Presentation on theme: "Chapter 31 Light Receptors and Pathogens. You Must Know How phototropism and photoperiodism use changes in the environment to modify plant growth and."— Presentation transcript:
Chapter 31 Light 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 (a) Before exposure to light Etiolation (b)After a week’s exposure to 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 curvature of coleoptiles. White light Refracting prism (a)Light wavelengths below 500nm induce curvature. Wavelength (nm) nm Phototropic effectiveness
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 Far-redDark RedFar-redDarkRed Far-redRedFar-red
light Enzymatic destruction Slow conversion in darkness (some species) PrPr Synthesis 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 P r to P fr triggering many developmental responses. Red light P fr Responses to P fr : Seed germination Inhibition of vertical growth and stimu- lation of branching Setting internal clocks Control of flowering The conversion of P r to P fr is relatively fast.
RedFar-redRedFar-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-red light Enzymatic destruction Slow conversion in darkness (some species) PrPr Synthesis Setting internal clocks Control of flowering Red light P fr The conversion of P r to P fr is relatively fast. The increase of P fr 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 (a) Short-day (long-night) plant (b) Long-day (short-night) plant DarknessFlash of light Light Critical dark period Flash of 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.
C oncept 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 Wounding Chemical in saliva Signal transduction pathway Recruitment of parasitoid wasps that lay their eggs within caterpillars Synthesis and release of volatile attractants
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 Infected tobacco leaf with lesions Signal transduction pathway Acquired resistance Systemic acquired resistance R protein Avr effector protein R-Avr recognition and hypersensitive response Avirulent pathogen Signal transduction pathway Hypersensitive response Signal
Systemic Acquired Resistance Systemic acquired resistance – Causes plant-wide expression of defense genes – Protects against a diversity of pathogens – Provides a long-lasting response