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Chapter 39 Plant Responses to Internal and External Signals Fig. 39-1
Figure 39.1 Can flowers tell you the time of day?
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39.2 Regulation of Plant Growth
The Search for a Plant Hormone Charles Darwin and his son Francis-late 1800s Phototropism-a plant’s response to or from light grass seedling could bend toward light only if the tip of the coleoptile was present Coleoptile senses light! *But the growth response occurred some distance from the tip, how was the coleoptile sending the message?
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Fig. 39-5 RESULTS Shaded side of coleoptile Control Light Illuminated side of coleoptile Darwin and Darwin: phototropic response only when tip is illuminated Light Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Site of curvature covered by opaque shield Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted? Boysen-Jensen: phototropic response when tip separated by permeable barrier, but not with impermeable barrier Light Tip separated by gelatin (permeable) Tip separated by mica (impermeable)
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2. Frits Went-1926 a. Cut off coleptile and put an agar block between the tip and the rest of the plant. b. Hypothesized that the chemical would diffuse into the agar. c. Substituted coleptile tip with agar block d. Agar block caused phototropism! i. Hormone = Auxin.
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RESULTS Excised tip placed on agar cube Growth-promoting
Fig. 39-6 RESULTS Excised tip placed on agar cube Growth-promoting chemical diffuses into agar cube Agar cube with chemical stimulates growth Control (agar cube lacking chemical) has no effect Offset cubes cause curvature Control Figure 39.6 Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?
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A Survey of Plant Hormones
Chemical produced in one tissue and targeted for use in another. *must be small molecules to pass through cell walls!* b. Usually target growth and development tissues; no specialized organs for hormone production. c. A [hormone] relative to other hormones and development stage of the plant are important to the result = gene expression!
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1. Auxins Indoleacetic acid (IAA)
Synthesized at the apical meristem (root formation and branching) Stimulates elongation by making cell wall more plastic Supresses growth of lateral buds Stimulates activity of vascular cambium and influencing differentiation of secondary xylem f. Synthetic auxins- 1. Can induce fruit formation without pollination seedless tomatoes. 2. Prevents abscission layer fruits remain on tree longer 3. Cause broadleaf weeds (eudicots like dandelions and weeds) to grow to death-herbicides!
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3 4 2 1 5 Expansins separate microfibrils from cross-
Fig. 39-8 Expansins separate microfibrils from cross- linking polysaccharides. 3 Cell wall–loosening enzymes Cross-linking polysaccharides Expansin CELL WALL Cleaving allows microfibrils to slide. 4 Cellulose microfibril H2O Cell wall Cell wall becomes more acidic. 2 Plasma membrane Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis Auxin increases proton pump activity. 1 Nucleus Cytoplasm Plasma membrane Vacuole CYTOPLASM 5 Cell can elongate.
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2. Cytokinins-stimulate cytokinesis
Produced in root Works together with auxin to stimulate cell division and differentiation Stimulates growth of lateral branches (antagonistic to auxin) Stimulates RNA and protein synthesis and prohibit protein breakdown (anti-aging effects)
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3. Gibberellins Causes bolting (growth of floral stock) Work with auxin to control stem elongation Produced in roots and young leaves Helps break down starch in germinating seeds Discovered in rice attacked by a fungus
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-amylase and other enzymes. Sugars and other nutrients are consumed.
Fig Gibberellins (GA) send signal to aleurone. 1 Aleurone secretes -amylase and other enzymes. 2 Sugars and other nutrients are consumed. 3 Aleurone Endosperm -amylase Sugar GA GA Figure Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Water Radicle Scutellum (cotyledon)
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*Also found in mammalian brain-possible a gene repressor!*
4. Abscisic Acid Produced in the buds Antagonistic to gibberelins Induces formation of winter buds (Seed dormancy) Inhibits cell % in vascular cambiumannual rings. Causes stomata to close in low water conditions (drought tolerance) *Also found in mammalian brain-possible a gene repressor!*
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5. Ethylene Gas that diffuses through air spaces in cells Acts as an inhibitor to cell growth can trigger apoptosis. Causes defoliation (autumn) Ripens fruit (positive feedback loop!) Production increases in response to stress.
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39. 1 Signal transduction pathways in plant cells.
Reception-hormone binds to a specific receptor in or on target cell Receptor could be on membrane or in cytoplasm 2. Transduction-amplifies the stimulus and converts it to a chemical capable of changing the cells activities Second messenger (transfers and amplify) such as calcium-calmodulin complex 3. Induction- amplified signal induces a specific response by the cell. Ex. bend toward light, stomata open or close
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CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response
Fig. 39-3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and Activation of cellular responses second messengers Receptor Figure 39.3 Review of a general model for signal transduction pathways Hormone or environmental stimulus Plasma membrane
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Fig. 39-4-1 Reception Transduction CYTOPLASM NUCLEUS Plasma membrane
2 Transduction CYTOPLASM NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP Second messenger produced Phytochrome activated by light Cell wall Light Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response
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Ca2+ channel opened Ca2+ Fig. 39-4-2 Reception Transduction CYTOPLASM
1 Reception 2 Transduction CYTOPLASM NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP Second messenger produced Phytochrome activated by light Cell wall Specific protein kinase 2 activated Light Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response Ca2+ channel opened Ca2+
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Ca2+ channel opened Ca2+ Fig. 39-4-3 Reception Transduction Response
1 Reception 2 Transduction 3 Response Transcription factor 1 CYTOPLASM NUCLEUS NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP P Second messenger produced Transcription factor 2 Phytochrome activated by light P Cell wall Specific protein kinase 2 activated Transcription Light Translation Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response De-etiolation (greening) response proteins Ca2+ channel opened Ca2+
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Tropisms-growth to or away from a stimuli
39.3 Response-In most cases, these responses to stimulation involve increased activity of enzymes Tropisms-growth to or away from a stimuli a. Phototropism-to light; photoreceptors in shoot tip Auxin-induced acidification of cell walls causes cell elongation b. Gravitropism: positive gravitropism-roots negative gravitropism-shoots i. statoliths-specialized plastids redistribute calcium c. Thigmotropism is growth in response to touch ex. It occurs in vines and other climbing plants
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(a) Root gravitropic bending (b) Statoliths settling
Fig Statoliths 20 µm Figure Positive gravitropism in roots: the statolith hypothesis (a) Root gravitropic bending (b) Statoliths settling
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(a) Unstimulated state (b) Stimulated state
Fig ab Figure Rapid turgor movements by the sensitive plant (Mimosa pudica) (a) Unstimulated state (b) Stimulated state
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Responses to light are critical for plant success
Light cues many key events in plant growth and development Plants detect not only presence of light but also its direction, intensity, and wavelength (color)
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Circadian rhythms are cycles that are about 24 hours long and are
governed by an internal “clock” Noon Midnight
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*Night length is actually what is important!*
Photoperiodism-is a physiological response to day length Short-day plants-require a light period is shorter than a critical length in order to flower Flower in late summer, fall, winter Long-day plants-require a light period is longer than a critical period. Flower in spring or early summer *Night length is actually what is important!* c. Day-neutral plants is controlled by plant maturity, not photoperiod
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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
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d. Phytochromes Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues
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Long-night vs. Short-night Plants
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39.5 Plants respond to a wide variety of other stimuli
Environmental stressors: Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms Abiotic Biotic Drought/Flooding Heat/cold stress Salt stress Herbivores (thorns,Methyljasmonic acid) Pathogens (epidermis, hypersensitive response)
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4 3 1 1 2 Recruitment of parasitoid wasps that lay their eggs
Fig 4 Recruitment of parasitoid wasps that lay their eggs within caterpillars 3 Synthesis and release of volatile attractants Figure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 1 Wounding 1 Chemical in saliva 2 Signal transduction pathway
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Plants damaged by insects can release volatile chemicals to warn other plants of the same species
Methyljasmonic acid can activate the expression of genes involved in plant defenses
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Defenses Against Pathogens
A plant’s first line of defense against infection is 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 inherited ability to recognize certain pathogens
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A virulent pathogen is one that a plant has little specific defense against
An avirulent pathogen is one that may harm but does not kill the host plant
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Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes An R protein recognizes a corresponding molecule made by the pathogen’s Avr gene R proteins activate plant defenses by triggering signal transduction pathways These defenses include the hypersensitive response and systemic acquired resistance
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The Hypersensitive Response
Causes cell and tissue death near the infection site Induces production of phytoalexins and PR proteins, which attack the pathogen Stimulates changes in the cell wall that confine the pathogen
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hypersensitive response Systemic acquired resistance
Fig Signal Hypersensitive response Signal transduction pathway Signal transduction pathway Acquired resistance Figure Defense responses against an avirulent pathogen Avirulent pathogen R-Avr recognition and hypersensitive response Systemic acquired resistance
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