Plant phys exam: 8 phototropism

What is unique about blue light photoreceptors?

“three fingered” action spectra is unique for all blue light mediated responses

What are some of the responses to blue light?

• phototropism, chloroplast movements, open/closing of guard cells, solar tracking in leaves

Plants can ______and _______to the quantity and position of light source

perceive and respond

What are cotyledons?

embryonic leaves

Describe Darwin’s phototropism studies

1- no treatment: growth & curvature towards

2- cut off tip: little growth & no curvature

3- opaque sleeve at base coleoptile: growth and curvature

4- clear cap on top: growth & curvature

5- opaque cap on tip: growth but NO curvature

What was the conclusions to Darwin’s experiment

• Response (growth) site is separate from perception site

• Shaded side of coleoptile grows faster than the lighted side

• Coleoptile tip is site of phototropic perception

Describe Darwin’s second experiment

mica sheet: inserted across, inserted on shaded side, or inserted on light side

gelatin: placed between tip and stump= normal growth

conclusion: 

• Chemical signal moved down shaded side of coleoptile (if blocked-no growth)

• Chemical signal is water soluble and mobile

Overall conclusions from Darwin’s experiments

1. Growth regulator was present normally in tip

2. Growth regulator isn’t induced/synthesized by unidirectional light source

3. Proposed that light causes the accumulation of the chemical on the shaded side of the coleoptile (redistributed in response to unidirectional light)

What wavelengths produce optimal phototropic curvature?

blue wavelengths (440/480n)

Any substance that promotes coleoptile elongation is…

an auxin

• Using known concentrations one can determine how much curvature is produced at that concentration of auxin

• how much auxin was present in a tissue by comparing how much curvature was produced by that tissue coleoptile etc.

What is fluence

total number of photons provided to a 1 M2 surface area over a given time interval

• Lower and higher fluences produce curvature (first & second positive curvature)

What it the neutral zone in fluence?

the low point between the first positive and second positive

What is phototropic curvature correlated with?

protein kinase activity

PHOT1

Describe the photoreceptors in phototropism

PHOT1: mediates phototropism in response to low and high fluence blue light

PHOT2: mediates phototropism in response to high fluence blue light only

• has some redundant function with PHOT1

Phot 1 and Phot 2 are Phototropins:

light activated serine/threonine kinases associated with plasma membrane

• Phototropin is a dimer and each monomer has 3 important regions

What makes up the important regions of phototropin monomer?

1- photosensor

2- Ser/Thr Autophosphorylating Kinase Domain

3- La helix

Describe photosensors

light absorption

• Flavin mononucleotide (FMN- part of FADH2) bound to LOV domain

• LOV domains is the sensor; named for organisms that have a LOV domain.

• L - light (photosynthetic organisms)

• O - oxygen (bacteria)

• V - voltage (drosophila/vertebrates-K+ channels)

Describe the Ser/Thr Autophosphorylating Kinase Domain

activated when FMN absorbs blue light

• Blue Light Absorption leads to conformational change in protein

  • Activates kinase domain

  • autophosphorylates itself

  • phosphorylates substrates

Describe the role of the Ja helix

links the light absorption domain to kinase domain

Describe the Inactivation of phototropins:

Phosphatase (PP2A) removes the activating phosphate

What is the response to blue light absorption

• Flavin mononucleotide absorbs blue light

• Conformational change in protein occurs

• Activates kinase domain

• autophophorylates itself

• phosphorylates substrates that produce phototropic response

What happens during phototropic response?

1. PHOT1 phosphorylates and inhibits the activity of ABCB19 • ABC19 is a protein in membrane enhances PIN1 efflux from cells • PIN1 is destabilized - prevents auxin efflux from cells

2. Auxin accumulates above cotyledons • hypocotyls cease growing/elongating (due to decrease in cell elongation)

3. PIN3 is localized to lateral walls of shaded side of stem • auxin transport resumes along shaded side of hypocotyl

4. Growth resumes (takes 15-20 minutes

Describe the directional movement of auxin

Auxin: exists in two forms

• protonated (IAAH)

• unprotonated (IAA-)

• depends on pH

PIN transporters

Review slide for PIN and ABC transporters

Describe chloroplast movements

Chloroplasts can move in any direction: mediated by PHOT1 & PHOT2

• When a small amount of light is given to part of a cell, the cell’s chloroplasts move to the irradiated area (accumulation respone)

• But when part of a chloroplast is irradiated with strong light, the chloroplast escapes from the light (avoidance response

Describe phot1 and phot2 mutants

phot1- weak accumulation response/good avoidance response

phot2- good accumulation response/ no avoidance response

What happens in a phot1/phot2 double mutant?

no accumulation OR avoidance response

What can we conclude about phot1 and phot2?

PHOT2: Key role in avoidance response

PHOT1 and PHOT2 required for accumulation response

Chloroplast movement is mediated by:

• dynamic reorganization of short actin filaments, called chloroplast actin filaments (cp-actin)

• located between each chloroplast and the plasma membrane

• These actin filaments are distinct from the cytoplasmic microfilaments

Describe Chloroplast Unusual Positioning 1 (CHUP1)

involved in blue-light mediated chloroplast movements

• chup1 mutants are defective in chloroplast movement in response to blue-light

• cp-actin filaments are not detected in chup1 mutant cells,

• But cytoplasmic actin filaments are clearly observed

• CHUP1 is normally attached to the chloroplast outer envelope & plasma membrane

• • Functions as an actin polymerization factor

• Evolved to assemble cp-actin filaments during blue light mediated chloroplast movements

How is the chloroplast influenced by strong blue light?

• PHOT2 causes rapid severing of the cp-actin filaments

• CHUP1 is reorganized and becomes asymmetrically redistributed to the “front end” of the chloroplast

• cp-actin filaments are polymerized at the interface between the plasma membrane and the chloroplast

• Polymerized cp-actin filaments are bundled and anchored to the plasma membrane by a plasma membrane protein, THRUMIN1

• Actin polymerization itself push CHUP1 forward, thereby pushing the CHUP1-bound chloroplast – myosin doesn’t seem to be involved.

All plants have…

CHUP1 protein- it is conserved

What are the characteristics of CHUP1?

plant-specific actin polymerization factor that functions specifically in cp-actin-based chloroplast movement