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Plant Growth Regulators
Chemical Messengers
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They provide for plant growth and development
Hormones Hormones are “chemical messengers” directing plant development & survival. They provide for plant growth and development
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Other methods of regulation are accomplished by
Chemical reactions to stimuli Sunlight on shoots Water on roots Insects (chewing) on leaves Stimulate signal transduction pathways
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Hormones = Growth Regulators
6 main plant hormones 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene 5. Brassinosteroids 6. Abscisic acid
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A good example of hormones at work: Etiolation
Plant growth in the absence of sunlight Environmental “awareness” Accomplished by hormones.
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Auxins Stimulate cell growth thru cell elongation
Produced in apical & lateral buds & meristems Auxins move mainly from apex (top) down. Auxins
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Remember zones of elongation?
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To demonstrate: Cells elongate under the auxin
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Auxins Greatest concentration in terminal bud.
Ensure apical dominance. Lateral bud growth is inhibited by other hormones
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In addition…(auxin’s roles)
Flower development Root growth (Rootone) –
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Indoleactic acid (IAA)(natural).
examples of Auxin Indoleactic acid (IAA)(natural). 2,4-dichlorophenoxy acetic acid (2-4D)(synthetic)
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Next: Gibberellins (Gibberellic Acid GA)
Stimulates cell division in stem, root, & leaf growth
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Gibberellic acid LOOK! Functional groups!
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Brassinosteroids are also growth regulators
(no need to know) Brassinosteroids are also growth regulators Cell elongation ( sound familiar?) Vascular differentiation Stress tolerance
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Cytokinins regulate growth
Produced in roots Transported throughout plant. Affect cell division, apical dominance
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Cytokinins Esp. in root meristems
Interact with auxins, stimulate bud formation.
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Cytokinins inhibits leaf senescence (seasonal leaf drop).
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Ethylene gas Diffuses easily throughout a plant
Facilitates ripening of fruits, apoptosis (?) leading to loss of leaves & flowers.
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Ethylene (gas) “the aging hormone”
In fruits & leaves, breaks down chlorophyll and lets colors show through.
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Plant growth Inhibitor
Abscisic Acid (ABA) Widespread in plants. moves freely through plant ABA made by the leaves. Interacts with other hormones in the plant; counteracts the growth effects of auxins & gibberellins.
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Abscisic Acid Predominant during plant dormancy
Produced when plants are under stress
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Abscisic Acid Maintains seed dormancy Responsible for seed germination occurring only under favorable conditions
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Additional ways plants respond to changes in their environment: It ain’t all hormones!
The role of photoreceptors and signal transduction pathways
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PHYTOCHROMES: 1 isomer stops growth, the other (far red) starts it.
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Photoperiodism Short day plants Long day plants
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Experiments have confirmed that it is actually the length of darkness, not light, that affects flowering Greenhouses apply this science to get flowers to be ready for certain seasons
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Other environmental stimuli
(phototropism) Gravitropism Thigmotropism Temperature stresses Responses to insect infestations Plants produce R gene products
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