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246 > Photoreception > Flashcards

Photoreception Flashcards

1
Q

What forms of evolution have plants undergone to maximise solar radiation interception?

A

Chloroplast structure, architecture…

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2
Q

How does chloroplast topology change?

A

Cytoskeletal movement mediates translocation of chloroplasts to periclinal cell walls in response to unilateral blue light illumination.

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3
Q

What are the categories of movement response of chloroplasts?

A

Positive phototropism (accumulation towards light), or negative phototropism (movement from light)

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4
Q

Why do chloroplasts move from light?

A

Minimize photodamage and photoinhibition.

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5
Q

How does photoinhibition occur?

A

This is accumulation of more energy than the chloroplast can use, resulting in ROS production.

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6
Q

How do ROS form as a consequence?

A

Transfer of excited energy to O2, which can cause oxidative damage.

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7
Q

How has this been done on the micro scale?

A

Chlorophyll and accessory pigments called photoreceptors like phytochromes

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8
Q

What are photoreceptors?

A

This regulates physiology and developments of plants through light perception.

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9
Q

What is an example of developmental regulation?

A

Seed development on soils differs from beneath the soil due to light accessibility.

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10
Q

What co-ordinates developmental reassignment for seed development?

A

Phytochromes and blue-light receptors

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11
Q

How did the study of phytochromes begin?

A

The realisation of differential processes related to wavelength interpreted by the plant.

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12
Q

What is the structure of a phytochrome?

A

A protein linked to a chromophore, the latter absorbing photons causing conformational changes, altering absorption spectrum.

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13
Q

What is the structure of the chromophore domain?

A

A chromophore binding domain and a regulatory domain, undergoing conformational changes upon absorption.

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14
Q

What is the regulatory domain?

A

Series of domains regulating protein activity in response to lights, including PHY domain.

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15
Q

What is its primary structure?

A

PHY domain and PAS domain.

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16
Q

What does the PHY domain do?

A

Transmit light signals to downstream signalling components, undergoing CC upon light absorption by chromophore, altering protein structure.

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17
Q

How do PAS in regulatory domains and chromophore binding domain interact?

A

Former sense CC induced by CC (transmitting to downstream components) whilst the latter is where photoisomerization occurs.

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18
Q

What initiates conformational changes?

A

660nm (red light) absorption converts Pr to Pfr, whilst far-red light (730nm) converts Pfr back to Pr.

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19
Q

How do the secondary structures facilitate these changes?

A

Pr form the chromophore adopts cis-configuration, whilst Pfr form switches to trans-configuration.

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20
Q

How is interconversion maintained?

A

Pfr absorbs somewhat in the red region meaning 660nm absorption leads to absorption by both forms.

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21
Q

Which is the active form?

A

Conversion of Pr to Pfr is the basis of red-promoted processes.

22
Q

What processes are initiated through conversion to Pfr?

A

seed germination, flowering induction, shade avoidance.

23
Q

What is the photostationary eqfuilibrium?

A

The stable state of Pfr/Pr at about 85 and 15% respectively.

24
Q

What determines this?

A

Relative intensities of red and far-red light (high intensity red-light towards Pfr).

25
Q

What are the stages of PMG?

A

Etiolated growth, light perception/signal transduction, de-etiolation, morphological/physiological changes, and developmental programming.

26
Q

What is etiolation?

A

Seedling growth pattern having germinated/grown without light.

27
Q

What morphological/physiological adaptations do etiolated seedlings have?

A

Elongated hypocotyl, closed cotyledons, undeveloped chloroplasts…

28
Q

Why is the hypocotyl elongated?

A

Allows seedling to reach potential light sources.

29
Q

What are cotyledons closed

A

Protect shoot apical meristem and emerging leaves from mechanical damaging and desiccation during soil emergence.

30
Q

How does Pr/Pfr activate it?

A

Pr accumulation inhibits gene expression associated with PMG, leading to their characteristic features.

31
Q

How does transition to de-etiolation/PMG occur?

A

Exposure to red-light with photoconversion from Pr to Pfr

32
Q

What does Pfr do in this conversion?

A

Induce gene expression pattern changes, alleviating inhibition and activating light-related genes.

33
Q

What genes are activated?

A

Chlorophyll synthesis, chloroplast development, leaf expansion, and hypocotyl elongation inhibition.

34
Q

How does this conversion process relate to absorption spectra?

A

Pr/Pfr overlap but have different peak wavelengths, Pr absorbing 660nm promoting Pfr conversion

35
Q

What is the SAR?

A

Set of adaptive physiological/developmental changes in response to R/FR with competition for light within dense vegetation.

36
Q

What are the stages of SAR?

A

Perception of low R/FR ratios, physiological changes, leaf morphology alterations, branching suppression, reproductive timing modulation, resource allocaiton, and plasticity and acclimation.

37
Q

What initiates SAR?

A

Perception of decrease in ratio of R/FR wavelength, through phytochrome action.

38
Q

What physiological responses occur with low R/FR ratios?

A

Maximising light capture and resource acquisition, so internode length elongation to optimise light interception through stem elongation.

39
Q

How is auxin important here?

A

Indole-3-acetic acid regulates stem elongation.

40
Q

How is phyB important to auxin?

A

Stimulates synthesis and redistribution of auxin, promoting cell elongation in stem.

41
Q

What does increasing auxin lead to?

A

In elongation zone of stem, cell wall loosening enzymes activated like expansins and xyloglucan endotransglucosylase/hydrolases facilitate cell expansion/elongation.

42
Q

What other hormones are important?

A

Gibberellin and Brassinosteroid

43
Q

How do feedback mechanisms regulate this?

A

Negative feedback through hormone biosynthesis downregulation

44
Q

What leaf morphological changes occur?

A

Increased leaf area, leaf angle/orientation changes, leaf thickness/pigment content alterations to optimize absorption/utilization.

45
Q

What characterises etiolated seedlings?

A

Elongated embryonic stems, closed cotyledons, and undifferentiated chloroplasts.

46
Q

What are the various phytochromes important in PMG?

A

UVR8, Cry1/2, phototropins…

47
Q

How do they interact?

A

Either synergistically, additively, antagonistically to synchronise physiological and cellular processes, and time developmental transitions.

48
Q

What happens upon Red light absorption?

A

Conversion to Pfr, translocation to nucleus, and induction of signalling.

49
Q

Why is triggering by sunlight important?

A

Allows otherwise sessile organisms to optimise growth and survival, maximising fitness.

50
Q
A

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