(a) (2 marks) What is meant by the “photoreversibility” of phytochrome?
Phytochrome exists in 2 interconvertible forms: Pr and Pfr
Conversion of Pr (inactive form) to Pfr (active form) when red light is applied and the process is reversed when far red light is applied
(b) (3 marks) Indicate and explain 3 events involved in the signal transduction pathway leading to active Pfr upon red light illumination (eg. some of the structural and other changes to phytochrome)
Assembly of phytochrome which is a dimer composed of chromophore pigment + polypeptide
chromophore undergoes cis to trans isomerazation when red light is absorbed
Phy protein also undergoes photoreversible conformational change when light is absorbed, autophosphorylation at the Ser in hinge region to expose the PAS domain repeats that contains nuclear localization signals that directs the active form of Pfr into nucleus (regulates gene expression)
His-kinase-related domain mediates autophosphorylation
small amounts of phytochrome remains in the cytosol which mediates rapid responses
(c) (2 marks) What modifying enzymes subsequently affect the stability or activity of the physiologically active form (Pfr) and how do they act?
When Pfr is autophosphorylated at the Ser in the hinge region, it’s less active and less stable form, so phosphatase PAPP5 dephosphorylates the phytochrome making it fully active and stable
On the other hand, a kinase does the reverse process, it phosporylates Pfr to make Pfr less active and less stable so that it can be degraded by proteasome
(d) (2 marks) Once Pfr is activated indicate some processes at the cellular level that are triggered when Pfr remains in the cytosol (Hint: these are changes that mediate rapid responses, the answer is not changes in gene expression. You can indicate the types of responses that are triggered as part of your answer).
- when Pfr is activated, rapid responses (due to ion fluxes) at the cellular level are triggered which cause relatively rapid turgor changes (cell volume changes – swelling / shrinking), organelle movements, stem elongation in pigweeds and in sun plants as shade avoidance response
At equilibrium:
-Saturating red light
85% Pfr
15% Pr
-Saturating far red light
97% Pr
3% Pfr
functional domains of phytochrome:
1. N-terminal half
- Chromophore-binding bilin lyase domain
- PHY domain – stablizes phytochrome in Pfr form
2. Hinge region
- Separates N- from C-terminus; role in conversion: Pr Pfr (active form)
3. PAS domain repeats
- Mediates dimerization & interactions with downstream effector proteins
- Contains NLS (nuclear localization signals) – when exposed, NLSs direct
active form of phytochrome (Pfr) into nucleus
4. His-kinase-related domain
- Mediates autophosphorylation
- Important for stopping phytochrome responses
fluence = amount of light
Law of Reciprocity (VLFR - very low fluence response & LFR)
- Response can be induced by brief pulses of light, provided that the total
amount of light energy adds up to the required fluence
HIR - high irridiance (fluence rate) response
– Need long exposure to light of high irradiance
– Not photoreversible
– Reciprocity does not apply
• i.e. cannot provide dimmer light for a longer time
• Dependent on fluence rate NOT fluence
- eg. induction of flowering, production of ethylene, enlargement of cotyledons, inhibition of hypocotyl elongation
daylight = more red light
canopy and soil = more far-red light
sun plants exhibit shade avoidance in canopies (shade = far-red light only passes through, leaves filters out red-light) - Stem elongates to lift leaves towards light, so in shade, lots of Far-red light, so Pfr is converted to Pr so there's low Pfr:Ptotal ratio when stem elongates
shade-tolerant plants don't do shade avoidance
high Pfr:Ptotal (lots of light) promotes seed germination, optimal growth conditions
photomorphogenesis - driven by changes in gene expression (eg. MYB, LHCB), slower, long term process, convert etiolated (seedling grown in dark, soil - long hypocotyl, apical hook, no chlorophyll) to de-etiolated
in the dark, it's in inactive form, Pr
when red light is present, it is converted to Pfr
How phytochrome was discovered: seeds germinated when exposed to red-light and inhibited when exposed to far-red light (petri dish with seeds), R/FR/R = germinate, the last one determines
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