Plant growth hormones

Plant growth hormones

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  Plant Growth Hormones Presented By: DikshaKataria Asst. Prof. Lord Shiva College of Pharmacy
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  Introduction  These arethe organic compounds other than nutrients that affect the plant’s morphological structures and physiological processes in low concentrations.  These are also known as phytohormones or plant hormones and generally abbreviated as PGR’s.  They are signal molecules produced within the plant that regulate plant cell processes.  Plant hormones control all aspects of growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and through to reproductive development.  Each plant cell is capable of producing hormones.  The term ‘phytohormon’ was coined by Went and Thiman and used in the title of their book in 1937.
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  Plant growth hormones Nativesubstances are those substances which are produced within the plant and help in its growth and development. Synthetic substances have to be added from outside to the plant. Hence, they are called exogenous substances. Native substances (Endogenous) Synthetic substances (Exogenous)
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  Role of PlantGrowth Regulators: a) Regulates cell division and enlargement b) Helps in cell differentiation and organogenesis (organ synthesis) c) Senescence (state of ageing) d) Dormancy (plant part unable to develop like seed doesn’t germinate) e) Ripening of fruit f) Plant tissue culture g) Enhance the production of secondary metabolite like alkaloids, tannins, resins etc.
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  Five major classesof Plant Growth Regulators are: 1.Auxin 2.Cytokinins 3.Abscisic acid 4.Ethylene 5.Gibberellins
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  Auxins  Auxin isthe first plant hormone/ growth substance discovered. The term comes from the Greek word, “auxein” means to grow.  Auxins promote elongation of coleoptile tissues. It is of two types: a) Natural- These auxins are synthesized naturally in the plants. Example, Indole-3- acetic acid (IAA) a) Synthetic- These have the same action as natural auxins but not produced by plants and provided externally. Example, Indole-3-butyric acid (IBA), Naphthyl acetic acid (NAA), 2,4- dichlorophenoxy acetic acid (2,4-D).
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  Functions of Auxin: Thefollowing are some of the responses that auxin is known to cause- 1. Stimulates internode elongation 2. Apical dominance 3. Initiation of adventitious roots on stem cuttings. 4. Lateral root development in tissue culture. 5. Stimulates differentiation of vascular tissue (into phloem and xylem). 6. Stimulation of abscission of older plant parts or fruits 7. Delaying senescence 8. Mediates the tropistic response of bending in response to gravity and light
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  Applications of Auxins: 1.In vegetative propagation 2. Parthenocarpic fruit development 3. Used as weedicide (2,4-D) in high concentration 4. Dilute solution of auxin initiates flowering in plants
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  Cytokinins  Miller isolatedthe crystalline substance capable of inducing cell division in tobacco cultures and named it as ‘kinetin’, which was found to be 6- furfuryladenine.  Later, some other adenine derivatives were found having similar biological activity called ‘kinins’, collectively known as Cytokinins.  Cytokinins are of two types: a) Natural- Zeatin b) Synthetic- Kinetin These help in regulating cell growth activity.
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  Functions of Cytokinins: 1.Promotion of cell division 2. Embryo development 3. Expansion of cells in leaf discs and cotyledons 4. Breaks dormancy 5. Degradation of proteins in ageing leaves 6. Help in formation of certain amino acids 7. Nucleic acid metabolism
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  Ethylene  It isan organic molecule present in the form of a volatile gas.  It is present in low concentration in the plant, generally about 0.1ppm.  Ethylene is produced by burning carbon rich substances like natural gas, coal and petroleum.  Functions of Ethylene: 1. Ripening of fruits and flowers 2. Leaf bending 3. Flower discoloration 4. Breaking dormancy 5. Stimulation of latex flow in rubber trees
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  Abscsic Acid (ABA) A diffusible abscission accelerating substance was found by Osborne in 1955 in senescent leaves.  During maturation, ABA accumulates in seeds and helps in seed dormancy.  ABA concentrations are found to increase in stress conditions like injury, mineral deficiency, drought and flooding.  It acts as a potential anti-transpiration agent by closing the stomata, when applied to leaves.  It helps in shedding or retaining of leaves, stems, flowers and fruits.
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  Gibberlins  They area class of endogenous plant growth regulators  At present 50 gibberlins are known (40 are of plant origin, 10 are from fungi) and denoted as GA1, GA2, GA3 etc.  GA denotes gibberllic acid  Kurosawa, a Japanese physiologist, discovered gibberlins from fungus Gibberella fujikuroi grown on rice  Present in young tissue, root, embryo, germinating seeds, shoot buds etc.
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  ➢ The commercialformulation of gibberlins are used for promoting vegetative parts and fruit growth, breaking dormancy, flower initiation and induction of parthenocarpy. ➢ Functions of Gibberllins: ▪ Rapid expansion of plant cells ▪ Stimulation of seed germination ▪ Breaking dormancy ▪ Stem elongation ▪ Increase in size of leaves ➢ Gibberlins effect cell division similarly like auxins. It is observed that gibberlins act in intact plants while major auxin effects are on excised parts.