1. Plant Responses

2. Basic Signal Transduction Pathway:
1. Reception: chemicals (hormones) bind to receptor proteins and cause a conformational change
2. Transduction: conformational change stimulates an amplification pathway by causing the activation of secondary messengers
3. Response: secondary messengers trigger cellular changes that activate transcription of mRNA (act as transcription factors or enhancers – page 365 and 366) or directly activating existing enzymes
- activated enzymes leads to cellular activity

3. CELL
WALL
CYTOPLASM
  1 Reception
2 Transduction
3 Response
Receptor
Relay molecules
Activation
of cellular
responses
Hormone or
environmental
stimulus
Plasma membrane

4. 1 Reception 2 Transduction 3 Response
CYTOPLASM
Plasma
membrane
Phytochrome
activated
by light
Cell
wall
Light
cGMP
Second messenger
produced
Specific
protein
kinase 1
Transcription
factor 1
NUCLEUS P
Translation
De-etiolation
(greening)
response
proteins
Ca2+
Ca2+ channel
opened
kinase 2
factor 2

5. Plant Hormones Hormone Major Functions Auxins
Stem elongation, root growth, fruit development
Cytokinins
Root growth, stimulates germination
Gibberellins
Seed and Bud Germination, Stem elongation, fruit development
Brassinosteroids
Inhibit root growth & leaf abscission
Abscisic Acid
Inhibits growth, promotes seed dormancy
Ethylene
Fruit ripening, abscission of leaves

6. Plant Response to Auxin
Acid growth hypothesis
- auxin binds to cells and activates proton pumps
- H+ pumped into cell wall – cell wall becomes more acidic
- acid activates “expansin” proteins – separate the cellulose fibers in the wall – wall becomes less rigid
- turgor pressure expands the wall

7. Expansin
CELL WALL
Cell wall enzymes
Cross-linking cell wall polysaccharides
Microfibril H+
ATP
Plasma membrane
Plasma
membrane
Cell
wall
Nucleus
Vacuole
Cytoplasm
H2O

8. Auxin and Phototropism (growth toward light)
- light hits a stem and blocks auxin production on lit side
- shaded side produces auxin
– growth happens on shaded side causing the stem to bend toward the light - page 792
Auxins and Seedless Tomatoes – seeds produce auxins which promotes fruit development – spraying developing fruits stimulates fruit without seeds

9. Plant Responses to Cytokinins
Stimulate cell differentiation when in combination with auxins
- in undifferentiated tissue:
– high cytokinin and low auxin = shoots
- low cytokinin and high auxin = roots
- in developed tissues:
- levels of leads to the regulation of apical dominance in roots and stems
- high auxin in shoots (produced by the apical bud) migrates down the stem and promotes apical growth, but suppresses lateral growth
- removal of apical bud allows for the production of lateral buds because cytokinin levels become higher
- high levels of auxins in roots promotes lateral root growth which is inhibited by the high levels of cytokinins which promotes apical root growth

10. Anti-Aging: cytokinins stimulate protein synthesis and inhibit protein breakdown which prolongs the life of plant organs
Florists spray cytokinin solutions on cut flowers to help them last longer

11. Plant Responses to Gibberellins
Stem elongation – stimulate expansins without acidification
- extra gibberellins act on inhibited stems – Mendel’s pea plants
Fruit Growth – increase fruit size
Seed Germination – released when water is absorbed – water can remove abscisic acid (keeps seed dormant)

13. After water is imbibed, the release of gibberellins from the embryo
Signals the seeds to break dormancy and germinate
2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes
starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.)
Aleurone
Endosperm
Water
Scutellum
(cotyledon) GA
-amylase
Radicle
Sugar
1 After a seed imbibes water, the embryo releases gibberellin (GA)
as a signal to the aleurone, the thin outer layer of the endosperm.
3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling. 2
Figure 39.11

14. Plant Response to Brassinosteroids
- similar to auxin

15. Plant Responses to Abscisic Acid
WHAT IT DOES NOT DO – cause leaf abscission (loss of leaves)
What it does:
- slows growth
- causes seed dormancy – ABA removed when water is absorbed or inactivated by light or cold
- produced in dry conditions  closes stomata

16. Plant Response to Ethylene
- stress response – drought, flooding, mechanical pressure, injury and infection
-Triple Response to Mechanical Pressure:
1. Slowing of stem growth
2. Thickening of stem
3. Curving of stem
- allows germinating seeds to grow around rocks

17. Ethylene concentration (parts per million)
Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response.
CONCLUSION
Germinating pea seedlings were placed in the
dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene.
EXPERIMENT
All the treated seedlings exhibited the triple response. Response was greater with increased concentration.
RESULTS
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)

18. Leaf abscission: loss of leaves in deciduous trees
prevents desiccation during winter
As a leaf ages it produces less auxin and makes it more susceptible to ethylene
Ethylene causes the production of enzymes that break down the tissues at the abscission layer of the leaf (base of petiole)
Weakened tissue eventually breaks after a protective layer of cork forms

19. 0.5 mm
Protective layer
Abscission layer
Stem
Petiole

20. Fruit ripening – release ethylene
– causes break down of cells
– loss of chlorophyll
– break down of starch
– fruit becomes colored and sweet
- stimulates the production of more ethylene
- one bad apple …….

21. Effects of Light on Plants
- Photomorphogenesis – changes due to light
- based on Blue pigment photoreceptors called Phytochromes
- composed of two identical proteins with a blue pigment that absorbs red light (660 nm) or absorbs far red light (730 nm) depending on the isomer form
- two forms – Pr and Pfr
- Pr form absorbs red light and is converted to Pfr
- in the presence of far red light or in the absence of light, Pfr converts back to Pr
- Pr/Pfr ratio determines plant morphological changes

22. Synthesis
Far-red
light
Red light
Slow conversion
in darkness
(some plants)
Responses:
seed germination,
control of
flowering, etc.
Enzymatic
destruction
Pfr Pr

23. - Seeds that need light to germinate – get hit with light – Pr  Pfr
- Apical dominance and Branching
- Pr = apical growth
- Pfr = branching
- in light – Pr  Pfr = branching growth
- in shade – Pfr  Pr = apical growth (shade avoidance)

24. Photoperiodism:
- based on the amount of light and dark
Ex: Flowering – based on critical night length – interruption of dark period prevents response
- short “day” plants – (long night plants) – need long period of dark to produce correct levels of signals for flowering
- long “day” plants – (short night plants) – need short period of dark to produce the correct signals for flowering
- day neutral plants – flower when plant body is mature

25. (a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light.
(b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

26. Florigen – flowering hormone
Figure 39.24
To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced.
EXPERIMENT
RESULTS
CONCLUSION
Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant.
Plant subjected to photoperiod
that induces flowering
that does not induce flowering
Graft
Time (several weeks)

27. TROPISMS: plant responses to other signals
Gravitropism – statoliths (starch granules) and dense organelles – migrate to the lower portion of root cells and stimulate the growth of the root in that direction

28. Gravitropism
Statoliths
20 m
(a)
(b)

29. Thigmotropism/Thigmomorphogenesis– response to touch
(a) Unstimulated
(b) Stimulated
Side of pulvinus with
flaccid cells
turgid cells
Vein
0.5 m
(c) Motor organs
Leaflets after stimulation
Pulvinus (motor organ)

30. Thigmotropism

31. Phototropism – light

32. Environmental Stresses
Drought – ABA – close stomata
Flooding – aerial roots
- ethylene gas and apoptosis creates air spaces in roots

33. Vascular cylinder
Air tubes
Epidermis
100 m
(a) Control root (aerated)
(b) Experimental root (nonaerated)

34. Salt stress – pump extra solutes into cell and some have salt glands (modified hydathodes)
Heat stress – heat shock proteins – chaperonins – maintain protein shape
Cold stress – modifications of phospholipids bilayer
Herbivores – thorns, hairs (trichomes), poisons, hormonal signals
Pathogens – secrete PR proteins – pathogen resistant

35. Recruitment of parasitoid wasps that lay their eggs within caterpillars4 3
Synthesis and release of volatile attractants 1
Chemical in saliva
Wounding 2
Signal transduction pathway

36. 4 Before they die, infected cells release a chemical signal, probably salicylic acid.
3 In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf.
Signal
5 The signal is
distributed to the
rest of the plant. 4 5
Hypersensitive response
Signal transduction pathway 6 3
6 In cells remote from the infection site, the chemical initiates a signal transduction pathway.
Signal transduction pathway 2
Acquired resistance 7
2 This identification
step triggers a
signal transduction
pathway. 1
7 Systemic acquired
resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days.
Avirulent pathogen
1 Specific resistance is
based on the
binding of ligands
from the pathogen
to receptors in
plant cells.
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Figure 39.31