1. Plant Responses to Internal and External Signals
Chapter 39
Plant Responses to Internal and External Signals

2. Stimuli and a Stationary Life
Plants, being rooted to the ground must respond to whatever environmental change comes their way
For example, the bending of a grass seedling toward light begins with the plant sensing the direction, quantity, and color of the light

3. Signal Transduction and Plant Responses
Signal transduction pathways link signal reception to response in plants
Plants have cellular receptors that they use to detect important changes in their environment
For a stimulus to elicit a response certain cells must have an appropriate receptor

4. Light-Induced Greening of Dark-Sprouted Potatoes
A potato left growing in darkness will produce shoots that do not appear healthy, and will lack elongated roots
These are morphological adaptations for growing in darkness collectively referred to as etiolation
(a) Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots.
Figure 39.2a

5. Light-Induced Greening of Dark-Sprouted Potatoes
After the potato is exposed to light the plant undergoes profound changes called de-etiolation, in which shoots and roots grow normally
Figure 39.2b
(b) After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.

6. Cell Signaling in Plants
The potato’s response to light
Is an example of cell-signal processing
Figure 39.3
CELL
WALL
CYTOPLASM
  1 Reception
2 Transduction
3 Response
Receptor
Relay molecules
Activation
of cellular
responses
Hormone or
environmental
stimulus
Plasma membrane

7. Reception-Transduction-Response
Internal and external signals are detected by receptors
Proteins that change in response to specific stimuli
Second messengers
Transfer and amplify signals from receptors to proteins that cause specific responses
Ultimately, a signal transduction pathway leads to a regulation of one or more cellular activities
In most cases
These responses to stimulation involve the increased activity of certain enzymes

8. An Example of Signal Transduction in Plants
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
2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca2+ that activates another specific protein kinase.
3 Both pathways lead to expression
of genes for proteins that function in the de-etiolation (greening) response.
1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways.
Figure 39.4

9. Plant Hormones
Plant hormones help coordinate growth, development, and responses to stimuli
Hormones are chemical signals that coordinate the different parts of an organism
Any growth response that results in curvatures of whole plant organs toward or away from a stimulus is called a tropism
These responses are often caused by hormones

10. Early Experiments on Phototropism
In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.
EXPERIMENT
In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.
CONCLUSION
RESULTS
Control
Darwin and Darwin (1880)
Boysen-Jensen (1913)
Light
Shaded
side of
coleoptile
Illuminated
Tip
removed
Tip covered
by opaque
cap
covered
by trans- parent cap
Base covered
by opaque shield
Tip separated
by gelatin block
by mica
Figure 39.5

11. A Survey of Plant Hormones

12. In general, hormones control plant growth and development
Plant Hormones
In general, hormones control plant growth and development
By affecting the division, elongation, and differentiation of cells
Plant hormones are produced in very low concentrations
But a minute amount can have a profound effect on the growth and development of a plant organ

13. Auxin
The term auxin is used for any chemical substance that promotes cell elongation in different target tissues
Responsible for phototropisms due to unequal distribution of auxin
Plant stems bend toward light as a result of increased cell elongation on the side of the stem away from the light source
Enhances apical dominance (the growth of plants upward toward the sun)
Stimulates stem elongation and growth by softening the cell wall
Auxins are produced in the growing tip of a plant and transported downward by a process called polar transport, which requires energy.

14. Cell Elongation in Response to Auxin
3 Wedge-shaped expansins, activated
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
Expansin
CELL WALL
Cell wall enzymes
Cross-linking cell wall polysaccharides
Microfibril H+
ATP
Plasma membrane
Plasma
membrane
Cell
wall
Nucleus
Vacuole
Cytoplasm
H2O
4 The enzymatic cleaving of the cross-linking
polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand.
2 The cell wall becomes more acidic.
1 Auxin increases the activity of proton pumps.
5 With the cellulose loosened,
the cell can elongate.
Figure 39.8

15. Lateral and Adventitious Root Formation
Auxin
Is involved in the formation and branching of roots
An overdose of auxins can kill dicots and is therefore a popular herbicide
Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

16. Cytokinins stimulate cell division
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin to promote growth and cell division
Cytokinins retard the aging of some plant organs
By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

17. Control of Apical Dominance
Cytokinins, auxin, and other factors interact in the control of apical dominance
The ability of a terminal bud to suppress development of axillary buds
Axillary buds
Figure 39.9a

18. Control of Apical Dominance
If the terminal bud is removed plants become bushier
“Stump” after removal of apical bud
Lateral branches
Figure 39.9b

19. Gibberellins
Gibberellins have a variety of effects such as stem elongation, fruit growth, and seed germination
Gibberellins stimulate growth of both leaves and stems
In stems gibberellins stimulate cell elongation and cell division
In many plants both auxin and gibberellins must be present for fruit to set

20. Gibberellins and Fruit Development
Gibberellins are used commercially in the spraying of Thompson seedless grapes
Figure 39.10

21. Gibberellins and Seed Germination
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

22. Gibberellins and Seed Germination
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 2
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.

23. Brassinosteroids & Abscisic Acid
Are similar to the sex hormones of animals
Induce cell elongation and division
Two of the many effects of abscisic acid (ABA) are
Seed dormancy
Drought tolerance

24. Seed Dormancy & Drought Tolerance
Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions
Precocious germination is observed in maize mutants that lack a functional transcription factor required for ABA to induce expression of certain genes
ABA is the primary internal signal that enables plants to withstand drought
Figure 39.12
Coleoptile

25. Ethylene – A Gaseous Hormone
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Ethylene gas promotes fruit ripening (an example of positive feedback) which in turn triggers increased production of the gas – can spoil ripening fruit
Commercial fruit sellers pick perishable fruits BEFORE they ripen and spray them with ethylene at their destination to hasten ripening
Ethylene facilitates apoptosis and promotes leaf abscission

26. The Triple Response to Mechanical Stress
Ethylene induces the triple response which allows a growing shoot to avoid obstacles
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)
Figure 39.13

27. Apoptosis: Programmed Cell Death
A burst of ethylene
Is associated with the programmed destruction of cells, organs, or whole plants
Prior to death, cells break down many of their chemical components for the plant to salvage and reuse

28. A change in the balance of auxin and ethylene controls leaf abscission
The process that occurs in autumn when a leaf falls
0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Figure 39.16

29. A burst of ethylene production in the fruit
Fruit Ripening
A burst of ethylene production in the fruit
Triggers the ripening process

30. Plant Response to Light
Responses to light are critical for plant success
Light cues many key events in plant growth and development
Effects of light on plant morphology are what plant biologists call photomorphogenesis
Plants not only detect the presence of light
But also its direction, intensity, and wavelength (color)
A graph called an action spectrum
Depicts the relative response of a process to different wavelengths of light

31. Phototropic effectiveness relative to 436 nm
Action Spectra
Action spectra are useful in the study of any process that depends on light
Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.
EXPERIMENT
RESULTS
The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.
Wavelength (nm)
1.0
0.8
0.6
0.2
450
500
550
600
650
700
Light
Time = 0 min.
Time = 90 min.
0.4
400
Phototropic effectiveness relative to 436 nm
The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.
Figure 39.17
CONCLUSION

32. Action & Absorption Spectra of Pigments
Research on action spectra and absorption spectra of pigments
Led to the identification of two major classes of light receptors: blue-light photoreceptors and phytochromes
Various blue-light photoreceptors
Control hypocotyl elongation, stomatal opening, and phototropism
Phytochromes
Regulate many of a plant’s responses to light throughout its life

33. Biological Clocks and Circadian Rhythms
Many plant processes oscillate during the day
Many legumes lower their leaves in the evening and raise them in the morning
Noon
Midnight
Figure 39.21

34. The Effect of Light on the Biological Clock
Cyclical responses to environmental stimuli are called circadian rhythms
And are approximately 24 hours long
Can be entrained to exactly 24 hours by the day/night cycle
Phytochrome conversion marks sunrise and sunset
Providing the biological clock with environmental cues

35. Photoperiodism and Responses to Seasons
Photoperiod, the relative lengths of night and day is the environmental stimulus plants use most often to detect the time of year
Photoperiodism, such as flowering, is a physiological response to photoperiod
Long day plants will flower only when the light period is longer than a certain number of hours
Short day plants will flower regardless of the length of day

36. Plants and their Environment
Plants respond to a wide variety of stimuli other than light
Because of their immobility
Plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms
A tropism is the growth of a plant toward or away from some stimulus.

37. Response to gravity is known as gravitropism
Roots show positive gravitropism
Stems show negative gravitropism

38. Mechanical Stimuli - Thigmotropism
The term thigmomorphogenesis refers to the changes in form that result from mechanical perturbation
Rubbing the stems of young plants a couple of times daily results in plants that are shorter than controls
Figure 39.26

39. Environmental Stresses
Have a potentially adverse effect on a plant’s survival, growth, and reproduction
Can have a devastating impact on crop yields in agriculture
During drought
Plants respond to water deficit by reducing transpiration
Deeper roots continue to grow

40. Plant Defenses
Plants defend themselves against herbivores and pathogens
Plants counter external threats with defense systems that deter herbivory and prevent infection or combat pathogens
Herbivory, animals eating plants is a stress that plants face in any ecosystem
Plants counter excessive herbivory with physical defenses such as thorns and with chemical defenses such as distasteful or toxic compounds

41. Plant Defenses
Some plants even “recruit” predatory animals that help defend the plant against specific herbivores
Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3
Synthesis and release of volatile attractants 1
Chemical in saliva
Wounding 2
Signal transduction pathway
Figure 39.29

42. Defenses Against Pathogens
A plant’s first line of defense against infection
Is the physical barrier of the plant’s “skin,” the epidermis and the periderm
Once a pathogen invades a plant
The plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread