1. Plant Responses to Internal & External Signals

2. CELL WALL CYTOPLASM Reception Transduction Response Activation
LE 39-3
CELL
WALL
CYTOPLASM
Reception
Transduction
Response
Activation
of cellular
responses
Relay molecules
Receptor
Hormone or
environmental
stimulus
Plasma membrane

3. Reception Transduction Response Internal and external signals
Detection by receptors
proteins that change in response to specific stimuli
Transduction
Second messengers transfer and amplify signals from receptors to proteins that cause responses
Response
Usually increased activity of enzymes

4. The Discovery of Plant Hormones
Tropism
Any response resulting in curvature of organs toward or away from a stimulus
caused by hormones

5. Shaded side of Control coleoptile Light Illuminated side of coleoptile
LE 39-5a
Shaded
side of
coleoptile
Control
Light
Illuminated
side of
coleoptile

6. Darwin and Darwin (1880) Light Tip removed Tip covered by opaque cap
LE 39-5b
Darwin and Darwin (1880)
Light
Tip
removed
Tip
covered
by opaque
cap
Tip
covered
by trans-
parent
cap
Base covered
by opaque
shield

7. Boysen-Jensen (1913) Light Tip separated by gelatin block
LE 39-5c
Boysen-Jensen (1913)
Light
In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance
Tip
separated by
gelatin block
Tip separated
by mica

8. LE 39-6
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments
Agar block
with chemical
stimulates growth
Control
(agar block
lacking
chemical)
has no
effect
Offset blocks
cause curvature
Control

9. A Survey of Plant Hormones
control plant growth and development
affecting the division, elongation, and differentiation of cells
produced in very low concentration

10. Auxin promotes cell elongation in target tissues
stimulates proton pumps in the plasma membrane
lower the pH in the cell wall
activate expansins
enzymes that loosen the wall’s fabric
Cell elongates

11. Other Effects of Auxin affects secondary growth
An overdose of auxins can kill eudicots

12. Cytokinins stimulate cytokinesis (cell division)
produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin

13. Control of Apical Dominance
Cytokinins, auxin, and other factors interact in the control of apical dominance
terminal bud’s ability to suppress development of axillary buds
Removal of terminal bud causes bushier plant

14. Plant with apical bud removed
LE 39-9
Axillary buds
“Stump” after
removal of
apical bud
Lateral branches
Intact plant
Plant with apical bud removed

15. Anti-Aging Effects Cytokinins retard the aging of some plant organs
Inhibit protein breakdown
stimulate RNA and protein synthesis
mobilize nutrients from surrounding tissues

16. Gibberellins
stem elongation
fruit growth
seed germination

17. Stem Elongation Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division

18. Brassinosteroids similar to the sex hormones of animals
induce cell elongation and division

19. Abscisic Acid Two of the many effects of abscisic acid (ABA):
Seed dormancy
Drought tolerance

20. Ethylene Produced in response to stresses Drought Flooding
mechanical pressure
Injury
Infection

21. The Triple Response Ethylene induces the triple response
allows a growing shoot to avoid obstacles
triple response consists of:
slows of stem elongation
thickens the stem
horizontal growth

22. Ethylene concentration (parts per million)
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)

23. ein mutant ctr mutant LE 39-14
ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene.
ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene.

24. Apoptosis the programmed destruction of cells, organs, or whole plants
A burst of ethylene is associated with apoptosis,

25. Responses to light growth and development Photomorphogenesis
Effects of light on plant morphology

26. light receptors: blue-light photoreceptors Phytochromes
hypocotyl elongation
stomatal opening
phototropism
Phytochromes
Seed germination
opposing effects of red and far-red light
provides the plant with information about the quality of light
“shade avoidance” response

27. Biological Clocks and Circadian Rhythms
Many plant processes oscillate during the day
Ex. legumes lower their leaves in the evening and raise them in the morning
circadian rhythms
Cyclical responses to environmental stimuli
about 24 hours long

28. LE 39-21
Noon
Midnight

29. Photoperiodism and Responses to Seasons
relative lengths of night and day
environmental stimulus plants use most often to detect the time of year
Photoperiodism
physiological response to photoperiod

30. Photoperiodism and Control of Flowering
short-day plants
Plants that flower when a light period is shorter than a critical length
long-day plants
Plants that flower when a light period is longer than a certain number of hours
responses to photoperiod are actually controlled by night length, not day length

31. Darkness Flash of light 24 hours Critical dark period Light
LE 39-22
Darkness
Flash of
light
24 hours
Critical
dark
period
Light
“Short-day” plants
“Long-day” plants

32. Gravity gravitropism Response to gravity
Roots show positive gravitropism
Stems show negative gravitropism

33. Mechanical Stimuli thigmomorphogenesis growth in response to touch
changes in form that result from mechanical perturbation
Ie. Rubbing stems
growth in response to touch
occurs in vines and other climbing plants

34. LE 39-27 Unstimulated Stimulated Side of pulvinus with flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Pulvinus
(motor
organ)
Vein
Motor organs
0.5 mm

35. Environmental Stresses
Drought
reducing transpiration
Deeper roots continue to grow
Flooding
Enzymatic destruction of cells creates air tubes that help plants survive oxygen deprivation during flooding

36. Control root (aerated) Experimental root (nonaerated)
LE 39-28
Vascular
cylinder
Air tubes
Epidermis
100 µm
100 µm
Control root (aerated)
Experimental root (nonaerated)

37. Environmental Stresses
Salt Stress
producing solutes tolerated at high concentrations
keeps the water potential of cells more negative than that of the soil solution
Heat Stress
Heat-shock proteins help plants survive heat stress
Cold Stress
Altering lipid composition of membranes

38. Defenses Against Herbivores
thorns
chemical defenses
distasteful or toxic compounds

39. Defenses Against Pathogens
A plant’s first line of defense against infection is its “skin,” the epidermis or periderm
the second line of defense is a chemical attack that kills the pathogen and prevents its spread
This second defense system is enhanced by the inherited ability to recognize certain pathogens

40. Systemic Acquired Resistance
Systemic acquired resistance (SAR)
set of generalized defense responses in organs distant from the original site of infection
Salicylic acid
one of the hormones that activates SAR