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Plant Responses to Signals IV
Photomorphogenesis
Circadian Rhythms
Gravitropism
http://sunflower.bio.indiana.edu/~rhangart/plantsinmotion.html
Signal Transduction
general
Receptors,
Second messengers,
- kinase cascades,
- calcium concentration,
- etc.
Responses,
- G-protein linked,
- enzyme linked,
- protease linked,
- etc.
- gene expression (+/-),
- membrane dynamics,
- metabolism
- cytoskeleton
- etc.
Signals,
- hormone,
- light,
- temperature,
- gravity,
- etc.
General Signal Transduction
Fig. 39.2
Photomorphogenesis
• Light is used by plants
as a signal, as well as an
energy source,
• Germination
(+/-)
• Stem length
(-)
• Leaf expansion
(+)
• Flowering
(+/-)
• Phototropism
(+/-)
• Stomatal opening
(+)
– direction,
• Chloroplast development
(+)
– duration.
• Pigment synthesis
(+)
– quantity,
– quality (wavelength),
• and more...
Germination
Action Spectra
• Action Spectrum,
– graph of the magnitude of a
biological response to light,
– as a function of wavelength.
Stem elongation
(inhibition)
Molecular Switch
…looking for a photoreceptor,
hypothesis
Germination
…look for a photo-reversible pigment.
Fig. 39.18
Absorption vs. Action Spectra
…looking for a photoreceptor,
Germination
Phytochrome
photoreceptor molecule
red
light
dimer
Fig. 39.19
FR
light
Phytochrome
photoreceptor molecule
Quantity,
Time,
Quality.
Fig. 39.20
Phytochrome Location
Phtyochrome is a cytosolic protein.
Phytochrome
…has multiple functions,
• Seed Germination,
• Flowering time (photoperiodism),
• Entraining (setting) the biological clock,
• End of day,
• Stem elongation,
• Leaf Expansion,
• Pigment synthesis.
Photoperiodism
…flowering times,
Long-day plants,
Short-day plants,
…night breaks induce flowering.
…night breaks inhibit flowering.
Fig 39.22
Phytochrome
…photoperiodism,
…use photoreversibility
to establish phytochrome
function.
Fig 39.23
Germination
Phytochrome
Stem elongation
(inhibition)
absorbtion spectra,
…the wavelengths absorbed by
specific pigments.
not
phytochrome
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-lightdependent 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)
Phototropins
…mediate phototropism,
blue light photoreceptors II
…contribute to stem, root and leaf
movements in response to directional
information,
• to maximize light gathering
capacity,
• and to minimize light damage at
high irradiances.
Phototropism action spectrum
Concept Map
Functions
Photoreversible
Phytochrome
Cryptochrome
Signal transduction
Photoperiodism
Photomorphogenesis
Phototropin
Action Spectra,
Absorption spectra
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 movements
Fig 39.21
Circadian Rhythms
amplitude
…of response,
Circadian processes
continue even if light (or
dark) is continuous...
Entrainment
…amplitude and period
entrainment is continuous,
<- Period (24h) ->
- allows fine control.
~ 480 of 8,000 (tested) genes are under circadian
control,
• ~1,500 (estimated) Arabidopsis genes, or ~6%
follow circadian cycles of expression.
Photosynthesis genes...
Secondary metabolism (wood, defense).
shoots,
…are negatively gravitropic.
Gravitropism
… the gravity directed growth processes
that direct root and shoot orientation
during a plants life-cycle,
– about 1.7%, or roughly 500 genes, are
transcribed in Arabidopsis when it is
re-oriented 90o.
roots,
…are positively gravitropic.
Gravitropic Set Point
180o
90o
0o
Plant organs orient themselves to the gravity vector.
http://www.biosci.ohio-state.edu/~plantbio/Sacklab/timelapse.htmlMoss
Starch Statolith Hypothesis
Re-orientation of heavy starch grains signals gravity vector.
Fig 39.25