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 of lateral branches is inhibited by 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 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.
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.
Plant shoots display positive phototropism: when illuminated from one direction, the shoot proceeds to grow in that direction.
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_Mechanism_of_Phototropism
http://149.152.32.5/Plant_Physiology/morephototropism.html
http://www.biologie.uni-hamburg.de/b-online/e31/31b.htm
http://plantphys.info/Plant_Physiology/phototropism.html