Plant Growth Regulators new 1


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Plant growth regulators are described briefly here enlighting on the role of plant growth regulators for increasing secondary metabolites production


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Plant Growth Regulators : 

Plant Growth Regulators Mr. Mangesh D.Wandhare M.Pharm (Pharmacognosy & Phytochemistry) Assistant Professor, Channabaweshwar Pharmacy College ( Degree) Latur ( M.S.)

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The growth & development of plants is regulated by a number of chemical substances which together exert a complex interaction to meet the needs of the plant. These chemical substances regulates plant growth ( either stimulates or inhibits growth ) & as such commonly referred to as “Plant Growth Regulators” These may be Endogenous or Exogenous substances. Five groups of Plant hormones are well established Auxins Gibberellins Cytokinins Ethylene Abscisic acid & its derivatives

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Characteristics of Plant Growth Hormones Specific in their action Active in very low concentrations Regulation of cell enlargement, cell division, cell differentiation, organogenesis, senescence & cell dormancy. Role of Plant Growth Hormones In Cell & Tissue cultures Production of secondary metabolites ( better yield ) These substances occur in all higher plants.

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Auxins Studied in 1931 by Dutch workers by isolating two growth regulating acids ( auxin – a & auxin – b) obtained from human urine & cereals respectively These had similar properties to indole – 3 - acetic acid ( IAA) Indole – 3 - acetic acid ( IAA) found to be major auxin of plants, found particularly in actively growing tissues Precursors like indoleacetaldehyde, indoleacetonitrile & indolepyurvic acid These all are derived from Tryptophan.

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Effects of Auxins Cell elongation i.e. stem length Inhibition of root growth Adventitious root production Fruit - setting in the absence of pollination In Tissue culture technique auxins plays vital role since auxins is responsible for growth as well as secondary metabolite production in excised plants.

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Mechanism of action Oxidative degradation of IAA to give number of products is controlled by IAA Oxidase enzyme Ortho - diphenols inhibit the action of enzyme hence stimulation of growth by themselves Monophenols promote the action of the enzyme & thus inhibit growth.

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Presence of IAA in Plants IAA present in conjugation with aspartic acid, glutamic acid, glycine, sugars & cyclitols serving inactive storage forms of hormone for plant detoxification mechanism. Synthetic Auxins Naphthalene -1-acetic acid ( NAA), 2,4 – dichlorophenoxyacetic acid ( 2,4 –D), indole-3-butyric acid. Practical Uses IBA & NAA in combination are used in rooting of cuttings 2,4 –D & 2,4,5 –T are used both as plant growth regulators & in higher concentrations as selective weed killers especially for dicot plants The addition of IAA, NAA, 2,4 - D in tissue cultures of ergot has led to increase yield of indole alkaloids. IAA & derivatives of IAA are found to be most important Plant growth hormones in excised parts of plant.

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Gibberellins These are endogenous plant growth regulators About 40 gibberellins occur in plants while others are present in some fungi Kurosawa, a japanese physiologist is credited for initiating the discovery of gibberellin from fungus Gibberella fujikuroi grown on rice in 1926 According to Paleg , gibberellins are compounds having gibbane skeleton & biological activity in stimulating cell division or cell elongation or both Yabuta & Hayashi isolated a crystalline sample of of the active material which they were called “ Gibberellins”

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Chemistry of Gibberellins Gibberellins are tetracyclic diterpene acids

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Functions of Gibberellins Stem elongation Bolting Seed Germination Breaking of seed dormancy Parthenocarpy Increasing Fruit Size Flowering and sex expression Fruit growth and parthenocarpy Delayed ripening Flowering Malting

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Mechanism of action Acts by inducing activity of gluconeogenic enzymes during early stages of seed germination Gibberellins also induces the synthesis of α – amylase & other hydrolytic enzymes during germination of monocot seeds Gibberellins also mobilizes seed storage reserves during germination & seedling emergence.

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Effect of Gibberellins on Secondary Metabolite Production Kaul & Kapoor reported 33 % increase in Chenopodium ambrosioides while 50 % increase in Anethum spp. & 30 % increase in A.sowa At lower doses of GA treatment there was no significant changes in the carvone content of the oil, but at higher concentration there appeared to be a slight increase over the official limit ( 53%, B.P.1958) Reduction in alkaloid content has been noted in Vinca, Datura, Hyoscyamus, Duboisia species. No any change in Fixed oil content observed after treatment of Gibberellins.

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Cytokinins Cytokinins (CK) are a class of Plant growth substances that promote cell division or cytokinesis , in plant roots & shoots They are involved primarily in cell growth and differentiation but also affect apical dominance , axillary bud growth & leaf senescence Chemistry of Cytokinins

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The naturally occurring cytokinins are zeatin, N6 dimethyl aminopurine & N6 – Δ 2 - isopentenyl aminopurine The synthetic cytokinins are Kinetin , adenine, 6 – benzyl adenine , benzimidazole & N, N’ - diphenyl urea

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History In 1913, Gottlieb Haberlandt discovered that a compound found in phloem had the ability to stimulate cell division (Haberlandt, 1913). In 1941, Johannes van Overbeek discovered that the milky endosperm from coconut also had this ability. He also showed that various other plant species had compounds which stimulated cell division (van Overbeek, 1941). In 1954, Jablonski and Skoog reported that vascular tissues contained compounds which promote cell division (Jablonski and Skoog, 1954). The first cytokinin was isolated from herring sperm in 1955 by Miller and his associates (Miller et al., 1955). This compound was named kinetin because of its ability to promote cytokinesis. Hall and deRopp reported that kinetin could be formed from DNA degradation products in 1955 (Hall and deRopp, 1955). In 1961 Miller isolated natural cytokinin from corn ( Zeatin )

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Biosynthesis & Metabolism of Cytokinins A product of the mevalonate pathway called isopentenyl pyrophosphate is isomerized.  This isomer can then react with adenosine monophosphate with the aid of an enzyme called isopentenyl AMP synthase resulting to isopentenyl adenosine-5'-phosphate (isopentenyl AMP). This product can then be converted to isopentenyl adenosine by removal of the phosphate by a phosphatase and further converted to isopentenyl adenine by removal of the ribose group. Isopentenyl adenine can be converted to the three major forms of naturally occurring cytokinins. Other pathways or slight alterations of this one probably lead to the other forms. Degradation of cytokinins occurs largely due to the enzyme cytokinin oxidase.

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Mechanism of Action Kinetin are reported to play the role in nucleic acid metabolism & protein synthesis In plant metabolism ,it is proposed that some t – RNA contain cytokinin like activity They have an action on some enzymes responsible for formation of certain amino acids Cytokinins are reported to increase markedly Sennoside content in Tinnevelly Senna leaves In opium , reduce alkaloid content. In Duboisia hybrids, the cytokinin activity present in extract of a seaweed , shows marked increase in hyoscine content. Effects on production of secondary metabolites

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Cytokinins act in concert with auxin, another plant hormone stimulating Cell expansion

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Ethylene Ethylene serves as a hormone in plants.  It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves. Commercial ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (68F) has been seen to produce CO2 levels of 10% in 24 hours.

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History of ethylene in plant biology Ethylene has been used since the ancient Egyptians, who would gash figs in order to stimulate ripening (wounding stimulates ethylene production by plant tissues). In 1864, it was discovered that gas leaks from street lights led to stunting of growth, twisting of plants, and abnormal thickening of stems. In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene. Doubt discovered that ethylene stimulated abscission  in 1917.  In 1934 Gane reported that plants synthesize ethylene.  In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as senescence of vegetative tissues.

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Ethylene biosynthesis in plants The enzymatic oxidation of ACC produces ethylene in plants. Ethylene is biosynthesized from the amino acid methionine to S-Adenosyl – L - methionine  (SAM) by the enzyme Met Adenosyl transferase. SAM is then converted to 1- aminocyclopropane-1-carboxylic-acid (ACC) by the enzyme ACC Synthase (ACS) The final step requires Oxygen and involves the action of the enzyme  ACC Oxidase (ACO)

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Growth responses due to Ethylene effect Fruit ripening Leaf abscission Stem swelling Leaf bending Flower petal discoloration Inhibition of stem & root growth

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Commercial Uses For promotion of Flowering & fruit ripening. Induction of fruit abscission, breaking dormancy & stimulation of latex flow in rubber trees.

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Abscissic acid ( ABA) It is a Plant Growth Inhibitor A diffusible abscission accelerating substance was found by Osborne (1955) in senescent leaves Carns et al. isolated several abscission accelerating substances from cotton plants & named them as abscission I & abscission II

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Biosynthesis Abscisic acid (ABA) is an isoprenoid  plant hormone, which is synthesized in the  MEP Pathway The C15 backbone of ABA is formed after cleavage of C40 carotenoids MEP.  Zeaxanthin is the first committed ABA precursor; a series of enzyme catalyzed epoxidations and isomerizations  via  vialoxanthin Final cleavage of the C40 Carotenoid  by a dioxygenation reaction yields the proximal ABA precursor, xanthoxin, which is then further oxidized to ABA. Abamine has been designed, synthesized, developed and then patented as the first specific ABA biosynthesis inhibitor, which makes it possible to regulate endogenous level of ABA.

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Location and timing of ABA biosynthesis Released during desiccation  of the vegetative tissues and when roots encounter soil compaction Synthesized in green fruits at the beginning of the winter period Synthesized in maturing seeds, establishing dormancy Mobile within the leaf  and can be rapidly translocated from the roots to the leaves by the transpiration stream in the xylem Produced in response to environmental stress , such as heat stress, water stress, salt stress Synthesized in all plant parts, e.g., roots, flowers, leaves and stems.

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Mechanism of Action It inhibit the GA induced synthesis of α – amylase & other hydrolytic enzymes During maturation , ABA accumulates in many seeds & helps in seed dormancy ABA concentrations are found to be enhanced in stress conditions like , mineral deficiency , injury , draught & flooding ABA serves as an potential antitranspirant by closing the stomata, when applied to leaves

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Effects Antitranspirant - Induces stomatal  closure, transpiration to prevent water loss. Inhibits Fruit ripening Responsible for seed dormancy by inhibiting cell growth – inhibits seed germination Inhibits the synthesis of  Kinetin  nucleotide  Down regulates enzymes  needed for  Photosynthesis

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Other Plant Growth Retardants Maleic Hydrazide Daminozide etc. There commercial uses are yet to be reported Morphactins : Group of Synthetic substances which are potent inhibitor of auxin transport causing tropic responses , reduction of apical dominance & promoting lateral growth. e.g. Chloroflurecol methyl , Flurecol butyl & TIBA ( 2,3,5 – triiodobenzoic acid

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References Textbook of Pharmacognosy by Trease & Evans Sixteenth edition Textbook of Pharmacognosy by C. K .Kokate & S. B. Gokhale , Fourty Fourth edition Wikipedia

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Thank You