1. Plant Responses to Internal and External Signals

2. Overview: Stimuli and a Stationary Life
Plants, being rooted to the ground, must respond to environmental changes that come their way
For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light

4. Signal transduction pathways link signal reception to response
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor

5. A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation

6. LE 39-2
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.
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.

7. De-Etioloation (“Greening”) Proteins
Many enzymes that function in certain signal responses are directly involved in photosynthesis
Other enzymes are involved in supplying chemical precursors for chlorophyll production

8. Plant hormones help coordinate growth, development, and responses to stimuli
Hormones are chemical signals that coordinate different parts of an organism

9. The Discovery of Plant Hormones
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
Tropisms are often caused by hormones

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

11. 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

12. Boysen-Jensen (1913) Light Tip separated by gelatin block
LE 39-5c
Boysen-Jensen (1913)
Light
Tip
separated by
gelatin block
Tip separated
by mica

13. LE 39-6
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Agar block
with chemical
stimulates growth
Control
(agar block
lacking
chemical)
has no
effect
Offset blocks
cause curvature
Control

14. A Survey of 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 concentration, but a minute amount can greatly affect growth and development of a plant organ

16. Auxin
The term auxin refers to any chemical that promotes cell elongation in target tissues
Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell

17. The Role of Auxin in Cell Elongation
According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane
The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric
With the cellulose loosened, the cell can elongate

18. Cell wall enzymes Cross-linking cell wall polysaccharides Expansin
LE 39-8a
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Expansin
CELL WALL
Microfibril
ATP
Plasma membrane
CYTOPLASM

19. Lateral and Adventitious Root Formation
Auxin is involved in root formation and branching
An overdose of auxins can kill dicots
Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

20. Cytokinins
Cytokinins are so named because they stimulate cytokinesis (cell division)
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin

21. Control of Apical Dominance
Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds
If the terminal bud is removed, plants become bushier

22. 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

23. Anti-Aging Effects
Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

24. Gibberellins
Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division

25. Fruit Growth
In many plants, both auxin and gibberellins must be present for fruit to set
Gibberellins are used in spraying of Thompson seedless grapes

27. Germination
After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

28. Aleurone Endosperm a-amylase Sugar GA GA Water Radicle Scutellum
(cotyledon)

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

30. Seed Dormancy
Seed dormancy ensures that the seed will germinate only in optimal conditions
Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes

31. LE 39-12
Coleoptile

32. Ethylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection

33. The Triple Response to Mechanical Stress
Ethylene induces the triple response, which allows a growing shoot to avoid obstacles
The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth

34. LE 39-14
ein mutant
ctr mutant
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.

35. Apoptosis: Programmed Cell Death
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

36. Leaf Abscission
A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls

37. Fruit Ripening
A burst of ethylene production in a fruit triggers the ripening process

38. 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 called photomorphogenesis
Plants detect not only presence of light but also its direction, intensity, and wavelength (color)

39. Phototropic effectiveness relative to 436 nm
LE 39-17
1.0
0.8
0.6
Phototropic effectiveness relative to 436 nm
0.4
0.2
400
450
500
550
600
650
700
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.

40. There are two major classes of light receptors: blue-light photoreceptors and phytochromes

41. Blue-Light Photoreceptors
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

42. Phytochromes as Photoreceptors
Phytochromes regulate many of a plant’s responses to light throughout its life
Studies of seed germination led to the discovery of phytochromes

43. Dark (control) Red Dark Red Dark Red Red Dark Red Red
LE 39-18
Dark (control)
Red
Dark
Red
Far-red
Dark
Red
Far-red
Red
Dark
Red
Far-red
Red
Far-red

44. The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses

45. Pr Pfr Red light Responses: seed germination, control of Synthesis
LE 39-20 Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction

46. Phytochromes and Shade Avoidance
The phytochrome system also provides the plant with information about the quality of light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded

47. 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

48. LE 39-21
Noon
Midnight

49. Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long
Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle

50. The Effect of Light on the Biological Clock
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues

51. 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 is a physiological response to photoperiod

52. Photoperiodism and Control of Flowering
Some processes, including flowering in many species, require a certain photoperiod
Plants that flower when a light period is shorter than a critical length are called short-day plants
Plants that flower when a light period is longer than a certain number of hours are called long-day plants
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

53. 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

54. LE 39-23 24 FR R 20 R FR FR FR
Critical dark period R R R R 16
Hours 12 8 4
Short-day (long-night) plant
Long-day (short-night) plant

55. A Flowering Hormone?
The flowering signal, not yet chemically identified, is called florigen
Florigen may be a hormone or a change in relative concentrations of multiple hormones

56. LE 39-24
Graft
Time
(several
weeks)

57. Gravity Response to gravity is known as gravitropism
Roots show positive gravitropism
Stems show negative gravitropism
Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grainsVideo: Gravitropism

59. Mechanical Stimuli
The term thigmomorphogenesis refers to changes in form that result from mechanical perturbation
Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants

60. Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials
NASTIC RESPONSE-DUE TO CHANGE IN TUGOR PRESSURE

61. 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

62. Defenses Against Pathogens
A plant’s first line of defense against infection is its “skin,” the epidermis or periderm
If a pathogen penetrates the dermal tissue, 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

63. Gene-for-Gene Recognition
A virulent pathogen is one that a plant has little specific defense against
An avirulent pathogen is one that may harm but not kill the host plant
Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes

64. A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant

65. LE 39-30a
Signal molecule (ligand)
from Avr gene product
Receptor coded by R allele R
Avr allele
Avirulent pathogen
Plant cell is resistant
If an Avr allele in the pathogen corresponds to an R allele in the host plant, the host plant will have resistance, making the pathogen avirulent.

66. If the plant host lacks the R gene that counteracts the pathogen’s Avr gene, then the pathogen can invade and kill the plant

67. R No Avr allele; virulent pathogen R allele; plant cell
LE 39-30b R
No Avr allele;
virulent pathogen
R allele; plant cell
becomes diseased
Avr allele
Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
No Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen will be virulent, causing disease to develop.

68. Plant Responses to Pathogen Invasions
A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection

69. hypersensitive response Systemic acquired resistance
LE 39-31
Signal
Signal
transduction
pathway
Hypersensitive
response
Signal transduction
pathway
Acquired
resistance
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance

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