lec 25- light signal transduction

Question 1

what is light signal transduction?

Answer 1

the process by which plants sense their light level and adjust growth accordingly

Question 2

what are the four processes of light signal transduction?

Answer 2

1. germination
2. photomorphogenesis
3. regulation of the photosynthesis machinery
4. flowering

Question 3

what is photomorphogenesis?

Answer 3

it is when a seedling develops for photosynthesis right away due to the seed being in the presence of light

Question 4

what is skotomorphogenesis?

Answer 4

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.

Question 5

what happens to the seed during skotomorphogenesis once it is out of the soil?

Answer 5

the shoot assumes photomorphogenesis once in the light (de-etiolation)

Question 6

what is positive and negative phototropism?

Answer 6

-positive: plant grows towards light source (shoots reach light)

-negative: plants grow away from light source (roots stay in soil)

Question 7

how does the stomata open in the morning and close in the evening?

Answer 7

-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

Question 8

what are the receptors for different wavelengths that generate specific responses?

Answer 8

red and far red receptors- phytochromes (Phy)

blue, UV-A/B receptors- cryptochromes (CRY) and phototropins (PHOT)

Question 9

what are the processes regulated by light receptors?

Answer 9

germination–> de-etiolation–> shade avoidance–> phototropism–> chloroplast movement (face light if normal, avoid if too much)–> stomata opening–> circadian entrainment–> flowering

Question 10

what does phytochrome red (Pr) and phytochrome far red (Pfr) do to germination?

Answer 10

Pr = stimulates germination
Pfr = inhibits germination

-Pr turns into Pfr and vice versa and then a response occurs

Question 11

which phytochrome determines the outcome of germination?

Answer 11

the last phytochrome pulse in the pattern (e.g. R/FR/R/FR means inhibition of germination)

Question 12

what does darkness do to Pfr?

Answer 12

turns into Pr in the dark

Question 13

what happens to long hypocotyl (hy) mutants and cop mutants?

Answer 13

hy mutants grow as if they are in dark in the light (skotomorphogenesis) and cop mutants do photomorphogenesis in the dark

Question 14

what happens to non-phototrophic hypocotyl (nph) mutants?

Answer 14

dont show signs of phototropism even when grown in dark and followed with light (dont curve in direction of light)

Question 15

what do photoreceptors consist of?

Answer 15

consist of protein (apoprotein) and light sensitive chromophore pigments

apoprotein + chromophore = holoprotein

Question 16

what UV range do phytochromes, phototropins and cryptochromes absorb?

Answer 16

phytochromes absorb 600-700nm (red)

phototropins and cryptochromes absorb 320-500 nm (blue)

Question 17

what is the first step during the phytochrome B organization and how does it change conformation?

Answer 17

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

Question 18

what are the overall steps of seed germination by Pr, Pfr, and PB?

Answer 18

1. PB is made in chloroplast then binds to GAF of cystein residue to make holoprotein, two are formed to make a dimer
2. red light causes conformational change from Pr to Pfr
3. conformational change causes opening of holoprotein
4. opening helps expose NLS sequence
5. Most Pfr is imported into the nucleus, others remain in cytosol to mediate rapid responses
6. Pfr interacts with nuclear proteins
7. regulates gene expression
8. seed germination, photomorphogenesis

Question 19

what does the Pr/PFR ratio do?

Answer 19

provides plants with position and time information

Question 20

what does the leaf canopy absorb to cause shade-avoidance response?

Answer 20

low R/FR light ratio causes elongation of plants to avoid shade

Question 21

what ratio of R/FR is seen in daylight and evening and in short and tall plants?

Answer 21

daylight = 1.15
evening = 0.7
short = high R/FR
tall = low R/FR

Question 22

what does PB interact with to cause skotomorphogenesis in the dark and what causes switch into photomorphogenesis once in light?

Answer 22

-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

Question 23

what are the steps to phototropin signal transduction leading to stomata opening?

Answer 23

1. blue light causes autophosphorylation of phototropin
2. signal transduction leads to closure of anion channels and activation of potassium channels
3. intracellular ion concentration is high leading to water uptake via osmosis
4. swelling occurs
5. stomata opens allowing gas exchange

Question 24

what is the phototropism seen when activated by blue and UV light?

Answer 24

-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