Day 8: Case Study - and mechanisms

What are coleoptiles

the first part of the shoot to emerge in monocot seedlings

What was the case study example

coleoptile bending

Who investigate the location of light perception in coleoptiles

Charles Darwin and his son Francis

Question of perception of light in coleoptiles

where is light sensed by coleoptiles to allow phototropism

Conclusion of perception of light in coleoptiles

light is sensed by coleoptile tips only

Peter Boysen-Jensen

investigated possible pathways for messages

What possible pathways for messages did Peter Boysen-Jensen investigate

1. water-soluble (hydrophilic) chemical

2. lipid-soluble (lipophilic) chemical

3. electrical signal

How did Peter Boysen-Jensen test message pathways

1. control - block all possible message types

2. hydrophilic chemical - used permeable agar

3. lipophilic chemical - used butter

4. electrical signal - used foil

Peter Boysen-Jensen’s conclusion on chemical pathways

a water-soluble chemical carries the message from the tip to the growing cell

Arpad Paal

hypothesized differential distribution of messenger

Conclusion of Arpad Paal on differential growth

light causes the accumulation of the chemical on the shaded side of the coleoptile

What does the name auxin mean

to grow

How does auxin affect cells elongation

1. it influences growth in shoots and roots - but in opposite ways

2. differential accumulation controls direction of growth

Frits Went

looked at the effect of messenger concentration

How is degree of response controlled

1. auxin diffuses into agar block

2. more time or more tips on agar = greater chemical concentration

3. agar block placed on one side of coleoptile

4. angle of bending increased with greater concentration of messenger

Conclusion of how degree of response controlled

concentration of messenger affects strength of response

How does auxin move in shoots and type of transport

moves down the shoot from the tip, polar transport

Effect of increased auxin in shoots

increased auxin causes increased cell elongation in shoots

Does auxin create greater growth on sunny or shady side

shady side

How does auxin move and type of transport

auxin moves from the sunny side to the shady side, lateral transport

Result of auxin moving from sunny side to shady side

differential concentration

Current hypothesis for cell elongation

acid-growth pathway

How do roots perceive gravity

by using amyloplasts known as statoliths

What doe statoliths control

the distribution of auxin

How do statoliths create unequal distribution of auxin

by pressing the “sides” of cells

In roots lower auxin leads to 

more cell elongation

Does lower of higher auxin levers create more cell elongation

lower levels

What are the two possible hypotheses to explain why more auxin is on the dark side

1. shoot tips make more auxin on the dark side than on the light side

2. auxin is made equally by all cells, but transported from the light side to the dark side

Conclusion for why more auxin on dark side

auxin is made at equal rates throughout the tip but transported to shaded side

Does phototropism occur for all wavelengths(colors)

no

What is light sensed by

PHOT1 and PHOT2

What are PHOT1 and PHOT2

they are membrane-bound receptors called phototropins