Chapter 34
SENSORY SYSTEMS IN PLANTS.
Plants grow in a fixed location. If the environment becomes unfavorable, the plant must cope or
die.
Plants are capable of sensing environmental changes and make adjustments.
The ultimate control of plant growth and development is genetic.
Location of a cell in the plant body and environment influence gene expression in plants.
Chemical signals from adjacent cells may help the cell perceive its location in the plant body.
Environmental cues like changes in light and temperature influence gene expression.
Plant hormones are chemicals that are produce in one part of the plant and transported to
another part where they cause a physiological response.
TROPISMS
Tropism is growth response to an external stimulus from a specific direction.
Changes are permanent and irreversible.
Tropisms may be positive if the plant grows toward the stimulus or away from it.
Phototropism is a response to the direction of light.
Gravitropism (syn. geotropism) is a response to gravity.
Thigmotropism is a response to contact with a solid object.
SENSING LIGHT
Charles Darwin and his son Francis were the first to report on plants sensing light. They
conducted an experiment with the new shoots (coleoptiles) of canary grass.
They noted that the young shoots or coleoptiles bent toward the light: phototropism
An experiment conducted by Morgan and Smith showed that plants can sense infrared light.
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Morgan and Smith controlled the amount of infrared light in their experiment because in
shade areas of the forest there is more infrared light than in open sunny places.
They found that...
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Plants that are adapted to grow in open habitats elongate their stems more when they
are grown in the shade. This allows them to grow out of the shade into the sun.
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Plants that grow in the forest floor do not elongate their stems as much when they are
grown in the shade, as those plants adapted to sunny habitats.
Other experiments have shown that lettuce seed germinate readily if they are exposed to red
light of 660 nm wavelength, but are inhibited if they are exposed to infrared light.
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A lettuce seed germinating in the shade will be at a disadvantage.
Phytochromes
Sterling Hendricks and Vivian Toole proposed a mechanism to explain the sensing of light by
plants:
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Phytochrome occurs in two forms: one form, Pr , absorbs red light at 660 nm and the other
form, Pfr, absorbs far-red light at 730 nm.
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When either form absorbs its preferred wavelength, changes to the other form. They called
this phenomenon photoreversibility.
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Pfr was considered to be the active form and Pr the inactive form of the phytochrome.
Butler and colleagues confirmed the existence of phytochrome by isolating it in 1959. They also
were able to change the color of the solution by exposing to red and infrared light, thus
confirming the change in molecular configuration of the pigment protein.
Subsequent studies have shown that in Arabidopsis thaliana there are five loci that code for five
phytochrome proteins.
All of these phytochromes appear to be photoreversible when exposed to red and infrared light.
These phytochromes cause different responses in the plant.
It is possible that there are more than five phytochromes in plants.
Phytochrome is also involved in the germination of seeds:
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Exposure to red light converts Pr to Pfr and germination occurs.
Other physiological responses influenced by phytochrome include leaf abscission, pigment
formation in flowers and fruits, sleep movements, stem elongation, shade avoidance and
shoot dormancy.
Phytochromes monitor the amount of shade a plant receives.
Blue-light receptor: NPH1 protein.
Chlorophylls and carotenoids absorb blue light in the visible part of the spectrum during
photosynthesis.
Scientist theorized that there must be a blue-light receptor that triggers phototropism toward the
source of blue light.
Briggs and coworkers found a protein that is abundant in the membranes of cells in the tips of
emerging shoots. They hypothesized that this protein was involved in sensing blue light and
causing phototropism.
Christie and colleagues showed that Briggs' protein was in effect a photoreceptor of blue light.
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Many proteins switch from inactive to active when they receive a PO43+ from ATP.
Christie showed that NPH1 protein became autophosphorylated.
This autophosphorylation of NPH1 triggers the plant response toward blue light.
Several steps are involved between the sensing of a signal and the response of the organism.
This sequence of steps is called signal transduction.
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To transduce: to convert energy from one form to another.
A possible sequence of steps in transduction.
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Receptor changes configuration in response to signal, e.g. blue light.
A second protein called a kinase found next to the receptor is in turn activated by the
receptor's change in configuration.
This change in the kinase converts the kinase into a catalyst that causes the
phosphorylation of a protein that starts the response by activating other proteins.
These newly activated proteins increase or decrease the transcription of certain genes or
alter the translation of some mRNA.
Genes called cry1 and cry2 are involved in the phototropic response.
This mechanism is not fully understood.
SENSING GRAVITY
Gravitropism (syn. geotropism) is a response to gravity.
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Gravitropism may be positive (toward) or negative (away from).
The curvature that occurs in reaction to gravity is due to differences in cell elongation
on the opposite sides of a root or shoot.
The molecule called auxin promotes cell elongation in shoot and inhibits it in roots.
The Statolith Hypothesis
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Statoliths are gravity sensors. In this case amyloplasts play the role of statoliths.
The perception of gravity is correlated with the sedimentation of amyloplasts,
starch-containing plastids, within specific cells of the shoot and root.
Amyloplasts accumulate at the bottom of cells in the root cap in response to gravity.
Pressure receptors in the amyloplasts' membrane become activated
The side of the cell opposite to the amyloplasts elongates.
The Gravitational Pressure or Hydrostatic Pressure Hypothesis
This theory proposes that ...
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Plants sense gravity by the hydrostatic pressure exerted by the protoplast on its cell wall.
There are receptor proteins in the cell membrane and extracellular matrix.
Pressure receptors at the top of the cell sense tension, while pressure receptors at the
bottom sense compression.
Amyloplasts are pulled to the bottom of cells by gravity and compress the receptors.
Transmembrane proteins.
The plasma membrane of animals and amyloplasts contain transmembrane proteins called
integrins.
Integrin span the membrane and project far into the extracellular space.
They bind to components of the extracellular matrix and the cytoskeleton inside the cell.
If these sites are altered by pressure, the change could indicate the direction of gravity.
The function of integrins is still to be elucidated. It may not play a role in gravitropism at all.
SENSING TOUCH
Sensors transduce the kinetic or mechanical energy of the signal or stimulus into chemical
energy.
When plants are buffeted by wind, the mechanical energy is transduced to a chemical response.
Electrical signaling.
The interior of plant cells has a negative charge relative to the exterior.
This occurs because proton pumps are active in many cells creating a charge separation across
the membrane.
This charge separation is called polarization.
This separation creates potential energy called a voltage. Potential energy is a tendency to
move.
Most plant cells then have a membrane voltage or a membrane potential.
The voltage across the membrane is measured with voltmeters.
Membrane potential are small and are expressed in units called millivolts, mV
By convention, membrane potentials are expressed as the state of a cell's interior relative to the
exterior.
Thus the resting potential of a plant cell - its normal state - is usually negative.
Action potentials
Plants like the Venus flytrap can send messages similar to nerve impulses.
This impulse is a drastic voltage change across the membrane due to a rapid flow of charges in
the form of ions, from the outside of the cell to the inside.
This rapid, temporary voltage change is called an action potential.
The action potential is a rapid change of the inside of the cell from negative to positive then
back to negative.
The resting potential of cells is about -70 mV.
Depolarization occurs when positive charges begin to flow into the cell lowering the membrane
potential by making the both sides more alike in charges.
The mechanical signal of pulling or touching causes the depolarization of the hair cells at the
base of the trap leaves of the Venus flytrap.
These cells swell with water and their pH increases dramatically. The mechanism involved in
this change in size is not well understood.