Phytohormones Mode of Action

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Phytohormones - Mode of Action
Chapter 9b
Topics 1. Hormone Binding and Signal
Transduction 2. Hormone Biosynthesis and
Transport 3. Biochemical Effects and Gene Action
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Sequence of Events for a Hormone to Act
1. The initial signal is perceived by the
target cell. 2. A signal transduction pathway
begins and amplification occurs. 3. A response
occurs.
Hopkins, 1999
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What is Signal Perception?
The cell involved must have a receptor or what is
called a hormone-binding protein. Criteria for a
hormone-binding protein a. Specific b. High
affinity c. Can be saturated d. Binding is
reversible
Hopkins, 1999
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Hormone-Binding Proteins
Two auxin-binding proteins are associated with
membranes one is associated with cytoplasmic and
nuclear fractions.
A cytokinin-binding protein has been found to be
associated with ribosomes.
An ABA-binding protein appears to exist on the
apoplastic side of the guard cell plasma membrane.
Hopkins, 1999
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Second Messengers
In animals, hormones such as insulin and
epinephrine bind to the membrane and eventually
cause cyclic AMP synthesis. Cyclic AMP is the
second messenger that stimulates protein
kinases. Protein kinases phosphorylate proteins
which lead to the hormonal response. In plants,
Ca and phosphoinositides serve as second
messengers.
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How hormones amplify responses
Hopkins, 1999
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How Calcium Acts as a Second Messenger
1. Cytoplasmic Ca2 concentration is low.
2. Once inside the cytoplasm, Ca2 binds to a 17
to 19 kD protein called calmodulin.
3. Next, the calmodulin - Ca2 complex stimulates
NAD kinases and protein kinases.
NAD/NADP balance is critical in regulating many
plant redox reactions.
Hopkins, 1999
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Ca2-Calmodulin Action
Hopkins, 1999
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How Phosphoinositides Act asa Second Messenger
1. The hormone binds to a membrane receptor.
2. A G-protein activates phospholipase C. This
enzyme catalyzes the conversion of PIP2 to IP3
and DAG.
3. IP3 stimulates Ca2 release from the vacuole
and DAG activates protein kinase C.
Many of the details of phosphoinositide
metabolism have yet to be finalized.
Hopkins, 1999
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Phosphoinositide Action
Hopkins, 1999
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Directionality of Auxin Transport
1. More than just diffusion in effect.
2. IAA conjugates may be involved.
Indole-3-acetyl-myo-inositol has been proposed as
a transport form.
3. Basipetal transport - from morphological apex
toward base Acropetal transport - from
morphological base toward apex.
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Directionality of Auxin Transport (continued)
4. pH and charge of IAA (IAAH vs IAA-) is
involved.
5. IAA transport is energy dependent.
Respiratory inhibitors inhibit transport.
6. Noncompetitive inhibitors (analogues) such as
TIBA or NPA inhibit transport.
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Polar transport of auxin
Hopkins, 1999
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Auxin Movement in Coleoptile Segments
Hopkins, 1999