How hormones work - Signal Transduction Pathways
Light and the Growth Response
Other Plant Growth Hormones
Plant Growth Responses and MovementsGrowth Responses
- Apical Dominance and Auxin
- Roots and Gravitropism
Movements - Nastic Responses
- Solar Tracking
- Thigmonastic Response
Light and Flowering
- The Dark Clock and Flowering
- Phytochrome C
General model. Hormone binding to a specific receptor activates chemical and transport steps that generate second messengers, which trigger the cell's various responses to the original signal.
In the diagram above, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside. Environmental stimuli can also initiate signal pathways. For example, phytochrome conversion is the first step in the transduction pathways that lead to a cell's responses to red light.
Specific example: a hypothetical mechanism for auxin's stimulation of cell elongation. (1) The hormone binds to an auxin receptor, and (2) this signal is transduced into second messengers within the cell, inducing various responses. (3) Proton pumps are activated, and secretion of acid loosens the wall, enabling the cell to elongate. (4) The Golgi apparatus is stimulated to discharge vesicles containing materials to maintain the thickness of the cell wall. (5) The signal-transduction pathway also activates DNA-binding proteins that induce transcription of specific genes. (6) This leads to the production of proteins required for sustaining growth of the cell.
Charles Darwin was the first to discuss how plants respond to light. He found that the new shoots (coleoptile) of grasses bend toward the light because the cells on the dark side grow faster that the lighted side.
[If a seedling is allowed to grow in the dark it will become tall, skinny, and white. Such a seedling is said to be etiolated]
Darwin concluded that a plant hormone made in the coleoptile tip could somehow move down and induce other cells to elongate. Other researchers using a variety of cleaver experiments determined that this hormone was Auxin (IAA - indolacetic acid).
Auxin: Its Structure and Roles
Auxin's structure is very similar to the amino acid tryptophan.
Tryptophan..................... Indolacetic acid (auxin)
Auxin - its Role and Effects:
Auxin promotes cell elongation. Recently divided cells are small, square, and densely packed. Auxin causes these cells to pump H+ into their cell walls. The higher pH activates enzymes to break cross linkages in the wall allowing the ever present turgor pressure to elongate these cells.
Auxin is involved in absorption of vital minerals and fall color. As a leaf reaches its maximum growth auxin production declines. In deciduous plants this triggers a series of metabolic steps which causes the reabsorption of valuable materials (such as chlorophyll) and their transport into the branch or stem for storage during the winter months. Once chlorophyll is gone the other pigments typical of fall color become visible. Another hormone, ethylene, is important in the events necessary for leaf fall.
The base of the leaf's petiole
farthest from the blade, where the leaf attaches to its
branch has a layer of special cells called the abscission
Auxin and Apical Dominance. Auxin produced in the dominant growing tip (usually uppermost) inhibits growth in lateral branches (those below it) This results in the typical triangular shape of conifers.
Auxin's inhibitory role in gravitropism. Roots always grow down into the soil regardless of how the seed lands. Events which occur in root cells due to gravity result in the accumulation of auxin on the lower side of the root. The concentrations of auxin rise to a level that actually inhibit growth or elongation in the lower cells while the cells above outpace their counterparts causing the root to bend down.
How does auxin become concentrated along the lower side of a horizontal root?
Auxin it seems is actively pumped from cell to cell until it reaches its lower destination. The auxin pumps are activated by a calcium-protein complex, the protein being a well known activator called calmodulin. Large starch granules, amyloplasts, drift down through the cytoplasm coming to rest and touching the endoplasmic reticulum in the lower half of the cell. ER stores calcium ions which are released once contacted by the amyloplast. The free Ca2+ combines with calmodulin which finally activates auxin and calcium pumps.
Polar auxin transport: a chemiosmotic model.
In growing shoots, auxin is transported unidirectionally, from the apex down the shoot. Along this pathway, the hormone enters a cell at the apical end, exits at the basal end, diffuses across the wall, and enters the apical end of 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. (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 molecules of the same size. (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.
Gibberellin: Role and Effects
Originally gibberellin was found in a fungus, Gibberella fujikuroi, which caused rice seedlings to grow so tall and spindly that they fell over. Today gibberellin has many important commercial uses. Find 2 and attach to your notes for this section.
- Like auxin it promotes cell elongation.
- Gibberellin acts as a chemical messenger, hormone, to stimulate the synthesis of enzymes such as a-amylase and other hydrolytic enzymes important during germination of seedlings in order to insure release of stored nutrients.
Cytokinins: Role and Effects
Zeatin and several other related molecules were first found in coconut milk and corn kernels
[Plant cells exhibit totipotency - the ability to dedifferentiate and develop into any tissue necessary.]
Plant tRNA's contain cytokinins: but the exact molecular mode of action is not known.
Ethylene: Role and Effects
Ethylene CH2CH2 is a gas and is produced by ripening fruit from the amino acid methionine.
molecular structure of ethylene
1. Ethylene controls the ripening of fruits
2. During the initial growth of a seedling, ethylene appears to be responsible for the hooked shape of the developing shoot. This helps protect the young tender leaves from the wear and tear of growing up through the soil.
3. Ethylene may insure that flowers are carpelate (female) while gibberellin confers maleness on flowers
4. Ethylene promotes the production of cellulases prior to leaf fall and abscission.
Commercially ethylene is used to artificially speed ripening of fruit prior to marketing. Tomatoes if picked red and ripe would spoil before reaching market, so most are picked green then reddened with ethylene rather than the slow natural way.
Abscisic Acid (ABA) Role and Effects
1. ABA induces winter dormancy by suppressing mRNA production. Without mRNA auxin and gibberellins are no longer produced.
2. ABA enters guard cells during periods of water stress, and brings about the outward transport of potassium ions (K+). Where salt goes so goes water so guard cells lose turgor pressure and collapse which, because of their unique shape, closes the stomata, preventing the further loss of water.
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.
Gravitropism - non-reversible growth toward gravity. Young roots tend to grow down into the soil thanks to the combinations of gravity and inhibitory effects of high auxin concentrations (see auxin)
In a hypothetical cellular mechanism, falling (amyloplasts?) statoliths in the horizontal root cause the release of calcium ions, which activate calmodulin, a well known enzyme activator. The activated enzymes, in turn start calcium and auxin pumps working in the nearby plasma membrane. Both Ca2+ and auxin leave the columella cells and migrate to the lower margin of the root cap, whereupon the auxin begins its journey along the lower side of the root--its accumulation there leading subsequently to the inhibition of cell elongation.
Thigmotropism - nonreversible growth toward touch stimulus. Special young slender branches called tendrils respond to touch by growing toward the contacting surface and entwining around it. Both auxin , light, and cell division are probably involved. (See article-
"Sleep movements". In many plants, during the last glimmerings of day, leaves bend up and together vertically (like hands in prayer) This has been shown to reduce the loss of heat during the night. When light returns the leaves fold down again for maximum exposure to the sun. A special structure at the base of the leaf's petiole, called a pulvinus, contains motor cells specialized in pumping potassium ions into nearby tissues changing the turgor pressure. The result is the nastic movement.
Solar Tracking- "Heliotropism". This nastic response, often seen in sunflowers, causes the plant to act as if it were a astronomical tracking device aimed at the sun. In fact the flowers are often parabolic in shape, ideal for receiving the maximum amount of light energy. Growth is not involved in this fast and reversible response (which is repeated in an identical fashion day after day.)
Thigmonastic response. Some plants have the ability to respond quickly to touch by collapsing their leaves, often together (at the base of the petiole). The sensitivity plant, Mimosa pudica may be responding to hot, dry conditions in order to conserve water or prevent leave damage. Changes in turgor pressure is involved.
In many plants the time for flowering is critical for their reproductive success. (This is usually not the case for plants found in the tropics. Why?)
The response to changing lengths of light (day) and dark (night) is known as Photoperiodism. The question then is how do plants know when to bloom, since to flower in the dead of winter would be suicidal.
The critical factor is the length of night (uninterrupted darkness). The plant's metabolic cycles can somehow count the hours. This was confirmed by interrupting the period of darkness with a few minutes of light. Long Day-Short Night plants (treated with a flash of light to breakup a long night) will begin to flower during winter. Those plants which need Short Days (actually a Long Night) to flower may be prevented from doing so by the same procedure, fooling them by apparently resetting their counting (clock) mechanism after the flash of light.
The wavelength of the light is also critical. Red light (predominant during the day) is necessary to reset the clock which counts the hours of darkness. Far-red light will reverse the effects of red light, but has no direct effect by itself.
The chemical which detects and is changed by red or far-red light is called phytochrome. The form of phytochrome which absorbs red light is designated Pr, and the alternate form which is sensitive to far-red light is Pfr. The conversions between these two forms of phytochrome can be expressed by the following:
Pr is the more stable form and slowly accumulates spontaneously during the night (or is synthesized etc.). Pfr predominates during the day since sunlight has more red light than far-red.
The test tubes in this diagram contain solutions of the two photoreversible forms of phytochrome. Absorption of red light causes the bluish Pr to change to the blue-greenish Pfr. Far-red light reverses this conversion. In most cases, it is the Pfr form of the pigment that triggers physiological responses in the plant.
Phytochrome is only an entraining mechanism, a link between the actual clock (mechanism unknown) and the hormone (yet to be discovered, but presumptively called florigen) which activates the flowering process.
It has recently come to my attention that many plants are simply not light sensitive which means that the change in concentration of other hormones such as auxin are involved in the flowering process. This area of botany is under active investigation.
updated on Oct. 27, 2003