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|
|Biosynthesis → Secondary Metabolites Biosynthesis → Terpenoids Biosynthesis → Diterpenoids Biosynthesis → Gibberellins and Gibberellin Precursors Biosynthesis|
Note: This is a chimeric pathway, comprising reactions from multiple organisms, and typically will not occur in its entirety in a single organism. The taxa listed here are likely to catalyze only subsets of the reactions depicted in this pathway.
This super-pathway provides an overview of the complexity and interconnectivity of gibberellins biosynthesis. It is important to note that a given species shown here may not synthesize all the gibberellins presented. To know which gibberellins are found in a specfic species, refer to the subpathways given below.
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 C20-gibberellin skeleton and C19-gibberellin skeleton, respectively). The fifth ring is a lactone ring. GAs containing a tetracyclic ent-gibberellane structure (C20-gibberellin skeleton) are called C20-GAs, whereas GAs containing a pentacyclic 20-nor-ent-gibberellane structure (C19-gibberellin skeleton) 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 giberellin 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.
Subpathways: gibberellin biosynthesis II (early C-3 hydroxylation) , gibberellin biosynthesis III (early C-13 hydroxylation) , gibberellin biosynthesis I (non C-3, non C-13 hydroxylation) , gibberellin biosynthesis V , superpathway of gibberellin GA12 biosynthesis , GA12 biosynthesis , ent-kaurene biosynthesis I
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
Cho04: Cho EM, Okada A, Kenmoku H, Otomo K, Toyomasu T, Mitsuhashi W, Sassa T, Yajima A, Yabuta G, Mori K, Oikawa H, Toshima H, Shibuya N, Nojiri H, Omori T, Nishiyama M, Yamane H (2004). "Molecular cloning and characterization of a cDNA encoding ent-cassa-12,15-diene synthase, a putative diterpenoid phytoalexin biosynthetic enzyme, from suspension-cultured rice cells treated with a chitin elicitor." Plant J 37(1);1-8. PMID: 14675427
Davis99a: 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
Fleet03: Fleet CM, Yamaguchi S, Hanada A, Kawaide H, David CJ, Kamiya Y, Sun TP (2003). "Overexpression of AtCPS and AtKS in Arabidopsis confers increased ent-kaurene production but no increase in bioactive gibberellins." Plant Physiol 132(2);830-9. PMID: 12805613
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
Harris05: Harris LJ, Saparno A, Johnston A, Prisic S, Xu M, Allard S, Kathiresan A, Ouellet T, Peters RJ (2005). "The maize An2 gene is induced by Fusarium attack and encodes an ent-copalyl diphosphate synthase." Plant Mol Biol 59(6);881-94. PMID: 16307364
Hayashi06: Hayashi K, Kawaide H, Notomi M, Sakigi Y, Matsuo A, Nozaki H (2006). "Identification and functional analysis of bifunctional ent-kaurene synthase from the moss Physcomitrella patens." FEBS Lett 580(26);6175-81. PMID: 17064690
Helliwell01: Helliwell CA, Chandler PM, Poole A, Dennis ES, Peacock WJ (2001). "The CYP88A cytochrome P450, ent-kaurenoic acid oxidase, catalyzes three steps of the gibberellin biosynthesis pathway." Proc Natl Acad Sci U S A 98(4);2065-70. PMID: 11172076
Helliwell01a: Helliwell CA, Sullivan JA, Mould RM, Gray JC, Peacock WJ, Dennis ES (2001). "A plastid envelope location of Arabidopsis ent-kaurene oxidase links the plastid and endoplasmic reticulum steps of the gibberellin biosynthesis pathway." Plant J 28(2);201-8. PMID: 11722763
Helliwell98: Helliwell CA, Sheldon CC, Olive MR, Walker AR, Zeevaart JA, Peacock WJ, Dennis ES (1998). "Cloning of the Arabidopsis ent-kaurene oxidase gene GA3." Proc Natl Acad Sci U S A 1998;95(15);9019-24. PMID: 9671797
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
Kawaide00: Kawaide H, Sassa T, Kamiya Y (2000). "Functional analysis of the two interacting cyclase domains in ent-kaurene synthase from the fungus Phaeosphaeria sp. L487 and a comparison with cyclases from higher plants." J Biol Chem 275(4);2276-80. PMID: 10644675
Showing only 20 references. To show more, press the button "Show all references".
©2014 SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025-3493