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					Life Sciences Study Materials                                                                      Plant Biochemistry

                               Plant Growth Regulators
                                  (Phytohormones)
HISTORICAL ASPECTS:

          Plant hormone research has mostly been occupied with the hormones themselves, their synthesis, and their
distribution within tissues, their displacement, and their physiological effects. Plant hormone receptors, however, have
received little attention. As a result can some of the observations not be interpreted conceptionally, which means that they
hold just for certain plant species, may be contradictory to observations of other species, and cannot simply be transferred
to different species.

         J. TREWAVAS (Department of Botany; University of Edinburgh) pointed already in 1982, 1983 out that the
study of plant hormones itself has just limited significance. He considered the sensitivity of cells towards the hormones
(and other factors), i.e. their existence or their obtain ability for the hormone to be of far greater importance.

         Numerous synthetically produced growth regulators display hormone-like effects. They have a decisive economic
importance as herbicides or growth stimulators in modern agriculture and horticulture, and – due to their dangerousness
and the toxicity of their by-products (dioxin!) – An explosive political potential.

HORMONE:
“It is a substance which being produced by any one part of the organism, is transferred to another part and there it influences
                                              a specific physiological process”.

PHYTOHORMONE
         Phytohormone is an organic substance other than nutrients active in minute quantity (less than micromolar
[µM] concentration), which is formed in certain parts of a plant and is then usually translocated to some other parts
or sites where it evokes physiological, biochemical and/or morphological responses. (Moore, 1989).

PLANT GROWTH REGULATOR:
       A plant growth regulator is an organic substance, natural or synthetic, other than nutrients which in small
amount [less than millimolar (mIM) concentration] promotes or inhibits or quantitatively modifies plant growth and
development (Moore, 1989).
         While all hormones are plant growth regulators, all plant growth regulators are not hormones. Hundreds of compounds
have been discovered which can modify plant growth and development. Growth retardants (e.g.: Norfoxin) are also
plant growth regulators.

Classes of Phytohormones:
         Phytohormones are divided into five classes. There are:

         I) Auxins
                 a) Natural auxins, e.g.: Indole Acetic Acid, Phenyl acetic acid
                 b) Synthetic Auxin. E.g.: 2,4-dichlorophenoxy acetic acid, Picolinic acid
         II) Cytokinins
                 a) Natural Cytokinin, e.g.: Zeatin
                 b) Synthetic Cytokinin, e.g.: Kinetin


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        III) Gibberellins (GA)/ Gibberellic acids, e.g.: Gibberellin1 to Gibberellin 92.

        IV) Abscisic acid (ABA)

        V) Ethylene
There are some other natural compounds, which have hormonal effects:
                        (I)     Polyamines, e.g.: Spermine, Spermidine.
                        (II)    Brassinisteroid, e.g.: Stigmosterol, Cetosterol (steroid hormone of plants).




                                                   AUXINS
        Auxin is the earliest known plant growth regulator. Charles Darwin and his son Francis Darwin first time
discovered the hormone auxin in 1880. It is of universal occurrence in the plant kingdom.
Historical aspect:
         The Danish botanist P. BOYSEN-JENSEN interrupted the assumed substance flow by inserting a mica
sheet into the shielded side thus separating the coleoptile’s tip from the tissue below (1913). The water-
impermeable sheet interrupted the phototropic reaction. Consequently occurs no transport of the effector around
the small tile. The phototropic reaction remained intact when the mica sheet was inserted into the illuminated
side or along the coleoptile’s vertical axis.
         In the late twenties was the material nature of the effector finally proved by the Dutch plant
physiologist F. WENT. He assumed that a substance that flows from tip to bottom should also flow through a
small cube of agar. In order to test his assumption did he place cut coleoptile tips with the cutting side on top of
small cubes of agar. Some time later did he remove the tips and placed the agar cubes that he believed to
contain the effector onto the decapitated coleoptiles. He wrote about the carrying out of the decisive
experiment:

"When I removed the tip after an hour and placed the agar cube on one side of the seedling, nothing happened
at first. But in the course of the night, the stump started to curve away from the agar block. It had acquired the
                         capacity of the stem tip to grow! At 3:00 A.M. on April 17, 1926"

   (According to F. B. SALISBURY, C. W. ROSS: Plant Physiology.
       Belmont/Cal: Wadsworth Publ. Comp. 1978, 2. edition.)

Went called the effector auxin (or growth-regulating
substance). Its chemical name is indole-3-acetic acid
(IES). The formula shows that it is a tryptophan derivative.

Types of Auxins
Auxins occur in two forms,
   a) Natural auxins: Indole acetic acid (IAA), 4-
        chloroindole acetic acid phenyl acetic acid (PAA),
        Indole acetic aldehyde (IAAld), etc.

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    b) Synthetic Auxin: Napthalene acetic acid (NAA), Indole propanoic acid (IPA), 2,4-dichlorophenoxy acetic
       acid (2,4-D), 2,4,6-Trichloro benzoic acid, etc.

Structure of Auxins:

Transport:

         The IAA transport in plant is basipetal as well as acropet and the ratio of basipetal and acropetal transport
is 10:1. The speed of IAA transport is about 10-m/hr. the transport is mostly active, i.e., requires metabolic energy.
The path of transport is the paranchymatous cell surrounding vascular tissues, i.e., not through xylem or phloem.
However, lateral transport of IAA is regulating by light.


Mode of Action:
                  Like all other phytohormones, the mode of action auxin is a controversial issue is a controversial
        issue. How IAA regulates plant growth is not known with certainty. The widely accepted hypothesis in this
        field is the acid-growth hypothesis. The target cell of IAA activity is the differentiating cell of the meristem.
        According to acid-growth hypothesis auxins release hydrogen ions from the target cell. These hydrogen
        ions accumulate surrounding the primary walls of the target cell and cause a lowering of pH. Due to the
        acidic pH, breaking of hydrogen bonding of cell wall polysaccharides takes place. Thus cell elongation
        occurs following the stretching of the cell wall.
                 According to the receptor- protein hypothesis some specific auxin receptors are involved in the mode of
        action. The receptor protein-auxin complex induces transcription and translation of mRNA into proteins.
        These proteins constitute the key enzymes operated in the biosynthesis of cell wall components the key
        enzymes operated in the biosynthesis of cell wall components. However, the main drawback of this
        hypothesis is that these receptors are still unknown to us.

Physiological Role:
    1. Growth and Cell Elongation: The most profound effect of IAA is cell elongation. The concentration of
       IAA at which stimulation of growth takes place varies along with the type of the tissues. As for example,
       higher concentration of IAA is needed for the growth of stem but the same concentration inhibits growth
       in roots and buds. Another hormonal compound ethylene thwarts the action of IAA. Inhibition of growth
       in presence of high IAA concentration takes place due to ethylene production. The stimulatory effect of
       IAA on growth is masked by the ethylene activation.
    2. Cell division and tissue differentiation: IAA stimulates cambial activity. As a result the rate of cell
       division increases rapidly in a particular part of the plant. In the shoot apical meristem differentiation of
       vascular tissues are regulated by appropriate IAA concentration.
    3. Adventitious root formation: IAA stimulates adventitious root formation in stem, leaf and root cutting.
       As a result commercial IAA (synthetic auxin) is used by horticulturist and floriculturist for formation of
       roots. Higher concentrations of auxins are needed to induce adventitious root formation but the same
       concentration inhibits root elongation. Again the development of prop root system in horizontally
       spreading branches of Ficus is under the control of IAA. Stilt roots of Pandanus, Zea, Saccharum, etc. are
       produced by the activity of auxins.
    4. Phototrophism: Stem tips move towards the source of light It is referred to as phototrophism. IAA
       regulates phototrophism and light plays the key role. Growth curvature of the stem is due to unequal
       distribution of IAA in the organ. However, IAA activates in low light intensity. In the light intensity (also in

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         presence of riboflavin, xanthophylls, etc). IAA is photo oxidized to 3-methyl-2-auxoindole and loses its
         activity.
   5.    Geotrophism: Previously it was thought that auxins and thylene together cause geotrophism. Recent
         investigations have put forwarded the view that ABA produced within the root cap in the geotrophic
         responses. Fransses and Bruinsma (1981) suggested that the curvature is respondible for the unequal
         distribution of IAA and not the vice-versa.
   6.    Senescence and abscission: Ethylene promotes the synthesis of certain cell wall degrading enzymes e.g.:
         Cellulase, Pectinase, etc. These enzymes are responsible for the formation of abscission layer. But IAA
         reverses the effect of formation of abscission layer. But IAA retard senescence and abscission. Developing
         fruits, maturing seeds etc., synthesize large amount of IAA, which prevents the premature senescence of
         these plant parts.
   7.    Apical dominance: Auxins and several other substances are credited for the maintenance of apical
         dominance. Philips (1975), Rubinstein and Nago (1976). Hillman et al. (1984), reviewed the apical
         dominance phenomenon of plants. Higher concentration of IAA in plants inhibits the lateral bud
         formation. Apical meristems supply the higher amount of IAA to the lateral buds, which prevents further
         growth. Cytokinin probably plays the opposite role, i.e., inhibits apical dominance by preventing the
         basipetal growth of the vascular bundles of lateral buds.
   8.    Parthenocarpy: Fruit formation without the act of fertilization is called Parthenocarpy. Parthenocarpic
         fruits obviously lack seeds. Parthenocarpic fruits are usually rich in IAA content. It is indirect evidence in
         favor of the involvement of IAA in parthenocarpy.
   9.    Flowering: Auxin induces flowering in some long day plants under short day condition, e.g., Ananas. In
         general IAA inhibits flowering in short day plants.


           Antiauxin
                   Chemical substances that inhibit the auxin activity are called antiauxins. 2,3,5-triiodobenzoic acid
           (TIBA), Napthyl pthalmic acid (NPA), Indole isobutyric acid, N-acetyl indole acetic acid is few antiauxins.

           Agricultural Utilization

                     The commercial root setting powder contains IBA (Indole butyric acid). These root setting powder
           are sold in the trade names as Seradex, Suratex, Arodix, etc.


           What is the biological significance of auxins?

                      In lower concentrations do they aid the coleoptile’s elongation, that of the shoot and the roots. If the
           concentration becomes higher, the effect reverses and elongation of root and shoot is inhibited. The reason is a stimulation
           of ethylen production, a gaseous hydro-carbon that is a plant hormone, too. One of its effect is the inhibition of elongation.
           Auxins are involved in the differentiation of vascular bundles, they control abscission, induce beta-1,4-gluconases in pea
           roots, and stimulate the opening of tree buds as well as the rapid growth of young shoots. They do also increase the rate of
           cell division within the cambium, i.e. they stimulate secondary thickening. Furthermore do they aid the development of
           ovary into fruit, and they are responsible for the evolvement and the maintenance of apical dominance.

           Auxins increase the plasma current, the plasticity of the cell wall, and they cause a proton efflux out of the cell. This list of
           activities is far from being complete, but it shows how varied the effects of auxins are.




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                                        GIBBERELLIN
         It is of universal occurrence in the plant kingdom. Total 92 different types of gibberellins have been
described. Kurosawa (1926) first time noted the existence of gibberellin in a fungus Gibberella fuzikuroi of
Ascomycetes (imperfect stage of Fusarium moniliformae) infected rice plant. This is popularly known as Bakanae
disease of rice in which internodes of the infected plant become extensively long. However, commercially
gibberellin is isolated from Spheaceloma manihoticola of Ascomycetes. Gibberellic acid (GA) is generally designed GA3
and it is a very potent gibberellins. The designed GA3 and ii is C20(or 19) H19O6. It is a ditarpenoid containing four
isoprene units (building rocks of terpenes). Gibberellin has four rings (A, B, C, D) and sometimes another ring
(lactone ring) is also present.

Historical Aspect:

        In 1926 studied the Japanese E. KUROSAWA a rice disease that is known as the ‘foolish seedling’-
disease in Japan. The plants grow extremely fast, look spindly and pale and break off easily. KUROSAWA
detected that the reason for this abnormal growth is a substance that is secreted by a parasitic fungi (Fusarium
moniliforme = Gibberella fujikuroi). It was termed gibberellin.

        During the thirties was gibberellin isolated and crystallized by Japanese scientists from Tokyo
(YABUTA and SUMIKI), though it was almost forgotten in the following years. In 1956 isolated C. A. WEST
and B. O. PHINNEY a gibberellin from Phaseolus vulgaris and other plants, thus showing that these
compounds are far-spread in the plant kingdom. Today are more than 110 different gibberellins known (GA1,
GA2,....GA3, GA4.....GA110) that differ only little chemically but very much in their biological activities.

Structure of Gibberelline:




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Biosynthesis and Transport:
        Gibberellic acids are syntheses from mevalinic acids. Transport of gibberellin takes place through xylem
and phloem, i.e., through the vascular tissues.

Mode of action:
According to Salisbury and Ross (1992) these are three modes of action of gibberellins. These are:
       i)       Cell division takes place just below the apical region of the stem. Gibberellin reduces the cell cycle
                time and as a result cell elongation takes place.
       ii)      Gibberellin induces synthesis and activity of hydrolytic enzymes, which in turn hydrolyze cell wall
                polysaccharides and increase the amount of soluble sugars. Thus respiration rate and osmotic
                potential increases. It causes loosening of cell wall polysaccharides.
       iii)     Gibberelln induces the synthesis of cell wall plastic materials. As a result the plasticity of the cell
                wall increases.

Physiological role:

    1. Stem cell elongation: Gibberellin induces internodes elongation in plants. As a result the total length of a
       plan is increase within a short interval.
    2. Expansion of leaves: Gibberellin induces the increase in diameter as well as surface area of leaf blade.
    3. Synthesis of hydrolytic enzymes: Gibberellin increases the synthesis and activity of hydrolytic enzymes,
       which in turn hydrolyze the cell wall polysaccharides and help in cell elongation.
    4. Flowering: Gibberellins induces flowering in long day plants under short day condition e.g., Bryophyllum. It
       also induces flowering in some long day rosette plants (the process is botanically known as blotiing), e.g.,
       Nicotina plumbaginifolia.
    5. Seed and bud dormancy: Gibberellin induces the breaking of dormancy in buds and seeds of varius
       plants. In these cases they at as germination promoter and prevent the activities of abscisic acid.
    6. Senescence: Gibberellin prevents senescence of immature plant parts by preventing the activities of
       abscisic acid and ethylene.
    7. Adventitious root formation: In some plants it helps in adventitious root formation.
    8. α-Amylase activity: Gibberellins are synthesized within the tissues of the germinating embryo. It is then
       transported to the aleurone layer where they produce endospermic reticulum. It is the pertinacious part for
       nourishment of the embryo.
    9. Parthenocarpy: Gibberellins induces seedless fruit formation in some plant species.




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                                      CYTOKININ
   Historical Aspect:

                It has been tried for a long time to cultivate plant tissue on artificial nutrient medium. The first
       approaches go back to the Austrian plant anatomist G. HABERLANDT (1854-1945, professor at Graz,
       later at Berlin). At first posed the composition of a suitable nutrient medium large problems. It was the
       Dutch plant physiologist J. v. OVERBEEK who discovered in 1941 that the addition of coconut milk
       causes a drastic increase in the growth of plant embryos and tissue cultures. Coconut milk is an
       endosperm product that has under natural conditions, too, a growth-stimulating effect on the developing
       coconut embryo. The question which components cause the growth stimulation arose immediately.

              In contrast to auxin is not elongation but growth by cell division stimulated. In 1955 discovered
       C. O. MILLER and F. SKOOG from the University of Wisconsin at Madison that aged or autoclaved
       DNA preparations have the same effect, while fresh DNA preparations display no effect at all. In the
       end was the adenine derivative 6-furfurylaminopurin (= kinetin) identified as the effective substance.

            Haberlandt (1913) first time noted the presence of cell division promoting substance in phloem tissue
       s. skoog (1945-55) established the occurance of cytokinin in plant. Both natural (eg.: Zeatin) and synthetic
       (e.g. Kinetin) cytokines are known to occur. In plants cytokinin occurs in two forms viz, free form and the
       “cytokinin-tRNA conjugates”.
           Each cytokinin can exist in the ‘free-base’ form or as a ‘nucleoside’, in which a ribose group is attached
       to the nitrogen atom of position nine. Again, the nucleosides can be converted to ‘nucleottides’, in which
       phosphate is esterified to the five carbon of ribose. But all these nucleotides seem to be less abundant than
       the free-base or nucleoside form.


   Structure:




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   Synthesis and transport:
           Cytokinin is of universal occurrence in the plant kingdom including bacteria and fungi. Some occur in
       tRNA (sometimes also r-RNA), however, with an unknown function. The root0tip is the main site of
       cytokinin biosynthesis. Mevalonic acid and the nucleotide base Adenine are two precursor of cytokinin.

          Cytokinins contain adenine bases. So it is basic in mature. Since it is basic in nature, cell-to-cell
       movement of cytokinin is very slow, i.e., immobile.
   Physiological role:
       1. Cytokinesis: The action of cytokinin is specific in mitosis; the specificity of cytokinin in the induction
          of cytokinesis is well established.
       2. Morphogenesis: Cytokinin is essential for morphogenesis in tissue culture. Formation of shoots and
          adventitious roots by the callus is called Organogenesis. A proper ratio of cytokinin (as well as other
          hormones) is chosen for the development of an entire plant from a callus.
       3. Senescence: Cytokinin content of a leaf is decreased when a mature leaf is removed from a plant.
          Immediately senescence occurs in that particular leaf. But the application of kinetin retard senescence.
          Richmond and Lang (1957) noted this effect for the first time in Xanthium pennsylvannicum. It is
          popularly known as “Richard and lang effect” Richmann (1987) suggested that cytokinins play the
          key role in maintaining the integrity of the tonoplast membrane. Otherwise, proteases from the

                                                   ETHYLENE
        Ethylene (C2H4) is a tiny molecule and gaseous in nature. It is not basic or acidic in nature. Each and every
   cell has the ability to synthesize the hormone ethylene, i.e., it has no distinct site of synthesis. Define transport
   mechanism of ethylene is absent and it is taken place by cell to cell diffusion.

   Biosynthesis:
       Ethylene is synthesized from the carbons three and four of an amino acid ‘methionine’. Also, an unusual
   amino acid like compound ‘1-amino-cyclopropane-1-carboxylic acid’ (ACC), is known to involve as a close
   precursor of ethylene.

   Physiological role:
                 1. Fruit ripening: Ethylene promotes fruit ripening. On the basis of the respiration pattern,
                     Biade (1960) divided fruits into two categeries. These are ‘climacteric fruits’ and ‘non-
                     climateric fruits’. In climacteric fruit there is a gradual rise of respiration rate after harvest,
                     e.g., banana, apple, melon, mango. In non-climateric fruit there is no respiratory rise after the
                     harvest of ripe fruits, e.g., cherry, grape, fig, cirtrus, orange, pineapple.
                               Climatic Fruits                                Non-climatic fruits
                  I. Ripe fruits show respiratory rise after I. No respiratory rise is seen after
                  harvest.                                         harvest.
                  II. Endogenous ethylene production II. No such changes in the level of
                  increases as a result of the increase in endogenous ethylene production is
                  respiratory rate.                                taken place.
                  III. These fruits are insensitive to The endogenous ethylene level is low
                  exogenous ethylene because here the and for this reason fruits are sensitive
                  saturation level is rich.                        to exogenous supply of ethylene.



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                  2. Flowering: In general ethylene inhibits flowering but it induces flowering in mango, pineapple
                     and plumbago.
                  3. Sex expression: Ethylene affects on floral expression of monoecious species, e.g., cucurbits
                     like squash, melon, pumpkin, etc. Ethylene strongly promotes female flower formation in these
                     plants.
                  4. Inhibition of stem growth: Neljubow (1901) first established that ethylene affects stem
                     growth. He noted that ethylene inhibits stem elongation but increases stem thickening, i.e.,
                     radial expansion and leads to a horizontal growth habit of the plant. But interestingly, certain
                     dicots and acquatic ferns respond to ethylene by enhancing stem elongation, e.g., Ranunculus
                     scelerotus, Regnellidium diphyllum, etc.
                  5. Senescence and abscission: Ethylene promotes senescence and abscission of leaves, flowers,
                     buds, fruits, etc.
                  6. Leaf-growth: Leaf expansion is inhibited by ethylene. Ethylene also of leaves, flowers, buds,
                     fruits, etc.
                  7. Adventitious root formation: Ethylene also promotes adventitious root formation on stems,
                     especially in tomato.
                  8. Epinapsy: Ethylene causes the leaf epinasty i.e., drooping down of leaves. This is caused due
                     to the swelling of cells on the upper parts of the petiole, e.g, leaves of tomato, potato, pea,
                     sunflower, etc.


                                           ABSCISIC ACID
         Abscisic acid (ABA) is a fifteen-carbon bearing sesquiterpene. The empherical formula of ABA is
C15H20O4. Naturally occurring ABA is entirely dextro rotatory (+) in nature while the synthetic ABA is the resimic
mixture (±). Milborrow (1968) found that resemic mixture biologically active. ABA possesses an asymmetric carbon
at the carbon one position of the cyclohexane with a double bond to which the acidic side chain is attached.

Biosynthesis and transport:
          ABA is present in all angisperms, gymnospems. It is generally absent in algae, fungi, bryophyte,
pteridophytes and totally absent in bacteria. In algae and bryophytes the function of ABA is performed by ‘lunularic
acid’. Biosynthesis of ABA in most plants occurs indirectly by degradation of certain carotenoids presents in
chloroplastids or other plastids. ABAs also follow mevalonic acid pathway for their synthesis. The sites of synthesis
are fruit tissues, leaves, roots and seeds.
         ABA is transported through xylum and phloem tissues. Parenchymatous cells outside the vascular tissue
also transport ABA.

Physiological role:
    1. Stomatal Closure:
            Land plants protect themselves against excessive water loss by reducing the size of the stomatal
       opening and ABA plays the key role in this respect. Exogenous application of low concentration of ABA to
       leave causes stomatal closure within 3-9 minutes under water stressed condition. It is also noticed that ABA
       content of leaves rises substantially when leaves are subjected to water stress.
            ABA causes stomata to close by inhibition of an ATP-dependent proton pump in the plasma
       membrane of guard cells. This pump normally transfers protons out of the guard cells, leading to rapid
       influx and accumulation of K+ and then to osmotic water absorption and result stomatal opening.
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       However, ABA, acting in the free space on the outer surface of guard cell plasma membrane, shuts off K+
       influx. As a result, water leak out which reduce the turgor pressure and cause stomatal closure.

   2. Bud and Seed dormancy:
          ABA appears to play a role in the dormancy of seeds and buds. It has been shown that the dormant
      immature embryo of Taxus buccata can be induced to germinate without scarification if the embryo is placed
      in a nutrient solution which causes an ABA like compound to be leached from it. ABA also inhibits the
      sproughting of potato tubers.

   3. Growth aspect:
          ABA in general inhibits plant growth. However, it induces perthenocarpy in Rosa and adventitious root
      formation in poinsettia. It also causes elongation of the hypocotyle of cucumber.

   4. Apical dominance:
          Application of ABA on the apical portion causes growth of lateral branches and suspension of apical
      buds. So it may be concluded that ABA inhibits the effect of IAA in case of apical dominance.

   5. Abscission:
          ABA has regulatory role in abscission of flowers, fruits and leaves. However, Osorn (1989) concluded
      that ABA probably has no direct role in abscission as ethylene. It acts indirectly by causing premature
      senescence of the cells in the organ that is shed which in turn provokes are a rise the production of
      ethylene.

   6. Defense against cold and salt stress:
         ABA hardens plants against frost damage and against excess salt in soil.

   7. Tuberisation: Application of ABA induces tuberization in Dalia, potato, etc.

   8. Fruit ripening: Externally applied ABA accelerates the ripening of young fruits.

   9. Protein synthesis: ABA increases protein synthesis in immature embryo of Brassica napus.




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