Inhibition gibberellin biosynthesis

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. 

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). 

Figure 2. inhibition of gibberellin biosynthesis with paclobutrazol... 

Figure 3 The sites of inhibition by azole compounds in the ergosterol and gibberellin biosynthesis.

The above results strongly suggest that the N4 atom of the 1, 2,4-triazo 1 y1 moiety of (I) and (II) co-ordinates to the active site(s) of oxidase enzymes, which leads to the inhibition of ergosterol and gibberellin biosynthesis, respectively. 

Plant hormone biosynthesis and metabolism are influenced and modified, too, as already mentioned, by a large number of typical secondary plant constituents of quite different structures (Eef. 5, 6). For instance, it has been shown that some oi-disubstituted acetic acid derivatives are active both as antiauxins and as inhibitors of the biosynthesis of cyclic terpenoids. Thus, some more simple model compounds synthesized by ourselves, for example substituted a-phenoxy-isobutyrio acids, are not only competitive auxin inhibitors but they are also able to inhibit both the gibberellin and sterol biosynthesis. The same is true for some well known plant growth retardants, such as CCO or AMO 1618, which are inhibitors of gibberellin biosynthesis both in plants and in Fus ium moniliforme, but at the same time they are inhibiting and modifying also the biosynthesis of steroids in the respective organisms as well as in animal in vitro systems (Eef. 29) ... 

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... 

Ihe inhibition of gibberellin biosynthesis provides a means of developing novel plant growth regulators. As mi t be expected,the effect of exogenous... 

Inhibits gibberellin and ergosterol biosynthesis and hence the rate of cell division... 

This plant growth regulatory activity is due to inhibition of the biosynthesis of gibberellins (j>), at the three oxidation steps between ent-kaurene and ent-kaurenoic acid (Figure 2). 

Investigations on the biochemical mechanisms causing these symptoms in plants have revealed that the inhibition of the biosynthesis of the plant hormone gibberellin and of plant sterols seem to be the most important targets in plants [17, 18]. 

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