HORMONE
Greek: hormon |
to excite |
Naturally occurring, signalling molecules that exert a profound influence on physiological processes |
Produced in tiny amounts by one part of an organism's body and transported to other parts, where it binds to a specific receptor and triggers responses in target cells and tissues. |
Phytohormones
Principal means of intercellular communication within plants |
Produced within plants, and are effective at extremely low concentrations |
Transported to different parts of the plants to perform various physiological functions |
Plant hormones control growth, flowering, fruiting, aging, and even death. |
Effect of a particular hormone is concentration dependent; hormones may have different effects at different concentrations. |
Like animal hormones, plant hormones affect target cells via receptor proteins. |
Plants regulate levels of hormones by altering precursors, transport, inactivation, breakdown, or storage. |
TYPES OF PHYTOHORMONES
auxin |
gibberilins |
cytokinins |
Ethylene |
Abscissic Acid |
Brassinosteroids |
Strigolactones |
AUXIN
AUXIN: THE GROWTH HORMONES |
Auxin was the first plant hormone to be discovered. |
Greek word auxein |
means "to increase or to grow.“ |
Enlargement of plant cells. |
Indole-3-acetic acid (IAA) |
is the most widely distributed natural auxin |
Promotes production of |
Shoot apical meristems |
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young leaves |
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root tips |
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germinating seeds |
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fruits |
PHYSIOLOGICAL EFFECTS OF AUXIN
Auxins promote cell elongation of stems and coleoptiles |
Coleoptile |
is the pointed protective sheath covering the emerging shoot of monocots |
Phototropism is mediated by the lateral redistribution of auxin |
Gravitropism involves lateral redistribution of auxin |
Auxin promotes apical dominance |
Auxin promotes the formation of lateral and adventitious roots |
Auxin delays the onset of leaf abscision |
When the level of auxin declines, a special layer of cells — the abscission layer — forms at the base of the petiole. |
Auxin promotes fruit development |
Gibberellins
Gibberellins: Regulators of Plant Height |
discovered by |
Ewiti Kurosawa |
Causes |
Internodal elongation known as the ‘bakanae’ or ‘foolish seedling’ disease of rice |
Isolated from |
fungus (Gibberella fujikuroi) |
Stimulate stem elongation |
Promotes PRODUCTION of: |
Meristems of apical buds and roots, |
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young leaves |
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developing seeds |
PHYSIOLOGICAL EFFECTS OF GIBBERELLINS
Gibberellins Stimulate Stem Growth in Plants. |
Gibberellin application results in bolting (stem growth) |
"Foolish rice" seedlings, suffer from an overdose of gibberellins normally found in plants in lower concentrations |
Gibberellins promote fruit set and parthenocarpy |
GA promote early seed development and germination. |
Gibberellins mobilize nutrients during seed germination |
Cytokinins : Regulators of Cell Division
Discovered in the search for? |
factors that stimulate plant cells to divide |
The most common natural cytokinin is? |
Zeatin, |
because it was discovered first in? |
(Zea mays) |
PHYSIOLOGICAL EFFECTS OF CYTOKININS
Auxin :Cytokinin regulates root and short initiation in callus tissues |
Cytokinin stimulates the Growth Of Axillary Buds |
Cytokinins Delay Leaf Senescence |
Leaf senescence is delayed in a transgenic tobacco plant containing a cytokinin biosynthesis gene, ipt. The ipt gene is expressed in response to signals that induce senescence. |
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Ethylene: The Gaseous Hormone
Discovered in the early |
1900s |
as a ? |
fruit ripening |
Not required for? |
normal vegetative growth |
Synthesized primarily in? |
in response to stress and may be produced in large amounts by tissues undergoing senescence or ripening |
Promotes production of? |
Fruit ripening |
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Senescence |
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Leaf abscission |
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Wounds and stress |
PHYSIOLOGICAL EFFECTS OF Ethylene
Ethylene stimulates fruit ripening. |
Fruits that ripen in response to ethylene exhibit a characteristic respiratory rise called climacteric before the ripening phase |
Ethylene triggers ripening, and ripening triggers more ethylene production-a rare example of positive feedbackmechanism |
As apples ripen, they release ethylene. Over- ripened apples release the hormone in high amounts, causing other apples stored nearby to ripen faster and rot sooner. |
Ethylene promotes senescence and leaf abscission. |
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auxin from the leaf prevents abscission |
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the amount of auxin from the leaf decreases and the ethylene level rises |
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Synthesis of enzyme that hydrolyze the cell wall polysaccharides, resulting in cell separation and leaf abscission |
Ethylene instigates triple response |
slowing of stem elongation |
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thickening of the stem |
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Curvature that causes the stem to start growing horizontally |
Ethylene regulates epinasty |
Abscisic Acid
Abscisic Acid: A Seed Maturation and Antistress Signal |
Accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths |
PHYSIOLOGICAL EFFECTS OF AA
Abscissic acid induces seed and bud dormancy |
ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins |
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ABA accumulates in dormant buds as an adaptive feature in cold climates |
ABA Closes Stomata in Response to Water Stress |
ABA binding leads to influx of Calcium and the opening of potassium channel |
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Potassium ions exits the guard cells and water follows. Guard cells become flaccid, closing the stomatal aperture |
Strigolactones
Strigolactones are signaling compounds made by plants. |
2 Main functions: |
as endogenous hormones to control plant development |
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as components of root exudates to promote symbiotic interactions between plants and soil microbes. |
Some plants that are parasitic on other plants have established a third function, which is to stimulate germination of their seeds when in close proximity to the roots of a suitable host plant. |
Brassinosteroids
Brassinosteroids (BRs) as a class of steroid plant hormones participate in the regulation of numerous developmental processes, including root and shoot growth, vascular differentiation, fertility, flowering, and seed germination. |
Brassinosteroids (BR) and gibberellins (GA) promote seed germination of these species and counteract the germination-inhibition by abscisic acid (ABA). |
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