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Bacteria can produce gibberellin plant hormones. While the bacterial biosynthetic pathway is similar to that of plants, the individual enzymes are very distantly related and arose via convergent evolution. The cytochromes P450 (CYPs) that catalyze the multi-step oxidation of the alkane precursor ent-kaurene (1) to ent-kauren-19-oic acid (5), are called ent-kaurene oxidases (KOs), and in plants are from the CYP701 family, and share less than 19% amino acid sequence identity with those from bacteria, which are from the phylogenetically distinct CYP117 family. Here the reaction series catalyzed by CYP117 was examined by 18O2 labeling experiments, the results indicate successive hydroxylation of 1 to ent-kauren-19-ol (2) and then ent-kauren-19,19-diol (3) and most likely an intervening dehydration to ent-kauren-19-al (4) prior to the concluding hydroxylation to 5. Accordingly, the bacterial and plant KOs converged on catalysis of the same series of reactions, despite their independent evolutionary origin.

Three routes for the preparation of ent-7-nor-5β-gibberell-16-enes from fujenal are described. Evidence is presented for the stereochemistry of the products.

Isosteviol (ent-16-oxobeyeran-19-oic acid) and steviol acetate (ent-13-acetoxykaur-16-en-19-oic acid) are efficiently metabolised by cultures of resuspended mycelium of Gibberella fujikuroi, mutant B1–41a. Isosteviol is converted, inter alia, into ring-CD-rearranged derivatives of gibberellins A17, A19, A20, and 13-hydroxygibberellin A12. Steviol acetate is metabolised mainly to the 7β-hydroxy- and 6β, 7β-dihydroxy-derivatives and to the 13-acetates of gibberellins A17 and A20. The preparation of [3H]steviol acetate and its conversion into [3H]gibberellin A20 of high specific activity is described. These results provide further evidence that the fungal enzymes for gibberellin biosynthesis from ent-kaurenoic acid lack substrate specificity. They also reveal that structural changes in the CD ring system of ent-kaurenoid substrates suppress the 3-hydroxylating enzyme.

The formation of 7β-hydroxy-(–)-kaur-16-en-19-oic acid and its incorporation into gibberellic acid and the kauranoid metabolites of Gibberella fujikuroi are described. The results of incubation of 7β-hydroxy-[6β-3H,17-14C]-(–)-kaur-16-en-19-oic acid have shown that the 6β-hydrogen atom is lost in the formation of gibberellic acid. However 6β,7β-dihydroxy-(–)-kaur-16-en-19-oic acid is not incorporated into gibberellic acid. Experiments with [1-3H2,1-14C]geranyl pyrophosphate suggest that the 6β-hydrogen atom migrates to C-7 during ring contraction.

Gibberellin A13 and A14 7-aldehydes have been tentatively identified as metabolites of gibberellin A12 aldehyde in a cell-free system derived from Gibberella fujikuroi; gibberellin A13 7-aldehyde is incorporated into gibberellic acid, and the C-20 carbon atom which is removed at this stage has been isolated as carbon dioxide.

ent-3α-Hydroxy- and ent-2α,3α-dihydroxy-kaur-16-en-19-oic acids have been prepared from ent-kaur-16-ene-3β,19-diol, and their metabolites from cultures of the mutant B1-41a of Gibberella fujikuroi have been analysed by g.l.c.–mass spectrometry. The metabolism of ent-2α-hydroxy, ent-2β-hydroxy-, and ent-3α-hydroxykaur-16-en-19-ols has been similarly investigated in cultures of the parent wild-type strain, GF-1a, in which gibberellin biosynthesis was blocked by a synthetic plant growth retardant. The results show that the ent-3α-hydroxylated analogues of the normal gibberellin intermediates, ent-kaur-16-en-19-ol and ent-kaur-16-en-19-oic acid, are efficiently converted into 3-hydroxylated gibberellins. They also indicate that the ent-2β-hydroxy-analogue is converted into gibberellin A3 by dehydration and that the conversion of ent-kaurenoids into gibbereilins is reduced by the presence of ent-2α- and ent-2β-hydroxy-groups.

The conversion of ent-7α-hydroxykaurenoic acid (1) into gibberellin A12-aldehyde (2) and ent-6α, 7α- dihydroxykaurenoic acid (3) involves the loss of one hydrogen atom at C-6.

Gibberellin A14-aldehyde (4) has been synthesised from 3β,7β-dihydroxykaurenolide (10). In cultures of Gibberella fujikuroi, mutant Bl-41a, it is shown to be formed from gibberellin A12-aldehyde (1) and to be converted into fungal 3-hydroxylated gibberellins including gibberellin A3(8). Gibberellin A14(6) and gibberellin A14-alcohol (7) are also metabolised to 3-hydroxygibberellins, the former at a much lower rate. The biosynthetic pathway to gibberellins in the fungus is discussed in the light of these results.

The ring-contracted ent-6α-hydroxy-5β(H)-B-norkaur-16-en-19-oic acid (4) and the corresponding alcohol (6) have been shown to act as inhibitors of gibberellin plant growth hormone biosynthesis at the ring contraction stage and to act as plant growth regulators in rice.

7β-Hydroxy-(–)-kaur-16-en-19-oic acid has been shown to act as a precursor of the gibberellins and the kaurenolides.

Evidence presented suggests that the ring contraction of a kauranoid diterpene to a gibbane is accompanied by a hydrogen migration from C-6 to C-7.

The stereospecific labelling of the gibberellins and the kaurenolides with 2(R) and 5(R)-[3H]mevalonate leads to the conclusion that the dehydrogenation of ring A is a cis-process and that hydroxylation of ring B takes place with retention of configuration.