Gibberellins biosynthesis

These two groups have yet to have a specfic primary target site ascribed to them however, both types of herbicides are reported to inhibit lipid biosynthesis (see Chapter 3). Gibberellin biosynthesis is inhibited by the thiocarbamate herbicide EPTC at a step before enf-kaurene. In a cell-free preparation from Sorghum seedlings, the conversion of [ CJmevalonic acid (MVA) to kaurene was reduced and [ C]-GGPP accumulated. 

Recently, EPTC has been shown to inhibit the conversion of acetate and pyruvate to acetyl-CoA. However, this cannot directly explain the inhibition of enf-kaurene synthesis from MVA. It seems likely that the thiocarbamate effect on gibberellin biosynthesis is a secondary target. This was the conclusion from a study with diallate, which inhibited enf-kaurene synthetase only at high concentrations. Gibberellins may have a role in 

In some studies, retardants have been implicated in yield enhancement mediated by the manipulation of crop canopy structure and the removal of dominance characteristics, leading to a more uniform crop and potentially higher yields. Senescence may be delayed. Although the maintenance of green tissue is a clear advantage with respect to yield production, prolonged seasons can expose the crop to damaging episodes of adverse weather and counteract the potential benefits of PGR treatment. 

Some compounds, for example, ancymidol are used in the horticulture industry to induce dwarfism in pot plants. 

Mepiquat chloride Ethyl 4-cyclopropyl(hydroxy)methylene-3,5-dioxocyclohexanecarboxylate 

Isopentyl pyrophosphate Geranyl pyrophosphate Carotenoids ---Farnesyl pyrophosphate  

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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. 

Kim, H. Y., and Suzuki, Y. (1989). Changes in assimilated BC distribution and soluble acid invertase activity of Zinnia elegans induced by Uniconazol an inhibitor of gibberellin biosynthesis. Plant Physiol. 90, 316-321. 

Gibberellin Biosynthesis in the Fungus Gibberella fujikuroi and in Higher Plants... 

Bearder, J.R., MacMillan, J., Weis, C.M., Chaffey, M.B., Phinney, B.O. "Position of the metabolic block for gibberellin biosynthesis in mutant Bl-Ula of Gibberella fujikuroi." Phytochemistry, 19lh, 13, 911-917. 

Gibberellin biosynthesis in a cell-free system from immature seeds of Pisum sativum." Biochem. Biophys. Res. Commun.,... 

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... 

V. M. Sponsel P. Hedden, In Plant Hormones Biosynthesis, Signal Transduction, Action Gibberellin Biosynthesis and Inactivation P. J. Davies, Ed. Kluwer Academic Dordrecht, 2004 pp 63-94. 

Of the substances that interfere with gibberellin biosynthesis, mention should first be made of the so-called onium compounds, which are substances with a charged central atom (5, 6). They include the already extensively employed bioregulators chlormequat chloride (CCC) and mepiquat chloride (DPC) (Figure 1). 

The retardation effect of all these substances can be reversed by gibberellins. Furthermore, it has been possible, in the case of some of these substances, to detect precisely the point of attack or site of action in the gibberellin biosynthesis sequence. As can be seen from Figure 3, it is assumed that the onium compounds inhibit the cyclization of geranylgeranyl pyrophosphate to copalyl pyrophosphate (7), whereas it has been demonstrated in cell-free systems that pyrimidines, norbornenodiazetines, and triazoles inhibit the sequential oxidation of ent-kaurene to ent-kaurenoic acid (fj, 9). 

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