Plant Growth Regulators

Plants respond to the environment by:
    Sending signals between different parts of the plant.
    Tracking the time of day and the time of year.
    Senseing and responding to gravity and direction of light, etc.
    Responding to environmental cues by adjusting their growth 
      pattern and development.
These control systems evolved through interactions with the environment.

I.  Hormones and Their Actions
  A.  Hormone - An organic compound produced by one part of an 
       organism that is translocated to other parts where in low 
       concentrations it triggers a response in target cells and 
  B.  Hormones control plant growth and development by affecting 
       division, elongation, and differentiation of cells.
    1.  Effects depend on site of action, stage of plant growth 
         and hormone concentration.
    2.  Reaction to hormones depends on hormonal balance (relative 
         concentration of one hormone compared with others).
    3.  The hormonal signal is amplified, perhaps by affecting 
         gene expression, enzyme activity, or membrane 
  C.  The concept of differential target cell sensitivity to 
    1.  Anthony Trewayas in the early 1980’s proposed that target 
         cell sensitivity was more important in determining 
         the effects of a hormone than it’s concentration within 
         the target cell.
    2.  Since that time it is now excepted that both target cell 
         sensitivity and hormone concentration are important in 
         the action of a hormone.
    3.  There are three parts of a hormonal response system:
      a.  The hormone must be present in sufficient quantities in 
            the target cells.
      b.  The hormone must be recognized by and bound tightly to 
           the target cells that respond to the hormone.  This 
           action occurs through hormone-binding receptor 
           proteins in the membranes of the plant cell.  It 
           appears that the plasma membrane is most important, 
           but the tonoplast and endoplasmic reticulum also have 
           been noted to have hormone-binding receptor 
      c.  The receptor protein must cause other metabolic changes 
           that lead to amplification of the hormonal signal.  
           These receptor proteins function by changing their 
           conformation after they are activated by the hormone.  
           The altered conformation then allows the receptor 
           proteins to activate other proteins or nucleic acids 
           (DNA or RNA)
  D.  It has been found that hormones effect gene activity.
    1.  This gene activation can result in a large degree of 
         amplification of the hormonal signal.
    2.  There are various controls points at which a hormone could 
         affect the expression of a gene:
      a.  Transcription:  DNA to RNA
      b.  Translation:  RNA to Protein
      c.  Post-translational modification of proteins.
  E.  Sites of hormonal activity usually involve membranes:
    1.  Hormones act first in the plasma membrane where receptor 
         proteins are located.
    2.  In animals there are two such receptor protein process 
         that have been identified.  Each one involves a second-
      a.  Cyclic-AMP (cAMP):  involves the enzyme Adenylate 
           cyclase.  Adenylate cyclase uses ATP to form cAMP.  
           cAMP is then involved in activation of protein 
           kinases.  Protein kinases function by activating or 
           inactivating other enzymes by a process of 
           phosphorylation.  Thus far, the cyclic-AMP process has 
           not been found to be important in plants.
      b.  Inositol Triphosphate (IP3):  involves the binding of 
           the hormone to a receptor protein in the plasma 
           membrane of the target cell.  The bound hormone-
           receptor complex then activates phospholipase c (PLC).  
           Phospholipase c hydrolyzes phosphoinositol (a 
           nonabundant  membrane phospholipid) into inositol-
           1,4,5-trisphosphate (IP3) and diacylglycerol (DAG).  
           Both of these compound that cause a cascade effect.
        (1)  IP3 is water-soluble and moves to the vacuole and 
              causes the release of Ca2+ into the cytosol.  
              [NOTE: in animals IP3 stimulates Ca2+ release from 
              the endoplasmic reticulum.]  These increased Ca2+ 
              levels in the cytosol causes activation of enzymes, 
              including several different kinds of protein 
              kinases.  When Ca2+ concentration rise in the 
              cytosol, four Ca2+ combine to form a chelate with 
              calmodulin.  This Ca-calmodulin complex can 
              activate other enzymes.  such as protein kinases, 
              NAD+ kinase.  One enzyme that is activated is 
              ATPase which functions to transfer excess Ca2+ out 
              of the cell.
        (2)  DAG is not water-soluble, so it functions within the 
               plasma membrane where it activates Protein Kinase c 
              (PKC).   This kinase uses ATP to phosphorylate other 
              enzymes that regulate metabolism.
  F.  So far, five classes of plant hormones have been identified.
    1.  Auxin (such as IAA)
    2.  Gibberellins (such as GA,)
    3.  Cytokinins (such as kinetin)
    4.  Abscisic acid
    5.  Ethylene

II.  Auxin
  A.  Auxin = A hormone that promotes elongation of young 
       developing stems or coleoptiles.
    1.  The term "Auxin" comes from the Greek auxin with means 
        "to increase".  This terms was coined by Frits Went in 
         1926.  Frits Went had been working as a graduate 
         student on the phototropic responses of coleoptiles in 
         response to auxin application.
    2.  Phototropism = Growth toward or away from light (e.g. 
         growth of a shoot toward light). 
      a.  Results form differential growth of cells in opposite 
          sides of a shoot or, in the case of a grass seedling, 
      b.  Cells on the darker side elongate faster.
      c.  Experiments on phototropism led to the discovery of 
          auxin as a plant hormone.
    3.  Charles and Francis Darwin removed the tip of the 
        coleoptile from a grass seedling (or covered it with an 
        opaque cap) and it f ailed to grow toward light.
      a.  Concluded that the coleoptile tip was responsible for 
          sensing light, and, since the curvature occurs some 
          distance below the tip, the tip sends a signal to the 
          elongating region.
    4.  Peter Boysen-Jensen separated the tip from the remainder 
         of the coleoptile by a block of gelatin, preventing 
         cellular contact but allowing chemical diffusion.
      a.  Seedlings behaved normally.
      b.  If an impenetrable barrier was substituted, no 
           phototropic response occurs.
      c.  Demonstrated that the signal was a mobile substance.
    5.  F.W. Went removed the coleoptile tip, placed it on an agar 
         block, and then put the agar (without the tip) on 
         decapitated coleoptiles kept in the dark.
      a.  If block was placed off -center, the plant curved away 
           from the side with the block.
      b.  Concluded the agar block contained a chemical that 
           diffused into it from the coleoptile tip, and that 
           this chemical stimulated growth.
      c.  Went called this chemical an Auxin.
    6  Auxin was purified and characterized by Kenneth Thimann.
  B.  Natural Auxin is a compound named indoleacetic acid (IAA).
    1.  There are three other compounds that are structurally 
        similar to IAA that also act as auxins:
      a.  4-chloroindoleacetic acid (4-chloroIAA):  found in young 
           seeds of legumes.
      b.  Phenylacetic acid (PAA): widespread and more abundant 
           than IAA.
      c.  Indolebutyric acid (IBA):  found in corn leaves and many 
    2.  Auxin Precursors:  indoleacetaldehyde, indoleacetonitrile, 
          and indole ethanol.
    3.  Synthetic Auxins: a-naphthalene acetic acid (NAA), 2,4-
         dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-
         chlorophenoxyacetic acid (MCPA).
      a.  NAA: is used as an auxin source in tissue culture
      b.  2,4-D is used as a herbicide on broadleaf (dicot) weeds 
           in narrowleaf (monocot) crops.  It is especially 
           useful to eliminate broadleaf weeds in lawns.  
           Another related compound is 2,4,5-T.  During the Viet 
           Nam Conflict ("WAR") 2,4-D and 2,4,5-T were the 
           active ingredients for Agent Orange.  Since that time 
           the U.S. Environmental Protection Agency has forced 
           removal of 2,4,5-T from the market because it 
           contains dioxin (a mutagen and carcinogen) as a by-
           product of manufacturing.
      c.  Other herbicides that are auxin derivatives are:  MCPA, 
           Picloram, and Dicamba
      d.  The effect of these herbicides on broadleaf plants occur 
           by an alteration of DNA transcription and RNA 
           translation that disrupt enzymes needed for 
           coordination of plant growth.  As a result plants 
           treated with these compounds respond by producing 
           twisted and deformed leaves, petioles, and stems.
  C.  The synthesis, concentration control, and degradation of 
    1.  IAA is structurally similar to tryptophan (an amino acid) 
         and it is indeed the precursor of IAA.  The enzymes 
         necessary for the conversion of tryptophan to IAA are 
         found in young tissues such as meristems, growing leaves 
         and fruits.  It is not surprising that auxin 
         concentrations are highest in these tissues.
    2.  Concentration control of auxin in plant tissues is achieved 
      a.  The rate of synthesis.
      b.  Temporary inactivation by conjugate formation, or what 
           has often been called bound auxin.  There are 
           numerous IAA conjugates:  peptide (indoleacetyl 
           aspartic acid, esters (IAA-inositol and IAA-
           glucose).  IAA can be released from these conjugates 
           by hydrolase enzymes.
      c.  Conjugates are storage forms of IAA, and are important 
           in the transport of IAA.
    3.  Degradation of IAA:
      a.  Oxidation by O2 and loss of the carboxyl group as CO2 to 
           form 3-methyleneoxindole.  This reaction is 
           catalyzed by IAA oxidase.  There are several IAA 
           oxidase isozymes which are all nearly identical to 
           the peroxidase that is involved in lignin synthesis.  
           Synthetic auxins and auxin conjugates are not 
           degraded by IAA oxidase.
      b.  Another pathway of degradation involves the oxidation of 
           carbon 2 in the heteroxylic ring to form oxindole-
           3-acetic acid.
  D.  The transport of auxin in the plant.
    1.  IAA is translocated through parenchyma cells in contact 
         with vascular bundles.  IAA will move through sieve 
         tubes if it is applied to the surface of a leaf that is 
         exporting sugars.  But, the normal transport of IAA in 
         plant stems is along the petioles of the leaf to the 
         cells along the vascular bundles.
    2.  Auxin movement is slow, about 1 cm per hour.  This, 
         however, is 10X faster than diffusion.
    3.  Polar Auxin transport requires metabolic energy.
      a.  IAA is actively transported down a stem by Auxin 
           carriers located on the basal ends of cells 
           (carriers are absent on the apical ends).
      b.  Even in root the transport is in an acropetal (apex-
            seeking) direction.
      c.  Antiauxins such as 2,3,4-triiodobenzoic acid (TIBA) and 
           a-naphthylthalamic acid (NPA) inhibit this polar 
    4.  Mechanism for polar transport of auxin:
      a.  Cells use plasma membrane ATPase to pump H+ from the 
           cytosol into cells walls.  The lower pH of the cell 
           walls keeps the carboxyl group of auxin less 
           dissociated than in the cytosol.  The noncharged auxin 
           moves from the cell wall into the cytosol by a 
           cotransport mechanism with H+.  
      b.  Once inside the cytosol the higher pH causes the 
           carboxyl group on auxin to dissociate and become 
           negative.  As the concentration of charged auxin 
           builds up, the outward flow becomes favored 
      c.  Polar transport requires auxin to move out only at the 
           basal end of the cell.  It is assumed that a carrier 
           transports auxin basipitally within the cell and then 
           transports it out into the cell wall, where the low pH 
           starts the cycle again.
      d.  Some flavonoids (quercetin, apigenin, kaempferol) have 
           been found to inhibitors of  the basal transporter of 
      e.  Auxin transport is important for: (1) vascular cambium 
           activity in woody plants during the spring and early 
           summer, (2) differentiation of xylem and phloem in 
           leaf petioles, (3) stem cell growth, (4) lateral bud 
  E.  Roots and their responses to auxins:
    1.  Auxin concentrations in roots are similar to other parts 
         of the plant.
    2.  Auxin inhibits root growth at low concentrations.  For 
          many years it was thought that this was due to increased 
          ethylene production which is stimulated by increases in 
          auxin.  Ethylene retards root and stem elongation.  But, 
          recent research has shown that auxin inhibits growth of 
          roots by an unknown mechanism that is independent of 
    3.  Roots can synthesize enough auxin for their growth.  There 
         is probably a balance between growth of root and shoot 
         systems that is strongly influenced by auxin from the 
         stems.  Auxin from the stems influences root initiation 
         and adventitious root development on stems.
  F.  Lateral-bud development and auxin effects:
    1.  Apical Dominance:  the inhibitory influence the apical bud 
         exerts on the lateral (axillary) buds.  This apical 
         dominance slows the development of the lateral buds.  It 
         also causes lateral branches to grow horizontally.  This 
         horizontal growth reduces shading of the lower branches 
         and thus increases photosynthetic productivity of the 
    2.  The IAA concentration in lateral buds increases after the 
         plant is decapitated.  This is contrary to what would be 
         predicted - one would suspect that auxin concentration 
         would decrease when the apical bud is removed.
    3.  Lateral buds have been found to be deficient in 
         cytokinins.  There may be an interaction between these 
         two growth hormones.
    4.  Abscisic acid, ethylene, and gibberellins have also been 
         found to be important in apical dominance.
  G.  Auxin Mechanism of Action:
    1. Growth requires water absorption, cells must maintain a 
         water potential more negative than that of the 
         surrounding solution.  Cell walls of auxin-treated cells 
         yield more easily, so the pressure potential required 
         to force cell expansion is not as great as untreated 
         cells.  Auxins cause wall loosening, the more rapidly 
         extensible or plastic nature of cell walls.
    2.  The most studied mechanism of action has been: the acid-
         growth hypothesis that states that cell elongation is 
         due to stimulation of a proton pump which acidifies the 
         cell wall.  Auxin treated cells pump H+ into the cell 
         walls, thus lowering the pH so that the wall loosens and 
         can grow faster.
      a.  Acidification activates enzymes that break the 
           crosslinks between the walls cellulose myofibrils.
      b.  This loosens the wall, allowing turgor pressure to 
           elongate the cell.
      c.  There is a difference between growth promotion of cells 
           caused by cytokinins and cell wall acidification.  
           Other growth hormones cause wall loosening and cell 
           expansion in some species by  some unknown mechanism
    3.  Which cells respond to auxin?
      a.  Epidermal cells:  these elongate in response to auxin 
      b.  Subepidermal cell layers (hypodermis, cortex, pith) 
           contain cells that are under pressure.  These cells 
           are ready to elongate because of their internal 
           pressure.  Elongation is restricted because 
           their cell wall polysaccharides are continuous with 
           those of the epidermal cells that are unable to 
           stretch.  The subepidermal cell layer elongates just 
           enough to keep the epidermal cell walls under 
           tension.  Auxins cause epidermal cell wall 
           polysaccharides to loosen and elongate.  The 
           elongation of the epidermal cells is followed by the 
           elongation of the connecting subepidermal cells such 
           that the stem elongates in unison.
    4.  How do auxins control the cell responses:
      a.  Auxins have been found to cause rapid changes in genetic 
      b.  Auxins change the kinds of proteins formed in the cell.  
           The main control seems to be at the level of 
      c.  Auxin works by binding to an auxin receptor protein 
           found mainly in the endoplasmic reticulum and to a 
           lesser extend in the plasma membrane.  This receptor 
           protein is a dimer of two polypeptides of about 20 kDa 

III.  Cytokinins
  A.  A Very Brief History of Cytokinin Discovery:
      1913: Gottlieb Haberlandt first found a growth hormone that 
        stimulated cell division (cytokinesis) in plants.  These 
        substances are now called cytokinins.  In the 1940's 
        Johannes vanOverbeek found cytokinins in the endosperm of 
        immature coconuts.  In the 1950's Folke Skoog and F.C. 
        Steward used this information about cytokinins in 
        endosperm in their plant tissue culture work.  D.S. 
        Letham (1974) isolated two very active forms of 
        cytokinins, Zeatin and Zeatin-riboside, from the coconut 
        endosperm.  Carlos Miller (1954) found a substance that 
        was named Kinetin, in aged or autoclaved herring-sperm 
        DNA.  This work helped lead to the defining of the 
        structure of cytokinins.
  B.  Chemistry of Cytokinins:
    1.  Cytokinins are not found in DNA or breakdown products of 
         DNA, even though they are involved in cytokinesis.  Some 
         cytokinins do occur in transfer-RNA and ribosomal-RNA.
    2.  Cytokinins have been found in seed plants, mosses, brown 
         and red algae, diatoms, fungi, bacteria, and primates.  
         More than 30 free and bound forms of cytokinins have 
         been identified.  In pathogenic fungi and bacteria, 
         cytokinins influence the disease process.  In non-
         pathogenic fungi and bacteria cytokinins influence the 
         mutualistic relationship with plants (mycorrhizae and 
         root nodules).
    3.  Most common and physiologically active natural forms are:  
          zeatin, dihydrozeatin, isopentenyl adenine (IPA).
    4.  Synthetic cytokinins: Kinetin and benzyladenine (BA)
    5.  Cytokinin Structure:
      a.  Cytokinins can exists as free-bases or and nucleosides 
           or nucleotides
      b.  Substituted adenine compounds
  C.  Cytokinin metabolism:
    1.  Isopentenyl-AMP is the precursor of Cytokinins.  It is 
          formed by the enzyme isopentenyl AMP synthase from 
          isopentenylpyrophosphate and AMP.
    2.  Cellular levels of cytokinins are influenced by their 
         degradation and conversion to inactive derivatives 
         (other than nucleosides and nucleotides).  Cytokinins are 
         degraded by cytokinin oxidase.  The most common 
         conjugates are glucose (cytokinin glucosides) or 
    3.  The cytokinin glucosides may be storage forms or transport 
         forms of cytokinins.  The alanine conjugates are not 
         stored, but are formed irreversibly as removal products 
         of cytokinin.  Conjugates represent physiologically 
         inactive cytokinins.
  D.  Sites of Cytokinin Synthesis and Transport:
    1.  Cytokinin levels are highest in young organs such as 
         seeds, fruits, young leaves, and root tips.
    2.  Roots tips are active in synthesis of cytokinins.  The 
         cytokinins are transported from the root tips, through 
         the xylem, to all plant parts.
    3.  Shoot are also able to synthesize some of the cytokinins 
          they need.
    4.  Cytokinins are mainly transported through the xylem, 
         however, they have been identified in sieve tubes of the 
         phloem.  For example, when the petiole of a mature leaf 
         is cut, cytokinins move to the base of the petiole 
         through the phloem.
  E.  Promotion of Cell Division and Organ Formation by 
    1.  A high cytokinin-to-auxin ratio in tissue culture will 
         produce meristematic cells in callus culture.  These 
         cells will give rise to buds, stems, and leaves.  A low 
         cytokinin-to-auxin ratio will favor root formation.
    2.  Organogenesis:  formation of shoots or adventitious roots 
         by callus culture.
        Embryogenesis:  formation of embryos by callus culture.
    3.  Agrobacterium tumefaciens (bacterium):  contains the Ti 
         plasmids that codes for isopentenyl AMP synthase.  
         Infection of a plant with this bacterium will result in 
         tumorous outgrowths (galls) on stems called crown gall 
  F.  Delay of senescence and increase of nutrient-sink activity 
       by cytokinins:
    1.  If a mature leaf is detached from the stem of a plant it 
         will lose chlorophyll, RNA, proteins, and lipids from 
         the chloroplast membrane system.  This will cause the 
         leaf to turn yellow, a process associated with 
         senescence.  If the leaf is treated with cytokinins, 
         this senescence process will be delayed.
    2.  Cytokinins maintain the integrity of the tonoplast 
          (vacuole) membrane.  This is important since the vacuole 
          stores proteases that could hydrolyze proteins in the 
          cytoplasm and organelles, especially organelle 
    3.  Cytokinins are also able to protect membranes from 
         degradation by preventing oxidation of unsaturated fatty 
         acids.  Cytokinins inhibit formation of or speed the 
         breakdown of free radicals (superoxide and hydroxy 
         radicals) that would normally attach the lipids in 
    4.  Cytokinins stimulate transport of solutes from older plant 
         parts to younger plant parts.  Migration of solutes 
         occurs through the phloem.  Young plant parts are able 
         to mobilize solutes towards themselves because they have 
         high cytokinin concentrations.  Thus the young tissues 
         of a plant are "sinks" for solutes.
    5.  Some pathogenic fungi produce cytokinins in order to 
          mobilize nutrients from the plant to the fungi.
    6.  The ability of cytokinins to retard senescence has been 
          used in agriculture.  Cytokinins are used to retard the 
          senescence of cut flowers and fresh vegetables.  
          Cytokinins increase the storage life of Brussels spouts 
          and celery (not done in the U.S.).
  G.  Promotion of lateral-bud development in dicots by 
    1.  Application of cytokinin to a lateral bud that is under 
          the control of a shoot apex will cause the lateral bud 
         to grow.  It is not clear if this is what occurs in 
  H.  Enhancement of cell expansion in dicot cotyledons and 
         leaves by cytokinins:
    1.  When dicot seeds germinate, they stay yellow (etiolated) 
         until they emerge above ground.  When cotyledons are 
         exposed to light their growth increases as a response to 
         phytochrome and cytokinins.  Growth is by cell expansion 
         caused by water uptake because the dry weight of the 
         cotyledons does not change.
  I.  The normal growth of stems and roots requires cytokinins 
       even though application of exogenous cytokinins sometimes 
       decreased elongation.
  J.  Promotion of chloroplast development and chlorophyll 
       synthesis by cytokinins:
    1.  Cytokinins enhance the development of etioplasts 
         (proplastids that eventually will form chloroplasts upon 
         maturation) into chloroplasts by promoting grana 
         formation and increasing the rate of chlorophyll 
    2.  Cytokinins enhance the formation of one or more proteins 
         in the chloroplast membrane system to which chlorophyll 
         binds and is stabilized.
  K.  Mechanism of action for cytokinins:
    1.  It has been found that cytokinins may have a primary 
         effect that is followed by many secondary effects which 
         depend on the physiology of the target cell.
    2.  The hormonal signal is amplified by the cell.
    3.  Cytokinins promote RNA and enzyme formation.
    4.  Cytokinins promote cell division by decreasing the 
         transition time from G2 to mitosis by increasing the 
         rate of protein synthesis.  It is well known that there 
         is a control point at the end of G2 which is controlled 
         by the stockpiling of enzymes and proteins that will be 
         need during mitosis.  Benzyladenine (BA) may control DNA 
         synthesis since it causes a decrease in the length of 
         the S phase in the cell cycle.
    5.  Cytokinins also act on translation of mRNA.  Cells treated 
         with cytokinins form large polysomes that aid in mRNA 
    6.  Cytokinins also appear to effect mRNA levels by influences 
         in transcriptional control of some genes (turning on 
         some and turning off others).  Cytokinins may also 
         affect the stability of mRNA.
    7.  Cytokinins have been reported to affect the amount of 
      a.  Upregulation of chlorophyll a/b binding protein (part 
           of the LHCII thylakoid membrane)
      b.  Upregulation of small subunit or RuBisCo.
      c.  Downregulation of phytochrome (by cytokinins and red 
    8.  Cytokinins cause increased plasticity of cells walls.  The 
          cell walls are loosened so that turgor pressure can 
          expand them faster.  This is not an acidification 
          process in the cell walls as seen with auxin.

IV.  Gibberellins
  A.  More than 84 different gibberellins, many naturally 
          occurring, have been identified.
      1.  First discovered in Japan in the 1930’s where it caused 
          a disease in rice plants called "foolish seedling".  
          Eventually it was found that a fungus (Gibberella 
          fujikuroi) caused the disease through a compound named 
          gibberellin.  Although Gibberellin was found about the 
          same time as Auxin, it did not receive much publicity 
          in the west until after WWII.
      2.  All gibberellins are acidic and are derivative from the 
          ent-gibberellane structure.  (19 or 20 carbon atoms in 
          four or five rings with one carboxyl group).  The most 
          active form is GA3, which has been named gibberellic 
      3.  Gibberellins are found in angiosperms, gymnosperms, 
          ferns, mosses, algae, and fungi.  They have also been 
          reported in some bacteria.
  B.  Metabolism of Ginbberellins:
     1.  Gibberellins are isoprenoid compounds (diterpenes) that 
          are formed through the mevalonic acid pathway.  They 
          are formed in the endoplasmic reticulum.
     2.  The 19 carbon gibberellins are more active than the 20 
          carbon gibberellins.
     3.  There are several commercial growth retardants that 
          inhibit stem elongation (stunting) by blocking 
          gibberellin synthesis:  Phosphon D, Amo-1618, Cycocel, 
          ancymido, and paclobutrazol.
     4.  Conjugated gibberellins are inactive.  Glucoside 
          conjugates are stored or translocated before they are 
          released at the proper time and place.
     5.  Many time active gibberellins can be changed into less 
          active forms.
     6.  Sites of gibberellin synthesis in the plant:
       a.  Most plant cells have the ability to synthesize 
       b.  Immature seeds:  usually contain large amounts of 
            gibberellins that arise from biosynthesis in the 
            seed itself, not from transport.
       c.  Young leave are major sties for gibberellin 
       d.  Roots synthesize gibberellins.  Root gibberellins 
            have little effect on root growth but it inhibits 
            adventitious-root (roots arising from above ground 
            stems) formation.  There is evidence that roots 
            supply the stems with a large amount of gibberellin 
            that is translocated through the xylem.
      7.  Gibberellins are transported through the plant in the 
          xylem and phloem in a non-polar fashion.
  C.  Growth promotion by gibberellin in intact plants:
      1.  Gibberellins have been found to greatly increase the 
          stem growth of dwarf plants, or biennials that are in 
          the rosette stage.
      2.  Many of the gibberellin-synthesis mutants that have been 
          found are dwarf because one or more steps in the 
          synthesis of gibberellin is missing or non-functional.
      3.  Most plant species require GA1 for stem elongation.  
          Gibberellin-sensitivity mutants have sufficient 
          amounts of gibberellin in their tissues but cannot 
          respond to it.  It is possible that a receptor protein 
          is missing or non-functional in these mutants.
  D.  Promotion of germination in dormant seeds and growth of 
      dormant buds by gibberellins.
      1.  Buds in temperate zone plants become dormant in late 
          summer or early fall.  To break the dormancy of these 
          buds the plant must undergo a cold treatment in the 
          winter, or proper exposure to photoperiod.
      2.  Seeds often show dormancy when first shed from the 
          plant.  These seeds must also undergo some treatment 
         (cold temperature, scarification, acid treatment, 
         exposure to sufficient moisture or proper photoperiod, 
         etc.) before they can germinate.
      3.  Breaking of bud and seed dormancy can be overcome by 
          gibberellin treatment.  In some seeds, the elongation 
          of the radicle by gibberellin treatment allows it to 
          push through the seed coat.
  E.  Gibberellin affects on flowering:
      1.  Flowering in a plant depends on several factors:  plant 
          age, environmental cues (freezing), and photoperiod.  
          Some plants will flower only under short day, some 
          flower only during long days, and some flower during 
          any length of day (day neutral).
      2.  Gibberellins are able to cause flowering in long-day 
          plants and plants that need a cold period.
  F.  Mobilization of food and mineral elements in storage cells 
      of seeds is stimulated by gibberellins.
      1.  After seed germination, root and shoot systems use 
          mineral nutrients, fats, starch, and proteins present 
          in seed storage cells.  These reserves are used before 
          the seedling is able to absorb minerals from the soil 
          and before photosynthesis has begun.
      2.  Mineral nutrients are readily translocated via the phloem 
          into the roots and shoots.  Fats, polysaccharides, 
          and proteins are not readily translocated.  These 
          nutrients are metabolized into smaller, more mobile 
          molecules such as sucrose, amino acids, and amides.  
          Gibberellin is responsible for these conversions in 
          many seeds, especially monocots.
      3.  The embryo of monocots is surrounded by the endosperm.  
          The endosperm is a nutritive storage tissue with 
          little metabolic ability.  the endosperm is surround 
          by a metabolically active layer called the aleurone.  
          the aleurone layer provides hydrolytic enzyme that 
          digest starch, proteins, phytin, RNA, and cell-wall 
          materials in the endosperm.
      4.  Gibberellin stimulates the secretion of hydrolytic 
          enzyme into the endosperm from the aleurone layer.  
          Mineral elements in reserve in the endosperm are 
          also made available by gibberellin activity.
      5.  Gibberellins are synthesized in the scutellum 
          (cotyledon) and parts of the embryo.  The scutellum 
          is more important than the aleurone layer in 
          providing enzymes for digestion of the endosperm 
      6.  In breaking both seed dormancy and bud dormancy, 
          gibberellins act antagonistically with abscisic acid, 
          which inhibits plant growth.
  G.  Other effects of gibberellin:
      1.  Gibberellins cause parthenocarpy in fruits (seedless 
          fruit development)
        a.  Fruit development is controlled by both gibberellins 
            and auxins.
        b.  In some plants both must be present for fruit set.
        c.  Commercial application in spraying Thompson seedless 
      2.  Gibberellins in young leave help renew activity of the 
          vascular cambium in woody plants during the spring.
      3.  Gibberellins delay aging (senescence) in leaves and 
          citrus fruits
      4.  Gibberellins have been found to influence leaf shape, 
          especially in plants that show a difference between 
          their juvenile and adult forms (heterophylly).
      5.  Gibberellins have been implicated in flower petal 
    H.  The many a varied effects of gibberellins on plant growth 
         and development suggest that gibberellins have more than 
         one primary site of action.
      1.  Stimulation of cell division in shoot apexes:  
          gibberellins have been found to promote cell division 
          by stimulating cells in the G1 phase to enter into the 
          S phase.  Gibberellins have also been found to shorten 
          the time a cell stays in the S phase.
      2.  Gibberellins promote cell growth by affecting enzymes 
          that increase hydrolysis of starch, fructans, and 
      3.  Gibberellins can increase cell-wall plasticity
      4.  Gibberellins probably function by activating genes 
          responsible for enzyme that run these physiological 
    I.  Uses of gibberellins in agriculture:
      1.  Prevent rind disorders in navel-oranges.
      2.  Promote seed production in Pinaceae seed orchards.
      3.  Improve the size of Thompson seedless-grapes.
      4.  Increase the rate of malting in breweries.
      5.  Increase the length and crispness of celery stalks.
      6.  Increase sugarcane growth and sugar yields in Hawaii.

V.  Abscisic Acid (ABA)
  A.  Abscisic acid signals physiological stress such as: water, 
      salinity, cold, and frost.  Abscisic acid helps in 
      embryogenesis, formation of seed-storage proteins, and 
      prevention of seed germination and bud growth.  It is 
      universal in vascular plants, mosses, green algae, and 
  B.  Abscisic acid helps prepare plants for winter by 
      suspending both primary and secondary growth.
      1.  Directs leaf primordia to develop scales that protect 
          dormant buds.
      2.  Inhibits cell division in vascular cambium.
  C.  Effects of ABA on bud dormancy and leaf abscission:
      1.  ABA levels increase in leaves and buds when bud 
          dormancy occurs.  ABA synthesis in leaves is 
          controlled by day length, and translocated to buds.
      2.  ABA probably has no direct role in leaf abscission but 
          acts to cause premature senescence of cells in the 
          leaf which in turn causes production of ethylene.
  D.  ABA also acts as a stress hormone, closing stomata in 
      times of water-stress.
      1.  ABA content rises when leaves are under water stress.  
          Roots under water stress form ABA that is transported 
          through xylem to the leaves where it causes stomatal 
           closure.  ABA from water stressed roots comes mainly 
          from shallow roots, thus acting as an advanced signal 
          to the plant that water is in short supply.  
      2.  ABA works by inhibiting the ATP-dependent proton pump in 
          the plasma membrane of the guard cells.  ABA stops the 
          K+ influx into the guard cells, so K+ and water leak 
          out of the guard cells, reducing turgor, and closing 
          the stomates.
      3.  The water stress signals the plasma membranes to 
          activate certain nuclear genes that increase ABA 
          synthesis.  The plasma membrane responds to Ca+ and 
          phosphoinositols (IP3), indicating that this is a 
          calmodulin response.
  E.  Abscisic acid involvement in salt and cold stress:
      1.  Salt stress:  causes formation of different proteins 
          like osmotin.  These proteins help protect the plant 
          against salt stress.
      2.  Cold Stress:  activation of genes that control ABA in 
          cold and heat stress are similar to the responses 
          noted in water stress.
  F.  Effect of abscisic acid on embryo development in seeds:
      1.  Embryo development is divided into three stages:
        a.  mitosis and cell differentiation
        b.  cell expansion and accumulation of food reserves
           (protein, fat, starch)
        c.  maturation: dehydration and dormancy
      2.  ABA is linked to normal maturation pathway in seed 
          development.  ABA inhibits precocious germination of 
          developing seeds.
      3.  ABA stimulates storage of seed proteins that are needed 
          for later dehydration and dormancy.
      4.  The ratio of ABA:gibberellins determines whether seeds 
          remain dormant or germinate.
  G.  Abscisic acid metabolism and transport:
      1.  ABA is a 15-carbon sesquiterpenoid from the mevalonic 
          acid pathway.  It is synthesized in plastids like the 
          chloroplasts.  It has common metabolic intermediates 
          with gibberellins, sterols, and carotenoids.  ABA is 
          synthesized as a degradation product of carotenoids in 
          plastids (chloroplasts - leaves, chromoplasts-fruits, 
          leucoplast-roots, proplastids-seeds).
      2.  ABA is inactivated by attachment to glucose.  ABA-
          glucose esters are restricted to vacuole.  Also, ABA 
          is inactivated by oxidation with oxygen to form 
          phaseic acid and dihydrophaseic acid.
      3.  ABA is transported in the xylem and phloem.  Transport 
          has also been found in parenchyma outside the vascular 
          system.  There is no polarity in ABA transport.
      4.  ABA has three major effects on plant tissues:
        a.  effects plasma membrane of roots:  root membranes are 
            positively charged probably due to the inhibition of 
            a plasma membrane ATPase.  There is probably an 
            interaction with auxin-induced acid growth.
        b.  inhibition of protein synthesis:  results in effects 
            on growth and development such as control of seed 
            and bud dormancy. 
        c.  specific activation and deactivation of genes.

VI.  Ethylene
  A.  A gaseous hormone that diffuses through air spaces between 
      plant cells.  High auxin concentrations induce release of 
      ethylene, which acts as a growth inhibitor.
      1.  During fruit ripening, ethylene triggers senescence, and 
          then the aging cells release more ethylene.  The 
          signal to ripen spreads from fruit to fruit since 
          ethylene is a gas.
      2.  Fruit ripening phenomenon has been know for a long time. 
          Ancient Chinese knew that burning incense would ripen 
          fruit.  The Jamaican Agricultural Department knew in 
          1910 that oranges could not be stored with bananas on 
          ships because the oranges would cause the bananas to 
          overripen.  Bonfires where used to help ripen 
          pineapples in Hawaii and Puerto Rico, and mangos in 
          the Philippines.
      3.  Ethylene released by soil fungi promotes seed 
          germination, controls seedling growth, and retards 
          disease caused by soil-borne organisms.
  B.  Synthesis of ethylene in plants:
      1.  All parts of the seed plant can produce ethylene.  Stems 
          produce large amounts of ethylene when they are laid 
           horizontally.  Roots do not produce much ethylene.  
          Leaf ethylene production increases gradually over the 
          growing period, until they senesce and abscise.  
          Flowers synthesize ethylene just before they fade and 
          wither.  Fruits produce ethylene just before the 
          respiratory climacteric (a large increase in cellular 
          respiration just before the fruit ripens).  In non-
          respiratory climacteric fruits (citrus fruits, 
          cherries, grapes), ethylene has little effect on fruit 
          ripening.  However, ethylene is used in the citrus 
          industry to de-green oranges and lemons.
      2.  Stress will also cause a plant to produce ethylene.  
          This can come from mechanical stress such as rubbing 
          (wind) or, pressure (trees leaning on each other), 
          pathogens (fungi, bacteria, virus, insects), 
          environmental stress (waterlogging or drought).  Figs 
          can be ripened by slashing the fruit.
      3.  Ethylene is synthesized form methionine.  Its direct 
          precursor is 1-amino-cyclopropane-1-carboxylic acid 
      4.  Inhibitors of ethylene production:  
           aminoethoxyvinylglycine (AVG) and aminooxyacetic acid 
          (AOA).  Both block the synthesis pathway from 
           methionine to ethylene.
      5.  Control of ethylene synthesis:
        a.  Auxin promotes ethylene production
        b.  Auxin promotes ACC formation, which is the direct 
            precursor of ethylene.
      6.  Ethylene stimulates its own formation in senescing
          organs (autocatalytic).  Thus there is some truth to 
          the saying that "One rotten apple can spoil the whole 
          bunch."  Diffusion of ethylene through intercellular 
          spaces in a fruit coordinates ripening.
      7.  Light inhibits ethylene production in leaves while 
          carbon dioxide promotes ethylene synthesis.
  C.  Leaf abscission is an adaptation that prevents deciduous 
      trees from desiccating during winter when roots cannot 
      absorb water from the frozen ground.
      1.  Before abscission, the leaf’s essential elements are 
          shunted to storage tissues in the stem.
      2.  Environmental stimuli are shortening days and cooler 
      3.  When a leaf falls, the breakpoint is an abscission zone 
          near the petiole base.
        a.  Weak area since the small parenchyma cells have very 
            thin walls and there are no fiber cells around the 
            vascular tissue.
        b.  Mechanics of abscission controlled by a change in the 
            balance of ethylene and Auxin.
        c.  Auxin decrease initiates changes in the abscission 
            layer.  Cells then produce ethylene.
        d.  Ethylene induces synthesis of enzymes that digest the 
            polysaccharides in the cell walls, further 
            weakening the abscission zone.
        e.  Wind and weight cause the leaf to f all.
      4.  Even before the leaf falls, a layer of cork forms a 
          protective scar on the twig's side of the abscission 
  D.  Ethylene effects on plants in waterlogged soils and 
      submerged plants.
      1.  Waterlogged roots produce less ethylene since 
          waterlogged soils are hypoxic.  Ethylene synthesis is 
          inhibited because oxygen is required to convert ACC to 
      2.  The ethylene that is synthesized is trapped in the root 
          because the ethylene cannot escape readily though 
      3.  Ethylene causes cortical cells to synthesize cellulase 
          which hydrolyzes cellulose.  The cortical cells lose 
          their protoplast, forming air-filled aerenchyma 
      4.  As ACC accumulates it is transported in the xylem to the 
          shoots where it is metabolized into ethylene.
      5.  Leaf epinasty occurs because the parenchyma cells on the 
          upper surface of the  petioles elongate in the presence 
          of ethylene relative to the lower surface.  Ethylene 
          also retards stem elongation, increases radial 
          expansion, causes leaf senescence, and promotes 
          adventitious root formation.
  E.  Effect of ethylene on stem and root elongation:
      1.  Ethylene inhibits elongation of stems and roots and 
          subsequently causes them to become thicker by 
          increasing radial expansion.  This is caused by the 
          cellulose microfibrils being laid down in a more 
          longitudinal orientation.
      2.  Important response of dicot seedlings emerging from 
          soil.  The hook in the epicotyl or hypocotyl is formed 
          as a response to ethylene.  The hook allows the 
          seedling to push through the soil without damaging the 
          meristematic tissue at the tip.  The ethylene causes 
          the seedling to increase in thickness so that it is 
      3.  This increased thickening is also found in the 
           mesocotyls of monocots.
      4.  Ethylene causes loss of gravitropic response in 
           seedlings, which allows them to bend around 
           obstructions in the soil as they are growing.
      5.  Aquatic plants respond to ethylene production by causing 
          elongation of their stems and roots.  This allows 
          aquatic plants to have their leaves kept above the 
          water line.
  F.  Ethylene effects on flowering:
      1.  Normally, ethylene inhibits flowering in plants.  
          However, in mangos and bromeliads (example: 
          pineapple), ethylene induces flowering.
      2.  Commercially:  Ethrel (ethephon) is used to promote 
          flowering in Hawaiian pineapple fields which 
          facilitates a single mechanical harvesting.  Ethrel 
          also used in late season on tomatoes to cause uniform 
          ripening and enable mechanical harvesting.
  G.  Other ethylene effects:
      1.  Ethylene causes flower senescence, probably because 
          after pollination there is an increase in the rate of 
          ethylene production in flowers.
      2.  Ethylene promotes adventitious-root formation, also an 
          effect of auxin.
      3.  Ethylene causes changes in the expression of flower sex.  
          Ethylene promotes formation of female flowers.
      4.  Ethylene helps break seed dormancy.  Found as a function 
          of some soil fungi.
  H.  Relation of ethylene to auxin effects:
      1.  Auxin increases ethylene production.  Raises the 
          question of whether some plant responses to auxin are 
          merely responses to ethylene.  
      2.  Some ethylene responses that relate to auxin production: 
          leaf epinasty, inhibition of stem and leaf elongation, 
          flower induction (bromeliads and mangos), inhibition 
          of hook opening in dicot seedlings, female flower 
          production, senesces of flowers.
      3.  Some ethylene responses that relate to other hormones 
         (auxin, cytokinins, abscisic acid):  abscission of 
          leaves, flowers, and fruits.
  I.  Antagonists of ethylene action:
      1.  High concentrations of carbon dioxide inhibits ethylene 
          responses in the plant.  High carbon dioxide 
          concentrations are often used in fruit packing houses 
          to prevent overripening of fruits and vegetables.
      2.  Silver ion (Ag+) also inhibits the action of ethylene.  
          Silver ions in the form of silver nitrate (AgNO3) 
          inhibit abscission of leaves, flowers, and fruits in 
      3.  Synthetic antagonists of ethylene are: volatile olefin, 
          transcycloctene, 2,5-norbornadiene.
  J.  Ethylene mode of action:
      1.  Ethylene causes an increase in synthesis of enzymes in 
          different target tissues.  For example ethylene 
          stimulates cellulase and other cell wall degrading 
          enzymes in the leaf abscission.  Different enzymes are 
          produced that aid in fruit ripening and flower 
          senescence in the presence of ethylene.  During 
          wounding, presence of ethylene is related to 
          synthesis of phenylalanine ammonia lyase, an enzyme 
          active in phenolic compound synthesis needed for 
          healing.  In cells infected with fungi, ethylene 
          stimulates the production of b-1,3-glucanase and 
          chitinase which degrade fungal cell walls.
      2.  Ethylene appears to stimulate mRNA transcription.
      3.  Ethylene binds one or more membrane receptor proteins 
          that contain copper at their active sites.

VII.  Other plant growth substances (not necessarily hormones):
triacontanol, brassins, salicylic acid, turgorin, and polyamines
A. Triacontanol: 30-carbon, saturated primary alcohol. Found to increase growth of several agricultural plants. B. Brassins and Brassinosteroids: growth promoters. Act by increasing sensitivity to auxins. C. Salicylic Acid: Active ingredient in aspirin. Promotes cyanide-resistant respiration that causes heat production and volatilization of compounds in Arum lilies which attract insect pollinators. Important in causing resistance to certain plant pathogens (tobacco necrosis virus, and fungi) D. Turgorin: act on the turgor pressure in the pulvinus cells of plants that have nastic movements. E. Polyamines: polyvalent cations containing two or more amino groups. Examples are: putrescine, cadaverine, spermidine, spermine. Involved in promoting cell division, stabilizing membranes and protoplasts, promoting development of fruit, reducing water stress, and delaying leaf senescence. The positively charged amino groups allow them to combine to the phosphates in DNA and RNA, thus controlling transcription and translation. VIII. Other inhibitory plant growth regulators: A. Lunularic acid: found in the gemmae of liverworts as well as many species of lower plants, but not algae. Prevents germination of the gemmae and growth of the thallus as a response to day length. Long days: lunularic acid concentrations are high and growth is slow. Short days: lunularic acid concentrations are low and growth is rapid. B. Batasins: found in yams where it causes dormancy in bulbils (vegetative reproductive structures). C. Jasmonic acid: found in oil of jasmine. Inhibits the growth of plant parts and promotes leaf senescence. IX. Some Unanswered Questions About Plant Hormones A. So much is unknown about internal chemical signals of plants that some argue it is premature to call these growth regulators hormones. B. For example, an increase in Auxin concentration in the zone of elongation has not been detected. C. According to one hypothesis, responses to these growth regulators is due more to changes in the sensitivity of local cells to regulators already present than to the arrival of these regulators from other parts of the plant.