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Biology 103 - Main points/Questions
1. How can plants respond to stimuli?
2. What stimuli will they respond to?
3. What are some of the chemicals that they
use to communicate?
A season of change at a Rhode Island Stream...
Signal pathways link signals to response
• Plants have cellular receptors that detect changes in their environment
An example in potatoes• A potato left growing in darkness
produces shoots that look unhealthy and lacks elongated roots
• This helps the potato grow out of the soil into the light, called etiolation
• But after exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally
(a) Before exposure to light (b) After a week’s exposure to natural daylight
A potato’s response to light is an example of cell-signal processing
• Stimulus detected (increased light exposure)
• A signal is sent out and then…• Some response occurs – in this case
changes in growth patterns and greening• Response needs to be coordinated across
entire organism
Plant hormones
• Hormones are chemical signals that coordinate different parts of an organism
• In plants many different hormones coordinate a plants response to its environment
This plant is growing in a window and is exhibiting a
growth pattern called positive phototropism!
Light!
The Discovery of Plant Hormones
• Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
• Early experiments in tropisms led to the discovery of the first plant hormones
• In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light
• They observed that a growing grass seedling could bend toward light
Phototropism – growth in response to light
Opaque capover tip.
How could you tell where is light sensed?
Darwin and his son discovered that you could block phototropism if you covered the tip of the plant
Opaque sleeve over bendingregion.
Clear capover tip.
They performed this control experiment - Why do you think they did this? (what does it tell you?)
Porous gelatinplaced betweentip and shoot.
Other biologists showed that the signal could pass through gelatin from the tip to the lower part of the plant.
Porous gelatinplaced betweentip and shoot.
Impenetrablebarrier betweentip and shoot.
Light
But that the signal didn’t pass through an impenetrable barrier
Porous gelatinplaced betweentip and shoot.
Impenetrablebarrier betweentip and shoot.
What does this tell you?
Light
Porous gelatinplaced betweentip and shoot.
Impenetrablebarrier betweentip and shoot.
Scientists decided this meant there was a chemical signal that diffused through the gelatin!
Light
• Later, in the 1920’s, a biologist used a similar experiment to investigate the chemical signal
• He removed the tip of several growing plants then placed them on agar (a substance a little like gelatin)
Tips placedon agar.
• After some time he placed these agar blocks, now infused with the signal, on the plants that had had their tips removed
• What do you think will happen to these plants? Why?
Figure 24.10 How Went demonstrated the effects of auxin on plant growth
Agar without treatment has no effect on plants but agar that has been
under tips…
Figure 24.10 How Went demonstrated the effects of auxin on plant growth
Agar that has been treated causes cells to
elongate
Figure 24.10 How Went demonstrated the effects of auxin on plant growth
If these treated agar blocks are placed on the edge the shoot curves just like in
phototropism!
• A chemical signal, Auxin, is produced in the tip of the growing shoot.
• This signal causes cells below the tip to elongate
• If there is more light on one side the auxin moves to the shaded side of the stem
How phototropism works
Figure 24.11 Auxin causes cells to elongate
• Excess auxin on the shaded side causes the curving response
Other Plant Hormones
• While auxin was the first hormone discovered there are many other plant hormones including:
• Gibberellins are synthesized in the apical portions of roots and shoots and affect stem elongation
Figure 24.12 The effect of a gibberellin
The plant on the right was treated with giberellin the one on the left was not
Other Plant Hormones
• Gibberellins
• Cytokinins stimulate cell division in plants and help determine the course of differentiation
• Cytokinins work with Auxin to determine what cells differentiate into
Fig. 24.13.a
What are Axillary buds?• What keeps them
dormant (not growing)?• Apical dominance!• Auxin from the structures
above – so remove the structures…
Fig. 24.13.b
• The axillary buds grow!• Only with cytokinin
around though…
Fig. 24.13.b
• This shoot development occurs because there is excess cytokinin and no auxin.
• What do you think would happen if you cut the tip but added auxin?
Auxin added to decapitated stem
Apical bud removed
Lateral branches
“Stump” afterremoval ofapical bud
• Excess auxin on the tip blocks branch growth
• Without auxin the lateral branches form – what if you add auxin?
Other Plant Hormones
• Gibberellins
• Cytokinins – important for root shoot balance.
• Ethylene, when applied to fruit, hastens ripening and can cause leaf senescence
Ripening tomatoes• Depends on ethylene gas
• Tomatoes picked green are ripened after
shipping!
Figure 24.14 The effects of ethylene
Other Plant Hormones
• Gibberellins
• Cytokinins
• Ethylene
• One more…
• Just before dawn guard cells are closed – but light causes them to pump potassium into the cells. What will this do?
• Potassium draws water & cells swell open!
Figure 24.15
• But what if plants need to conserve water?
Other Plant Hormones
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic acid can cause plants close guard cells in response to drought stress
Photoperiodism and Dormancy
• Photoperiodism plants sense seasonal changes in day and night length
• three categories of plants
– long-day plants flower as days get longer
– short-day plants flower as days get shorter
– day-neutral plants use other cues to control flowering
How photoperiodism works• Long day plants flower as
nights get shorter and shorter
• Short day are opposite
How photoperiodism works• If you interrupt the night
though plants think it is two short nights… so who flowers?
Really it is night length that is key!
Photoperiodism
• Plants contain a pigment called phytocrome that influences flowering
• this pigment exists in two interconvertible forms Pr (inactive) and Pfr (active)
• in short-day plants, the presence of Pfr suppresses flowering
(a) Before exposure to light (b) After a week’s exposure to natural daylight
Remember de-etiolation?
• Light is detected by phytochrome!
CYTOPLASM
Reception
Plasmamembrane
Cellwall
Phytochromeactivated by light
Light
Signal transduction
Signal molecules insideThe cells of the potato
NUCLEUS
1 2
When phytochrome absorbes light…
It triggers changes in the cell that alter gene expression!
Signals in Animals
• Animals also need to coordinate activities in
a lot of different places
• As you know they can use the nervous
system to do this but…
• Animals use a large number of different
chemical signals
Signals in Animals
• Neurons use electrical changes for high
speed communication
• Diffusion of signal molecules important for
local communication
• Hormones are signal molecules that are
used over long distances
Local and long-distance cell communication in animals
Hormone Signals in Animals
• Used for longer term signals than neurons
• Different cells respond to different
hormones
• Hormones often key for homeostasis
33.02 The Timescale over Which Chemical Messengers
Work• CD33020.GIF
Hormone signaling is a series of simple steps
1. issuing the command
2. transporting the signal– most are transported through body by the blood
3. hitting the target– the hormone binds to a receptor on the target cell
4. having an effect– when the hormone binds, the protein changes shape
and triggers a change in cell activity
• Issuing the command
• “hit the target”
• Transport
Water vs. Lipid based
• Which is which?– Steroids are lipids– Peptide hormones are water soluble
NUCLEUS
Signalreceptor
(a) (b)
TARGETCELL
Signal receptor
Transportprotein
Water-solublehormone
Fat-solublehormone
• Peptide based– Bind to receptor
on cell membrane
• Steroid– Transported
attached to a protein
– Bind to receptor inside the cell
Signalreceptor
TARGETCELL
Signal receptor
Transportprotein
Water-solublehormone
Fat-solublehormone
Generegulation
Cytoplasmicresponse
Generegulation
Cytoplasmicresponse
OR
(a) NUCLEUS (b)
• Peptide based– Signals are
often more transient (just in the cytoplasm)
– May alter gene expression
• Steroid– Mostly alter
gene expression
– Tend to be long lasting effects
Hormones are produced in
glands throughout your body
Hormones are key players in maintaining homeostasis
• Commonly used as signals in negative feedback loops
• Remember Insulin & Glucagon?
• Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis using negative feedback
