Chapter 31Light Receptors and Pathogens

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)

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

Figure 31.11

(a) Before exposure to light

Etiolation

(b) After a week’s exposureto natural daylight

De-etiolation

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

(b) Blue light induces the most curvatureof coleoptiles.

Whitelight

Refracting prism

(a) Light wavelengths below 500nm inducecurvature.

Wavelength (nm)

400 450 500 550 650600 700

436 nm

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There are two major classes of light receptors

1. blue-light photoreceptors

2. phytochromes, photoreceptors that absorb mostly red light

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

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.)

Low light because of shade

Leaves in the canopy absorb red light

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.

Dark (control)

Phytochromes and seed germination: Many seeds remain dormant until light conditions are optimal.

DarkRed Red Far-red Dark

Red Far-red DarkRed Red Far-red Red Far-red

Far-redlight

Enzymaticdestruction

Slow conversionin darkness(some species)

PrSynthesis

This is how a plant “knows” the ratio of red to far red light it is receiving.

Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses.

Red light

Pfr

Responses to Pfr:• Seed germination• Inhibition of vertical

growth and stimu-lation of branching

• Setting internal clocks• Control of flowering

The conversion of Pr to Pfr is relatively fast.

Red Far-red Red Far-red

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.”

Noon 10:00 PM

Sleep movements of a bean plant

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.

Far-redlight

Enzymaticdestruction

Slow conversionin darkness(some species)

PrSynthesis• Setting internal clocks• Control of flowering

Red light

Pfr

The conversion of Pr to Pfr is relatively fast.

The increase of Pfr every day at dawn resents the biological clock and lets the plant “know” what season it is.

• 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.

Figure 31.16

(a) Short-day(long-night)plant

(b) Long-day(short-night)plant

DarknessFlashof light

Light

Criticaldark period

Flashof light

24 hours

These plants need more uninterrupted darkness to flower.

These plants need a minimum number of uninterrupted darkness to flower. .

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

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

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

Figure 31.23

Wounding Chemicalin saliva

Signal transductionpathway

Recruitment ofparasitoid waspsthat lay their eggswithin caterpillars

Synthesisand releaseof volatileattractants

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• 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

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

The Hypersensitive Response

• 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

Figure 31.24

Infected tobacco leaf with lesions

Signaltransduction

pathway

Acquiredresistance

Systemic acquiredresistance

R protein

Avr effector protein

R-Avr recognition andhypersensitive response

Avirulentpathogen

Signal transduction pathway

Hypersensitiveresponse

Signal4

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Systemic Acquired Resistance

• Systemic acquired resistance – Causes plant-wide expression of defense genes – Protects against a diversity of pathogens– Provides a long-lasting response