5.3 Plant Response to Light_Phototropism & Germination
5.4 Plant Response to Gravity_Gravitropism
5.5 Other Plant Growth Responses
5.6 Plant Response to Herbivore & Pathogen Attack
5_Plant-Hormones
Exercise 2a Photosynthesis and Cellular Respiration
Exercise 2b Plant Hormones
Plant Hormones
Overview
Daisies (Bellis perennis) facing the sun after opening in the morning, and they follow the sun through the day
böhringer friedrich, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons
Georgia Tech Organismal Biology
https://organismalbio.biosci.gatech.edu/
Did you have an idea for improving this content? We’d love your input.
Introductions
Learning Objectives
- list the five major plant hormones, including auxin, cytokinins, gibberellins, abscisic acid, and ethylene.
- List some of the plant behaviors regulated by these hormones including phototropism, gravitropism, germination, shade avoidance, thigmotropism, water stress, and pathogen/herbivory defense.
Key Terms
abscisic acid (ABA) - a plant hormone that induces dormancy in seeds and other organs
apical dominance - a phenomenon where the growth of lateral branches is inhibited by the main stem of a tree
auxin - a plant hormone that influences cell elongation (in phototropism), gravitropism, apical dominance, and root growth
cytokinin - a plant hormone that promotes cell division
dormancy - the inert state in an organism when most functions are suspended
ethylene - volatile plant hormone that is associated with fruit ripening, flower wilting, and leaf fall
germination - the process of a seedling emerging from a seed
gibberellin (GA) - is a plant hormone that stimulates shoot elongation, seed germination, and the maturation and dropping of fruit and flowers
gravitropism - growth toward Earth’s gravitational center
phototropism - directional bending of a plant toward a light source
plant hormone - signaling molecules that are produced at different parts of a plant. The interaction of these hormones regulates growth, development, and many other functions.
thigmotropism - directional growth of a plant in response to constant contact
Shade avoidance - plant response to grow away from shade saturated with far-red light
water stress - a condition caused due to drought, flooding, salinity, or acidity of water available to plant
A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Just as in animals, hormones are signaling molecules that are present in very small amounts, transported throughout the plant body, and only elicit responses in cells that have the appropriate hormone receptors. In plants, hormones travel large throughout the body via the vascular tissue (xylem and phloem) and cell-to-cell via plasmodesmata.
Potentially every cell in a plant can produce plant hormones. In contrast, many animal hormones are produced only in specific glands. Plants do not have specialized hormone-producing glands.
Hormones in Plant
Hormones regulate a variety of plant behaviors in response to different stimuli or environmental conditions. Let us first look at the most common plant hormones and their effects. Thereafter, we will look at the stimuli that provoke these responses and the pathways that regulate the response. Throughout this reading, you should aim to recognize both the stimuli that provoke a specific behavior, as well as the hormones and (when described) the signaling pathway that mediates the response.
Auxins: The Master Growth Regulator
The term auxin is derived from the Greek word auxein, which means “to grow.” Auxins are the main hormones responsible for cell elongation in phototropism (movement in response to light) and gravitropism (movement in response to gravity). Apical dominance (inhibition of lateral bud formation) is triggered by auxins produced in the apical meristem. Flowering, fruit ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of blue light (via phot1 and phot2) and red/far-red responses (via phytochrome). Synthetic auxin is used as a rooting hormone to promote the growth of roots on cuttings and detached leaves.
Cytokinins: Cell Division
Cytokinins promote cytokinesis or cell division. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins also delay senescence (aging) in leaf tissues, promote mitosis (cell division), and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, often in combination with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds, and cytokinins that promote bushier growth.
Gibberellins: Stem, Fruit, and Seed Growth
Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. Maturing grapes are routinely treated with GA to promote a larger fruit size. GAs is synthesized in the root and stem apical meristems, young leaves, and seed embryos. Gibberellins were first identified in the fungus, Gibberella fujikuroi.
GAs also delays senescence (aging) in leaves and fruit and break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Abscisic acid (described next) is a strong GA antagonist (works against it).
Abscisic Acid (ABA): Dormancy
Abscisic acid (ABA) causes abscission (dropping) leaves. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counteracts many of the growth-promoting effects of GAs and auxins. ABA also inhibits stem elongation, induces dormancy in lateral buds and seeds, and closes stomata in drought conditions.
ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Many plants require a long period of cold temperature before seeds germinate, which protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring.
ABA also regulates the short-term drought response: low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss.
This video describes the activities of both auxins: gibberellins and abscisic acid:
Ethylene: Aging
Ethylene promotes fruit ripening, flower wilting, and leaf fall. Ethylene is unusual as a hormone because it is a volatile gas (C2H4). Aging tissues (especially older leaves) and nodes of stems produce ethylene. The best-known effect of the hormone is the promotion of fruit ripening: ethylene stimulates the conversion of starch and acids into simple sugars. Ethylene also triggers leaf and fruit abscission, flower fading, and dropping. Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with the application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.
This video provides a quick summary of the different roles of ethylene in plants:
Two other plant hormones—jasmonic acid and salicylic acid—participate in plant-pathogen interactions.
Systemin: Anti-Herbivory
Systemin, named for the fact that it is distributed systemically (everywhere) in the plant body upon production, activates plant responses to wounds from herbivores. Systemin initiates the production of compounds, like jasmonic acid, which tastes bad and inhibits digestion by herbivores (causing a stomachache!) to deter them from continuing to eat the plant.
Methyl Salicylate (MeSa): Immune Response
Methyl salicylate (MeSa) helps regulate responses to infection by parasites or pathogens. When a parasite or pathogen infects a cell, there is a specific, localized response called the hypersensitive response (HR). Following this very localized response, the plant initiates a systemic (whole body) response called the systemic acquired response (SAR). MeSA is responsible for inducing the SAR in response to the HR.
This section provided a brief description of plant hormones. The next section focuses to explain how these hormones regulate the different plant behaviors.
Access for free at https://openstax.org/books/biology-2e/pages/30-6-plant-sensory-systems-and-responses
Plant Response to Light: Phototropism & Germination
Plants are generally capable of detecting and responding to at least three wavelengths of light: blue light, red light, and far-red light. The different wavelengths are detected by different photoreceptors, which are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. Together, the two are called a chromoprotein. The behaviors regulated by light stimuli include:
phototropism (movement toward the light),
stem elongation (growth),
shade avoidance (growth)
germination(seed sprouting).
Blue light: Phototropism
Plants are dependent on access to sunlight in order to fix carbon dioxide into sugars; thus, as stationary organisms, they must grow toward the sunlight in order to survive. Phototropism is a movement toward or away from light. Tropism means movement, and photo means light, so “phototropism” is “movement in response to light.”
Phototropins are the chromoproteins responsible for mediating the phototropic response. Other responses under the control of phototropins are leaf opening and closing, chloroplast movement within cells, and the opening of stomata to permit gas exchange (and thus photosynthesis).
Charles Darwin and his son Francis determined that light was perceived by the tip of the plant (the apical meristem), but that the response (bending) took place in a different part of the plant. They concluded that the signal had to travel from the apical meristem to the base of the plant to cause the bending. We now know that the detection of light in the apical meristem occurs via phototropin called phot1 and phot2, which specifically detect blue light (Figure 2.5.1.).
In 1913, Peter Boysen-Jensen cut off the tip of a seedling, covered the cut section with a layer of gelatin (essentially jello), and then replaced the tip (Figure 2.5.2.). The cut seedling bent toward the light. However, when he inserted an impermeable barrier between the tip and the cut base, the seedling could no longer bend in response to light. Later experiments showed that the signal traveled on the shaded side of the seedling. When the barrier was inserted only on the illuminated side, the plant could still bend towards the light. Therefore, the chemical signal was a growth stimulant because the phototropic response involved faster cell elongation on the shaded side than on the illuminated side. We now know that the chemical signal is the plant hormone auxin, also called indole acetic acid or IAA.
Auxin stimulates cell elongation on the shady side of the stem through a process called the acid growth hypothesis where auxin causes cells to activate proton pumps, which then pump protons out of the cells and into the space between the plasma membrane and the cell wall. The movement of protons into the extracellular space does two things:
- The lower pH activates expansin, which breaks the links between the cellulose fibers in the cell walls, making them more flexible.
- The high concentration of protons causes sugars to move into the cell, which then creates an osmotic gradient where water moves into the cell causing the cell to expand.
To sum up, the phototropic response works in the following fashion. The phototropin phot1 and phot2 are present in the plant apical meristem. When activated by blue light, phot1 and phot2 cause an accumulation of auxin on the shaded side of the plant. Auxin promotes cell elongation due to the weakening of the cell wall combined with an influx of water (which literally stretches the cells). Because the cell expansion occurs only on the shaded side of the stem, the plant bends away from the shade and toward the light.
This video provides a concise summary of auxin’s role in phototropism and the acid growth hypothesis.
Red Light: Growth, Germination, and Photoperiodism
Blue light promotes stem bending, but red light (as opposed to far-red light) promotes stem elongation or growth. This happens because red light indicates the full sun to a plant, while far-red light indicates that a plant is being shaded out by another plant, which is due to the fact that unfiltered, full sunlight contains much more red light than far-red light. Chlorophyll absorbs strongly in the red region of the visible spectrum, but not in the far-red region, so any plant in the shade of another plant on the forest floor will be exposed to light that has been depleted of red light and enriched for far-red-light. The non-shaded areas on the forest floor have more red light, and red light triggers plant growth. In other words, plants use the red vs far-red light detection to grow away from shade and towards the light. This response is called Shade avoidance.
The chromoproteins responsible for red/far-red light detection are called phytochromes (Figure 2.5.3). Phytochromes have two photo-interconvertible forms: Pr (phytochrome red) and Pfr (phytochrome far-red). The forms are named for what they are capable of absorbing next: the Pr form is capable of absorbing red light (~667 nm), and the Pfr form is capable of absorbing far-red light (~730 nm). When the Pr form absorbs red light, it is immediately converted to Pfr; and when Pfr absorbs far-red light, it is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of protein. Because phytochrome is in the Pfr state after exposure to red light, this means that exposure to red light turns the phytochrome “on.” Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system.
Plant Response to Gravity: Gravitropism
Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and roots grow down into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas, growth of the roots downward is called positive gravitropism.
Amyloplasts (also known as statoliths) are specialized cellular compartments that contain starch granules that move in response to gravity. The starch granules are heavy and literally fall to the bottom of the cellular compartment in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall, which causes auxin (produced by the root apical meristem just like at the shoot apical meristem) to redistribute to the new bottom of the root. In roots, a high concentration of auxin inhibits cell elongation, slowing growth on the lower side of the root, while cells develop normally on the upper side and causing the root to bend toward the high concentration of auxin and thus causing the root to be grown down. Note that this is the exact opposite of auxin’s effect on shoots, where a higher concentration stimulates cell expansion, causing the shoot to bend away from the higher concentration of auxin. After the root begins to grow vertically again, the amyloplasts return to their normal position, and auxin is equally distributed on both sides of the root tip (Figure 2.5.5.).
Other Plant Growth Responses
Other plant responses to different growth-related stimuli include apical dominance, leaf abscission, fruit growth, and fruit ripening.
Many plants grow primarily at a single apical meristem and have limited lateral branches (which would result in multiple meristems). This phenomenon is called apical dominance and is regulated by the presence of auxin at the apical meristem. Auxin is required for the function of other growth-regulating hormones such as cytokinins, which promote cell division but only in the presence of auxin. Abscisic acid in the lateral buds inhibits the production of auxin, and removal of the apical bud will release this inhibition of auxin, allowing the lateral buds to begin growing (Figure 2.5.6.).
Some plants drop leaves in response to changing seasons (based on temperatures, photoperiod, water, or other environmental conditions). This process is called leaf abscission and is regulated by interactions between auxin and ethylene. During the growing season, the leaf produces high levels of auxin which blocks the activity of ethylene; however, as the seasons change the leaf produces lower levels of auxin. Lower levels of auxin permit ethylene to initiate senescence (aging) and ultimately programmed cell death at the site of leaf attachment to the stem, allowing the leaf to fall off in a controlled manner without harming the rest of the plant.
The growth of fruits in size is promoted by gibberellins. The artificial addition of gibberellins to fruits while still on the plant will cause them to grow larger than they ordinarily would. And once fruits have grown to the appropriate size, they begin ripening; this process is stimulated by ethylene. Fruit ripening is a form of senescence (aging), so the role that ethylene plays in fruit ripening is very similar to its role in leaf abscission.
Plant Responses to Water or Water Stress (Drought)
Though germination can be controlled by the phytochrome system, the seeds of some plant species instead rely on the imbibition (intake) of water to initiate germination (shown below). Intake of water activates the hormone gibberellin, which then signals to transcribe the gene encoding amylase, an enzyme that breaks down starches stored in the seed into simple sugars, and then germination proceeds (note these final steps are identical to what occurs in phytochrome-regulated germination). When water is absent, germination in this pathway is blocked by a hormone called abscisic acid (also called ABA), which inhibits the activity of gibberellins. Thus, gibberellins and abscisic acid act in opposition to regulating the germination response (Figure 2.5.7.).
As briefly noted above, activation of phot1 and phot2 by blue light cause stomata to open to permit gas exchange so that photosynthesis can occur. But in addition to sunlight and carbon dioxide, photosynthesis also requires water. When the plant is dehydrated due to drought, the hormone abscisic acid (ABA) causes stomata to close, preventing gas exchange and halting photosynthesis. This is the stomata closing response to stress, and this response to abscisic acid occurs even if blue light is present (Signaling from drought via ABA overrides the signaling from blue light via phot1) (Figure 2.5.8.).
In drought conditions, the immediate response is closing stomata, as noted above. However, because closed stomata prevent gas exchange, plants will die if the stomata remain closed for too long. Thus, if the drought persists for too long, the plant will begin sacrificing certain areas by allowing the leaves or stems to die in localized regions—also known as localized cell death. This process may be regulated by the hormone ethylene, which can induce localized cell death under certain conditions.
Plant Responses to Touch: Thigmotropism
Thigmotropism is movement in response to touch. Different plant species have different types of responses to touch, including slow thigmotropism and fast thigmotropism.
Slow thigmotropism describes a plant response to a touch stimulus that affects the direction of growth, such as vines that wrap around or grow along with structures. Slow thigmotropism is regulated by auxin, which redistributes in the elongating stem in response to the touch, ultimately resulting in differential cell elongation (much like the role auxin plays in phototropism).
Fast thigmotropism only occurs in a few plant species and describes a rapid plant response to touches, such as the way the Venus flytrap snaps shut to trap an insect or the way mimosa plants clamp their leaves closed in response to touch. This response occurs as a result of an electrical signal that causes rapid changes in cell turgor pressure and thus rapid movement of structures associated with those cells.
This video shows an example of slow thigmotropism (mediated by auxin) in morning glory plants, which require a support structure of some type to grow optimally. The time-lapse images were taken at 10-minute intervals.
And this video shows an example of fast thigmotropism (mediated by membrane potential) in a Venus flytrap.
Access for free at https://openstax.org/books/biology-2e/pages/30-6-plant-sensory-systems-and-responses
Plant Response to Herbivore & Pathogen Attack
Plants face two types of enemies: herbivores and pathogens. Herbivores both large and small use plants as food and actively chew them. Pathogens are agents of disease. These infectious microorganisms—such as fungi, bacteria, and nematodes—live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers.
The first line of defense in plants is an intact and impenetrable barrier. Bark and the waxy cuticle can protect against predators. Other adaptations against herbivory include thorns—modified branches—and spines—modified leaves. They discourage animals by causing physical damage and inducing rashes and allergic reactions. A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes.
Herbivory
When herbivores breach a plant’s physical defenses, chemical responses are induced to deter further herbivory through a couple of different mechanisms, depending on the plant species: volatile compounds or systemin and jasmonic acid.
In some plant species, damage to tissues from insects induces the production of a hormone called systemin. Systemin is named for the fact that it travels throughout the plant (systemic) after it is produced locally (though this is true of all hormones, of course). Systemin activates the production of jasmonic acid, which induces the transcription of proteinase inhibitors. Proteinase inhibitors both taste bad and prevent the breakdown of proteins in the herbivore’s gut, thus making the insect sick and deterring further herbivory.
In some plant species, damage to tissues from insects induces the production of volatile chemical attractants—or volatile compounds—that are released into the air and attract certain parasites of the insects that are eating the plant. These parasites are often parasitoid wasps that lay their eggs inside of the insect-eating the plant; when the eggs hatch into larvae, the larvae eat the insect from the inside out and kill it.
This video describes some of the chemical signalings that can occur between plants to communicate about herbivory and other threats.
Pathogens and Parasites
Plants demonstrate two sequential responses to parasites and pathogens: the hypersensitive response, which then induces the systemic acquired response (SAR).
The hypersensitive response occurs when a pathogen infects a plant cell. The response occurs via recognition of specific pathogen molecules, meaning the plant has specialized pathogen-specific receptors capable of detecting the pathogen molecules. Activation of these receptors induces a three-pronged, localized stress response:
- the plant produces phytoalexins, toxins that help kill the pathogen,
- the cells infected by the pathogen are physically walled off to prevent pathogen escape,
- and the cells infected by the pathogen undergo programmed cell death, removing those cells as a food source for the pathogens.
In addition, activation of the hypersensitive response induces the production of the hormone methyl salicylate (MeSA), which then induces activation of the systemic acquired response (SAR). The SAR is only induced in response to the hypersensitive response. The SAR activates transcription of general “pathogenesis-resistance” genes, which are not pathogen-specific (unlike in the hypersensitive response) but serve as a general defense against pathogenic infection. The SAR is slower than the hypersensitive response, and it also differs in that it is systemic instead of localized to the site of the infection.
This video describes a general overview of the HR and SAR, though it does not mention the signaling molecules by name.
Access for free at https://openstax.org/books/biology-2e/pages/30-6-plant-sensory-systems-and-responses
Unit 2 Lab Exercises
Exercise 2a: Photosynthesis and Cellular Respiration
Students focus on the processes by which plants convert light energy into chemical energy and how they utilize this energy for growth and maintenance. It includes detailed explanations and diagrams to illustrate these essential biological functions.
Exercise 2b: Plant Hormones
Students focus on understanding the roles and effects of different plant hormones, such as auxins, gibberellins, and cytokinins, on plant growth and development. It includes activities to observe and analyze these hormones’ impact on various plant processes.
Attributions
Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Introduction to Organismal Biology at https://sites.gatech.edu/organismalbio/ is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Botany (Ha, Morrow, and Algiers) is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers.