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Light-induced de-etiolation (greening) of dark-grown potatoes
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes (a) Before exposure to light (b) After a week’s exposure to natural daylight
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Signal Transduction Pathways
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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Signal Transduction in plants: the role of phytochrome in the de-etiolation (greening) response
1 2 Reception 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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Signal Transduction in plants: the role of phytochrome in the de-etiolation (greening) response
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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Signal Transduction in plants: the role of phytochrome in the de-etiolation (greening) 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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Signaling and Phototropism
RESULTS Shaded side of coleoptile Control Light Illuminated side of coleoptile 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? 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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What part of a grass coleoptile senses light, and how is the signal transmitted?
RESULTS Shaded side of coleoptile Control Light Illuminated side of coleoptile Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted?
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Phototropic response only when tip is illuminated
RESULTS Phototropic response only when tip is illuminated Light Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted? Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Site of curvature covered by opaque shield
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Boysen-Jensen: phototropic response when tip is separated
RESULTS Boysen-Jensen: phototropic response when tip is separated by permeable barrier, but not with impermeable barrier Light Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted? Tip separated by gelatin (permeable) Tip separated by mica (impermeable)
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Cell elongation in response to auxin: acid growth hypothesis
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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Effects of gibberellins on stem elongation and fruit growth
(b) Gibberellin-induced fruit growth Figure Effects of gibberellins on stem elongation and fruit growth Gibberellin-induced stem growth
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Mobilization of nutrients by gibberellins during the germination of seeds such as barley
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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ethylene-induced triple response
Figure The ethylene-induced triple response 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million)
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Abscission of a maple leaf
0.5 mm Figure Abscission of a maple leaf Protective layer Abscission layer Stem Petiole
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Phototropic effectiveness
1.0 436 nm 0.8 0.6 Phototropic effectiveness 0.4 0.2 400 450 500 550 600 650 700 Wavelength (nm) (a) Action spectrum for blue-light phototropism Light Figure Action spectrum for blue-light-stimulated phototropism in maize coleoptiles Time = 0 min Time = 90 min (b) Coleoptile response to light colors
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Structure of a phytochrome
Two identical subunits Chromophore Photoreceptor activity Figure Structure of a phytochrome Kinase activity
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Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses. Red light Pr Pfr Far-red light
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Phytochrome: a molecular switching mechanism
Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Figure Phytochrome: a molecular switching mechanism Slow conversion in darkness (some plants) Enzymatic destruction
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Sleep movements of a bean plant
Figure Sleep movements of a bean plant (Phaseolus vulgaris) Noon Midnight
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Photoperiodic control of flowering
24 hours (a) Short-day (long-night) plant Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Figure Photoperiodic control of flowering Photoperiodic control of flowering Flash of light
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Short-day (long-night) plant Long-day (short-night) plant
Reversible effects of red and far-red light on photoperiodic response. 24 hours R RFR Figure Reversible effects of red and far-red light on photoperiodic response RFRR RFRRFR Short-day (long-night) plant Long-day (short-night) plant Critical dark period
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Graft Short-day plant Long-day plant grafted to short-day plant
Experimental evidence for a flowering hormone 24 hours 24 hours 24 hours Graft Figure Experimental evidence for a flowering hormone Short-day plant Long-day plant grafted to short-day plant Long-day plant
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(a) Root gravitropic bending (b) Statoliths settling
Positive gravitropism in roots: the statolith hypothesis Statoliths 20 µm Figure Positive gravitropism in roots: the statolith hypothesis (a) Root gravitropic bending (b) Statoliths settling
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Rapid turgor movements by the sensitive plant (Mimosa pudica)
(a) Unstimulated state (b) Stimulated state Side of pulvinus with flaccid cells Leaflets after stimulation Side of pulvinus with turgid cells Figure Rapid turgor movements by the sensitive plant (Mimosa pudica) Pulvinus (motor organ) Vein 0.5 µm (c) Cross section of a leaflet pair in the stimulated state (LM)
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A developmental response of maize roots to flooding and oxygen deprivation
Vascular cylinder Air tubes Epidermis Figure A developmental response of maize roots to flooding and oxygen deprivation 100 µm 100 µm (a) Control root (aerated) (b) Experimental root (nonaerated)
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A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore. 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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hypersensitive response Systemic acquired resistance
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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Review: Signal Transduction Pathway
CELL WALL CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Hormone or environmental stimulus Relay proteins and Activation of cellular responses second messengers Receptor
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Review Photoreversible states of phytochrome Pr Pfr Red light
Responses Far-red light
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You should now be able to:
Compare the growth of a plant in darkness (etiolation) to the characteristics of greening (de-etiolation). List six classes of plant hormones and describe their major functions. Describe the phenomenon of phytochrome photoreversibility and explain its role in light-induced germination of lettuce seeds. Explain how light entrains biological clocks.
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Distinguish between short-day, long-day, and day-neutral plants; explain why the names are misleading. Distinguish between gravitropism, thigmotropism, and thigmomorphogenesis. Describe the challenges posed by, and the responses of plants to, drought, flooding, salt stress, heat stress, and cold stress.
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