lec 25- light signal transduction Flashcards
what is light signal transduction?
the process by which plants sense their light level and adjust growth accordingly
what are the four processes of light signal transduction?
- germination
- photomorphogenesis
- regulation of the photosynthesis machinery
- flowering
what is photomorphogenesis?
it is when a seedling develops for photosynthesis right away due to the seed being in the presence of light
what is skotomorphogenesis?
in the absence of light, the embryonic stem (hypocotyl) elongates rapidly to break through soil and litter. root and cotyledons stay undeveloped to save resources.
what happens to the seed during skotomorphogenesis once it is out of the soil?
the shoot assumes photomorphogenesis once in the light (de-etiolation)
what is positive and negative phototropism?
-positive: plant grows towards light source (shoots reach light)
-negative: plants grow away from light source (roots stay in soil)
how does the stomata open in the morning and close in the evening?
-in the morning: the guard cells take up potassium ions in response to the light, which helps create a gradient for water to be taken in by osmosis. The guard cells swell causing curvature
-in the evening: the process is reversed, potassium and water leaving to cause elongation
what are the receptors for different wavelengths that generate specific responses?
red and far red receptors- phytochromes (Phy)
blue, UV-A/B receptors- cryptochromes (CRY) and phototropins (PHOT)
what are the processes regulated by light receptors?
germination–> de-etiolation–> shade avoidance–> phototropism–> chloroplast movement (face light if normal, avoid if too much)–> stomata opening–> circadian entrainment–> flowering
what does phytochrome red (Pr) and phytochrome far red (Pfr) do to germination?
Pr = stimulates germination
Pfr = inhibits germination
-Pr turns into Pfr and vice versa and then a response occurs
which phytochrome determines the outcome of germination?
the last phytochrome pulse in the pattern (e.g. R/FR/R/FR means inhibition of germination)
what does darkness do to Pfr?
turns into Pr in the dark
what happens to long hypocotyl (hy) mutants and cop mutants?
hy mutants grow as if they are in dark in the light (skotomorphogenesis) and cop mutants do photomorphogenesis in the dark
what happens to non-phototrophic hypocotyl (nph) mutants?
dont show signs of phototropism even when grown in dark and followed with light (dont curve in direction of light)
what do photoreceptors consist of?
consist of protein (apoprotein) and light sensitive chromophore pigments
apoprotein + chromophore = holoprotein
what UV range do phytochromes, phototropins and cryptochromes absorb?
phytochromes absorb 600-700nm (red)
phototropins and cryptochromes absorb 320-500 nm (blue)
what is the first step during the phytochrome B organization and how does it change conformation?
apoprotein + phytochromobilin chromophore = holoprotein
-phytochrome B is synthesized in plastids in the chloroplast, then it attaches to GAF of a cystein to produce a holoprotein, 2 holoproteins form a dimer
-phytochrome B changes conformation in response to red and far red light (red light converts cis to trans, Pr –> Pfr) which exposes its NLS sequence
what are the overall steps of seed germination by Pr, Pfr, and PB?
- PB is made in chloroplast then binds to GAF of cystein residue to make holoprotein, two are formed to make a dimer
- red light causes conformational change from Pr to Pfr
- conformational change causes opening of holoprotein
- opening helps expose NLS sequence
- Most Pfr is imported into the nucleus, others remain in cytosol to mediate rapid responses
- Pfr interacts with nuclear proteins
- regulates gene expression
- seed germination, photomorphogenesis
what does the Pr/PFR ratio do?
provides plants with position and time information
what does the leaf canopy absorb to cause shade-avoidance response?
low R/FR light ratio causes elongation of plants to avoid shade
what ratio of R/FR is seen in daylight and evening and in short and tall plants?
daylight = 1.15
evening = 0.7
short = high R/FR
tall = low R/FR
what does PB interact with to cause skotomorphogenesis in the dark and what causes switch into photomorphogenesis once in light?
-PB interacts with PIFs which activate genes for skotomorphogenesis
-once in light, PIFs are degraded by Pfr induced degradation, which triggers activation of genes for photomorphogenesis
what are the steps to phototropin signal transduction leading to stomata opening?
- blue light causes autophosphorylation of phototropin
- signal transduction leads to closure of anion channels and activation of potassium channels
- intracellular ion concentration is high leading to water uptake via osmosis
- swelling occurs
- stomata opens allowing gas exchange
what is the phototropism seen when activated by blue and UV light?
-redistribution of auxin efflux carriers to dark side of cells
-polar auxin transport to shady side
-auxin induced elongation on the shady side
-bending towards light