.
• Ex: chrysanthemums, poinsettias, and some soybean varieties.
• Long-day plants will only flower when the light
period is longer than a critical number of hours.
• Ex: include spinach, iris, and many cereals.
• Day-neutral plants will flower when they reach
a certain stage of maturity, regardless of day
length.
• Ex: tomatoes, rice, and dandelions.
• Short-day plants are actually long-night plants,
requiring a minimum length of uninterrupted
darkness.
• Cocklebur is actually unresponsive to day length, but it
requires at least 8 hours of continuous darkness to flower.
• Red light is the most effective color in interrupting
the nighttime portion of the photoperiod.
• Action spectra and photoreversibility experiments show
that phytochrome is the active pigment.
• If a flash of red light
during the dark period is
followed immediately by
a flash of far-red light,
then the plant detects no
interruption of night
length, demonstrating
red/far-red
photoreversibility.
Fig. 39.23
• A higher proportion of FR light allows plants to
detect when they are shaded.
• Plants adapted for growth in full sun will display
greater stem elongation when they are transferred
to shade. They also develop smaller leaves and
less branching. This change is due to greater
proportion of Pr to Pfr.