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.
Synonyms: gibberellin 2β-hydroxylation
|Superclasses:||Activation/Inactivation/Interconversion → Inactivation|
|Degradation/Utilization/Assimilation → Hormones Degradation → Plant Hormones Degradation → Gibberellins Degradation|
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.
At the time of this review, over 130 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
The irreversible deactivation of GAs is achieved by 2β-hydroxylation of the active form. This reaction is performed by dioxygenases known as gibberellin 2-oxidases (GA 2-oxidases) which are essential for effective regulation of GA concentration in planta. In some tissues such as the cotyledons and the testae of some developing seeds, 2β-hydroxylation is pushed further to form a ketone, hence giving rise to compounds known as 'GA-catabolites'. These catabolites appear as dicarboxylic acids with an open lactone ring. It has however been postulated that this open form might be artefactual and the result of the analytical procedure. Study of recombinant GA 2-oxidases expressed in E .coli showed that ketone formation is relatively inefficient and only occurs when the enzymes is present at high concentrations [Sponsel04]. Deactivation of gibberellins by 2β-hydroxylation is not observed in Fusarium fujikuroi.
Unification Links: AraCyc:PWY-102
Thomas99a: Thomas SG, Phillips AL, Hedden P (1999). "Molecular cloning and functional expression of gibberellin 2- oxidases, multifunctional enzymes involved in gibberellin deactivation." Proc Natl Acad Sci U S A 96(8);4698-703. PMID: 10200325
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
Itoh01a: 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
Sakai03: Sakai M, Sakamoto T, Saito T, Matsuoka M, Tanaka H, Kobayashi M (2003). "Expression of novel rice gibberellin 2-oxidase gene is under homeostatic regulation by biologically active gibberellins." J Plant Res 116(2);161-4. PMID: 12736788
Sakamoto01: Sakamoto T, Kobayashi M, Itoh H, Tagiri A, Kayano T, Tanaka H, Iwahori S, Matsuoka M (2001). "Expression of a gibberellin 2-oxidase gene around the shoot apex is related to phase transition in rice." Plant Physiol 125(3);1508-16. PMID: 11244129
Schomburg03: Schomburg FM, Bizzell CM, Lee DJ, Zeevaart JA, Amasino RM (2003). "Overexpression of a novel class of gibberellin 2-oxidases decreases gibberellin levels and creates dwarf plants." Plant Cell 15(1);151-63. PMID: 12509528
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