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 carboxymethylation, gibberellin methylation
|Superclasses:||Activation/Inactivation/Interconversion → Inactivation|
|Degradation/Utilization/Assimilation → Hormones Degradation → Plant Hormones Degradation → Gibberellins Degradation|
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), ( [Lee02c, Thomas99], et al.)), epoxidation (see gibberellin inactivation III (epoxidation)) and methylation (this pathway) [Varbanova07] may be used to reduce the levels of bioactive GAs.
About This Pathway
Gibberellin methylation is believed to inactivate bioactive GAs and to lead to their further catabolism [Varbanova07]. The existence of this pathway is supported by the identification of two enzymes from Arabidopsis, GAMT1 and GAMT2, that can catalyze GA methylation in vitro [Varbanova07]. Although no methylated forms of GA have been detected in Arabidopsis plants to date, over-expression of the enzymes reduced the levels of active GAs while a gamt1 gamt2 double mutant had elevated levels of bioactive GAs arguing that these enzymes do play a role in gibberellin metabolism in vivo. In addition, there is evidence for the existence of methylated GAs as these have been observed in at least one plant species, namely, the fern Lygodium circinatum [Yamauchi96]. 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 methyl ester 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].
Several lines of evidence suggest that GA methyl esters are not biologically active. Importantly, direct application of exogenous GA methyl esters are not nearly as effective at inducing GA-mediated biological responses as non-methylated GAs in several plant species including Arabidopsis thaliana [Weiss95, Cowling98]. In addition, the putative GA receptor in rice, GID1, does not bind to GA methyl esters significantly [UeguchiTanaka05]. On the other hand, in some assays, methyl or dimethyl-GAs appear to cause biological responses [Smith93, Yamauchi96]. It is possible that specific GA methyl esters will have different biological activities in different species.
The role of GA methylation in various plant processes will also depend on the tissues and organs in which this process occurs. In Arabidopsis, expression of the two GA carboxymethyl transferases is largely limited to siliques during the later stages of development indicating that methylation of GAs might play a limited role in other GA-regulated processes that occur in other organs or in different developmental stages [Varbanova07]. Additional studies on different plant species will reveal if expression during seed development is a generalized trend or if this expression pattern is limited to Arabidopsis thaliana.
Several other plant hormones including jasmonic acid and salicylic acid may be reversibly methylated and later demethylated to be reactivated. There is no evidence to date that GA methyl esters can be cleaved by methylesterases, but, it is interesting to note that the GID1 receptor in rice is part of the superfamily that includes the methyl salicylate and methyl jasmonate methylesterases [Varbanova07, UeguchiTanaka05].
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
Smith93: Smith, S.J., Walker, R.P., Beale, M.H., Hooley, R. (1993). "Biological activities of some gibberellins and gibberellin derivatives in aleurone cells and protoplasts of Avena fatua." Phytochemistry. 33(1):17-20.
Thomas99: 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
UeguchiTanaka05: Ueguchi-Tanaka M, Ashikari M, Nakajima M, Itoh H, Katoh E, Kobayashi M, Chow TY, Hsing YI, Kitano H, Yamaguchi I, Matsuoka M (2005). "GIBBERELLIN INSENSITIVE DWARF1 encodes a soluble receptor for gibberellin." Nature 437(7059);693-8. PMID: 16193045
Varbanova07: Varbanova M, Yamaguchi S, Yang Y, McKelvey K, Hanada A, Borochov R, Yu F, Jikumaru Y, Ross J, Cortes D, Ma CJ, Noel JP, Mander L, Shulaev V, Kamiya Y, Rodermel S, Weiss D, Pichersky E (2007). "Methylation of gibberellins by Arabidopsis GAMT1 and GAMT2." Plant Cell 19(1);32-45. PMID: 17220201
Weiss95: Weiss, D., Vanderluit, A., Knget, E., Vermeer, E., Mol, J.M.N., Kooter, J.M. (1995). "Identification of endogenous gibberellins in petunia flowers - induction of anthocyanin biosynthetic gene-expression and the antagonistic effect of abscisic-acid ." Plant Physiology. 107(3):695-702.
Yamauchi96: Yamauchi T, Oyama N, Yamane H, Murofushi N, Schraudolf H, Pour M, Furber M, Mander LN (1996). "Identification of Antheridiogens in Lygodium circinnatum and Lygodium flexuosum." Plant Physiol 111(3);741-745. PMID: 12226326
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