Movement in Plants
I. Some basic principles about movement in plants: A. There are many kinds of plants and therefore, many ways that plants can move. B. Most movements in plants can be classified in two categories: 1. Tropisms (Greek: trope = "turn"); in this case the direction of an environmental stimulus determines the direction that a plant will move. 2. Nastic Movements (Greek astos="pressed close"): here the environmental stimulus triggers a response by the plant. The direction of the stimulus does not control the direction of movement in response from the plant. 3. Both of these types of movements can be either the result of differential growth or the reversible uptake of water into specialized cells called motor cells that collectively form the pulvinus. 4. Taxis: a movement toward or away from a stimulus in a single or few-celled organism. Most common in lower plants. 5. Stimulus: an environmental change that elicits a response from an organism. In the case of plant movement, the induced response from the stimulus can continue after the stimulus is no longer present. a. The stimulus acts on some part of the plant. That plant part that perceives the stimulus is the receptor. After a stimulus has been received, it is transduced into another form. This transduction is usually in the form of a signal that can lead to a motor response, or, an actual movement by the plant. Thus, there are three steps in the movement of a plant that need to be studied. (1) Perception: how can a plant or plant part detect the environmental stimulus in order to respond? (2) Transduction: how is the perception changed into a stimulus that can be perceived by the cells in the plant where the movement will occur? (3) Response: what mechanism does the plant use to respond to the stimulus? b. By asking these questions two generalizations concerning plant movements have been identified. (1) Similar mechanism of perception can lead to different responses in the plant. (2) Different mechanism of perception can lead to similar responses in the same or different plant. II. Nastic Movements: A. Definitions: 1. Epinasty: downward bending of a plant part 2. Hyponasty: upward bending of a plant part 3. Pulvini: group of motor cells found at the base of petioles, blades, or leaflets that are responsible for many nastic plant movements. These nastic movements are reversible. B. Nyctinasty (Greek, nux ="night"): leaf movements that occur on a daily basis. These movements are rhythmic and controlled by environment and the biological clock. For example, leaves of many plants show sleep movements where during the day the leaves are horizontal and vertical at night. 1. Control of sleep movements is by the pulvinus a. Swelling of extensor cells in the pulvinus causes leaf opening b. Closing of the flexor cells in the pulvinus causes leaf closing. 2. The swelling is caused by water movement in response to osmotic gradients developed by ion transport. For example, K+ movement out of a cell causes the lose water. a. There are depolarization-activated K+-selective channels in membranes of shrinking cells that allow the outward diffusion of K+. b. There is a cotransport of Cl- into motor cells via H+/anion symport. c. These mechanisms are activated by phytochrome and a blue-absorbing pigment through the phosphatidylinositol cycle as seen in plant hormone response. 3. Light can cause opposite responses in extensor and flexor cells. Changing from white light to darkness activates the H+ pump in flexor cells and inactivates the H+ pump in extensor cells. C. Hydronasty: folding or rolling of leaves in response to water stress. The rolling of leaves reduces the exposed leaf area to dry air. This response is usually coupled with closure of stomata. The folding or rolling is caused by turgor loss in the bulliform cells of the leaf. Bulliform cells will lose water before the rest of the leaf because they have little or no cuticle to protect them from dehydrating. Bulliform cells are located on the only one side of the leaf. As the bulliform cells dehydrate they lose turgor pressure which causes the leaf to close. D. Thigmonasty (Greek, thigma="touch"): nastic movements that result from touch. Example: Mimosa or Sensitive Plant that responds to touch, shaking, heat, or rapid cooling by folding its leaflets. If only one leaflet is touched, an electrical stimulus will travel to the other leaflets of the plant, causing them to close. This may be a protection mechanism which would effectively startle insects. 1. Mechanism of movement involves transport of water out of motor cells in the pulvini as a result of an efflux of K+. 2. Signal transmission occurs by two different mechanisms: a. Electrical signal: an action potential created by a charge (voltage) differential across the cell membrane as a response to fluxes of ions. Electrical signals travel through the plasmodesmata of parenchyma cells in the xylem and phloem at velocities of 2 cm per second. b. Chemical signal: the action potential will not pass to the pulvinus from leaflet to leaflet until the chemical response occurs. Turgorin, the chemical signal, must move through the xylem in the transpiration stream in order for the electrical response to travel from one leaflet to anther. (1) Periodic Leaf Movement Factors (PLMFs): beta- glucosides of gallic acid. These are a new class of plant hormones that act on the turgor pressure of pulvinus cells. The two most active gallic acid glucosides are: Beta-D-glucoside-6-sulfate (PLMF1) and Beta-D-glucoside-3,6-disulfate(PLMF2). These hormones do appear to have protein receptors on the plasma membranes of pulvini cells in Mimosa. 3. Venuss-Flytrap: Excitation of sensory epidermal hairs by an insect causes an action potential to move from the hairs to the bilobed leaf tissues causing them to shut. The rapid closing is a result of acid growth. In response to the touch of an insect, hydrogen ions are pumped into the walls of cells on the outside (underside) of the leaf. The hydrogen ions loosen the cellulose in the cells walls which quickly take up water from the apoplast. This causes the leaves to shut, trapping the insect. Digestive enzymes dissolve the insect which provides the plant with supplemental nitrogen and phosphorous. The trap opens gradually as the inside cells grow, forcing the leaves open. 4. Thigmomorphogenesis and Seismomorphogenesis: a. Both of these plant morphological responses are the result of mechanical stimulation. The typical response involves slower stem elongation and an increase in the diameter of the stem. This results in shorter, stronger plants that are less easily damaged by such natural mechanical stresses as wind and strong rains. b. Commonly observed in greenhouse plants that are not affected by wind. The same plant variety will tend to be taller if greenhouse grown as compared to non- greenhouse grown. c. The response is probably caused by a change in growth hormone patterns, especially ethylene. Ethylene will cause an increase in stem thickening and a decrease in stem elongation. Also, it has been found that auxin production is inhibited and gibberellic acid activity is decreased in mechanically stimulated plants. d. The amount of hormone is changed: (1) by in membrane permeability affects the amount of hormone at the site of action (2) production of growth regulator by making available precursor molecules e. The hormones probably work by the calcium-calmodulin secondary messenger since calcium levels increase in stressed cells. III. Tropisms: differential growth that results in a directional response. A. Plants respond to the direction of an environmental stimulus by unequal or differential growth. The most common tropisms are phototropism (response to light), gravitropism (response to gravity), and thigmotropism (response to touch). B. Phototropism: 1. Coleoptiles and Stems: a. Perception: (1) Light has two effects in phototropism. It acts as the trigger for the bending response, and it decreases the organ sensitivity to subsequent light. This is a nondirectional effect, referred to as a tonic effect. Phytochrome has been found to be important in the sensitivity of coleoptile bending in response to blue light. Etiolated coleoptiles and stems (grown in the dark ) can pipe light like a fiber-optic cable. As the light moves through the stem or coleoptile it quantity and quality (wavelength) can change. This can cause different tissue responses to the same light treatment. (2) The phototropic response in monocots and dicots have identical action spectra in. The active pigment is Cryptochrome, a flavin pigment that absorbs blue light controls phototropism. b. Transduction in phototropism: Auxin migrates from the irradiated side of the coleoptile or stem to the shaded side. Inhibitors of this hormone movement have been identified. c. Growth response: there is an inhibition of growth on the dark side of the coleoptile or stem. 2. Leaf Mosaics: a. One response: if a leaf is partly in the sun and partly in the shade, the leaf will respond to the light by elongating the side of the petiole that corresponds to the shaded part of the leaf. This causes to leaf petiole to bend which moves the leaf toward the light. b. A second response: the upward bending (hyponasty) of the shaded side of a leaf. c. Both of these responses will place the in full sun (if possible). Thus, if you stand under a tree you will notice that the leaves barely overlap, creating a mosaic. This is the plants response to maximization of photosynthesis. 3. Solar Tracking: a. Another way that many plants maximize photosynthesis is by keeping their leaves at right angles to the sun during the day. b. Diaphototropism (diaheliotropism): since the leaves are neither positive or negative phototropic since the orientation is at a right angle. Orientation is controlled by motor cells in the pulvinus. Water movement controlled by K+ fluxes is responsible for the leaf movement. The lamina follows the sun during the day. At sunset the lamina is vertical, facing the western horizon. Within an hour the lamina assumes a resting position at right angles to the petiole. An hour before sunrise, the lamina move to face east. Thus, the lamina responds to both the direction of the sun and a circadian rhythm. c. Lamina are most sensitive to blue light. There is some evidence that auxin may play a role in transmission of the solar stimulus. d. Negative Solar Tracking: found in desert plants. These plant maintain their leaves parallel to the suns rays to protect themselves from direct sunlight. This minimizes water lose and heat gain, and protects against photooxidation of photosynthetic pigments. 4. Skototropism: growth of some vines toward darkness. This enables vines to find trees to "clime". Once the vine reaches the tree it becomes positively phototropic and grows toward the light. Gravitropism and thigmotorpism may also plant a role in the climbing of vines. C. Gravitropism: plant growth in response to the earths gravitational field. Roots are positively gravitropic, while stems are negative gravitropic. 1. Terms: Orthogravitropism: vertical growth Diagravitropism: horizontal growth Plagiotropism: growth at any distinct angle Agravitropic: no response to gravity 2. Roots: Primary roots are orthogravitropic Secondary roots may be plagiotropic Tertiary roots may be agravitropic a. Perception: Gravity perception is in the root cap. Amyloplasts which contain two or more starch grains that settle in response to gravity are the perception mechanism. The amyloplasts are the statoliths in statocytes that are responsible for perception of gravity. b. Transduction: the root cap sends an inhibitor to the lower side of the root. this inhibitor slows growth so that the root bends downward. Was thought that ABA was the inhibitor, but it now seems that IAA is the inhibitor. If IAA is added to one side of the root, the root bends toward that side. c. Calcium and electric currents: Calcium is important in gravitropism. If an agar block with calcium is applied to the side of the root cap, the root will bend in the direction of the agar block. Electric currents have been measured in roots that are responding to gravity. It is possible that these electric currents are caused by counter flow of H+ with Ca2+ across membranes. There may be an interaction between the statoliths and calcium. As the statolith move within the cell, they interact with the endoplasmic reticulum (ER) (bumping into it, or pulling on the cytoskelleton that is connected to the ER). As the ER is distorted by the action of the statoliths, it releases Ca2+ which can activate calmodulin. Calmodulin is important in activation of many biochemical systems within plants (and animals). The calmodulin could activate auxin pumps in the cell membranes on the lower part of the root. This would increase the auxin concentration in the lower half of a horizontal root. 3. Stems and Coleoptiles a. RESPONSE: mechanics of stem bending: If a stem that is responding to gravity is tied down (restrained) the cells on the bottom of the stem increase in diameter and the top cells decrease in diameter. If the restraint is released the bottom cells elongate and become thinner while the top cells shorten and become thicker. This results in the stem bending upward. These findings have shown that the bottom cells of a stem grow in response to gravity while the top cells cease growth. b. PERCEPTION: The site of response and perception are the same in stems. In stems, the amyloplasts are the statoliths. They are located in a one or two cell layer just outside the vascular bundles called the starch sheath. The starch sheath is the inner most layer of the cortex. c. TRANSDUCTION: Only a small gradient of auxin has been found across gravitropically stimulated stems. Sensitivity of stem tissues to auxin in the epidermal cells of the stem seems to be the important factor in transduction of the gravitropic response. Epidermal cells are much more responsive to auxin than the underlying subepidermal tissue. Auxin transported during gravity response may be limited. The auxin may only move from the cortex to the epidermal tissue on the bottom side of the stem, and from the epidermal tissues to the cortex on the top side of the stem. (1) Gravitropic Memory: bending of stems does not occur when sufficient auxin is not present. If the tip of a hypocotyl (auxin source) is removed, and then gravistimulated, the hypocotyl will not respond. If the decapitated hypocotyl is returned to the vertical and auxin applied, the hypocotyl will bend. Thus, the stems are able to "remember" the gravity stimulus. (2) Other transduction substances: (a) Gibberellins have been found in high concentrations on the bottom of gravistimulated stems. (b) Ethylene has been found to reduce the rate of gravitropic bending. Ethylene may be responsible for inhibition of growth on the top side of a stem placed in a horizontal position. However, ethylene concentrations have found to be increased in the bottom tissues of gravistimulated stems. (c) Calcium ions are also important in the gravitropic response. Ca2+ concentrations are higher in cells on the top of horizontal stems. It is known that Ca2+ inhibits cell elongation, possibly by overcoming the effect of auxin. (3) Changing sensitivity to auxin in gravistimulated stems: Sensitivity: the capacity of a cell, tissue, organelle, or organism to respond to a stimulus. Many time the sensitivity of a physical system to a stimulus is analogous to classical Michaelis-Menten reactions for enzymes. This can be seen in the changing sensitivity of stems to auxins under influence of gravity. In stems of plants, the tissues on the lower side of a horizontal stem may become more sensitive to auxin while those tissues on the top may become less sensitive. This would cause the lower tissues to respond more to auxin, increasing cell elongation, and thus bend the stem upward. Research has shown that the lower surface of a horizontal stem shows the greatest growth in buffer and low concentrations of auxin. As auxin concentrations increase the amount of stem growth decreases. The reverse is true for the upper surface of a horizontal stem. 4. Special cases in gravity response: a. Much of the organs on a plant are sensitive to gravity in that the organs grow at some orientation other than vertical. Flower parts, fruits, and leaves all respond to gravity. The actual angle involved depends on the plant species, age, organ, and environmental conditions. b. False pulvini of grasses: this is a meristematic zone located at the node of grasses. Under gravistimulation, the meristematic cells of the false pulvini elongate on the lower side, producing parenchyma and collenchyma cells with some vascular tissue. These are false pulvini because pulvini show reversible responses to water uptake, the responses shown here are permanent. c. In experiments under weightless conditions such as the space shuttle, plants respond by showing epinasty (down bending of leaves). IV. Special Tropisms and Other Related Plant Responses A. Other Tropisms: 1. Hydrotropism: growth toward water 2. Hygrotropism: growth in response to humidity 3. Chemotropism: growth toward chemicals 4. Electrotropism: growth toward electrical fields 5. Thigmotropism: growth in response to touch B. Circumnutation: as a stem grows the stem tip traces an elliptical path. 1. Occurs in response to internal and rhythmical controls. 2. Evidence supports the idea that circumnutation is caused by gravitropic overshoot of the plant as it grows. This means that the stems grows in one direction until gravity makes it grow in another direction. C. Reaction Wood: Compression Wood and Tension Wood: 1. As the branches of a tree grow, the weight of the wood and the interaction with gravity causes differences in the xylem of the stem. 2. Conifers: reaction wood forms on the lower side of the stem. This pushes the stem upward. This type of wood is called compression wood. 3. Angiosperms: reaction wood forms on the top side of the stem. This contracts to hold the stem up. This type of wood is called tension wood. 4. Auxin and ethylene seem to play a role in the formation of reaction wood.