Plant phys exam: 7 responses to Light

What is photomorphogenesis?

light induced phenotypic changes distinct from photosynthesis

What changes happen with varying light quality?

stem elongation rates, leaf shape, surface area, time to flowering number of leaf hairs etc.

Compare and contrast the differences between light and dark grown seedlings

light: cotyledons open, short/thick stem, apical hooks open, direction of growth determined by light

dark: stem elongates rapidly, leaves don’t grow/expand, root growth is inhibited, direction of growth determined by gravity

What is the benefit of sunlight?

• regulates cellular response (guard cell opening)

• provides directional and nondirectional cues for growth (solar tracking)

• regulates aspects of development (flowering/circadian rhythms)

Plants can perceive the spectral ___________of light in its environment

quality and quantity

What are the types of photoreceptors in plants?

A. Red Light Photoreceptors •

• phytochromes (PHYA-E)-photomorphogenesis

B. Blue Light Photoreceptors

• Cryptochromes (CRY1 and CRY2)- photomorphogenesis

• phototropins (PHOT1 and PHOT2) – phototropism, guard cell opening, chloroplast movements

• Zeitlupe (ZTL) – day length perception/circadian rhythms

C. UV Photoreceptors

UV Resistance Locus 8 (UVR8) - tends to inhibit elongation and counteract the shade-avoidance responses mediated by phytochrome

_____ are involved in lots of morphological changes. Give examples

phytochromes

• seed germination, flowering, inhibin of stem elongation, leaf unfolding during de-etiolation, shade avoidance responses in “sun” plants

How does seed germination relate to light intensity. 

red light promotes germination

• far red light immediately following red light: inhibits germination

• seed germination exhibits “far-red photo reversibility

Seed germination experiment

Last light treatment determines outcome

• Red light – germination

• Far red light – no germination

What are the 2 forms of photoconvertible phytochromes?

Pr : red absorbing form (650-680 nm) – absorption converts it to Pfr form

Pfr : far-red absorbing form (710-740 nm) – absorption converts it to Pr form

• Phytochrome is synthesized in Pr form (dark grown tissues- only have Pr form)

• In darkness: Pfr slowly reverts to Pr form

What is the absorbance spectra of the phytochromes?

Maximum Absorbance:

Pr = 666 nm
Pfr = 730 nm

** never have 100% of one form due to the overlap in the absorption spectra

What is the active form of phytochrome?

Pfr

• able to promote/inhibit a response

Example:

• Pfr promotes flowering in plants that respond to short nights (called long day plants)

• Pfr inhibits flowering in plants that respond to long nights (called short day plants)

Describe phytochromes

they are homodimers of 2 identical subunits

• homodimer: 2 of the same polypeptides

What is each phytochrome subunit made of?

• chromophore (light absorption)

• apoprotein (chromophore binding site) with hinge region= mediates conformational changes in protein shape

What happens in the absorption of red light?

1. conformational change via hinge region

2. exposes nuclear localization signal

3. Pfr form brings about large global changes in transcription

What rapid cellular responses does phytochrome cause?

activation of ion channels… changes in membrane potential

What longer slower changes does phytochrome mediate?

i.e. Flowering, germination, etc.

Transcriptional regulation

• In dark, phytochrome remains outside the nucleus

• Under white light (or red light-move into nucleus)

Describe the types of multigene family photochromes are encoded by

Type I phytochrome: PHY A

Type II phytochrome: PHY B-E

• Dark grown seedlings: 23X more phytochrome than in light grown seedling

  •   85% Type I: 15% Type II

  • Massive amounts of PHYA in DG tissues 

• Light grown seedlings: less total phytochrome present than in dark grown seedling & type II is more abundant

  • 5% Type I: 95% Type II

  • Massive loss of PHYA in light

What factors influence the abundance of PHYA?

1. Darkness promotes transcription of PHYA mRNA (phytochrome is synthesized in Pr form)

2. Levels of PHYA protein regulated by 3 factors

• A. mRNA degradation in light

• B. proteolysis of PHYA Pfr protein by ubiquitin mediated degradation • PHYA has a PEST sequence (pro glu ser thr)

• C-terminal end of protein has UB site C. PHYA Pfr represses transcription of PHYA gene

some proteins are very stable while others are not

Describe type II phytochromes.

1. transcription not regulated by light/dark

2. more stable in light (degraded more slowly, no pest sequence)

3. lower transcription rates

What is the result of a high concentration of PhyA in dark grown tissues?

an adaptation for maximized sensitivity to miniscule amounts of light

What are the 3 types of possible phytochrome responses?

based on the amount of light required to evoke the response

1. VLFR - very low fluence response

2. LFR - low fluence response

3. HIR – high fluence response

Define fluence

μmoles photons/M2

Define total fluence

μmoles photons/M2 x length of time (duration)

Define fluence rate

(μmoles/M2/Sec) # of photons absorbed per unit surface area per unit time

Describe the first response type of phytochrome

A. Initiated at very low fluences… Responses saturate at very low fluence (reach max @ low light (2 minutes of moonlight)

B. VLFR’s occur in dark-grown tissues

• germination in some seeds

• Initiate de-etiolation responses

C. VLFR’s are mediated by PHYA: high concentration of PhyA in DG tissues is an adaptation for maximized sensitivity to miniscule amounts of light

D. VLFR responses are NOT far-red photoreversible… phytochrome synthesized in Pr form

E. Obey Law of Reciprocity: reciprocal relationship between fluence rate and time

• If response occurs under both conditions then the Law of Reciprocity holds

Describe the second response type of phytochrome

A. Initiated at low fluences (10,000X greater than VLFR) •

• saturate at energy that is about 1% of the energy provided by 1 minute of full sunshine (0.6 seconds)

B. R/FR reversible (like the classic seed germination experiment) • red promotes response • far red light inhibits response

C. Law of Reciprocity holds (total amount of light)

D. Mediated by PhyB

What are some physiological responses that are LFRs?

• seed germination (i.e. lettuce)

• leaf unrolling

• leaf movements

• flowering (in some species)

• Apical hook opening

Can a seed germinate if under the soil?

Light is enriched in far-red light

*** inhibits germination

Can a seed germinate under a plant canopy(shade)?

blue and red light is absorbed by surrounding plants for photosynthesis

• so most light that reaches ground is enriched in far-red light

• Far red light-prevents seed germination

BUT-

if you place far-red filter over leaf: ( i.e. block all FR light)

• seed germinates, even if red light fluence is really low

• blocking far-red light produces really large Pfr:Pr ratio

** ensures germination when close to surface of soil/in open sunlight/space

Describe the third response type of phytochrome

A. Require at least 100X more energy than LFR’s

B. Require prolonged/continuous light • remove light, response stops • law of reciprocity doesn’t apply/hold

C. Response is proportional to irradiance level (up to a maximum)

D. Responses saturate at very high fluences

E. Not R/FR reversible

What is the action spectra for HIP?

1. Light grown tissues - Ex. Inhibition of stem elongation

• Red light (680 nm) evokes response (promotes inhibition)

• PHYB mediated

•  As soon as you remove the red light-stems start elongating

What are physiological Responses that are HIRs

• inhibition of hypocotyl elongation in mustard (Arabidopsis), lettuce and petunia • anthocyanin synthesis in some dicot species

• induction of flowering in henbane

• apical hook opening in lettuce

• enlargement of cotyledons in mustard (Arabidopsis)

How can Phytochrome mediate a response?

1- a specific threshold level of Pfr causes a response • all or nothing type response

• i.e. Seed germination

2- the amount of Pfr formed determines the magnitude of the response (more Pfr-bigger response)

• Flowering or pigment synthesis

3- it is the ratio of Pr/Pfr that determines response: plants can use to ratios: Pfr/total phytochrome (Pr + Pfr) or Pr/Pfr

How does shade avoidance work in response to sun plants

monitoring the Pfr/total

red light is absorbed by leaves of canopy/ shade is enriched in far-red light

if “sun” plant is in shade, absorbs more far-red light than red light • converts more phytochrome to Pr form

Being in the shade changes Pfr:Ptotal ratio (decreases it) and stem begins to elongate

What do plants use the Pr:Pfr ratio for?

sense the change of seasons

• At dawn, the phytochrome molecules in a leaf quickly convert to the active Pfr form,

• In the dark, the Pfr form takes hours to slowly revert back to the Pr form.

By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the days/nights

Mechanism for inhibition and promotion of stem elongation in the light

• Pfr decreases the synthesis of gibberellins in hypocotyls

• In the shade-when Pfr levels are non-existent or low then GA has a greater effect on hypocotyls

• shade avoidance is mediated by PHYB

Define PIFS

Phytochrome Interacting Factors

How is Photomorphogenic development (de-etiolation) regulated by ubiquitin mediated proteolysis in darkness:

COP1 is E3 ligase

• RING domain E3 ligase

• Highly conserved across organisms • SPA1 is required for its function

• COP1 Functions as repressor for photomorphogenic development by degrading transcription factors that promote light mediated development

How is Photomorphogenic development (de-etiolation) regulated by ubiquitin mediated proteolysis in light:

E3 ligase moves out of nucleus

• Transcription factors promote photomorphogenesis

• PHYA Pfr moves into nucleus faster than COP1 leaves

• PHYA is degraded by COP1 – decreasing the total PhyA protein levels

• (remember PHYA levels decrease in light)