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Lecture20 - MCDB321 MCDB321 Introductory Plant Physiology...

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Unformatted text preview: MCDB321 MCDB321 Introductory Plant Physiology Lecture 20 Gibberellins March 31, 2011 1. History of GA discovery 2. Physiological effects of GAs 3. Biosynthesis A. three stages of synthesis A. B. sites of synthesis B. 4. GA receptor/signaling History: History: •1926 E. Kurosawa – “foolish seedling disease” in rice Makes rice grow tall but eliminates seed production caused by a fungus pathogen Gibberella fujikuroi culture medium causes disease symptoms •1935 T. Yabuta named substance "gibberellin" •1938 isolated crystals of GA-A and GA-B •War-effort stops work and communications •Mid-1950s, two western research groups identified gibberellic acid •1956 West and Phinney (UCLA) isolate GA from plants •1958 MacMillan Bristol (England) identifies GA1 from beans 1. Gibberellins are classified on the basis of structure as well as function. 2. All gibberellins are derived from the ent-gibberellane skeleton. 3. All gibberellins are acidic compounds and are therefore also called gibberellic acids (GA) with a different subscript to distinguish between them. 4. GA3 has historically been called gibberellic acid but the term is often used in describing all gibberellins. 5. There have been over 136 GA's isolated, all of which are most likely not essential to the plant. Instead, these forms are probably inactive precursors or breakdown products of active gibberellins Gibberellin biosynthesis has three stages. 1.) biosynthesis of ent-kaurene from GGPP in plastids (cyclization) 2.) ent-kaurene converts to GA12/GA53 in ER (oxidation-ringB shrinking) 3.) biosynthesis of bioactive GAs from GA12. Assembled from 4 isoprenoid units he Stage III reactions ccur in the cytosol ccur two pathways: 13OH pathway&non-13OH pathway) two 13OH our enzymatic reactions our . GA13 Oxidase catalyzes C13 hydroxylation GA13 catalyzes . GA20 Oxidase C20H3-->CH2OH-->CHO-->ring formation GA20 . GA 3β -oxidase is essential for making a biologically active GAs GA . GA 2β -oxidase is the enzyme that inactivates GAs GA Both GA1 & GA4 are active hormones Mendel’s dwarf gene encodes GA3ox Mendel’s Sprayed with GA20 2. Measuring levels of Gas 3. Feeding radiolabelled GA 3. intermediates. intermediates. Sprayed with GA1 Genetically engineered dwarf wheat plants Genetically The three transgenic wheat plants on the right are The transformed with a GA2ox genes transformed 1. GA biosynthesis occurs at multiple cellular sites 1. (GA1:GUS) Shoot apices/root tips, vascular tissues, anthers, immature seeds, embryos (GA1:GUS) 2. GA downregulates expression of both GA4&GA5 genes (feedback regulation) 3. Photoperiod/temperature affects GA biosynthesis genes 3. Physiological Effects of GAs Physiological 1. Stimulate stem elongation in dwarf and rosette plants 1. Stimulate GA stimulates stem elongation by : A.stimulating cell division. B. increase cell wall plasticity (by a mechanism other than how auxin works) 2. Promote seed germination 2. Promote A. overcome seed dormancy A. B. mobilization of food reserves in grass seed germination. 3. Regulate the transition from juvenile to adult phases 3. 4. Promote flowering GA can substitute for long-days or cold temperature requirement for flowering in many plants Gibberellins Stimulate Cell Elongation Gibberellins They do so by enhancing cell wall extensibility They without cell wall acidification (unlike auxin) Growth Rate= m(Ψw-Y) m: wall extensibility Y: yield threshold Y: Continued recording of the growth of the upper Continued internode of deep-water rice in the presence or absence of exogenous GA1. and cell division and Promote seed germination 2. GAs diffuse into 2. the aleurone layer the 1. GAs are synthesized by 1. the embryo and released into the the starchy endosperm via the scutellum scutellum 5. The endosperm solutes are absorbed by the scutellum and transported to the growing embryo. 3. Aleurone layer cells are 3. induced to synthesize and secret α -amylase and other hydrolases into the endosperm 4. Starch and other macromolecules are broken down to small molecules 5. Influence sex expression in maize, high [GA] causes feminization of the tassel flowers maize GA-deficient mutants develop anthers in their ears which should contain only female flowers 6. Promote fruit setting Affects development of fruits (used commercially on some varieties of grapes for larger berries) cause parthenocarpic (seedless) fruit development [fruit development in the absence of fertilization] Mutant Screening for GA Signaling Mutants Identified Several Signaling Components Identified Of GA Signaling Pathway Of Three Main Classes of GA Signaling Mutants Three 1. GA-insensitive dwarfs (gai) 2. Suppressor mutants of GA-deficient mutation (rga) 3. Constitutive GA signaling mutants (spy) GAI and RGA are homologous proteins How can this happen??? In rice, mutations of the rice homolog of GAI/RGA, SLN, In give rise to two opposite phenotypes The GAI is an inhibitor of GA signaling. The Removing the inhibitor could increase signaling, but a mutation Removing that makes the inhibitor stable/stronger will inhibit GA signaling, that causing a dwarf phenotype. causing GA treatment destabilizes RGA and GAI 2 hour hour GA GA treatmen treatmen t 48 hour treatment treatment with GA inhibitor inhibitor SLEEPY (SLY) is also a SLEEPY F-Box protein known to be Involved in protein degradation Mutations in rice GID2 lead to Mutations accumulation of SLR1 accumulation [SLR1/SLN is the rice homolog of GAI/RGA [SLR1/SLN while GID2 is a homolog of SLEEPY] gid2-1 Mutations in GID1 are also defective in SLR1 degradation Mutations 1. 1. 2. 3. GID1 binds active GAs (with a Kd of ~10-7M) GID1 binds SLR1 (the rice homolog of RGA/GAI) in a GA-dependent manner An Arabidopsis mutant lacking all 3 GID1 homologs is a GA-insensitive dwarf mutant ...
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