Tropisms and Nastic Movements

Unlike animals, plants cannot move to more favorable locations. Instead, plants survive by adjusting their growth to their local environment. A major way this is done is by sensing the directions of environmental signals such as light and gravity. This sensory information is then used to orient the direction of growth toward or away from a stimulus in a process called a tropism. By these mechanisms, shoots grow up from the ground and into the light. This enhances photosynthesis and biomass by increasing the amount of sunlight absorbed by chlorophyll. The raised stature of the plant also promotes pollination and seed dispersal, and increases plant competitiveness.

Tropisms are different from nastic movements. Like tropisms, these plant movements are influenced by environmental cues. But the direction of a nastic movement is independent of where the signal comes from, and most such movements are temporary. Nastic movements are more specialized in function and distribution than tropisms. For example, some insectivorous plants capture prey by moving trap organs together.

Tropisms should not be confused with tactic movements found in many microorganisms, such as a unicellular green alga that moves toward the light (phototaxis). Because plants are not motile , only part of their body grows in the direction of a stimulus.

Tropisms

In tropisms, the growth of a plant organ is oriented by an environmental signal. Usually the organ grows toward or away from the stimulus. The former is considered to be a positive tropism, whereas growth away from a stimulus is a negative tropism. Houseplants on a windowsill grow toward the light by positive phototropism. Stems that emerge from seeds buried in the soil grow upward away from gravity by negative gravitropism. Tropisms can also occur at other angles with respect to a stimulus. Modified stems, called rhizomes, grow along the surface of the soil at right angles to gravity, such as in iris plants. Phototropism and gravitropism are by far the most important and widespread of tropisms in plants. In some plants and organs, other physical stimuli, including touch, temperature, and water, can orient growth as well.

Tropisms allow plants to adjust the direction of growth when their environment changes. For example, when a seedling is turned on its side, the root grows gradually downward creating a curvature, an example of positive gravitropism. This occurs in the growing region of the root, a region located close to the root tip. Once the root tip points downward again, the root stops curving and the subsequent growth is straight. After a region responds to a stimulus its orientation is usually permanent. For example, the curvature remains for the life of the root.

In most cases, only the growing regions of the plant are capable of tropisms. There are many such regions in a plant. The tips of stems and roots contain meristems, regions where new growth occurs. Cell divisions in meristems contribute to the elongation of stems and roots, and form new stem branches. Each stem branch is capable of phototropism and gravitropism. Their collective responses help determine the overall shape of the shoot.

By definition, a tropism involves a stimulus that contains some directional information. With gravity, both the direction and strength of the stimulus are uniform. In contrast, the direction of illumination constantly changes, and even heavily shaded plants receive at least some light from all directions. Two conditions must be fulfilled for a tropism to occur in such situations. First, the stimulus must exist in a gradient with respect to the plant. Thus the light is brighter on one side of a shoot, or there is more water on one side of a root. Second, this gradient must last long enough to influence growth. Because a tropism results in a permanent change in position, it would be wasteful for plants to respond to short-lived changes in the environment.

All tropisms include two major stages, sensing and the growth response. The direction of the signal must first be sensed. Sensing means that physical information in the environment is somehow converted into biological information in the plant. This biological information is then interpreted in the growing zone resulting in guided growth. When a root is placed on its side, it curves downward because the upper side of the root grows faster than the lower side. The end result is that directional information about a physical signal is translated into different rates of elongation to produce directional growth.

Much of the research done on tropisms tries to understand how sensing and differential growth take place. Although scientific understanding of these processes is still incomplete, there have been some important advances, especially since the late 1980s and the development of genetic analytical techniques.

Phototropism.

Phototropism is one of the most significant tropisms for plant survival because it positions shoots where more light for photosyn-thesis is available. It is especially important during seedling emergence and when plants are shaded unequally.

Sensing. One of the first important studies of plant tropisms was of phototropism by Charles Darwin and his son Francis over a century ago. They tried to determine where light is sensed in the coleoptile, which is a leaflike sheath that covers and protects emerging grass seedlings. As in current phototropism experiments, they exposed the seedling to light from just one side. Coleoptiles whose tips were cut off or were kept dark by a hood did not grow toward the light. However, if the tips were covered by transparent material or if the base of the coleoptile was kept dark, then the coleoptiles grew towards the light. They concluded that the tip of the coleoptile is largely responsible for sensing the light during phototropism.

However, coleoptiles are an unusual organ found only in grasses. In stems, the most common phototropic organ, the site of sensing seems to be more spread out. Even stems whose tips were covered and darkened were capable of bending toward a light from the side.

One reason why the precise site of stem sensing is unclear is that the pigment responsible was unknown for many years. Light acts when it is absorbed by a pigment-containing molecule. For many years, scientists knew that blue light is the most effective color in causing phototropism. Green or red light were either ineffective or caused only slight bending. Scientists tried to find pigments isolated from plants that especially absorbed blue light, but that did not absorb green or red light. Two types of pigments had these characteristics, carotenoids and flavin-containing molecules. But this information was not enough to identify the particular molecule responsible. One reason for this is that there are many types of blue-light responses in plants in addition to phototropism.

As in so many other areas in plant biology, recent studies using the model plant Arabidopsis resulted in rapid progress in phototropism. A mutant was isolated, non-phototropic 1 (nph1), which fails to grow towards blue light. The affected gene was found to code for a protein that binds to a flavin molecule. It is likely that the combination of this protein and a flavin pigment molecule is responsible for phototropism. The identification of this molecule provides an important foundation for future research.

Growth response. The Darwins showed that light sensing takes place in the tip of the oat coleoptile and that phototropic curvature occurs several millimeters below the tip. This separation suggests that there is communication between these two regions. Subsequent studies provided evidence for a chemical signal that moves from the tip to the base. This signal can move through the water in a gelatin block. In 1926 Fritz Went isolated this chemical and named it "auxin." In the 1930s, auxin was identified as the molecule indoleacetic acid (IAA). IAA was found to cause many effects in plants in addition to phototropism, and auxin is now recognized as a major plant hormone.

Auxin moves from the tip of a coleoptile towards the base. Under some circumstances, auxin can also move from one side of the organ to the other. A major effect of auxin is to stimulate stem elongation. When coleoptiles are illuminated, auxin moves from the lighted side to the dark side. This lateral movement occurs in or close to the tip of the coleoptile. The auxin then moves down to the growing part of the coleoptile. The presence of more auxin in the dark side causes that side to grow more than the illuminated side. This causes the coleoptile to curve towards the light. Similar events appear to occur in stems.

In summary, a light gradient is sensed by a flavo-protein pigment molecule. Light absorption somehow increases the amount of auxin on the dark side of the growing zone, which causes more growth on that side and curvature towards the light.

Phototropism and solar tracking. It is often easy to detect the effects of phototropism in nature. It can be seen when stems emerge from the ground, or when part of a plant is more shaded than another. Unequal shading can be produced by other plants, or by objects such as rocks, logs, and walls. In contrast, phototropism is rare in a mature plant that is growing in an open, sunny area. This is because the movement of the sun during the day and the season is too fast and variable to allow phototropism to develop.

However, in a few plants, the leaves do follow the sun during the day. The leaf stalk twists during the day so that the leaf blade keeps facing the sun. The result is an increase in photosynthesis. This phenomenon, known as solar tracking, is not a tropism because there is no permanent change in the direction of growth.

Gravitropism.

Gravitropism helps plants flourish. The importance of gravitropism can be illustrated by the maize (corn) plant. The upward growth of the stem raises the leaves up. The base of each leaf is also oriented by gravitropism at a set angle. The result is that the leaves become located in the position that exposes them to the most light. The raised stature also allows the plant to compete with other plants for sunlight. The upward growth also positions the pollen-producing flowers at the top of the plant where it can be carried by the wind to pollinate the female flowers (the silks). Some roots grow straight down, but others only do so after the roots reach a certain length. The result is well-branched root system that is distributed throughout the soil in a coordinated manner. This positions the roots near new supplies of water and minerals. It also anchors the plant to prevent it from falling over. Gravitropism helps optimize the growth of all of parts of this maize plant.

Gravitropism has a profound effect on the shape and form of many plants in addition to maize. Its influence is obvious in plants with pronounced vertical stems such as pine trees. Careful observation will also reveal subtler effects of gravitropism in other plants. Many stems and branches that are not vertical still grow at a more or less set angle with respect to gravity. This angle may vary with age and lengthwise position, but if a regularity is observed, it is likely to represent gravitropism. Indeed, gravitropism probably shapes plant life more than any other tropism.

Sensing. How might a plant sense the direction of gravity? Unlike light, gravity cannot be absorbed by a molecule. Instead it must act on some dense structure. A century ago, German scientist Gottlieb Haberlandt observed that gravitropic organs contain heavy starch-filled bodies that fall or sediment. These bodies are organelles called amyloplasts. They are a type of plastid, special organelles in plants that include chloroplasts . Starch is dense and thus amyloplasts are heavy. Amyloplasts are found in many different locations in plants. But they only sediment in specific locations, such as the rootcap at the tip of roots and the starch sheath in the growing zone of stems. Haberlandt proposed that the falling of amyloplasts triggers gravity sensing. Most data support Haberlandt's hypothesis. For example, all natural, gravitropic organs have sedimented amyloplasts. And stems of the Arabidopsis "scarecrow" mutant are not gravitropic probably because they lack both a starch sheath and sedimented amyloplasts.

But this hypothesis was challenged when several mutants were found that do not have any starch but that are still gravitropic. This shows that starch is not necessary for gravitropism. However, the gravitropism in these mutants is defective, and they are much less sensitive to gravity than normal plants. This suggests that starch normally plays a role. But how might starchless mutants sense gravity, albeit poorly? Even these mutants still have plastids. Perhaps the starchless plastids can still function mechanically in sensing, but more poorly because they are lighter. When starch is present, there is more mass and probably a stronger signal as well.

Growth response. The mechanisms of gravitropic curvature are thought to be similar to that of phototropic curvature. The lower side of a stem probably has more auxin than the upper side, resulting in faster growth on the lower side and upward curvature. In roots it is thought that a higher concentration of auxin inhibits rather than stimulates growth. Thus more auxin on the lower side would cause it to grow more slowly than the upper side, causing downward curvature. The involvement of auxin in gravitropism is strongly supported by the isolation of two different types of mutants in Arabidopsis whose roots are not gravitropic. In each case, the mutated gene was found to disrupt the function of proteins that are probably necessary for auxin transport.

Thigmotropism.

Thigmotropic organs grow around an object that touches them. When a tendril on a pea leaf or the stem of a vine come in contact with an object, they cling to and wrap around it. Thigmotropic shoots avoid the expense of making their own support tissue. Instead, they depend on an object to help them climb and position their leaves into the light. Vines and tendrils typically locate a support through slow sweeping movements. These movements stop when contact is made with a support. Pea tendrils respond to contact in two stages. They first quickly coil around the object through changes in water pressure of the cells. The outer part of the tendril then grows much faster than the inner part. The mechanism of sensing is not known. But in some plants, contact induces a wave of electrical signals down the organ. And thigmotropic organs might contain specialized stretch receptors such as membrane proteins that allow ions to pass through them when they are mechanically stimulated. Roots are also capable of thigmotropism. This probably helps them grow around hard objects in the soil.

Roots also exhibit hydrotropism and electropism, which are growth responses to gradients in water and voltage. Electropism probably does not operate in nature, but scientists have used it to study root growth, including on NASA's Space Shuttle. Hydrotropism is obviously adaptive to guide root growth towards water. But root growth is much more affected by gravitropism, and in some plants by negative phototropism, than by hydrotropism.

Nastic Movements

In tropisms the direction of the stimulus controls the orientation of growth, and the effect is more or less permanent. In nastic movements, the direction of the response results from the structure of the organ, and it is only the quality, rather than the direction, of the stimulus that triggers a response. Most nastic movements are reversible. Unlike tropisms, which are found in virtually all plants, nastic movements are mostly found in specialized plants and organs.

Nyctinasty and Photonasty.

The quality and intensity of light can cause organ movements. In many plants, such as legumes (members of the bean family), the leaves move downward or fold at the end of the day. In other plants such as tulips, it is the flowers that close as night approaches. These nyctinastic or "sleep" movements are triggered by changes in the color and intensity of sunlight at the end of the day. The organs open up again around dawn.

The "light-off" and "light-on" signals interact with internal rhythms in plants. The light signals are used to set the internal clock, but are not actually required for movement. Once the rhythm is set, the leaves open and close in the dark at the correct times for several days.

Sleep movements help protect leaves and flowers from damage at night, but many plants survive quite well without such movements. Some plants that grow in the shade show photonastic movements. The leaves of wood sorrel (Oxalis ) fold up during the day if the sun gets too strong This protects the pigments in the leaves from sun-induced damage. Leaves can also curve downwards in response to other signals (epinasty) such as when roots are flooded. Although light and dark can cause nastic movements in plants, there are no gravinastic movements. Gravity is not useful as a signal of changes in the environment because it is constant in presence and extent.

Thigmonasty and Seismonasty.

Organ movements can also be induced by touch (thigmonasty) or shaking (seismonasty). These two types of nastic movements are related but distinct. The leaves of the sensitive plant, mimosa, fold up when they are touched by a falling object or an animal. They also close when they are shaken such as by the wind. This closing may reduce evaporation from the leaf in a strong wind, or discourage an animal from eating it.

In contrast, the closing of the specialized leaves of the Venus's-flytrap plant is only triggered by touch, not shaking. The inner surface of this trap has several trigger hairs. These hairs will only activate the closing of the trap if they are touched several times in rapid succession. One touch has no effect. In this way, the trap is only likely to close if an insect is exploring the leaf, but not if the insect flies away after a single contact. Trap closure in turn triggers the release of enzymes that break down the prey in the trap. Many insectivorous plants grow in bogs where little combined nitrogen (nitrate, nitrite, or ammonia) is available because the water is so acidic that decomposition is slowed. The digestion of the prey in traps is important to the plant because it provides a source of combined nitrogen for protein synthesis.

The rapid movements of the sensitive plant and the Venus's-flytrap share similar mechanisms. Stimulation causes a wave of electrical signals called action potentials that are similar to, but slower than, nerve impulses in animals. These signals change the turgor (water) pressure inside cells causing some cells to expand and other cells to contract. After a certain amount of time without any further stimulation, the turgor changes again and the organs open up.

Thigmonastic movements also help spread seeds. When ripe fruits of touch-me-not plants (impatiens) are touched, they snap open with such force that they spread seeds into areas where the new seedlings will not be shaded by the parent plant.

see also Carnivorous Plants; Flavonoids; Hormonal Control and Development; Hormones; Plastids; Rhythms in Plant Life.

Fred D. Sack

Bibliography

Chen, Rujin, Elizabeth Rosen, and Patrick Masson. "Gravitropism in Higher Plants." Plant Physiology 120 (1999): 343-50.

Hart, James W. Tropisms and Other Plant Growth Movements. London: Unwyn Hyman Ltd., 1990.

Motchoulski, Andrei, and Emmanuel Liscum. " Arabidopsis NPH3: A NPH1 Photoreceptor-Interacting Protein Essential for Phototropism." Science 286 (1999): 961-64.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, 1998.

Wilkins, Malcolm. Plant Watching. New York: Facts on File, 1988.

Tropisms and Nastic Movements

Tropisms are growth responses of plants that result in curvatures of plant organs toward or away from certain stimuli. Tropisms can be positive, in which case the plant will bend toward a stimulus, or negative, in which case the plant will bend away from a stimulus. Important tropisms in plants include phototropism, gravitropism, and thigmotropism.

Phototropism is the tendency for plant organs to bend in response to a directional light source. For example, light streaming in a window from one direction will often cause the stems of plants placed nearby to bend toward the window, a positive phototropism. Gravitropism is the tendency for plant organs to bend in response to gravity. In most plants, roots grow downward with gravity while shoots grow upward against gravity. Within hours, the shoot of a plant placed on its side will usually bend upward and the roots will bend downward as the plant reorients its direction of growth in response to gravity. Thigmotropism is the tendency for a plant organ to bend in response to touch. For example, the specialized touch-sensitive tendrils of many vining plants, such as pea, will bend toward the side receiving a touch stimulus. Continual stimulation can lead to the coiling of the tendril around an object, which enables vining plants to grasp objects on which they can climb.

For a plant organ to bend in response to a stimulus, differential growth of cells on either side of the organ is required. For example, for the stem of a plant to bend toward a light source, cells on the shaded side of the stem near the shoot tip must elongate faster than cells on the lighted side. Differential cell growth results from either the accumulation of growth-promoting substances on the shaded side, accumulation of growth inhibitors on the lighted side, or both. One substance that appears to mediate many tropisms is auxin, a plant hormone that promotes cell elongation. When the tip of a plant is lighted from one side only, auxin appears to accumulate on the shaded side of the tip, where it promotes more rapid cell elongation than occurs on the lighted side, resulting in the bending of the stem toward the light source.

Nastic movements are rapid movements of plant organs in response to a stimulus that results from alterations in cell volume in a specialized motor organ called a pulvinus. For example, handling of the touch-sensitive leaves of Mimosa pudica results in the folding of its leaflets within a few seconds and is an example of a thigmonastic movement. Leaf folding is due to the rapid uptake of water and increase in volume of some cells in the pulvinus located at the base of each leaflet, coupled with the rapid water loss and collapse of adjacent cells. Because nastic movements occur so rapidly, the movement of plant hormones (which can be slow) does not appear to be involved. Instead, rapidly propagated bioelectrical signals appear to mediate many nastic movements.

see also Hormones, Plant; Rhythms of Plant Life

Donald F. Cipollini

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Benjamin Cummings, 1999.

Hopkins, William J. Introduction to Plant Physiology, 2nd ed. New York: John Wiley & Sons, 1999.

tropism The directional growth of a plant organ in response to an external stimulus, such as light, touch, or gravity. Growth towards the stimulus is a positive tropism; growth away from the stimulus is a negative tropism. See also geotropism; hydrotropism; orthotropism; phototropism; plagiotropism; thigmotropism. Compare nastic movements; taxis.

tropism(tropic movement) A directional response by a plant to a stimulus. It may be positive or negative (i.e. towards or away from the source of the stimulus). The plant may respond by growth or turgor changes so that parts of it bend towards, away from, or at right angles to the direction of the stimulus. The suffix -tropism is used in relation to responses to particular stimuli (e.g. phototropism is a response to light, geotropism to gravity, and chemotropism to a chemical substance).

tropism (tropic movement) A directional response by a plant to a stimulus. It may be positive or negative (i.e. towards or away from the source of the stimulus). The plant may respond by growth or turgor changes so that parts of it bend towards, away from, or at right angles to the direction of the stimulus. The suffix-tropism is used in relation to responses to particular stimuli; e.g., phototropism is a response to light, geotropism to gravity, and chemotropism to a chemical substance.