Chapter 20

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 1-10 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

Diterpenes – 19 or 20 Cs 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 3--hydroxylation or 3--1,3-

dihydroxylation 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 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

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IKI stained starch agar

A – controlB -- GA

Essential for germination -- seeds of some gibberellin mutants cannot germinate

Complements the roles of auxins and brassinsteroids in seed germination.

Involves the activation enzymes. Following imbibition – synthesis of

gibberellins. Diffuse to the aleurone layer -- induce

the synthesis of -amylases and proteases.

Turn endosperm into useful nutrients for developing embryo

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Stimulate stem and root growth Dwarf mutants 1st year biennials (bolting)

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

Sex determination in imperfect flowers Cucumber, hemp & spinach -> formation of

staminate flowers 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 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

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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18

<-pyruvate/ G3P

(also plastid membrane)

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) 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

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

Both loss of function mutants are recessive Negative regulator made active – dwarf

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 tryptophan-dependent 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

** contents from Table 27-1 from Raven!

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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.

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