1. Light Receptors and Pathogens
Chapter 31
Light Receptors and Pathogens

2. You Must Know
How phototropism and photoperiodism use changes in the environment to modify plant growth and behavior.
How plants respond to attacks by herbivores and pathogens. (For the AP Test only)

3. Concept 31.2: Responses to light are critical for plant success
Light triggers many key events in plant growth and development, collectively known as photomorphogenesis 3

4. Etiolation De-etiolation (b) After a week’s exposure
Figure 31.11
Etiolation
De-etiolation
(b) After a week’s exposure
to natural daylight
A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots.
These are morphological adaptations for growing in darkness, collectively called etiolation.
After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally.
(a) Before exposure to light 4

5. Plants detect not only presence of light but also its direction, intensity, and wavelength (color).
Light wavelengths below 500nm induce
curvature.
Wavelength (nm)
400
450
500
550
650
600
700
436 nm
Phototropic effectiveness
1.0
0.8
0.6
0.4
0.2
Blue light induces the most curvature
of coleoptiles.
White
light
Refracting prism
A graph called an action spectrum depicts relative response of a process to different wavelengths.
Action spectra are useful in studying any process that depends on light. 5

6. There are two major classes of light receptors
blue-light photoreceptors
phytochromes, photoreceptors that absorb mostly red light 6

7. Various blue-light photoreceptors control
phototropism (movement in response to light)
stomatal opening,
hypocotyl elongation 7

8. Phytochrome Photoreceptors
Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life.
These responses include seed germination, shade avoidance and flowering (which needs to be done at the time of year.)
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. 8

9. Low light because of shade
Leaves in the canopy absorb red light
Low light because of shade
Plants shaded by other plants receive more far-red than red light. When a plant senses a high ratio of far-red light it “knows” it is in a competitive environment.
The ratio of red to far-red light is unaffected for plants that are shaded by non-plants.

10. Phytochromes and seed germination: Many seeds remain dormant until light conditions are optimal.
Dark
Red
Red
Far-red
Dark
Dark (control)
Red light increased germination, while far-red light inhibited germination.
The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome.
Red
Far-red
Dark
Red
Far-red 10

11. This is how a plant “knows” the ratio of red to far red light it is receiving.
The conversion of Pr to Pfr is relatively fast. Pr
Synthesis
Red light
Pfr
Responses to Pfr:
• Seed germination
• Inhibition of vertical
growth and stimu-
lation of branching
• Setting internal clocks
• Control of flowering
Far-red
light
Enzymatic
destruction
Slow conversion
in darkness
(some species)
Red light triggers the conversion of Pr to Pfr.
Far-red light triggers the conversion of Pfr to Pr .
The conversion to Pfr is faster than the conversion to Pr .
Sunlight increases the ratio of Pfr to Pr and triggers germination.
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses. 11

12. These seeds “know” they are being shaded by a plant and so will “wait for another time to start growing.”
Once a seed has germinated, if it “knows” it is being shaded by another plant it will grow tall “as fast as it can to beat the competition.”
Red
Far-red
Phytochromes and shade avoidance: The phytochrome system also provides the plant with information about the quality of light.
Leaves in the canopy absorb red light.
Shaded plants receive more far-red than red light.
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded.
This shift induces the vertical growth of the plant.

13. Sleep movements of a bean plant
Noon
10:00 PM
Figure (Phaseolus vulgaris)
Many plant processes oscillate during the day.
Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions.
Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”.
Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle.
Circadian rhythms
Plants will go through their sleep cycle even when kept in total darkness or continuous light, but the cycle will start to drift. 13

14. The conversion of Pr to Pfr is relatively fast.
Synthesis
Red light
Pfr
• Setting internal clocks
• Control of flowering
Far-red
light
Enzymatic
destruction
Slow conversion
in darkness
(some species)
The increase of Pfr every day at dawn resents the biological clock and lets the plant “know” what season it is.
“Phytochrome is synthesized in its Pr form and enzymes destroy pre Pfr than Pr. In some plant species, Pfr present at sundown slowly converts to Pr. In darkness there is no means for the Pr to be reconverted to Pfr, but upon illumination, the Pfr level suddenly increases again as Pr is rapidly converted. This increase in Pfr each day at dawn resent the biological clock.”
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues that keep the cycle in synchrony with the environment. 14

15. Which is a plant less likely to experience in nature?
A moment of darkness during the day or
A moment of daylight in the night?
Critical night length: In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length.
That is, plants respond to a certain amount of uninterrupted darkness.

16. These plants need more uninterrupted darkness to flower.
Figure 31.16
24 hours
(a) Short-day
(long-night)
plant
These plants need more uninterrupted darkness to flower.
Light
Flash
of light
Darkness
Critical
dark period
(b) Long-day
(short-night)
plant
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.
Flowering in day-neutral plants is controlled by plant maturity, not photoperiod.
Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness.
Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness.
Some plants flower after only a single exposure to the required photoperiod.
Other plants need several successive days of the required photoperiod.
Still others need an environmental stimulus in addition to the required photoperiod.
For example, vernalization is a pretreatment with cold to induce flowering
These plants need a minimum number of uninterrupted darkness to flower. .
Flash
of light 16

17. You might need the following information for the AP Test
You might need the following information for the AP Test. I won’t test you on it.

18. Concept 31.4: Plants respond to attacks by herbivores and pathogens
Through natural selection, plants have evolved defense systems to deter herbivory, prevent infection, and combat pathogens 18

19. Defenses Against Herbivores
Herbivory, animals eating plants, is a stress that plants face in any ecosystem
Plants counter excessive herbivory with physical defenses, such as thorns and trichomes, and chemical defenses, such as distasteful or toxic compounds
Some plants even “recruit” predatory animals that help defend against specific herbivores 19

20. 4 Recruitment of parasitoid wasps that lay their eggs
Figure 31.23 4
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars 3
Synthesis
and release
of volatile
attractants 1
Wounding 1
Chemical
in saliva
Figure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 2
Signal transduction
pathway 20

21. Plants damaged by insects can release volatile chemicals to warn other plants of the same species
Arabidopsis can be genetically engineered to produce volatile components that attract predatory mites 21

22. Defenses Against Pathogens
A plant’s first line of defense against infection is the barrier presented by the epidermis and 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 plant’s ability to recognize certain pathogens 22

23. The Hypersensitive Response
Causes localized cell and tissue death near the infection site
Induces production of phytoalexins and PR proteins, which attack the specific pathogen
Stimulates changes in the cell wall that confine the pathogen 23

24. Infected tobacco leaf with lesions
Figure 31.24
Infected tobacco leaf with lesions
Signal 4 5 6
Hypersensitive
response
Signal
transduction
pathway 3 2
Signal transduction pathway
Figure Defense responses against an avirulent pathogen 7
Acquired
resistance 1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance 24

25. Systemic Acquired Resistance
Causes plant-wide expression of defense genes
Protects against a diversity of pathogens
Provides a long-lasting response 25