1. Chapter 39.
Plant Response

2. Plant Reactions 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. What mechanism causes this response?
grown in dark
1 week exposure to light

4. 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?

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

7. Plant hormones auxins cytokinins gibberellins brassinosteroids
abscisic acid
ethylene

8. Phototropism
Went 1926
Growth towards light

9. Auxin Indolacetic acid (IAA) stimulates cell elongation
near apical meristems
enhances apical dominance
classical explanation of phototropism
asymmetrical distribution of auxin
cells on darker side elongate faster than cells on brighter side

10. Zones of meristem growth
shoot
root

11. Polarity of auxin transport
Auxin picks up H+ between cells & is neutralized
Neutral auxin passes through membrane
Cellular pH 7 causes auxin to ionize & is trapped in cell
Auxin stimulates proton pump
Auxin leaves through carriers

12. Auxin response Acid growth hypothesis
According to a model called the acid growth hypothesis, proton pumps play a major role in the growth response of cells to auxin. In a shoot’s region of elongation, auxin stimulates the plasma membrane’s proton pumps. This pumping of H+ increases the voltage across the membrane (membrane potential) and lowers the pH in the cell wall within minutes
Acidification of the wall activates enzymes called expansins that break the cross–links (hydrogen bonds) between cellulose microfibrils and other cell wall constituents, loosening the wall’s fabric. (Expansins can even weaken the integrity of filter paper made of pure cellulose.) Increasing the membrane potential enhances ion uptake into the cell, which causes osmotic uptake of water and increased turgor. Increased turgor and increased cell wall plasticity enable the cell to elongate.

13. Cytokinins Family of hormones Effects modified forms of adenine
produced in roots, fruits & embryos
Effects
control of cell division & differentiation
enhances apical dominance
interaction of auxin & cytokinins

14. Gibberellins Family of hormones Effects
over 100 different gibberellins identified
Effects
stem elongation
fruit growth
seed germination

15. Brassinosteroids Steroids Effects similar to auxins
cell elongation & division in shoots & seedlings

16. Abscisic acid (ABA) Effects slows growth seed dormancy
high concentrations of ABA
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

17. Ethylene Ethylene is a gas released by plant cells Multiple effects
response to mechanical stress
triple response
slow stem elongation
thickening of stem
curvature to horizontal growth
apoptosis
leaf abscission
fruit ripening

18. Apoptosis & Leaf abscission
What is the evolutionary advantage of loss of leaves in autumn?
Balance of ethylene & auxin
many events in plants involve 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).

19. Fruit ripening
Hard, tart fruit protects developing seed from herbivores
Ripe, sweet, soft fruit attracts animals to disperse seed
burst of 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

20. 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

21. Responses to light Photomorphogenesis Light detection
effect of light on plant growth
Light detection
intensity
direction
wavelength
blue-light receptors
phytochromes (red-light receptors)
Why does it make “biological sense” that red & blue light have greater effects on plants response that other wavelengths?

22. Phytochrome photoreceptors
Molecular switch reaction to red light
conversion of Pr  Pfr in sunlight stimulates germination, flowering, branching…
conversion of Pfr  Pr in dark inhibits response, & stimulates other responses: growth in height
Chromophore
Photorecptor
Light induced
Kinase activity
Phytochrome
Phytochrome
Response:
Vertical growth

23. Practical Application
Why do you plant lettuce seed by scattering them on the ground instead of burying seed?
What is the evolutionary advantage to lettuce seeds?

24. 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

25. Is there a flowering hormone?
Plant on left is induced to flower & then grafted onto plant on right
plant on right is triggered to flower
What can you conclude?

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

27. Responses 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

28. Responses 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

29. Plant defenses
Defenses against herbivores

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

31. Plant defenses
Defenses against pathogens

32. Help Me!!!