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 
    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 
    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 
         (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 
  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 
   1.  Control of sleep movements is by the pulvinus
      a.  Swelling of extensor cells in the pulvinus causes leaf 
      b.  Closing of the flexor cells in the pulvinus causes leaf 
   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 
      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 
        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.  Venus’s-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 
        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 
  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 
       (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 
     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 
   3.  Solar Tracking:
    a.  Another way that many plants maximize photosynthesis is by 
        keeping their leaves at right angles to the sun during the 
    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 sun’s 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 earth’s 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 
     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 
        (2)  Other transduction substances:
           (a)  Gibberellins have been found in high 
                concentrations on the bottom of gravistimulated 
           (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 
      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.