Effect of light on Growth and Development
-Light quality and quantity are the most significant environmental factors affecting plant development.
-Light induces dramatic changes in morphology and biochemical (protein) composition.
How does light induce such changes?  e.g. increase in rubisco, LHC

A Simple Model of Signal-induced Responses
1. Signal perception by a receptor
2. Signal transduction
    a) Communcate signal to other cell parts
    b) Amplify the signal
    c) Network and cross-talk
3. Primary response
    a)  Increase or decrease in gene expression
    b) Change from inactive protein ---> active protein
4. Physiological or cellular response.

Plants have 3 types of Photoreceptors
1. Phytochrome            660 /730 nm
2. Blue light receptor    400-500 nm
3. UV-B Receptors     260-350 nm

Phytochrome plays a critical role in every stage of development.
e.g. seed germination (lettuce)

Different Intensities of light induce different responses
    a.  Very low fluence (VLF) response :
    b.  Low fluence (LF) responses
    c.  High irradiation reactions (HIR)

I.  PHYTOCHROME : A red light receptor
Absorbs in the Red 660 nm, Red induces change
Absorption in the Far red 730 nm reverses it.

Red light causes many changes: e.g.
    Seed germination
    Inhibition of stem elongation
    Leaf expansion

Present in all tissues:
        Roots, Shoots, Leaves
        Phy is found in the nucleus and cytoplasm, and it is not embedded in the membrane.

1.  CHEMISTRY
Chromophore is attached to a protein of 124 kD.

Phy has two interconvertible states.
Pr ----RED LIGHT ---> Pfr --->   --->    ----> RESPONSE
    <---FAR RED -------
    <---DARK ----------
Stable                          Unstable (Degraded)
Inact.                         Active

2.  PHYTOCHROME acts as a light-activated switch.

A unique trait of phytochrome-regulated response is PHOTOREVERSIBILITY.

Three kinds of phytochrome responses:
    VLF,               1-100 nmol/m²            Not reversible
    LF,                 1-100 umol/m²             Reversible
    HIR                10 mmol/m²                  Not reversible

Phytochrome can sense light changes during the day
    -photon density (quantity)
    - Ratio of Pr/Pfr (quality)
Multiple forms of phytochrome
    Phy A:                  labile form, present in dark grown seedlings
    Phy B, C, D, E:     stable form found in green seedlings and plant.

3.  How is a light signal transduced to give a response?
a.  Physiology and cell biol. approaches : electrical potential, [Ca] changes
b.  Biochemical approaches: enzyme activity changes
c.  Molecular methods: changes in gene expression (mRNA)
d.  Genetic approach (v. powerful):  find mutants that fail to respond to signal
    Identify gene product to deduce its role in signaling

Models: mode of Action
1. Regulate ion transport
2. Regulate gene expression
        Early response genes
        Late response genes
3.  Regulate metabolic pathway
4.  Change cytoskeleton

II.  Blue Light-induced Responses
1.   Responses are triggered by light of 450 nm.
e.g.    phototropism, inhibition of hypocotyl elongation, induction of stomatal opening

2.  Several blue-light receptors have been identified. Receptors affect different responses.
    a.  Cryptochrome or Cry1                           inhibit stem elongation
    b.  Cry2                                                        cotyledon expansion, clock, flower induction
    c.  Nph1 at the PM = Phot1 (phototropin1)    phototropism
    d.  Npl 1= Phot2                                            chloroplast- high light avoidance
    e.  Flavin
    f.  Carotenoids

3.  How is a Blue light signal received and transduced?  Mode of Action:
Many responses are regulated by both Red and blue light.
1. Signal Perception by a Receptor
2. Second messengers transduce and amplify the signal
3. Final Responses
    a. Regulate Activity of a Membrane Transporter:
         e.g. H+ pump activated, or Anion channel opens
    b.   Activation/repression of Gene expression through proteins interacting with light-regulated elements

Crytochromes found in the retina of mammals have a role in circadian rhythm.

Gaps in our knowledge:
What are the signal transduction components that act early in the photoreceptor action pathways?
Especially what are the components that interact directly with a photoreceptor?
                --------------                    -----------------                -----------------
Plants in Motion: Movies
Google: Plants in Motion by R. Hangarter, Indiana Univ.

Movies: Arabidopsis & other plants
   Germination in Darkness
    Elongation in Light
    Phototropism
    Gravitropism: flowers get up
    Nutation
    Root growth

            ----------------            -----------------------                ----------------------

References:

Shimazaki K, Doi M, Assmann SM, Kinoshita T. 2007. Light regulation of stomatal movement.
Annu Rev Plant Biol. 2007;58:219-47. Review.

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Lecture Review
I.  General
a. How can one distinguish between a phytochrome-induced response and a blue light induced response?  Give an example of each.

b.  Distinguish between low fluence response and high irradiance response.  Give an example of each.

c.  In what way is phytochrome particularly suited to monitor the natural light environment?

d.  Why are there multiple phytochromes in plants?
Why are there multiple blue light receptors?

e.  Most of the phytochrome in dark-grown seedlings is lost following a brief exposure to low fluence light.  If so, how can phytochrome control development in plants normally growing in the light?

II. Sample exam question.
(30) Lemna, a small aquatic plant, can be grown heterotrophically in the dark by culturing in a petri dish containing sucrose.  Plant grows slowly and is pale yellow.  If plants are given 10 min of 660 nm light every 8 hours, growth rate increases, and rbsS (rubisco Small subunit) mRNA increase.  The response is reversed by 10 min 730 light given right after 660 nm light. rbsS protein increases when higher level of light is given.
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Treatment                                                 Growth                             rbsS mRNA                 rbsS protein
---------------------------------------------------------------------------------------------------------
1. Complete Dark                                    v. slow                                  0                                    0
2. 10 min 660 nm light, 8 h Dark              fast                                       30                                  0
   Repeated
3. 10 min 660 nm, 10 min 730 nm, 8 h     Dark slow                               0                                  0
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(a) (4) What molecule senses the light and why?

What is the absorption spectrum (wavelength of light it absorbs) of this molecule?
 

(b)  (6) Why does 660 nm light increase rbsS RNA levels even though 10 min light was followed by 8 h dark?
 
 
 

(c) (6)  if 660 nm light was immediately followed by 730 nm light, why is rbsS RNA not made?
 
 
 

d) ( 8) Propose a model specifically showing how 660 nm light leads to an increase in rbsS RNA?
[rbsS gene is encoded by the _________ (chloroplast or nuclear or mitochondrial) genome.
 
 
 
 
 

(e)  (6) What is the purpose and significance of inducing rbsS RNA?  What is the specific reaction catalyzed by the assembled protein complex?
 
 

(f)  (2) rbsS protein is synthesized in the
 i) chloroplast ,  ii) nucleus iii) cytoplasm or iv) ER
 
 

PBIO 442