This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Hormones Biosynthesis → Plant Hormones Biosynthesis → Gibberellins and Gibberellin Precursors Biosynthesis → Gibberellins biosynthesis|
|Biosynthesis → Secondary Metabolites Biosynthesis → Terpenoids Biosynthesis → Diterpenoids Biosynthesis → Gibberellins and Gibberellin Precursors Biosynthesis → Gibberellins biosynthesis|
At the time of this review, 136 fully characterized gibberellins (starting with GA1 [MacMillan68]) have been identified in more than a hundred vascular plant species, seven bacteria and seven fungi [Sponsel04, MacMillan01]. Of these gibberellins only a few have biological activity. Many of the GAs identified early in the history of the discovery of these hormones are the ones which possess the highest biological activity. These include GA1, GA3, GA4, GA5, GA6 and GA7. GA1 is the most active GA for stem elongation in Zea mays and Pisum sativum, while GA4 is the most active GA in Cucurbitaceae and in Arabidopsis thaliana. GA3 (gibberellic acid), on the other hand, has been identified in more than 40 plants and is the major GA in the fungus Fusarium fujikuroi. GA3 is used commercially to promote seed germination, stem elongation and fruit growth. In Lolium, GA5 and GA6 have been shown to enhance flowering, whilst GA1 and GA4 enhanced stem elongation [King03].
Gibberellins are diterpenes which, in higher plants, are synthesized in the plastids from glyceraldehyde-3-phosphate and pyruvate via the isopentenyl diphosphate (IPP). They all have either 19 or 20 carbon units grouped into either four or five ring systems (see gibberellin A12 and gibberellin A9, respectively). The fifth ring is a lactone ring. GAs containing a tetracyclic ent-gibberellane structure (gibberellin A12) are called C20-GAs, whereas GAs containing a pentacyclic 20-nor-ent-gibberellane structure (gibberellin A9) are called C19-GAs. C20-GAs do not usually have biological activity but can be metabolized to active C19-GAs (note that not all C19-GAs are bioactive).
Gibberellins are believed to be synthesized in young tissues of the shoot and also the developing seed. It is uncertain whether young root tissues also produce gibberellins. There is also some evidence that leaves may be the source of some biosynthesis.Active gibberellins show many physiological effects, each depending on the type of gibberellin present as well as the species of plant. Some of the physiological processes stimulated by gibberellins are: i) stimulation of stem elongation by stimulating cell division and elongation, ii) stimulation of bolting/flowering in response to long days, iii) interruption of seed dormancy in plants requiring stratification or light to induce germination, iv) stimulation of enzyme production (α-amylases) in germinating cereal grains for mobilization of seed reserves, v) induction of maleness in dioecious flowers (sex expression), vi) parthenocarpic (seedless) fruit development, and vii) delay of senescence in leaves and citrus fruits.
Certain commercial chemicals which are used to stunt growth do so in part because they block the synthesis of gibberellins. Some of these chemicals are phosphon D, AMO-1618, cycocel, ancymidol and paclobutrazol (more inhibitors of GA biosynthesis can be found in [Sponsel04]). During active growth, the plant will maintain gibberellin homeostasis by metabolizing most gibberellins by rapid hydroxylation to inactive conjugates. Active gibberellin A3 is degraded much slower which helps to explain the symptoms observed in the disease bakanae caused by the rice pathogen Fusarium fujikuroi. This pathogen produces large amounts of GA3 which it secretes. Inactive conjugates might be stored or translocated via the phloem and xylem before their release (activation) at the proper time and in the proper tissue.
About This Pathway
GA12, the first true gibberellin is synthesized in plastids (see GA12 biosynthesis). It is then further oxidized in the cytosol to a wide range of gibberellins. Depending on the sequence of hydroxylation at the 3β- and 13-positions, parallel pathways branch from GA12 to the C19-GAs, the number and prevalence of these branch pathways varying from species to species. The presence of the early 13-hydroxylation branch pathway, a pathway originating from GA12 via GA53 and leading to the hydroxylated C19 GA1 has been demonstrated in many species including maize, rice, pea and Arabidopsis thaliana [Davis99, Kobayashi00a, GarciaMartinez97, Phillips95]. So far, however, there is no evidence that these gibberellins exist in Bryophyta
Enzymes: The reactions of this pathway are catalyzed by 2-oxoglutarate-dependent dioxygenases. The first step of this pathway is performed by a gibberellin 13-hydroxylase. This enzyme allows for the conversion of GA12 to GA53. The following enzyme of this pathway, gibberellin 20-oxidase, catalyzes the removal of C-20, thereby forming the C19-skeleton in GA20. This enzyme performs the sequential oxidation of C-20 to the alcohol and aldehyde, followed by the removal of the C atom through formation of the 4, 10-lactone ring. The alcohol intermediate (GA44) must be in its open lactone form (free alcohol) to be catalyzed by the enzyme. It is however extracted from plants in its closed δ-lactone form (gibberellin A44 (closed lactone form)) which is no longer oxidized by GA20-oxidases. Although it is not certain whether this form occurs naturally in planta or is an artifact of the extraction procedure, it is interesting to note that spinach cell-free extracts contain an enzyme able to catalyze the conversion of the δ-lactone form to its free alcohol form [Ward97]. Additional oxidation reactions leading to the formation of growth-active GAs such as GA1 are performed by gibberellin 3-oxidases which, depending on the species, are varyingly regiospecific [Sponsel04].
Superpathways: superpathway of gibberellin biosynthesis
Variants: gibberellin biosynthesis I (non C-3, non C-13 hydroxylation) , gibberellin biosynthesis II (early C-3 hydroxylation) , gibberellin biosynthesis IV (Gibberella fujikuroi) , gibberellin biosynthesis V
Unification Links: AraCyc:PWY-5035
Davis99: Davis G, Kobayashi M, Phinney BO, Lange T, Croker SJ, Gaskin P, MacMillan J (1999). "Gibberellin Biosynthesis in Maize. Metabolic Studies with GA(15), GA(24), GA(25), GA(7), and 2,3-Dehydro-GA(9)." Plant Physiol 121(3);1037-1045. PMID: 10557253
GarciaMartinez97: Garcia-Martinez JL, Lopez-Diaz I, Sanchez-Beltran MJ, Phillips AL, Ward DA, Gaskin P, Hedden P (1997). "Isolation and transcript analysis of gibberellin 20-oxidase genes in pea and bean in relation to fruit development." Plant Mol Biol 33(6);1073-84. PMID: 9154988
Kobayashi00a: Kobayashi M, MacMillan J, Phinney B, Gaskin P, Spray CR, Hedden P (2000). "Gibberellin biosynthesis: metabolic evidence for three steps in the early 13-hydroxylation pathway of rice." Phytochemistry 55(4);317-21. PMID: 11117879
Phillips95: Phillips AL, Ward DA, Uknes S, Appleford NE, Lange T, Huttly AK, Gaskin P, Graebe JE, Hedden P (1995). "Isolation and expression of three gibberellin 20-oxidase cDNA clones from Arabidopsis." Plant Physiol 108(3);1049-57. PMID: 7630935
Ward97: Ward J.L., Jackson G.J., Beale M.H., Gaskin P., Hedden P., Mander L.N. (1997). "Stereochemistry of the oxidation of gibberellin 20-alcohols, GA15 and GA44, to 20-aldehydes by gibberellin 20-oxidases." Chem. Comm. (1): 13-14.
Appleford05: Appleford NE, Evans DJ, Lenton JR, Gaskin P, Croker SJ, Devos KM, Phillips AL, Hedden P (2005). "Function and transcript analysis of gibberellin-biosynthetic enzymes in wheat." Planta NIL;1-15. PMID: 16160850
Gilmour86: Gilmour SJ, Zeevaart JA, Schwenen L, Graebe JE (1986). "Gibberellin metabolism in cell-free extracts from spinach leaves in relation to photoperiod." Plant Physiol 82(1);190-5. PMID: 16664991
Itoh01: Itoh H, Ueguchi-Tanaka M, Sentoku N, Kitano H, Matsuoka M, Kobayashi M (2001). "Cloning and functional analysis of two gibberellin 3 beta -hydroxylase genes that are differently expressed during the growth of rice." Proc Natl Acad Sci U S A 98(15);8909-14. PMID: 11438692
Lange94a: Lange T, Hedden P, Graebe JE (1994). "Expression cloning of a gibberellin 20-oxidase, a multifunctional enzyme involved in gibberellin biosynthesis." Proc Natl Acad Sci U S A 91(18);8552-6. PMID: 8078921
Magome13: Magome H, Nomura T, Hanada A, Takeda-Kamiya N, Ohnishi T, Shinma Y, Katsumata T, Kawaide H, Kamiya Y, Yamaguchi S (2013). "CYP714B1 and CYP714B2 encode gibberellin 13-oxidases that reduce gibberellin activity in rice." Proc Natl Acad Sci U S A 110(5);1947-52. PMID: 23319637
Sasaki02: Sasaki A, Ashikari M, Ueguchi-Tanaka M, Itoh H, Nishimura A, Swapan D, Ishiyama K, Saito T, Kobayashi M, Khush GS, Kitano H, Matsuoka M (2002). "Green revolution: a mutant gibberellin-synthesis gene in rice." Nature 416(6882);701-2. PMID: 11961544
Solfanelli05: Solfanelli C, Ceron F, Paolicchi F, Giorgetti L, Geri C, Ceccarelli N, Kamiya Y, Picciarelli P (2005). "Expression of two genes encoding gibberellin 2- and 3-oxidases in developing seeds of Phaseolus coccineus." Plant Cell Physiol 46(7);1116-24. PMID: 15894806
Talon90: Talon M, Koornneef M, Zeevaart JA (1990). "Endogenous gibberellins in Arabidopsis thaliana and possible steps blocked in the biosynthetic pathways of the semidwarf ga4 and ga5 mutants." Proc Natl Acad Sci U S A 87(20);7983-7. PMID: 2236013
Williams98: Williams J, Phillips AL, Gaskin P, Hedden P (1998). "Function and substrate specificity of the gibberellin 3beta-hydroxylase encoded by the Arabidopsis GA4 gene." Plant Physiol 1998;117(2);559-63. PMID: 9625708
Yamaguchi98a: Yamaguchi S, Smith MW, Brown RG, Kamiya Y, Sun T (1998). "Phytochrome regulation and differential expression of gibberellin 3beta-hydroxylase genes in germinating Arabidopsis seeds." Plant Cell 10(12);2115-26. PMID: 9836749
©2014 SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025-3493