Download Plant Hormones

Biotechnology II
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What is a plant hormone?
How are plant hormones are involved in the
plant’s signal transduction mechanism?
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Compound produced by one part of an
organism that is transported to other parts
where it triggers a response in target cells
and tissues
Considered to be a chemical signal that
stimulates/inhibits biological response(s)
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Growth and development by controlling
 Division
 Elongation
 Cell differentiation
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Involves four stages:
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Signal- environmental or chemical
Reception- signal detection; commonly, on cell membrane
Transduction – amplification of the signal within the cell
Cellular Response –
▪ gene expression- turning on/off transcription
▪ changes in enzyme activity- more active/less active
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Chemical – hormones,
nutrients (e.g.
glucose)
Effect of hormone Gibberellic Acid on
Internode elongation in plants
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Present in low concentration (10-10M)
Differ in where they are made and their mode
of delivery
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Detect by the presence of receptor
Receptor has a binding site complementary
to signaling molecule, often referred to as a
ligand
Binding of the signal generally causes the
receptor protein to change in shape
Triggers the production of secondary
messengers, which amplify the signal
Allows large biological response
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Stimulate seed germination
Inhibit seed germination
Promote cell elongation
Promote fruit ripening
Phototropism – bending towards the light
Came from series of classic experiments
studying phototropism
 Plant used for these studies: grass seedlings,
specifically oats
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Charles Darwin and son, Francis (1880)- observed
that the tip of coleoptile was required to bend
towards light
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Boysen-Jensen’s Experiment (1913) – showed
that the signal was a chemical passing down
from the coleoptile tip
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F. W. Went (1926) – extracted the growth
stimulus.
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Amount of auxin in coleoptile tips too small to purify;
Went developed bioassay to determine relative amounts
Determined that the degree of curvature was
proportional to amount of auxin activity
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Auxin (IAA)
Cytokinins (zeatin)
Gibberellins (GA3)
Abscisic Acid
Ethylene
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Apical meristem major site of auxin
production
Stimulates cell growth at low concentrations
10-8 to 10-3 M
Moves from apex (tip of plant) to zone of
elongation at a rate of 10 mm per hr.
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In region of elongation, proton pump pumps H+ into
cell wall. Result:
 Lowers pH of cell wall
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Acidification activates enzymes called expansins
 break crosslinks in cell wall
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Change in pH causes ion (charged elements) uptake;
Drives uptake of water and cell elongation
 Analogous to blowing up a balloon
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Induce fruit production without pollination
Promote growth of roots
Inhibits loss of leaves
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Several companies produce a synthetic form
of auxin – 1-napthaleneacetamide
 Active ingredient in many commercial rooting
compounds
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Cytokinin
 Promote cell division
 Prevents senescence
 Used commercially to prolong the shelf life of
flowers and vegetables
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Types of organs that
form depends on ratio
of auxin to cytokinin
concentration
 Auxin > Cytokinin
▪ Rooting on Callus
 Auxin = Cytokinin
▪ Callus formation
 Auxin < Cytokinin
▪ Axillary bud development
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Gibberellin
 Promotes stem elongation
 Used commercially
▪ to increase growth and size of plants
▪ Break seed dormancy
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Abscisic Acid
 Initiates and maintains seed and bud dormancy
 Used commercially for thinning of fruits
▪ Promote growth of remaining fruit
+ GA
-GA
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Ethylene
 Involved in abscission and ripening of fruit
 Used commercially to
▪ Ripen fruit and vegetables for market
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What four stages are involved in plant signal
transduction mechanism?
Explain how hormones stimulate biological
response?
Which hormones would produce an in
increase in the size of plant’s organs
Explain why abscisic acid is sometimes
referred to as an inhibitory hormone.