Chapter 20

 Found in both xylem and phloem  Appears to be source/sink  Free or conjugated? 2

 Gibberella fujikuroi pathogen of rice  Western science 1950s  Higher conc. in immature seeds than vegetative tissue (1 ppm vs ppb)  Stimulate both cell division and cell elongation 3

 More than 135 identified.  Few with biological activity.  Most are intermediates or inactivated  Diterpenes – 19 or 20 Cs  C 19 -GAs or C 20 -GAs  Identified based on order of discovery:  GA 1 = the first gibberellic acid  GA 3 = a natural fungal gibberellic acid  GA 4 = another bioactive plant gibberellin 4

 Gibberellins -- terpenes.  Gibberellins are plant hormones with notable effects on:  Stem elongation  Seed germination  Reproductive processes, such as flower and fruit development 5

 Subtle differences influence bioactivity:  Carboxyl at c-7 for bioactivity  C 19 more bioactive than C 20  Most potent  3-  -hydroxylation or 3-  -1,3- dihydroxylation  1,2-unsaturation  Both hydroxylation and unsaturation (highest activity) 6

 Auxins – based on biological properties  Gibberellins – based on structure 7

 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 8 IKI stained starch agar A – control B -- 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 9

 Stimulate stem and root growth  Dwarf mutants  1 st year biennials (bolting)  Transition from juvenile to adult 10

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 The signals that trigger flowering -- trigger conversion of inactive to active forms  Active gibberellins promotes elongation of stems. 12

 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 13

 Promote pollen development & pollen tube growth  GA deficient dwarf mutants have impaired anther development  Blocked GA response – defects not reversible 14

 Promote fruit set & parthenocarpy  In grapes, also makes longer pedicels & reduces fungal infections b/c less “cramped” 15

 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 16

 Tetracyclic diterpenes  Homeostasis – biosynthesis + deactivation  Use of mutants important for determining pathways  Pathway – 3 stages  Plastid  ER  cytosol 17

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! 19

 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 20

 Light and Temperature – profound effects on metabolism and response  Day-length on flowering  Seed germination  Etiolation 21

 Precursors are not bioactive  3 genes  LE/le – studied by Mendel  NA/na – production pathway  SLN/sln – impaired deactivation 22

 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 23

 Auxins can regulate GA biosynthesis  Stem elongation  Fruit development  Different in different species  Different in different organs/tissues 24

 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 25

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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 28

 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! 29

 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. 30