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

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

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

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+

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+

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)

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?

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

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)

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.

Effects of gibberellins on stem elongation and fruit growth (b) Gibberellin-induced fruit growth Figure 39.10 Effects of gibberellins on stem elongation and fruit growth Gibberellin-induced stem growth

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 39.11 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Water Radicle Scutellum (cotyledon)

ethylene-induced triple response Figure 39.13 The ethylene-induced triple response 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million)

Abscission of a maple leaf 0.5 mm Figure 39.15 Abscission of a maple leaf Protective layer Abscission layer Stem Petiole

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 39.16 Action spectrum for blue-light-stimulated phototropism in maize coleoptiles Time = 0 min Time = 90 min (b) Coleoptile response to light colors

Structure of a phytochrome Two identical subunits Chromophore Photoreceptor activity Figure 39.18 Structure of a phytochrome Kinase activity

Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses. Red light Pr Pfr Far-red light

Phytochrome: a molecular switching mechanism Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Figure 39.19 Phytochrome: a molecular switching mechanism Slow conversion in darkness (some plants) Enzymatic destruction

Sleep movements of a bean plant Figure 39.20 Sleep movements of a bean plant (Phaseolus vulgaris) Noon Midnight

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 39.21 Photoperiodic control of flowering Photoperiodic control of flowering Flash of light

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 39.22 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

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 39.23 Experimental evidence for a flowering hormone Short-day plant Long-day plant grafted to short-day plant Long-day plant

(a) Root gravitropic bending (b) Statoliths settling Positive gravitropism in roots: the statolith hypothesis Statoliths 20 µm Figure 39.24 Positive gravitropism in roots: the statolith hypothesis (a) Root gravitropic bending (b) Statoliths settling

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 39.26 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)

A developmental response of maize roots to flooding and oxygen deprivation Vascular cylinder Air tubes Epidermis Figure 39.27 A developmental response of maize roots to flooding and oxygen deprivation 100 µm 100 µm (a) Control root (aerated) (b) Experimental root (nonaerated)

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 39.28 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

hypersensitive response Systemic acquired resistance Signal Hypersensitive response Signal transduction pathway Signal transduction pathway Acquired resistance Figure 39.29 Defense responses against an avirulent pathogen Avirulent pathogen R-Avr recognition and hypersensitive response Systemic acquired resistance

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

Review Photoreversible states of phytochrome Pr Pfr Red light Responses Far-red light

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.

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.