plant responses to signals iv photomorphogenesis circadian rhythms gravitropism...
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Plant Responses to Signals IV
Photomorphogenesis
Circadian RhythmsGravitropism
http://sunflower.bio.indiana.edu/~rhangart/plantsinmotion.html
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Signal Transductiongeneral
Fig. 39.2
General Signal Transduction
Signals,
- hormone, - light, - temperature, - gravity, - etc.
Receptors,
- G-protein linked, - enzyme linked, - ion channel, - etc.
Second messengers,
- kinase cascades, - calcium concentration, - etc.
Responses,
- gene expression (+/-), - membrane dynamics, - metabolism - cytoskeleton - etc.
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ein,
…blocks pathway.
ethylene,
…or ctr mutant,
no ethylene
…no triple response.
active
inactive
induces transcription,erf: ethylene response factor.
?no transcription
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• Light is used by plants as a signal, as well as an energy source,
– quantity,
– quality (wavelength),
– direction,
– duration.
Photomorphogenesis
• Germination (+/-)
• Stem length (-)
• Leaf expansion (+)
• Flowering (+/-)
• Phototropism (+/-)
• Stomatal opening (+)
• Chloroplast development (+)
• Pigment synthesis (+)
• and more...
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• Action Spectrum,
– graph of the magnitude of a biological response to light,
– as a function of wavelength.
Action Spectra
Germination
Stem elongation(inhibition)
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Molecular Switch…looking for a photoreceptor,
Germination
hypothesis
…look for a photo-reversible pigment.
Fig. 39.18
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Phytochromephotoreceptor molecule
dimer
Pfr
redlight
FRlight
Pr
Fig. 39.19
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Pfr
Phytochromephotoreceptor molecule
Pr
Quantity,Time,Quality.
Fig. 39.20
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Phytochrome Location
Phtyochrome is a cytosolic protein.
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Phytochrome…has multiple functions,
• Seed Germination,
• Flowering time (photoperiodism),
• Entraining (setting) the biological clock,
• End of day,
• Stem elongation,
• Leaf Expansion,
• Pigment synthesis.
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Photoperiodism…flowering times,
Long-day plants,
…night breaks induce flowering.
Short-day plants,
…night breaks inhibit flowering.
Fig 39.22
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Phytochrome…photoperiodism,
…use photoreversibility to establish phytochrome function. Fig 39.22
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Phytochrome
absorbtion spectra,
…the wavelengths absorbed by specific pigments.
Germination
Stem elongation(inhibition)
not phytochrome
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Cryptochromes
blue light photoreceptors (I),
…evolved from a light dependent DNA repair enzyme,
...across phylogeny, these proteins have been used for many functions,
• ranging from blue-light-dependent development in plants,
• blue-light-mediated phase shifting of the circadian clock in insects,
• to a core circadian clock
component in mammals.
Stem elongation(inhibition)
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Phototropins…mediate phototropism,
blue light photoreceptors II,
…contribute to stem, root and leaf movements in response to directional information,
…also contributes to the alignment of chloroplasts within mesophyll cells,
• to maximize light gathering capacity,
• and to minimize light damage
at high irradiances. Phototropism action spectrum
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Concept Map
Phytochrome
Action Spectra
Photoperiodism
Photomorphogenesis
Cryptochrome
Phototropin
PhotoreversibleFunctions
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Circadian Rhythms
• Relating to, or exhibiting approximately 24-hour periodicity,
– circa around + dies day.
• Internal Biochemical Oscillators,
– found in all eukaryotes,
– eubacteria as well. sleep movementsFig 39.21
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Circadian Rhythms
<- Period (24h) ->
amplitude
Entrainment
Circadian processes continue even if light (or
dark) is continuous...
…amplitude and period entrainment is continuous,
- allows fine control.
…of response,
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~ 480 of 8,000 (tested) genes are under circadian control,
• ~1,500 (estimated) Arabidopsis genes, or ~6% follow circadian cycles of expression.
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Photosynthesis genes...
Secondary metabolism (wood, defense).
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Gravitropism
… the gravity directed growth processes that direct root and shoot orientation during a plants life-cycle,
roots,
…are positively gravitropic.
shoots,
…are negatively gravitropic.
– about 1.7%, or roughly 500 genes, are transcribed in Arabidopsis when it is re-oriented 90o.
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Gravitropic Set Point
0o
90o
180o
Plant organs orient themselves to the gravity vector.
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Starch Statolith Hypothesis
Re-orientation of heavy starch grains signals gravity vector.
http://www.biosci.ohio-state.edu/~plantbio/Sacklab/timelapse.html
Fig 39.25
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Final
• All material since Lecture 11 (Reproduction),
– lecture, book and other assigned readings (i.e.
• Review: 5 pm Monday, (will post room on WEB),
• Final, here in this room…
– Tuesday 11/9, 10:30 – 12:30.
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Assigned Essays
• Explain the importance of auxin in plants. How it is signaled, what are some of its function, where is it made, how is it transported? Give an example how it interacts with other hormones.
• Why would a plant want to prevent self pollination? Discuss two mechanisms used by angiosperms to avoid self fertilization.
• Give examples of heterospory and homospory and explain the differences between the two, mentioning the evolutionary significance.
• Describe how phase changes are used by developmental biologists to uncover biological processes.
• Describe how plants use light and hormones to influence the germination of the seed.
• What is phytochrome?
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Assigned Essays
• Explain the importance of auxin in plants. How it is signaled, what are some of its function, where is it made, how is it transported? Give an example how it interacts with other hormones.
• Why would a plant want to prevent self pollination? Discuss two mechanisms used by angiosperms to avoid self fertilization.
• Give examples of heterospory and homospory and explain the differences between the two, mentioning the evolutionary significance.
• Describe how phase changes are used by developmental biologists to uncover biological processes.
• Describe how plants use light and hormones to influence the germination of the seed.
• What is phytochrome?