Transcript 07 Gibberellins
Chapter 20 2 Found in both xylem and phloem Appears to be source/sink Free or conjugated? 3 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 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 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 C19 more bioactive than C20 Most potent 3-b-hydroxylation or 3-b-1,3dihydroxylation 1,2-unsaturation Both hydroxylation and unsaturation (highest activity) 6 7 Auxins – based on biological properties Gibberellins – based on structure 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 IKI stained starch agar A – control B -- GA 8 Essential for germination -- seeds of some gibberellin mutants cannot germinate Complements the roles of auxins and brassinsteroids in seed germination. Involves the activation enzymes. 9 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 Stimulate stem and root growth Dwarf mutants st 1 year biennials (bolting) 10 Transition from juvenile to adult 11 12 The signals that trigger flowering -- trigger conversion of inactive to active forms Active gibberellins promotes elongation of stems. Induce floral initiation Q.v. bolting Long day requirements Sex determination in imperfect flowers Cucumber, hemp & spinach -> formation of staminate flowers 13 GA inhibitors -> formation of pistillate flowers Corn -> GA promotes pistil formation 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 15 In grapes, also makes longer pedicels & reduces fungal infections b/c less “cramped” 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 <-pyruvate/ G3P (also plastid membrane) 18 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 24 Different in different species Different in different organs/tissues 3 kinds Non-functional positive regulator -- dwarf Non-functional negative regulator – overly tall Negative regulator made active – dwarf 25 Both loss of function mutants are recessive Semidominant 26 27 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 29 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 30 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.