Plant hormones and phototropism
Introduction
When a plant bends towards the light, we call that a positive phototropic
response. The structural and physiological mechanisms involved in this response
still pose questions for plant researchers. Our lab today will help us explore what
is happening in this response. The hormone involved in this response is an
auxin. This chemical messenger is found in the apical meristem regions of both
root and stem tips. The hormone is associated with plant growth. Its chemical
structure you see below is similar to the amino acid tryptophan.
What do we know about how this chemical messenger works?
1. Auxin is involved in the elongation of cells in plants. Auxin causes a drop in
the pH in the region of the cell walls between plant cells. This drop in pH
triggers an enzyme reaction that breaks the carbohydrate linkages of the cell
wall. This permits the turgor pressure in the cell to increase, thus elongating
the cell.
2. All plant cells must absorb minerals needed for metabolic processes such as
magnesium for chlorophyll, phosphorus for ATP, etc. As a plant begins to
decrease its metabolic processes for winter, auxins trigger a series of metabolic
steps that reabsorb valuable minerals into storage spaces during the dormant
months. One result of this auxin effect is the breakdown of chlorophyll and the
increased visibility of the other pigments during the fall.
3. Auxin is produced in the growing tip of the plant. As a result, excessive growth
this apical dominance.
4. Phototropism - plants grow in response to light (nonreversible growth toward
light stimulus. Light causes auxin to move laterally in the apical meristem. An
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of lateral branches is inhibited by
unknown (yellow pigment) receptor absorbs blue light and helps transport auxin
to the unlighted side where cells respond by elongating auxin is quickly
inactivated by enzymes further down the stem.
Polar auxin transport: a chemiosmotic model.
Auxin is transported unidirectionally when a shoot is exposed to light from one
direction only. Along this pathway, the hormone enters a cell at the lighted end,
exits at the shaded end, diffuses across the wall, and enters the next cell. A pH
difference between the cell wall (acidic at about pH 5) and the cytoplasm (pH 7)
contributes to auxin transport. In the pH 7 environment of the cell, auxin is an
anion. Here is a description of a proposed step-by-step process.
1. When auxin encounters the acidic environment of the wall, the molecule picks
up a hydrogen ion to become electrically neutral.
2. As a relatively small, neutral molecule, auxin passes across the plasma
membrane.
3. Once inside a cell, the pH 7 environment causes auxin to ionize. This
temporarily traps the hormone within the cell, because the plasma membrane is
less permeable to ions than to neutral (aka hydrophobic) molecules of the same
size. (remember the nonpolar nature of the cell membrane)
4. ATP-driven proton pumps maintain the pH difference between the inside and
outside of the cell.
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Plant shoots display positive phototropism: when
illuminated from one direction, the shoot proceeds to
grow in that direction.
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5. Auxin can only exit the cell at the basal end, where specific carrier proteins
are built into the membrane. The proton pumps contribute to this auxin efflux by
generating a membrane potential (voltage) across the membrane, which favors
transport of anions out of the cell. Now in the
acidic environment of the wall again, auxin picks
up a hydrogen ion and enters the next cell as an
electrically neutral molecule. Polar auxin
transport is one specific application of a basic
mechanism of energy coupling in cells. This
mechanism, chemiosmosis, uses proton pumps
to store energy in the form of an H+ gradient
and membrane potential, and then taps this
energy source to drive cellular work.
Procedure:
Day One:
1. Obtain three egg cartons and seeds. The types of seeds available are oat, wheat and
barley.
2. Place an equal amount of soil in each of the egg cup. Add 10 ml of water to each egg
cup.
3. Place two seeds into each egg cup
Each team should have a total of three egg cartons.
Be sure that any opening of the carton is covered. No light should enter the egg
carton.
Day Two:
1. In a darkened room, open the egg cartons and check on the growth of the
seedlings. When the seeds germinate and are about 1-2 cm in length, add the
auxin to ONE side of each seedling (coleoptile). Apply the auxin using a
toothpick. Seedlings to which auxin is applied should be fairly vertical.
Egg Cartons 1, 2, 3
Cups A: add auxin full strength to one side of each coleoptile
Cups B: add auxin in a 5000:1 strength to one side of each coleoptile
Cups C: NO auxin added
Replace cartons in the darkened area. Again be sure that no light enters the egg cartons.
Day Three:
Check and measure the curvature of each seedling. Use a protractor to measure the
degrees of curvature from vertical. Record your data in a Data Table. Take photographs.
References
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Tropisms.html#The_Mechanis
m_of_Phototropism
http://149.152.32.5/Plant_Physiology/morephototropism.html
http://www.biologie.uni-hamburg.de/b-online/e31/31b.htm
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http://plantphys.info/Plant_Physiology/phototropism.html