1. Plant Response to Signals
Ch 39

2. Plant Response Stimuli & a Stationary Life
animals respond to stimuli by changing behavior
move toward positive stimuli
move away from negative stimuli
plants respond to stimuli by adjusting growth & development

3. Signal Transduction Pathway model
signal triggers receptor
receptor triggers internal cellular messengers & then cellular response
receptor
signal pathway (2° messengers)
response
General model for signal transduction pathways:
A hormone or another signal binding to a specific receptor stimulates the cell to produce relay molecules, such as second messengers. These relay molecules trigger the cell’s various responses to the original signal. In this diagram, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside.
What kinds of molecules are the receptors?

4. Signal Transduction Pathway example
1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
Signal Transduction Pathway controlling greening (de-etiolation) of plant cells, like in a sprouting potato tuber.
Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
One pathway uses cGMP as a 2nd messenger to activate a protein kinase.The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.
Both pathways lead to expression of genes for proteins that function in greening response of plant.
2. One pathway uses cGMP as a 2nd messenger to activate a protein kinase.
The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.
3. Both pathways lead to expression of genes for proteins that function in greening response of plant.

5. Signal Transduction Pathway example
1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
Signal Transduction Pathway controlling greening (de-etiolation) of plant cells, like in a sprouting potato tuber.
Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
One pathway uses cGMP as a 2nd messenger to activate a protein kinase.The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.
Both pathways lead to expression of genes for proteins that function in greening response of plant.
2. One pathway uses cGMP as a 2nd messenger to activate a protein kinase.
The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.
3. Both pathways lead to expression of genes for proteins that function in greening response of plant.

6. Plants do not have brains
Or nervous systems for that matter,
So how do they communicate with itself and coordinate beneficial responses?

7. Plant hormones
Chemical signals that coordinate different parts of an organism
only tiny amounts are required
produced by 1 part of body
transported to another part
binds to specific receptor
triggers response in target cells & tissues

8. Plant hormones auxins cytokinins gibberellins abscisic acid ethylene
Plant hormones
auxins
cytokinins
gibberellins
abscisic acid
ethylene

9. Hormones in review:
AUXINS __ Promote cellular elongation __ by softening of cell walls __ Involved in phototropism __ Involved in geotropism __ Involved in apical dominance
CYTOKININS __ Promotes lateral growth_ growth in size of leaf cells __ Stimulate cell division (hence name) __ Release buds from apical dominance
GIBERELLINS __ seasonal growth___ Stimulate cell elongation __ Produce bolting in biennials __ Stimulate production of starch digestion enzymes in some seeds
ABSCISIC ACID __ opposite of giberellins__causes slow down or “cut off” of growth__Promotes stomatal closure __ __ Promotes seed and bud dormancy _
ETHYLENE __ Promotes ripening of fruit

10. Response to light: Phototropism
Growth towards light
Hormone: Auxin
asymmetrical distribution of auxin, moves away from sunny side of stem (-ve phototropism, -ve gravitropism)
cells on darker side elongate faster than cells on brighter side

11. Apical dominance Controls cell division & differentiation
Controls cell division & differentiation
axillary buds do no grow while apical bud exerts control
shoot
root

12. Cell elongation in response to auxin: the acid growth hypothesis
Figure 39.8
Cell elongation in response to auxin: the acid growth hypothesis
Cross-linking polysaccharides
Cell wall–loosening enzymes
Expansin
CELL WALL
Cellulose microfibril
H2O H
Plasma membrane H H
Cell wall H H H H H
Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis.
Nucleus
Plasma membrane
Cytoplasm
ATP H
Vacuole
CYTOPLASM

13. Gibberellins Family of hormones Effects
over 100 different gibberellins identified
Effects
fruit growth
seed germination
plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

14. Abscisic acid (ABA) Effects slows growth seed dormancy
high concentrations of Abscisic acid
germination only after ABA is inactivated down or leeched out
survival value: seed will germinate only under optimal conditions
light, temperature, moisture
drought tolerance
rapid stomate closing

15. Ethylene Ethylene is a hormone gas released by plant cells
Multiple effects
response to mechanical stress
triple response
slow stem elongation
thickening of stem
curvature to stem growth
leaf drop (like in Fall)
apoptosis
fruit ripening

16. Apoptosis & Leaf drop: combination of hormones
What is the evolutionary advantage of loss of leaves in autumn?
Ethylene & auxin
many events in plants involve apoptosis (pre-programmed cell death)
death of annual plant after flowering
differentiation of xylem vessels
loss of cytosol
shedding of autumn leaves
The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn.
Photo: Abscission of a maple leaf.
Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM).

17. Fruit ripening Adaptation Ethylene
hard, tart fruit protects developing seed from herbivores
ripe, sweet, soft fruit attracts animals to disperse seed
Ethylene
triggers ripening process
breakdown of cell wall
softening
conversion of starch to sugar
sweetening
positive feedback system
ethylene triggers ripening
ripening stimulates more ethylene production

18. Applications Truth in folk wisdom…..
Applications
Truth in folk wisdom…..
one bad apple spoils the whole bunch
ripening apple releases ethylene to speed ripening of fruit nearby
Ripen green bananas by bagging them with an apple
Climate control storage of apples
high CO2 storage = reduces ethylene production

19. Plant stimuli

20. Flowering Response Triggered by photoperiod
relative lengths of day & night
night length—“critical period”— is trigger
Plant is sensitive to red light exposure
What is the evolutionary advantage of photoperiodism?
Synchronizes plant responses to season
Short-day plants
Long-day plants

21. Circadian rhythms Internal (endogenous) 24-hour cycles 4 O’clock
Noon
Midnight
Morning glory

22.
Response to gravity
How does a sprouting shoot “know” to grow towards the surface from underground?
environmental cues?
roots = positive gravitropism
shoots = negative gravitropism
settling of statoliths (dense starch grains) may detect gravity

23. Response to touch Thigmotropism
Response to touch
Thigmotropism
Mimosa (Sensitive plant) closes leaves in response to touch
Caused by changes in osmotic pressure =
rapid loss of K+ =
rapid loss of H2O =
loss of turgor in cells

24. Plant defenses
Defenses against herbivores

25. Plant defenses coevolution Defenses against herbivores
Parasitoid wasp larvae emerging from a caterpillar

26. Any Questions??