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Plants and Light Flashcards

1
Q

What regulates plant development?

A

Light

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2
Q

What is photomorphogenesis?

A

Collective regulation of plant growth and development by light

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3
Q

What type of movement do sunflowers show?

A

Heliotropism

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4
Q

When do chrysanthemums flower more quickly?

A

In short day length

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5
Q

What is an etiolated seedling

A

Grows in dark
Closed cotyledon
Long hypercotyl
This is to grow up through soil to light

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6
Q

What type of growth is seen in the dark?

A

Skotomorphogenesis

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7
Q

What portion of the electromagnetic spectrum is active in plant photomorphogenesis?

A

400-700nm

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8
Q

What are the functions of phytochromes?

A

Influence germination, seedling de-etiolatjon, plant architecture, reproductive development

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9
Q

How to use genetics to determine the functions of individual photoreceptors

A
  1. Create or identify mutants that carry defective genes for photoreceptor proteins
  2. Compare the photoresponses of the mutants with those of the wild type
  3. From the mutant phenotype, deduce the functions of the wild type photoreceptors
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10
Q

What does a phytochrome consist of?

A

An apophytochrome protein component and a light-absorbing chromophore (phytochromobilin)

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11
Q

Responses of wild type to different light

A

Dark: etiolated, long hypercotyl, small cotyledons
Red: de-etiolated, short hypercotyl, large cotyledons (green)
Far-red: de-etiolated, short hypercotyl, medium cotyledons (yellow)
Blue: de-etiolated, short hypercotyl, medium cotyledons (green)

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12
Q

Forward genetics

A 
Mutate WT seed
Look for plant with phenotype you want
Sequence
Identify gene
You know the phenotype you are looking for but don’t know what gene is involved
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13
Q

Reverse genetics

A

Obtain specific mutant from stock centre
Screen for phenotypes
Identify gene function
You have a mutant with a known mutated gene but do not know what the gene does

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14
Q

phyA mutant in different light

A

Dark: etiolated, long hypercotyl, small cotyledons
Red: de-etiolated, short hypercotyl, large cotyledons (green)
Far-red: eiolated, long hypercotyl, small cotyledons
Blue: de-etiolated, medium hypercotyl, medium cotyledons (yellow)
Not sensitive to far-red light

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15
Q

Unique properties of phytochrome A

A
  1. Phytochrome A is rapidly degraded in the Pfr form (the more Pfr, the faster the degradation). Red light = fast degradation
  2. Phytochrome A accumulates to high levels in dark-grown seedlings (phyB in light-grown plants)
  3. FR is very inefficient at converting Pr to Pfr. A small pool of phyA Pfr is therefore produced. This is protected from degradation and ‘cycles’ between Pr and Pfr. This cycling generates a signal which drives photomorphogenesis
    phyA acts as a highly sensitive light ‘antenna’, rapidly degrading and triggering photomorphogenesis following soil emergence
    phyA can inhibit elongation growth in far-red rich light environments
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16
Q

phyB mutant different light responses

A

Dark: etiolated, long hypercotyl, small cotyledons
Red: de-etiolated, long hypercotyl, small cotyledons (green)
Far-red: de-etiolated, short hypercotyl, medium cotyledons (yellow)
Blue: de-etiolated, short hypercotyl, medium cotyledons
phyB mutants insensitive to red light

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17
Q

What do light-grown phyB mutants look like?

A

They are elongated and show early flowering - resembles shade avoidance of wild type plants

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18
Q

In general, what do phytochromes do?

A

Suppress elongation growth and flowering

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19
Q

What do light signals provide plants?

A

The quality of light signal provides information about the presence and density of neighbouring vegetation

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20
Q

Difference between shade tolerance and shade avoidance

A

Shade tolerance: thinner leaves with more chlorophyll, increase photosynthetic efficiency
Shade avoidance: elongate leaves and stems to overtop competitors

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21
Q

What can be inferred from the red:far-red ratio?

A

It is a signal that other plants are nearby

If reflected R:FR in a ratio of <0.2 (ratio low as far-red wavebands reflected), shade avoidance is observed

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22
Q

Rapid responses to facilitate shade avoidance

A
  • Gene expression
  • Leaf hyponasty (increased lead angles)
  • Increased internode extension
  • Increased petiole (leaf stem) extension
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23
Q

Longer term responses to facilitate shade avoidance

A 
Reduced branching
Reduced leaf area
Reduced leaf thickness
Reduced chlorophyll synthesis
Accelerated flowering
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24
Q

Shade avoidance in grasses

A

Eg sorghum

  • Increased stem elongation
  • Reduced tillering (branching)
25
Q

What is required for shade avoidance?

A

Auxin synthesis and transport

IAA transported down seedling from cotyledons to hypercotyl

26
Q

Why must shade avoidance be taken into account when planting crops?

A

Determines crop density

Crops planted too close together have lower yield as they put energy into shade avoidance

27
Q

Shade avoidance risk in cereals

A

Excessive elongation leads to weakening of stems

Stem weakening can lead to lodging (falling over) and considerable crop losses

28
Q

What does phyA do?

A

Signalling limits shade avoidance to prevent over-elongation

29
Q

What initiates leaf elevation?

A

Touching leaf tips

de Wit et al. 2012

30
Q

Blue light photoreceptors in plants

A

Cryptochrome 1 & 2
Phototropin 1 & 2
FKF1
ZTL

31
Q

hy4 mutant response to different light - what does it encode?
What is it also called?

A 
Dark: etiolated, long hypercotyl, small cotyledons
Red: de-etiolated, shirt hypercotyl, medium cotyledons (green)
Far-red: de-etiolated, short hypercotyl, medium cotyledon (yellow)
Blue: de-etiolated, long hypercotyl, small cotyledons (green)
HY4 gene encodes the protein component of cryptochrome 1
Renamed CRY1 (for cryptochrome 1)
32
Q

What light are cry1 mutants insensitive to?

A

High irradiance blue light

33
Q

What light are cry2 mutants insensitive to?

A

Low irradiance blue light

34
Q

What is the phenotype of cry2 mutants under long day conditions?

A

They are late flowering

35
Q

What does green light do to cryptochromes?

A

It can reverse cryptochrome function

cry1 and cry2 display some blue/green reversibility

36
Q

General functions of cryptochromes

A

Inhibition of hypocotyl elongation

Promotion of flowering

37
Q

Conclusions of Darwin’s experiments on phototropism

A

1880

  • Stimulus (light) is detected at one location (root tip)
  • Response (bending) is carried out at another location (region of elongation)
  • Tip must therefore be communicating with cells in region of elongation
38
Q

Screening for mutants with defective phototrophic responses

A

Expose shoots to low irradiance blue light from an angle

Non-phototrophic hypercotyl 1 mutants (nph1) mutants will not bend towards light

39
Q

What does the NHP1 green encode?

A

The protein component of phototropin 1 (PHOT1)

40
Q

What does each LOV domain in the phototropin 1 protein bind?

A

Bind non-covalently to a single flavin (FMN) molecule

41
Q

What does blue light regulate in leaf mesophyll cells?

A

Chloroplast movement
Low irradiance - chloroplasts spread across top and bottom surfaces of cells to absorb maximum light
High irradiance - causes plants to bleach as chloroplasts move to edges of cells to protect from UV damage

42
Q

What does npl1 gene encode?

A

NPL1 = phot2 (phototropin 2)

43
Q

What do phot1 and phot2 mediate?

A

Blue light-dependent stomatal opening

Leaf development

44
Q

Functions of the phototropins

A
  • Phototrophic curvature
  • Chloroplast movement
  • Stomatal opening
  • Leaf development
45
Q

What are PIFs?

A

Phytochrome interacting factors

Sub-family of bHLH transcription factors originally identified by Peter Quail

46
Q

Model for PIF-mediated phytochrome signalling

A

Pr activated to Pfr
Pfr going into nucleus and prevents PIF from binding to DNA
PIF is an transcriptional repressor so inhibits transcription of target gene
Pfr therefore allows transcription of target gene, causing gene expression

47
Q

What is COP signalling?

A

Constitutive photomorphogenesis signalling

48
Q

What growth do COP mutants exhibit in the dark?

A

De-etiolated

Photomorphogenesis in the dark

49
Q

What is COP1?

A

Ubiquitin ligase

50
Q

What is HY5 and what does HY5 do?

A

HY5 8: a Basic Leu-Zipper (bZIP) transcription factor that activated many light regulated genes
Promotes photomorphogenesis

51
Q

What is the link between COP1 and HY5?

A

COP1 binds to and degrades HY5 through the COP9 complex

52
Q

Regulation of gene expression by:

A
  1. Nuclear-cytoplasmic partitioning of phytochrome Pfr and it’s interaction with the PIF family of transcriptional regulators. Interaction leads to PIF degradation
  2. Phytochromes and cryptochomes together inactivate COP1. This promotes HY5 accumulation in the light. COP1 is active in the dark and degraded key photomorphogenesis transcription factors (eg HY5)
53
Q

How do plants protect themselves from UV damage?

A
  • Reflectance (hairs and waxes)
  • Efficient DNA repair
  • Sunscreen biosynthesis
54
Q

How does UV-B exposure promote plant survival?

A
  1. Protection: activation of genes involved in UV-B protection eg. flavonoid, biosynthesis enzymes, DNA photolyases, antioxidant enzymes
  2. Regulatory signal: affects photosynthesis, metabolism, photomorphogenesis, pathogen resistance
55
Q

What photoreceptor detects UV-B?

A

UV RESISTANCE LOCUS 8 (UVR8)

56
Q

What happens to uvr8 and hy5 mutants when they are exposed to UV-B?

A

UV-B damage occurs

57
Q

How does UVR8 signal?

A
  • Tryptophan residues within UVR8 molecule are thought to act as a ‘chromophore’ (absorb UV-B)
  • UV-B stimulates interaction of UVR8 with COP1
  • UVR8/COP1 interaction leads to HY5 stabilisation which leads to photomorphogenesis
58
Q

What are the 5 phytochromes?

A

phyA - phyE

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