Gibberellin pathways

FRAY R, WALLACE A, FRASER P D, VALERO D, HEDDEN P, BRAMLEY P M and GRIERSON D (1995) Constitutive expression of a fruit phytoene synthase gene in transgenic tomatoes causes dwarfism by redirecting metabolites from the gibberellin pathway . Plant J, 8, 696-701. 

Since carotenoids are isoprenoids, they share a common early pathway with other biologically important isoprenoids such as sterols, gibberellins, phytol and the terpenoid quinones (Fig. 13.3). In all cases, these compounds are derived from the C5 isoprenoid, isopentenyl diphosphate (IPP). Until a few years ago it was believed that a single pathway from the Cg precursor mevalonic acid (MVA) formed IPP, which itself was synthesised from hydroxymethylglutaryl coenzyme A (HMG CoA) by the action of HMG... 

Since carotenoids are derived for the central isoprenoid pathway (Fig. 13.3), the regulation of their formation must involve a co-ordinated flux of isoprenoid imits into this branch of the pathway as well as into others such as the biosynthesis of sterols, gibberellins, phytol and terpenoid quinones. An imderstanding of the complexities of regulation of the pathway is necessary in order to target the regulatory steps for genetic manipulation. 

The most commercially successful PGRs are those that operate either through the inhibition of gibberellin biosynthesis, from mevalonic acid (Knee, 1982) or through the production of ethylene. These represent the two most valuable pathways for growth modification since they have key roles in extension growth, ripening, fruit set and dominance. 

U. Bearder, J.R., MacMillan, J., Phinney, B.O. "Fungal products. Part XIV. Metabolic pathways from ent-kaurenoic acid to the fungal gibberellins in mutant Bl-J+la of Gibberella fujikuroi." J. . . Perkin Trans. 1975, 1, 721-726. 

W. Rademacher, Growth retardants effect on gibberellin biosynthesis and other metabolic pathways, Annu. Rev. Plant Physiol. Plant Mol. Bio. 51 (2000) 501-531. 

HELLIWELL, C.A., SULLIVAN, JA. MOULD, R.M., GRAY, J.C., PEACOCK W.J., DENNIS, E.S., A plastid envelope location of Arabidopsis enf-kaurene oxidase links the plastid and endoplasmic reticulum steps of the gibberellin biosynthesis pathway., Plant J., 2001,28,201-208... 

Figure 25 Biosynthetic pathways of gibberellins and diterpenoid phytoallexins in rice.

Growth retardation in host plants has been associated with the use of a number of these EBI s. This effect results from the inhibition of a reaction in the gibberellin biosynthetic pathway that involves cytochrome P-450 enzymes (24). 

Leubner-Metzger, G. (2000). Brassinosteroids and gibberellins promote tobacco seed germination by distinct pathways. Planta, 213, 758-763. 

Unlike primary metabolites, the genes that regulate the formation of the enzymes of seeondary metaboUte biosynthesis are often clustered. In several eases the loei of these genes have been determined. This has considerable sig-nifieanee in the control of secondary metabolite biosynthesis. The genes that eode for several important polyketide pathways such as those leading to the aflatoxins and the statins have been identified. Similar work has also been reported for penicillin biosynthesis and some non-ribosomal peptides as well as terpenoid pathways such as that leading to the gibberellins. 

The carbon skeleton of the ent-kaurenoid precursors of the gibberellins is closely related to that of the diterpenoids of the ent-atiserene (10.29), ent-beyerene (10.30) and ent-trachylobane (10.31) series. The dilferences lie in the structure of ring D. The flexibility of the gibberellin biosynthetic pathway in... 

Like all steroids, brassinosteroids derive from a single common precursor mevalonic acid. Some phytohormones are synthesized totally or in part via the isoprenoid pathway, such as absdsic acid, gibberellins and cytokinins. The importance of this biosynthetic pathway in processes involved with cell cycle regulation and tumorigenesis in mammals is well documented. Having in mind the similarities between certain regulatory systems in plants and animals the question arose whether brassinosteroids as putative plant steroid hormones would show a specific effect on plant tumor cells. 

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