BIO-0014 Lecture Notes - Lecture 19: Phototropism

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30 Jan 2014
Bio 14 Lecture 19 Notes
Auxin as a signal molecule
When a plant is harmed, sugar can be diverted by the phloem to an area that is working
properly instead of an area that is damaged and not working properly
Darwin and his wife were the first to study the seeds and realize that the plants were
If they wanted to study plant growth, they should study something that is growing
very quickly
Found that when the seed is underground, there is a flimsy, delicate, very sugary
leaf coming out of the cell
oTend to be degraded by sand particles and environmental factors
To deal with this, evolution has made it so that new grass leaves that are produced
are covered with a sheath – sheath is called a coleoptile
oProtects the grass leaf from environmental factor
oHas to grow faster than the grass leaf – creates a tunnel for the leaf to
grow up in to
Coleoptile Growth
Usually, an intact seedling will have a curvature because they grow towards a
light source – illuminated to the side, bent to the side
oStudied by Darwin – 1880
oColeoptiles act as a function of light
oIf you cut off the tip of one or put an opaque cap on the tip, there will be
no curvature
oThis led to the conclusion that growth stimulus is produced in the
coleoptile tip and is transmitter to the growth zone
Cells that respond to light are much below the tip, known as the
growth zone
Light is perceived in one part of the plant, and is communicated to
another part of the plant
Following experiments
o1913 – Boysen-Jensen
oWhat kind of info is this transmission from tip to growth zone
(mechanically, physical, chemical?)
oCut off tip, placed an agar block in the gap, and then put tip back
Hypothetically, the message should pass through the agar
oFound that the plant still curved
oThe only time it wouldn’t work is if there was a water-impermeable
The Agar Block induces plant growth
Auxin is a plant growth hormone – initially discovered in coleoptiles
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