Download Bio3460-22 Hormones

Major Plant Hormones
• Auxin (IAA)
• Cytokinin (Kinetin, Zeatin)
• Gibberellin
• Ethylene
• Abscisic Acid (ABA)
Hormones:
• chemical signal produced in one part of the
organism & transported to other parts where
it binds to a specific receptor and initiates
a response (signal transduction)
• small amounts can induce a substantial change
in growth and development (signal amplified)
• small molecules required for easy passage
across cell walls
Phototropism: A grass seedling growing toward a candle’s light
Hormones:
• plant hormones can have multiple effects
• relative concentration of two or more hormones
often control the growth and development of cells
Early experiments of phototropism
Coleoptile = a modified en-sheathing leaf.
It protects primary leaves as seedling grows
through soil.
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Frits Went’s experiments: Discovery of Auxin
Auxin is from Greek auxein, “to increase” or “to grow”
Auxin Transport:
• most Auxin movement is too slow
for phloem transport
• Auxin normally moves from shoot tip
to base NOT in reverse direction
• Unidirectional transport is called
Polar Transport (one cell to next cell)
• Polar transport requires ATP
Polar auxin transport: a chemiosmotic model (Layer 1)
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Polar auxin transport: a chemiosmotic model (Layer 2)
Asymmetric location of PIN proteins
Normal
Polar auxin transport: a chemiosmotic model (Layer 3)
To modify localization of PIN proteins
Treatment
Main function of Auxin is to:
• Stimulate the elongation of developing shoot cells
Role of Auxin in Cell Elongation
Acid Growth Hypothesis
Cell elongation in response to auxin:
the acid growth hypothesis
AUXIN and the statolith hypothesis for root gravitropism
Statoliths are starch storing plastids
that serve as gravity sensors
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Auxin – can stimulate or inhibit cell elongation depending on concentration
Statoliths are starch storing plastids
that serve as gravity sensors
Other Uses of Auxins
• used commercially in vegetative propagation
of plants by cuttings
• treat a detached leaf or stem with rooting
powder containing Auxin results in the
formation of adventitious roots
Other Uses of Auxins
• used commercially in vegetative propagation
of plants by cuttings
• treat a detached leaf or stem with rooting
powder containing Auxin results in the
formation of adventitious roots
Extreme proliferation of roots in an Arabidopsis
mutant that produces 17-fold higher Auxin conc.
Auxin promotes the development of adventitious roots
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Synthetic Auxins
Other Uses of Auxins
• 2,4-D is a synthetic Auxin that kills dicot weeds
• synthetic Auxins sprayed on Tomato vines
induce fruit development without pollination
(seedless tomatoes)
Cytokinin
Was discovered during trial and error attempts
to find chemical additives to enhance growth
and development of cells in
Tissue Culture
• liquid endosperm from coconut
• degraded samples of DNA
active ingredient was modified form of adenine
Effects of Cytokinin
Kinetin is a heat-induced
degradation product of DNA
(adenine base)
Naturally occurring cytokinin
first found in Corn (Zea mays),
is the most prevalent cytokinin
in higher plants.
1) control of cell division and differentiation
(tissue culture)
2) apical dominance
Naturally occurring cytokinin
in other plants
Named Cytokinins because they stimulate cell division
or cytokinesis
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Effects of Cytokinin
1) control of cell division and differentiation
• when stem tissue is cultivated without cytokinin
cells grow very large but do not divide
• adding cytokinins alone has no effect
• cytokinins along with auxin control cell
division and differentiation in tissue culture
Effects of Cytokinins
2) Apical dominance
• Auxin and Cytokinin have counteracting
effects on apical dominance
IAA is a
natural Auxin
Kinetin is an
artificial Cytokinin
• plants that over produce cytokinins tend to
have “bushier” growth - more than one
main stem
Apical dominance: with apical bud (left), apical bud removed (right)
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“Foolish seedling disease” in rice
Gibberellins
• first discovered as a chemical produced by
a fungus (Gibberella fujikuroi)
• fungus was the cause of “foolish seedling disease”
• infected seedlings grew tall & spindly - they
toppled over before they could mature and flower
Three of the more
than 110 Gibberellins
Gibberellin Effects
1) Stem Elongation
2) Fruit growth (grapes)
3) Seed germination
Gibberellin Effects
Gibberellin Mutants
1) Stem Elongation
• stimulate growth in leaves and stems
• stimulate cell elongation and cell division
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Figure 20.5 The three stages of GA biosynthesis
Non-13-Hydroxylation pathway
occurs in some crop plants and
Arabidopsis plants
13-Hydroxylation pathway is
the major pathway in many
plants
This pathway results in the
synthesis of bioactive GA1
We will focus on this pathway
Figure 20.7 Portion of GA biosynthetic pathway showing mutation-blocked metabolic steps
GA1 is active form -->
Figure 20.7 Portion of GA biosynthetic pathway showing mutation-blocked metabolic steps
Figure 20.9 Phenotypes and genotypes of peas that differ in GA 1 content of their vegetative tissue
Treating pea dwarfism with a growth hormone (GA1)
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Figure 20.10 Genetically engineered dwarf wheat plants
GA1 is active form -->
Figure 20.7 Portion of GA biosynthetic pathway showing mutation-blocked metabolic steps
DELLA Protein Components:
GA Signal Transduction Pathway:
• GID1 hormone receptor binds GA
• After binding, GA-GID1 interacts with DELLA protein
and results in the degradation of the DELLA protein
• DELLA protein is a NEGATIVE regulator of
plant growth - stem elongation
• GA binding then removes a restriction on growth
• DELLA Domain – regulatory domain – GA binding
• GRAS Domain – functional domain
prevents gene expression controlling
stem elongation
Effect of Mutations in:
• DELLA Domain > makes dwarf plants & GA insensitive
• GRAS Domain > makes tall plants & will NOT exhibit
reduced growth even if GA
biosynthesis is stopped
Figure 20.14 Structure of the GA-GID1a-DELLA complex
Part of the Gibberellin Biosynthesis Pathway
and Signal Transduction Pathway
GA8
GA2ox
sln
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Figure 20.16 Domain structures of the RGA and GAI repressor proteins
Figure 20.15
DELLA-domain (N-terminal end of protein - the first 5 amino acids are:
Aspartic Acid (D), Glutamic Acid (E), Leucine (L), Leucine (L), Alanine (A)
Figure 20.19 Degradation of the DELLA protein
Figure 20.15
DELLA-domain (N-terminal end of protein - the first 5 amino acids are:
Aspartic Acid (D), Glutamic Acid (E), Leucine (L), Leucine (L), Alanine (A)
GA1 receptor
-A mutation in this receptor
will result in dwarf plants even
when GA levels are high
Part of the Gibberellin Biosynthesis Pathway
and Signal Transduction Pathway
GA8
GA2ox
sln
Other Effects of Gibberellin
2) Fruit growth
• commercial application to “Thompson”
seedless grapes produces larger grapes
• internodes of grape stem elongates and
provides more air circulation and less
yeast and other microbe pest growth
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The effect of gibberellin treatment on seedless grapes
Figure 20.4 Gibberellin induces growth in “Thompson Seedless” grapes
Other Effects of Gibberellin
3) Seed germination
• developing embryos produce GA
• GA stimulates synthesis of a amylase
(a digestive enzyme) that mobilizes
stored nutrients (breaks down starch)
Ethylene (H2C=CH2)
Ethylene (H2C=CH2)
• has numerous effects on plants - many effects
that were previously ascribed to other hormones
(particularly Auxin)
• gas formed from plant biosynthetic pathways
Physiological Effects
(Amino Acid)
1) Fruit ripening
2) Triple response to mechanical stress
3) Flooding induced aerenchyma production
4) Leaf abscission & programmed cell death
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Ethylene
Triple Response to Mechanical Stress
Physiological Effects
• reduced stem elongation
• thickening of stem
• horizontal growth
1) Fruit ripening (Bananas, Tomatoes)
• Ethylene causes production of enzymes
(Cellulase & Pectinase) that breakdown
cellulose and pectin - structural components
of cell walls
• Ethylene causes starches to be converted to sugar
Ethylene induces the triple response in pea seedlings
Ethylene signal-transduction mutants can be distinguished by their different
responses to experimental treatments
Ethylene triple response in mutants: ein mutant (left), ctr mutant (right)
A developmental response of corn roots to flooding and oxygen
deprivation involves ethylene
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Abscission of a maple leaf
Abscisic Acid (ABA)
Physiological Effects
1) Stomatal Closure
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Fig. 1 Contrasting stomatal responses to exogenous ABA applied to diverse lineages of
vascular plants.
T J Brodribb, S A M McAdam Science 2011;331:582-585
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