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|
Expected Taxonomic Range: Viridiplantae
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 C3-hydroxylation branch pathway, a pathway originating from GA12 and leading to the hydroxylated C19 GA4 via formation of GA14 has been demonstrated in particular in the developing seedlings and endosperm of the pumpkin Cucurbita maxima [Pun, Lange97]. It was also shown that GA15 could be the subject of C3-hydroxylation, thereby linking the C-3 hydroxylation pathway to the gibberellin biosynthesis I (non C-3, non C-13 hydroxylation).
Enzymes: The reactions of this pathway are catalyzed by 2-oxoglutarate-dependent dioxygenases. The first enzyme, gibberellin 3β-hydroxylase catalyzes the hydroxylation of the C-3 carbon of GA12 or GA15. These enzymes generally have broad spectrum of substrates and do not therefore specifically catalyze the early C-3 hydroxylation but also the 3β-hydroxylation reactions of late C-3 hydroxylation pathways. (Note: one pumpkin enzyme encoded by GA3ox3, however, was shown to specifically catalyze the late C-3 hydroxylation of GA9 to GA4). Gibberellin 20-oxidase, catalyzes the removal of C-20, thereby forming the C19-skeleton in GA4. 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 intermediates (GA15 and GA37) must be in their open lactone form (free alcohol) to be catalyzed by the enzyme. Those gibberellins are however extracted from plants in their closed δ-lactone form ( gibberellin A15 (closed lactone form) and gibberellin A37 (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 some free alcohol forms [Ward97].
Superpathways: superpathway of gibberellin biosynthesis
Variants: gibberellin biosynthesis I (non C-3, non C-13 hydroxylation), gibberellin biosynthesis III (early C-13 hydroxylation), gibberellin biosynthesis IV (Gibberella fujikuroi), gibberellin biosynthesis V
Unification Links: AraCyc:PWY-5036
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
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.
Lange94b: Lange T., Schweimer A., Ward D.A., Hedden P., Graebe J.E. "Separation and characterization of three 2-oxoglutarate-dependent dioxygenases from Cucurbita maxima L. endosperms involved in gibberellin biosynthesis." Planta (1994) 195 : 98-107.
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
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
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