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Chapter 39: Plant Responses to Internal & External Stimuli
1. How was it determined that the plant tip controlled phototropism?
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Figure 39.5 What part of a coleoptile senses light, and how is the signal transmitted?
In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. EXPERIMENT In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. CONCLUSION RESULTS Control Darwin and Darwin (1880) Boysen-Jensen (1913) Light Shaded side of coleoptile Illuminated Tip removed Tip covered by opaque cap covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block by mica
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Figure 39.6 Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light? Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Control Offset blocks cause curvature RESULTS CONCLUSION In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. EXPERIMENT EXPERIMENT
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? Hormone Site of Production Effect Auxin (IAA) embryo of seed germination apical meristems apical dominance Cytokinins roots stimulates cell division & growth, delays aging
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Figure 39.9 Apical dominance
Axillary buds “Stump” after removal of apical bud Lateral branches (a) Intact plant (b) Plant with apical bud removed
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? Hormone Site of Production Effect Auxin (IAA) embryo of seed germination apical meristems apical dominance Cytokinins roots stimulates cell division & growth, delays aging Gibberellins apical meristems elongation & differentiation, flowering fruit development embryo seed germination
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Figure 39.10 The effect of gibberellin treatment on Thompson seedless grapes
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Figure 39.11 Gibberellins mobilize nutrients during the germination of grain seeds
2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling. 2
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? Hormone Site of Production Effect Auxin (IAA) embryo of seed germination apical meristems apical dominance Cytokinins roots stimulates cell division & growth, delays aging Gibberellins apical meristems elongation & differentiation, flowering fruit development embryo seed germination Abscisic acid leaves, stems, roots, inhibits growth green fruit prepares for winter Ethylene ripening fruit ripens fruit triple response
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Info Essays 1999 – 1 & 2 2003 – 2 i & ii 2005 – 3 a & c 2006 – 3 2008 – 4 2011 – 4 2007 – 3 Transpiration data is on-line People needing “transport credit” – help with lab clean-up Review session – 7 AM
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Ethylene concentration (parts per million)
Figure How does ethylene concentration affect the triple response in seedlings? Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response. CONCLUSION Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the triple response. Response was greater with increased concentration. RESULTS 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million) Slowing elongation, stem thickening, & stem curvature
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? How does auxin control cell elongation?
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Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis
1 Auxin increases the activity of proton pumps. Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril 2 The cell wall becomes more acidic. H+ ATP Plasma membrane 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2O 5 With the cellulose loosened, the cell can elongate. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes.
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? How does auxin control cell elongation? Why do leaves change colors & fall off trees? New pigments made during fall (yellow & orange carotenoids, red pigment) Chlorophyll no longer produced
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Figure 39.16 Abscission of a maple leaf
0.5 mm Protective layer Abscission layer Stem Petiole Aging leaves produce less auxin so abscission layer is more sensitive to ethylene Abscission layer has thin walls Weight of leaf causes separation
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? How does auxin control cell elongation? Why do leaves change colors & fall off trees? How do plants “move?” Tropisms – toward or away from stimuli Photo – light Gravi – gravity Thigmo – touch Turgor movements – changes in turgor pressure in specialized cells How are plants able to respond to light? Blue-light photoreceptors Phytochromes
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Phototropic effectiveness relative to 436 nm
Figure What wavelengths stimulate phototropic bending toward light? EXPERIMENT Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light. RESULTS The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light. Wavelength (nm) 1.0 0.8 0.6 0.2 450 500 550 600 650 700 Light Time = 0 min. Time = 90 min. 0.4 400 Phototropic effectiveness relative to 436 nm CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.
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Figure 39.19 Structure of a phytochrome
A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore. Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase.
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Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes
(a) Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots. (b) After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.
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Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response 1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 NUCLEUS
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Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response 1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 NUCLEUS Ca2+ Ca2+ channel opened kinase 2
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Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response 1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 Transcription factor 1 NUCLEUS P Translation De-etiolation (greening) response proteins Ca2+ Ca2+ channel opened kinase 2 Transcription factor 2
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Phytochromes are sensitive to 2 different wavelengths
Red light converts the phytochrome to be far-red sensitive Far-red converts the phytochrome to be red light sensitve Red light Far-red light Pr Pfr
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Figure 39.20 Phytochrome: a molecular switching mechanism
Pr Pfr Synthesis Red light Responses: seed germination, control of flowering, etc. Far-red light Slow conversion in darkness (some plants) Enzymatic destruction
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Figure 39.18 How does the order of red and far-red illumination affect seed germination?
EXPERIMENT During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right). RESULTS Dark (control) Dark Red Far-red Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible. CONCLUSION
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? How does auxin control cell elongation? Why do leaves change colors & fall off trees? How do plants “move?” How are plants able to respond to light? What controls a plant’s biological clock? Photoperiodism – a physiological response to the duration of night & day Flowering
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Figure 39.22 How does interrupting the dark period with a brief exposure to light affect flowering?
During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants. 24 hours Darkness Flash of light Critical dark period Light (a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example). Day neutral plants are unaffected by photoperiod….maturity important.
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Figure Is phytochrome the pigment that measures the interruption of dark periods in photoperiodic response? A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods. 24 20 16 12 8 4 Hours Short-day (long-night) plant Long-day (short-night) plant R FR Critical dark period
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Figure 39.24 Is there a flowering hormone?
To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced. EXPERIMENT RESULTS CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant. Plant subjected to photoperiod that induces flowering that does not induce flowering Graft Time (several weeks) YES!!! Florigen – flowering signal
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? How does auxin control cell elongation? Why do leaves change colors & fall off trees? How do plants “move?” How are plants able to respond to light? What controls a plant’s biological clock? How does gravitropism work? - Statoliths
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Figure 39.25 Positive gravitropism in roots: the statolith hypothesis
Statoliths 20 m (a) (b)
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Chapter 39: Plant Responses to Internal & External Stimuli
How was it determined that the plant tip controlled phototropism? What are the primary plant hormones? How does auxin control cell elongation? Why do leaves change colors & fall off trees? How do plants “move?” How are plants able to respond to light? What controls a plant’s biological clock? How does gravitropism work? What’s the difference between thigmomorphogenesis & thigmotropism? Thigmomorpho – permanent change in shape (p 1087) Thigmo – growth in response to touch - vines
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Figure 39.26 Altering gene expression by touch in Arabidopsis
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Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)
(a) Unstimulated (b) Stimulated Side of pulvinus with flaccid cells turgid cells Vein 0.5 m (c) Motor organs Leaflets after stimulation Pulvinus (motor organ)
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