Movement in Plants
I. Some basic principles about movement in plants:
A. There are many kinds of plants and therefore, many ways that
plants can move.
B. Most movements in plants can be classified in two
categories:
1. Tropisms (Greek: trope = "turn"); in this case the
direction of an environmental stimulus determines the
direction that a plant will move.
2. Nastic Movements (Greek astos="pressed close"): here the
environmental stimulus triggers a response by the plant.
The direction of the stimulus does not control the
direction of movement in response from the plant.
3. Both of these types of movements can be either the result
of differential growth or the reversible uptake of water
into specialized cells called motor cells that
collectively form the pulvinus.
4. Taxis: a movement toward or away from a stimulus in a
single or few-celled organism. Most common in lower
plants.
5. Stimulus: an environmental change that elicits a
response from an organism. In the case of plant movement,
the induced response from the stimulus can continue after
the stimulus is no longer present.
a. The stimulus acts on some part of the plant. That
plant part that perceives the stimulus is the receptor.
After a stimulus has been received, it is transduced
into another form. This transduction is usually in the
form of a signal that can lead to a motor response, or,
an actual movement by the plant. Thus, there are three
steps in the movement of a plant that need to be
studied.
(1) Perception: how can a plant or plant part detect
the environmental stimulus in order to respond?
(2) Transduction: how is the perception changed into a
stimulus that can be perceived by the cells in the
plant where the movement will occur?
(3) Response: what mechanism does the plant use to
respond to the stimulus?
b. By asking these questions two generalizations
concerning plant movements have been identified.
(1) Similar mechanism of perception can lead to
different responses in the plant.
(2) Different mechanism of perception can lead to
similar responses in the same or different plant.
II. Nastic Movements:
A. Definitions:
1. Epinasty: downward bending of a plant part
2. Hyponasty: upward bending of a plant part
3. Pulvini: group of motor cells found at the base of
petioles, blades, or leaflets that are responsible for many
nastic plant movements. These nastic movements are
reversible.
B. Nyctinasty (Greek, nux ="night"): leaf movements that occur
on a daily basis. These movements are rhythmic and
controlled by environment and the biological clock. For
example, leaves of many plants show sleep movements where
during the day the leaves are horizontal and vertical at
night.
1. Control of sleep movements is by the pulvinus
a. Swelling of extensor cells in the pulvinus causes leaf
opening
b. Closing of the flexor cells in the pulvinus causes leaf
closing.
2. The swelling is caused by water movement in response to
osmotic gradients developed by ion transport. For example,
K+ movement out of a cell causes the lose water.
a. There are depolarization-activated K+-selective channels
in membranes of shrinking cells that allow the outward
diffusion of K+.
b. There is a cotransport of Cl- into motor cells via
H+/anion symport.
c. These mechanisms are activated by phytochrome and a
blue-absorbing pigment through the phosphatidylinositol
cycle as seen in plant hormone response.
3. Light can cause opposite responses in extensor and flexor
cells. Changing from white light to darkness activates
the H+ pump in flexor cells and inactivates the H+ pump in
extensor cells.
C. Hydronasty: folding or rolling of leaves in response to
water stress. The rolling of leaves reduces the exposed
leaf area to dry air. This response is usually coupled with
closure of stomata. The folding or rolling is caused by
turgor loss in the bulliform cells of the leaf. Bulliform
cells will lose water before the rest of the leaf because
they have little or no cuticle to protect them from
dehydrating. Bulliform cells are located on the only one
side of the leaf. As the bulliform cells dehydrate they
lose turgor pressure which causes the leaf to close.
D. Thigmonasty (Greek, thigma="touch"): nastic movements that
result from touch. Example: Mimosa or Sensitive Plant that
responds to touch, shaking, heat, or rapid cooling by
folding its leaflets. If only one leaflet is touched, an
electrical stimulus will travel to the other leaflets of the
plant, causing them to close. This may be a protection
mechanism which would effectively startle insects.
1. Mechanism of movement involves transport of water out of
motor cells in the pulvini as a result of an efflux of
K+.
2. Signal transmission occurs by two different mechanisms:
a. Electrical signal: an action potential created by a
charge (voltage) differential across the cell membrane
as a response to fluxes of ions. Electrical signals
travel through the plasmodesmata of parenchyma cells
in the xylem and phloem at velocities of 2 cm per
second.
b. Chemical signal: the action potential will not pass
to the pulvinus from leaflet to leaflet until the
chemical response occurs. Turgorin, the chemical
signal, must move through the xylem in the
transpiration stream in order for the electrical
response to travel from one leaflet to anther.
(1) Periodic Leaf Movement Factors (PLMFs): beta-
glucosides of gallic acid. These are a new class
of plant hormones that act on the turgor pressure
of pulvinus cells. The two most active gallic acid
glucosides are: Beta-D-glucoside-6-sulfate (PLMF1)
and Beta-D-glucoside-3,6-disulfate(PLMF2). These
hormones do appear to have protein receptors on the
plasma membranes of pulvini cells in Mimosa.
3. Venus’s-Flytrap: Excitation of sensory epidermal hairs
by an insect causes an action potential to move from the
hairs to the bilobed leaf tissues causing them to shut.
The rapid closing is a result of acid growth. In
response to the touch of an insect, hydrogen ions are
pumped into the walls of cells on the outside
(underside) of the leaf. The hydrogen ions loosen the
cellulose in the cells walls which quickly take up water
from the apoplast. This causes the leaves to shut,
trapping the insect. Digestive enzymes dissolve the
insect which provides the plant with supplemental
nitrogen and phosphorous. The trap opens gradually as
the inside cells grow, forcing the leaves open.
4. Thigmomorphogenesis and Seismomorphogenesis:
a. Both of these plant morphological responses are the
result of mechanical stimulation. The typical
response involves slower stem elongation and an
increase in the diameter of the stem. This results in
shorter, stronger plants that are less easily damaged
by such natural mechanical stresses as wind and strong
rains.
b. Commonly observed in greenhouse plants that are not
affected by wind. The same plant variety will tend to
be taller if greenhouse grown as compared to non-
greenhouse grown.
c. The response is probably caused by a change in growth
hormone patterns, especially ethylene. Ethylene will
cause an increase in stem thickening and a decrease in
stem elongation. Also, it has been found that auxin
production is inhibited and gibberellic acid activity
is decreased in mechanically stimulated plants.
d. The amount of hormone is changed:
(1) by in membrane permeability affects the amount of
hormone at the site of action
(2) production of growth regulator by making available
precursor molecules
e. The hormones probably work by the calcium-calmodulin
secondary messenger since calcium levels increase in
stressed cells.
III. Tropisms: differential growth that results in a directional
response.
A. Plants respond to the direction of an environmental stimulus
by unequal or differential growth. The most common tropisms
are phototropism (response to light), gravitropism (response
to gravity), and thigmotropism (response to touch).
B. Phototropism:
1. Coleoptiles and Stems:
a. Perception:
(1) Light has two effects in phototropism. It acts as the
trigger for the bending response, and it decreases the
organ sensitivity to subsequent light. This is a
nondirectional effect, referred to as a tonic effect.
Phytochrome has been found to be important in the
sensitivity of coleoptile bending in response to blue
light. Etiolated coleoptiles and stems (grown in the
dark ) can pipe light like a fiber-optic cable. As
the light moves through the stem or coleoptile it
quantity and quality (wavelength) can change. This
can cause different tissue responses to the same light
treatment.
(2) The phototropic response in monocots and dicots have
identical action spectra in. The active pigment is
Cryptochrome, a flavin pigment that absorbs blue light
controls phototropism.
b. Transduction in phototropism: Auxin migrates from the
irradiated side of the coleoptile or stem to the shaded
side. Inhibitors of this hormone movement have been
identified.
c. Growth response: there is an inhibition of growth on the
dark side of the coleoptile or stem.
2. Leaf Mosaics:
a. One response: if a leaf is partly in the sun and partly
in the shade, the leaf will respond to the light by
elongating the side of the petiole that corresponds to the
shaded part of the leaf. This causes to leaf petiole to
bend which moves the leaf toward the light.
b. A second response: the upward bending (hyponasty) of the
shaded side of a leaf.
c. Both of these responses will place the in full sun (if
possible). Thus, if you stand under a tree you will
notice that the leaves barely overlap, creating a mosaic.
This is the plants response to maximization of
photosynthesis.
3. Solar Tracking:
a. Another way that many plants maximize photosynthesis is by
keeping their leaves at right angles to the sun during the
day.
b. Diaphototropism (diaheliotropism): since the leaves are
neither positive or negative phototropic since the
orientation is at a right angle. Orientation is
controlled by motor cells in the pulvinus. Water movement
controlled by K+ fluxes is responsible for the leaf
movement. The lamina follows the sun during the day. At
sunset the lamina is vertical, facing the western horizon.
Within an hour the lamina assumes a resting position at
right angles to the petiole. An hour before sunrise, the
lamina move to face east. Thus, the lamina responds to
both the direction of the sun and a circadian rhythm.
c. Lamina are most sensitive to blue light. There is some
evidence that auxin may play a role in transmission of the
solar stimulus.
d. Negative Solar Tracking: found in desert plants. These
plant maintain their leaves parallel to the sun’s rays to
protect themselves from direct sunlight. This minimizes
water lose and heat gain, and protects against
photooxidation of photosynthetic pigments.
4. Skototropism: growth of some vines toward darkness. This
enables vines to find trees to "clime". Once the vine
reaches the tree it becomes positively phototropic and
grows toward the light. Gravitropism and thigmotorpism may
also plant a role in the climbing of vines.
C. Gravitropism: plant growth in response to the earth’s gravitational field. Roots are positively gravitropic, while stems are negative gravitropic.
1. Terms:
Orthogravitropism: vertical growth
Diagravitropism: horizontal growth
Plagiotropism: growth at any distinct angle
Agravitropic: no response to gravity
2. Roots:
Primary roots are orthogravitropic
Secondary roots may be plagiotropic
Tertiary roots may be agravitropic
a. Perception: Gravity perception is in the root cap.
Amyloplasts which contain two or more starch grains that
settle in response to gravity are the perception
mechanism. The amyloplasts are the statoliths in
statocytes that are responsible for perception of
gravity.
b. Transduction: the root cap sends an inhibitor to the
lower side of the root. this inhibitor slows growth so
that the root bends downward. Was thought that ABA was
the inhibitor, but it now seems that IAA is the
inhibitor. If IAA is added to one side of the root, the
root bends toward that side.
c. Calcium and electric currents: Calcium is important in
gravitropism. If an agar block with calcium is applied
to the side of the root cap, the root will bend in the
direction of the agar block. Electric currents have been
measured in roots that are responding to gravity. It is
possible that these electric currents are caused by
counter flow of H+ with Ca2+ across membranes. There may
be an interaction between the statoliths and calcium. As
the statolith move within the cell, they interact with
the endoplasmic reticulum (ER) (bumping into it, or
pulling on the cytoskelleton that is connected to the
ER). As the ER is distorted by the action of the
statoliths, it releases Ca2+ which can activate
calmodulin. Calmodulin is important in activation of
many biochemical systems within plants (and animals).
The calmodulin could activate auxin pumps in the cell
membranes on the lower part of the root. This would
increase the auxin concentration in the lower half of a
horizontal root.
3. Stems and Coleoptiles
a. RESPONSE: mechanics of stem bending: If a stem that is
responding to gravity is tied down (restrained) the
cells on the bottom of the stem increase in diameter and
the top cells decrease in diameter. If the restraint is
released the bottom cells elongate and become thinner
while the top cells shorten and become thicker. This
results in the stem bending upward. These findings have
shown that the bottom cells of a stem grow in response
to gravity while the top cells cease growth.
b. PERCEPTION: The site of response and perception are the
same in stems. In stems, the amyloplasts are the
statoliths. They are located in a one or two cell layer
just outside the vascular bundles called the starch
sheath. The starch sheath is the inner most layer of
the cortex.
c. TRANSDUCTION: Only a small gradient of auxin has been
found across gravitropically stimulated stems.
Sensitivity of stem tissues to auxin in the epidermal
cells of the stem seems to be the important factor in
transduction of the gravitropic response. Epidermal
cells are much more responsive to auxin than the
underlying subepidermal tissue. Auxin transported
during gravity response may be limited. The auxin may
only move from the cortex to the epidermal tissue on the
bottom side of the stem, and from the epidermal tissues
to the cortex on the top side of the stem.
(1) Gravitropic Memory: bending of stems does not occur
when sufficient auxin is not present. If the tip of
a hypocotyl (auxin source) is removed, and then
gravistimulated, the hypocotyl will not respond. If
the decapitated hypocotyl is returned to the vertical
and auxin applied, the hypocotyl will bend. Thus,
the stems are able to "remember" the gravity
stimulus.
(2) Other transduction substances:
(a) Gibberellins have been found in high
concentrations on the bottom of gravistimulated
stems.
(b) Ethylene has been found to reduce the rate of
gravitropic bending. Ethylene may be responsible
for inhibition of growth on the top side of a stem
placed in a horizontal position. However, ethylene
concentrations have found to be increased in the
bottom tissues of gravistimulated stems.
(c) Calcium ions are also important in the gravitropic
response. Ca2+ concentrations are higher in cells
on the top of horizontal stems. It is known that
Ca2+ inhibits cell elongation, possibly by
overcoming the effect of auxin.
(3) Changing sensitivity to auxin in gravistimulated stems:
Sensitivity: the capacity of a cell, tissue, organelle,
or organism to respond to a stimulus. Many time the
sensitivity of a physical system to a stimulus is
analogous to classical Michaelis-Menten reactions for
enzymes. This can be seen in the changing sensitivity
of stems to auxins under influence of gravity.
In stems of plants, the tissues on the lower side of a
horizontal stem may become more sensitive to auxin
while those tissues on the top may become less
sensitive. This would cause the lower tissues to
respond more to auxin, increasing cell elongation, and
thus bend the stem upward. Research has shown that
the lower surface of a horizontal stem shows the
greatest growth in buffer and low concentrations of
auxin. As auxin concentrations increase the amount of
stem growth decreases. The reverse is true for the
upper surface of a horizontal stem.
4. Special cases in gravity response:
a. Much of the organs on a plant are sensitive to gravity
in that the organs grow at some orientation other than
vertical. Flower parts, fruits, and leaves all
respond to gravity. The actual angle involved depends
on the plant species, age, organ, and environmental
conditions.
b. False pulvini of grasses: this is a meristematic zone
located at the node of grasses. Under
gravistimulation, the meristematic cells of the false
pulvini elongate on the lower side, producing
parenchyma and collenchyma cells with some vascular
tissue. These are false pulvini because pulvini show
reversible responses to water uptake, the responses
shown here are permanent.
c. In experiments under weightless conditions such as the
space shuttle, plants respond by showing epinasty
(down bending of leaves).
IV. Special Tropisms and Other Related Plant Responses
A. Other Tropisms:
1. Hydrotropism: growth toward water
2. Hygrotropism: growth in response to humidity
3. Chemotropism: growth toward chemicals
4. Electrotropism: growth toward electrical fields
5. Thigmotropism: growth in response to touch
B. Circumnutation: as a stem grows the stem tip traces an
elliptical path.
1. Occurs in response to internal and rhythmical controls.
2. Evidence supports the idea that circumnutation is caused by
gravitropic overshoot of the plant as it grows. This means
that the stems grows in one direction until gravity makes
it grow in another direction.
C. Reaction Wood: Compression Wood and Tension Wood:
1. As the branches of a tree grow, the weight of the wood and
the interaction with gravity causes differences in the
xylem of the stem.
2. Conifers: reaction wood forms on the lower side of the
stem. This pushes the stem upward. This type of wood is
called compression wood.
3. Angiosperms: reaction wood forms on the top side of the
stem. This contracts to hold the stem up. This type of
wood is called tension wood.
4. Auxin and ethylene seem to play a role in the formation of
reaction wood.