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 16α, 17-epoxidation
|Superclasses:||Activation/Inactivation/Interconversion → Inactivation|
|Degradation/Utilization/Assimilation → Hormones Degradation → Plant Hormones Degradation → Gibberellins Degradation|
Some taxa known to possess this pathway include : Oryza sativa Japonica Group
Expected Taxonomic Range: Viridiplantae
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 [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).
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.
Given the important roles that GAs play in plant growth and development, several levels of regulation are used to carefully control the level and types of bioactive GAs present in various tissues. Direct chemical modifications to gibberellins including oxidation (see gibberellin inactivation I (2β-hydroxylation)) methylation (gibberellin inactivation II (methylation)), and epoxidation (this pathway) may be used to reduce the levels of bioactive GAs.
About This Pathway
Gibberellin epoxidation is believed to inactivate bioactive GAs and their precursors [Zhu06a]. The existence of this pathway is supported by the identification of an enzyme from rice, CYP714D1, that can catalyze GA epoxidation in vitro [Zhu06a]. In addition, there is also evidence that the transcript expression pattern of a putative 16α, 17 gibberellin epoxidase in Lolium temulentum may be correlated with a selective reduction in the level of active GA1 and GA4 just below the shoot apex in this species [King08]. Although the epoxy forms of GA have not been detected in rice plants to date, their putative dihydroxy derivatives, such as, 16,17-dihydro-16α,17-dihydroxy gibberellin A4, have been found in a number of plant species, including rice, Pisum sativum, Lupinus albus, Malus domestica (see refs in [Zhu06a]) and Lolium temulentum [King08]. Although these dihydroxy compounds can be produced spontaneously in vitro by exposure to acids such as acetic acid during the compound analysis process [Zhu06a], it is likely that they are also produced in vivo through the activity of an epoxy-hydrolase or other enzyme.
Although gibberellins are produced by a number of fungi (for example, see gibberellin biosynthesis IV (Gibberella fujikuroi)) and bacteria, there is no evidence to date of GA epoxide formation in these species, and it may be unlikely to occur in organisms where GA does not play a role as an endogenous signaling molecule that needs to be selectively inactivated [Bomke09].
It is interesting to note that CYP714D1 -mediated epoxidation can occur in vitro both on the biologically active gibberellin A4 as well as the gibberellin A12 precursor compound that is produced very early in the GA biosynthetic pathway. This indicates that epoxidation may be used to inactivate specific GAs that are involved in particular processes, or it may be used to more globally shut down GA signaling by acting on upstream compounds. Work in additional plant species will be required to gain a better understanding of the specificity and functional role of GA epoxidation in GA metabolism. Given the important role that the CYP714D1/EUI GA epoxidase plays in regulating internode elongation and facilitating hybrid rice production (see refs in [Zhu06a]) this mechanism of GA regulation is likely to be subject to further study.
Unification Links: PlantCyc:PWY-6494
Bomke09: Bomke C, Tudzynski B (2009). "Diversity, regulation, and evolution of the gibberellin biosynthetic pathway in fungi compared to plants and bacteria." Phytochemistry 70(15-16);1876-93. PMID: 19560174
King08: King RW, Mander LN, Asp T, MacMillan CP, Blundell CA, Evans LT (2008). "Selective deactivation of gibberellins below the shoot apex is critical to flowering but not to stem elongation of Lolium." Mol Plant 1(2);295-307. PMID: 19825541
Zhu06a: Zhu Y, Nomura T, Xu Y, Zhang Y, Peng Y, Mao B, Hanada A, Zhou H, Wang R, Li P, Zhu X, Mander LN, Kamiya Y, Yamaguchi S, He Z (2006). "ELONGATED UPPERMOST INTERNODE encodes a cytochrome P450 monooxygenase that epoxidizes gibberellins in a novel deactivation reaction in rice." Plant Cell 18(2);442-56. PMID: 16399803
Kobayashi93: Kobayashi M, Gaskin P, Spray CR, Suzuki Y, Phinney BO, MacMillan J (1993). "Metabolism and Biological Activity of Gibberellin A4 in Vegetative Shoots of Zea mays, Oryza sativa, and Arabidopsis thaliana." Plant Physiol 102(2);379-386. PMID: 12231829
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