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Green planet > L2 and 3 Kerry Franklin - Light responses > Flashcards

L2 and 3 Kerry Franklin - Light responses Flashcards

1
Q

What is photomorphogenesis?

A

Plant development controlled by light quantity, direction, periodicity, and quality.

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2
Q

what is a photoperiod?

Give an example of photoperiod affecting photomorphogenesis

A
  • the length of light (of a ‘day’)

- Chrysanthemums are short day plants - flowers in 8h days. no flowers in 16h days.

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3
Q

describe etiolated and de-etiolated apearence of arabidopsis seedlings

A

Etiolated - dark grown - Long hypocotyl, cotyledons folded up in apical hook. If cotyledons were open, would be damaged in soil.
De-etiolated - light grown - large cotyledons, short hypocotyl.

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4
Q

What is a monocot version of the apical hook?

A

coleptie

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5
Q

Which photoreceptors detect red/far red light?

A

Phytochromes

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6
Q

Which photoreceptors detect blue light?

A

Cryptochromes, phototropins

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7
Q

What two components are in a phytochrome molecule?

A

Protein - Apophytochrome

Chromophore

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8
Q

What is a chromophore?

A

A light absorbing molecule, formed by Protoporphyrin in the chloroplast.

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9
Q

Wha does the chromophore and the Apophytochrome together make?

A

holophytochrome

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10
Q

Which is the active and inactive phytochrom form?

A

Pr- inactive
Pfr - active, regulated gene expression.
Red light favours activation, and far red light favours inactivation.

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11
Q

Describe the flip flop germination experiment.

A
  • Under red light, arabidopsis seedlings germinated (Pfr)
  • Under red then FR light, Pfr converted into Pr so no germination
  • Red, FR then Red again, germination, as the final molecule was Pfr
  • Shows he last molecule present caused germination or not germination.
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12
Q

What phytochrome types do dicots and monocots have?

A

Dicots - (A,C) (E, B, D)

Monocots - (A, C) B

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13
Q

What are the functions of phytochromes?

A

Germination timing
Suppress flowering, plant energy is used for leaves and photosynthesis.
Regulate development through lifecycle
Regulate plant architecture

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14
Q

How can you use genetics to determine phytochrome function?

A
  • create mutants with genes defective in phytochrome function
  • Compare photoresponses with wild type, to determine what the phenotype is of the mutated gene
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15
Q

What does a light grown phya mutant look like?

A

Long hypocotyl.

Phy A supresses hypocotyl elongation in light.

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16
Q

Describe photoresponses in Dark, Red, FR and blue light

A

dark - long hypocotyl, cotyledons inside apical hook
Red - small plant, cotyledons open
FR - small, open cotyledons and yellow
Blue - Taller, small cotyledons

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17
Q

Why is it weird that phyA signals in far red light?

A

It gets inactivated in far red light, Pfr -> Pr

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18
Q

How does phy a manage to signal in FR light even though it gets inactivated?

A
  • Pfr is rapidly degraded, and more Pfr = faster degradation.
  • accumulates to high levels in dark grown seedlings
  • FR is very inefficient at producing Pfr, so cycling between Pr and Pfr causes a signal and protects it from degradation.
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19
Q

Simply, what does Phy A do?

A

Acts as a sensitive light antenna, which rapidly degrades and triggers photomorphogenesis following soil emergence.

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20
Q

What do plants with less phytochromes look like?

A

Long and spindly, early flowering

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21
Q

How do plants adapt in an enclosed habitat to low light conditions?

A

Shade tolerance mechanisms. Adapted to endure shade, with thinner leaves, higher chlorophyll content, increase photosynthetic efficiency.

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22
Q

How do plants adapt in an open habitat to low light conditions?

A

Adapted to escape, shade avoidance techniques.

Elongate leaves and stems to overlap competitors

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23
Q

in sunlight are phytochromes mainly active or inactive?

A

Active, more Pfr, as sunlight has more red than FR light.

24
Q

Why is having more Pfr in sunlight beneficial?

A

Keeps stems short so energy and resources are directed at leaf development, for photosynthesis.

25
Q

what causes shade avoidance photomorphism?

A

Plants grown around neighbours receive mainly red light from sun, and green and FR reflected off other plants. Lowers R:FR ratio compared to just sunlight.
This shifts Phytochrome back to Pr form.
low R:FR triggers IAA production and it is pumped down the plant, driving shade avoidance.

26
Q

what are rapid responses of shade avoidance?

A 
Leaf hyponasty - increased leaf angles
increased internode extension
increased petiole (leaf stem) extension.
27
Q

what are long term responses of shade avoidance?

A

Reduced: branching, leaf area, leaf thickness, reduced chlorophyll synthesis, accelerated flowering

28
Q

Why does crop density affect shade avoidance?

A

If dense, more FR light is reflected so more shade avoidance

29
Q

What is lodging?

A

Falling over of stems in cereal crops. Caused by excessive elongation of stems due to excessive shade avoidance

30
Q

What limits shade avoidance

A

Phy A signalling

Shown by shaded phy A mutants which over elongated and died. WT was fine.

31
Q

What else causes leaf elevation?

A

Touch - of other plants
Shown by Wil et al 2012
one leaves are elevated, plants start reflecting FR light and shade avoidance begins.

32
Q

What is skotomorphogenesis

A

development in dark

33
Q

What 6 hours of blue light photoreceptors are there?

A

Cryptochromes 1 and 2
Phototropins 1 and 2
ZTL
FKF1

34
Q

What gene defines a blue light photoreceptor?

A

HY4

The HY4 mutant was long and spindly in blue light

35
Q

What does the HY4 gene encode?

A

The protein component of Cyptochrome 1

36
Q

What was HY 4 renamed?

A

CRY1

37
Q

describe the structure of CRY1

A

2 chromophores: Pterin, FADH-

attached to photolyase-like protein. also has a Tropomyosin like protein.

38
Q

What does CRY2 do?

A

enhances sensitivity to low levels of blue light, also promotes flowering in long days.
Mutant has elongated hypocotyl in high irradiance blue light, whereas WT is short.

39
Q

what partially inactivates chryptochromes?

A

green light

40
Q

What type of light regulates phototropism?

A

Blue light

41
Q

describe Darwin’s experiment on phototropism

A

1880
flame put next to grass coleoptile
- coleoptile curves towards light
- with cap on tip, no curvature
- with shield lower down coleoptile, still curves.
Shows tip perceives light and communicates with another location where response is carried out.

42
Q

what is nph1 mutant and how was it found?

A

non phototropic hypocotyl 1
Screened plants by first growing in the dark, then applying blue irradience light from the side
Mutants did not bend towards light.
Found nph1 gene encodes protein component of phototropin 1

43
Q

describe the structure of phot1

A

2 chromophores - flavins, non covaletly bonded to LOV 1 and 2 domains on protein. also has a kinase domain on protein.

44
Q

What are 4 functions of phototropins?

A
  1. phototropism
  2. regulates chloroplast movement
  3. regulates stomatal opening
  4. Leaf development
45
Q

how does blue light stimulate choroplast movement?

A

In low irradiance- chloroplasts on top and bottom of cells. to get max light for ph
In high irradiance - chloroplasts move to sides of cell to protect from damaging light levels
phot2 mostly responsible, encoded for by npl1 gene.

46
Q

how do phototropins regulate stomatal opening?

A

blue light dependent, drives phototropin action
In double PHOT1 and PHOT2 mutants, stomatal aperture is similar in blue light to dark. no response to blue light in double mutants.

47
Q

how do phototropins regulate leaf development?

A

in either phot1 or 2 mutants, same as WT - leaves flattened
in double mutant, eaves curled
Only one phototropin is needed for leaf development.

48
Q

what was seen when using GFP to study photoreceptors?

A

Can attach GFP to phytochrome
in dark, phytochrome is equally dispersed through cell.
In red light Pfr aggregates in the nucleus of the cell. Pr cannot get into nucleus.

49
Q

What happens to phytochrome in the nucleus, in Red light?

A

PIFs - phytochrome interacting factors bind to Pfr.

Promotes dark deveopment, then Pfr degraded and PIF destroyed. When PIF is destroyed, cant regulate gene expression.

50
Q

What is an example of constitutive photomorphogenesis?

A

COP signalling

51
Q

how were COP’D mutants found?

A

Screened for de-etiolated mutants when seedlings grown in dark (WT have etiolated appearance).

52
Q

COP signaling???

A

In the dark: CRY1 is in nucleus but in active.
COP1 degrades HY5 transcription factor, forming COP9 complex. Seedlings in dark can use a different transcription factor to promote etiolated growth.

In light: Pfr active. binds to COP1 in nucleus. COP1 can no longer degrade HY5. HY5 switches on all genes needed for photomorphogenesis.

53
Q

Why dont plants get sunburnt?

A

UVB is reflected by hairs and waxes.
Effective DNA repair - cryptochromes continuously repair damaged DNA. similar to DNA photolyases.
UVB stimulates upregulation of ‘sunscreen’

54
Q

How does UVB exposure promote plant survival?

A

activation of genes involved in UVB protection

eg - antioxidant enzymes, DNA photolyases, flavenoid biosynthesis enzymes

55
Q

how was the UVB photoreceptor found?

A

mutants defective in UVR8 and HY5 gene could not survive as they couldnt perceive UVB and make sunscreen.

56
Q

Describe UVR8 structure and signalling

A

It has an internal chromophore made of a cluster of tryptophan molecules which absorb UVB.
UVB stimulates UVR8 + COP1 interaction = HY5 stabilisation = photomorphogenesis

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