Presentation on theme: "Light Receptors and Pathogens"— Presentation transcript:
1Light Receptors and Pathogens Chapter 31Light Receptors and Pathogens
2You Must KnowHow 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)
3Concept 31.2: Responses to light are critical for plant success Light triggers many key events in plant growth and development, collectively known as photomorphogenesis3
4Etiolation De-etiolation (b) After a week’s exposure Figure 31.11EtiolationDe-etiolation(b) After a week’s exposureto natural daylightA 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 light4
5Plants detect not only presence of light but also its direction, intensity, and wavelength (color). Light wavelengths below 500nm inducecurvature.Wavelength (nm)400450500550650600700436 nmPhototropic effectiveness1.00.80.60.40.2Blue light induces the most curvatureof coleoptiles.WhitelightRefracting prismA 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
6There are two major classes of light receptors blue-light photoreceptorsphytochromes, photoreceptors that absorb mostly red light6
7Various blue-light photoreceptors control phototropism (movement in response to light)stomatal opening,hypocotyl elongation7
8Phytochrome 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
9Low light because of shade Leaves in the canopy absorb red lightLow light because of shadePlants 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.
10Phytochromes and seed germination: Many seeds remain dormant until light conditions are optimal. DarkRedRedFar-redDarkDark (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.RedFar-redDarkRedFar-red10
11This 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.PrSynthesisRed lightPfrResponses to Pfr:• Seed germination• Inhibition of verticalgrowth and stimu-lation of branching• Setting internal clocks• Control of floweringFar-redlightEnzymaticdestructionSlow conversionin 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
12These 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.”RedFar-redPhytochromes 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.
13Sleep movements of a bean plant Noon10:00 PMFigure (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 rhythmsPlants will go through their sleep cycle even when kept in total darkness or continuous light, but the cycle will start to drift.13
14The conversion of Pr to Pfr is relatively fast. SynthesisRed lightPfr• Setting internal clocks• Control of floweringFar-redlightEnzymaticdestructionSlow conversionin 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
15Which is a plant less likely to experience in nature? A moment of darkness during the day orA 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.
16These plants need more uninterrupted darkness to flower. Figure 31.1624 hours(a) Short-day(long-night)plantThese plants need more uninterrupted darkness to flower.LightFlashof lightDarknessCriticaldark period(b) Long-day(short-night)plantSome 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 floweringThese plants need a minimum number of uninterrupted darkness to flower. .Flashof light16
17You 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.
18Concept 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 pathogens18
19Defenses Against Herbivores Herbivory, animals eating plants, is a stress that plants face in any ecosystemPlants counter excessive herbivory with physical defenses, such as thorns and trichomes, and chemical defenses, such as distasteful or toxic compoundsSome plants even “recruit” predatory animals that help defend against specific herbivores19
204 Recruitment of parasitoid wasps that lay their eggs Figure 31.234Recruitment ofparasitoid waspsthat lay their eggswithin caterpillars3Synthesisand releaseof volatileattractants1Wounding1Chemicalin salivaFigure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore2Signal transductionpathway20
21Plants 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 mites21
22Defenses Against Pathogens A plant’s first line of defense against infection is the barrier presented by the epidermis and peridermIf a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spreadThis second defense system is enhanced by the plant’s ability to recognize certain pathogens22
23The Hypersensitive Response Causes localized cell and tissue death near the infection siteInduces production of phytoalexins and PR proteins, which attack the specific pathogenStimulates changes in the cell wall that confine the pathogen23
24Infected tobacco leaf with lesions Figure 31.24Infected tobacco leaf with lesionsSignal456HypersensitiveresponseSignaltransductionpathway32Signal transduction pathwayFigure Defense responses against an avirulent pathogen7Acquiredresistance1R proteinAvirulentpathogenAvr effector proteinR-Avr recognition andhypersensitive responseSystemic acquiredresistance24
25Systemic Acquired Resistance Causes plant-wide expression of defense genesProtects against a diversity of pathogensProvides a long-lasting response25