07 Gibberellins

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Transcript 07 Gibberellins

Chapter 20
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Found in both xylem
and phloem
Appears to be
source/sink
Free or conjugated?
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Gibberella fujikuroi pathogen of rice
Western science 1950s
Higher conc. in immature seeds
than vegetative tissue (1 ppm vs 110 ppb)
Stimulate both cell
division and cell
elongation
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More than 135 identified.
Few with biological activity.
 Most are intermediates or inactivated
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Diterpenes – 19 or 20 Cs
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C19-GAs or C20-GAs
Identified based on order of discovery:
GA1 = the first gibberellic acid
 GA3 = a natural fungal gibberellic acid
 GA4 = another bioactive plant gibberellin
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Gibberellins -- terpenes.
Gibberellins are plant hormones with
notable effects on:
Stem elongation
 Seed germination
 Reproductive processes, such as flower and
fruit development
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Subtle differences influence
bioactivity:
Carboxyl at c-7 for bioactivity
 C19 more bioactive than C20
 Most potent
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3-b-hydroxylation or 3-b-1,3dihydroxylation
 1,2-unsaturation
 Both hydroxylation and
unsaturation (highest activity)
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Auxins – based on biological properties
Gibberellins – based on structure
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Promote seed germination
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Relative amounts of ABA and GA can determine dormancy
Treatment of dormant seeds can bypass after-ripening
conditions
GA induces synthesis of hydrolytic enzymes (amylase) –
provide nourishment from endosperm
IKI stained
starch agar
A – control
B -- GA
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Essential for germination -- seeds
of some gibberellin mutants
cannot germinate
Complements the roles of auxins
and brassinsteroids in seed
germination.
Involves the activation enzymes.
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Following imbibition – synthesis of
gibberellins.
Diffuse to the aleurone layer -induce the synthesis of a-amylases
and proteases.
Turn endosperm into useful
nutrients for developing embryo
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Stimulate stem and root growth
Dwarf mutants
st
 1 year biennials (bolting)
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Transition from juvenile to adult
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The signals that trigger flowering -- trigger
conversion of inactive to active forms
Active gibberellins promotes elongation of
stems.
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Induce floral initiation
Q.v. bolting
 Long day requirements
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Sex determination in imperfect flowers
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Cucumber, hemp & spinach -> formation of
staminate flowers
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GA inhibitors -> formation of pistillate flowers
Corn -> GA promotes pistil formation
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Promote pollen development & pollen tube
growth
GA deficient dwarf mutants have impaired
anther development
 Blocked GA response – defects not reversible
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Promote fruit set & parthenocarpy
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In grapes, also makes longer pedicels & reduces
fungal infections b/c less “cramped”
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Promote early seed development
Commercial uses
Growth of fruit crops
 Stimulate barley malting
 Increase sugar yield in sugarcane
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Commercial uses of inhibitors
Reduce some grain height
 Make container-grown ornamentals shorter;
more compact
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Tetracyclic diterpenes
Homeostasis – biosynthesis + deactivation
Use of mutants important for determining
pathways
Pathway – 3 stages
Plastid
 ER
 cytosol
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<-pyruvate/
G3P
(also plastid membrane)
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Bioactivity controlled through deactivation
and reduced synthesis
Negative feedback control – inhibition of gene
expression
 Positive feedforward control – enhanced
deactivation
 Importance feedback/feedforward varies with
species/tissue!
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Cell-free preparations can also show
gibberellin synthesis.
Three principle sites of gibberellin synthesis
Developing seeds and fruits
 Young leaves of developing apical buds and
elongating shoots
 Root apex
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Light and Temperature – profound effects
on metabolism and response
Day-length on flowering
 Seed germination
 Etiolation
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Precursors are not bioactive
3 genes
LE/le – studied by Mendel
 NA/na – production pathway
 SLN/sln – impaired deactivation
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Not just short!
Some dormant, non-germinatable seeds
 Male sterile (GA needed for anther/pollen
development)
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Two different kinds
GA deficient -- effects reversible
 Blocked GA response – effects not reversible
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Auxins can regulate GA biosynthesis
Stem elongation
 Fruit development
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Different in different species
Different in different organs/tissues
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3 kinds
Non-functional positive regulator -- dwarf
 Non-functional negative regulator – overly tall
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Negative regulator made active – dwarf
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Both loss of function mutants are recessive
Semidominant
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Stimulate both cell division and cell elongation
Obserced to cause in increase in both mechanical
extensibility and stress relaxation
 Auxins  cell wall acidification
 GA NEVER present without Auxin
 Lag time longer
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Chemical Nature: Indole-3-Acetic Acid (IAA) – principal
naturally occuring auxin. Synthesized via tryptophandependent and tryptophan independent pathways
Sites of Biosynthesis: primarily in leaf primordia and
young leaves and in developing seeds
Transport: both polarly (unidirectionally) and nonpolarly
Effects: Apical dominance; tropic responses; vascular
tissue differentiation; promotion of cambial activity;
induction of adventitious roots on cuttings; inhibitions of
leaf and fruit abscission; stimulation of ethylene
synthesis; inhibition or promotion (in pineapples) of
flowering; stimulation of fruit development
First found: coleoptiles
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Chemical Nature: Gibberellic acid, a fungal produce, is the
most widely studied. Synthesized via the terpenoid
pathway
Sites of Biosynthesis: in young tissues of the shoot and
developing seeds. It is uncertain whether synthesis also
occurs in roots
Transport: probably transported in the xylem and phloem
Effects: hyperelongation of shoots by stimulating both cell
division and cell elongation, producing tall, as opposed to
dwarf plants; induction of seed germination; stimulation
of flowering in long-day plants and biennials; regulation of
production of seed enzymes in cereals.