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WVirwille and Mitchell reported that applications of 2'-isopropyl-4'(trimethylammonium chloride) -5'methylphenyl piperidine-1-carboxylate (Amo 1618) (fig 1, I) to several types of flowering plants led to an abnormal growth habit characterized by shorter stems and internodes (17). Amo 1618 is one of the most active of several carbamate esters containing a quarternary ammonium group which produce this effect (12). Cathey has recently reviewed the physiological effects of these and other plant growth retardants (2). Sachs et al. (15) have shown that application of Amo 1618 leads to a drastic reduction of mitotic activity in the subapical meristem of vegetative, rooted cuttings of Chrysanthemum morifolium. In this case, as in numerous other instances reported in the literature, added gibberellins partly or completely overcome the action of plant growth retardants. [see (2) for a review]. Kende, Ninnemann and Lang (11) have provided evidence for the proposal that at least some of these plant growth retardants may function by inhibiting the production of gibberellins in the plant. This proposal was based initially on their demonstration that the addition of Amo 1618 at 100 ug per ml or of f-chloroethyltrimethylammonium chloride (CCC) at 300 ,ug per ml to the culture medium of Fusarium moniliforme Sheld. leads to a complete inhibition of gibberellin production without affecting mycelial growth (11). Several tvpes of evidence were presented to show that CCC inhibited the biosynthesis of gibberellins rather than causing destruction of preformed gibberellins (13). More recently Baldev et al. have also shown Amo 1618 to be an inhibitor of the accumulation of substances with gibberellin-like activity in developing pea seeds (1). The evidence to be discussed in this paper shows that an enzymic reaction resulting in the production of (-)-kaurene (fig 1, Ia) in a flowering plant system is inhibited in the presence of Amo 1618 or certain other plant growth retardants. (-)-Kaurene is a probable intermediate in the biosynthesis of gibberellins. Not only did Cross et al. (5, 6) isolate this compound from culture filtrates of F. noniliforme along with gibberellins, but they also demontrated in these cultures a conversion of (-) -kauretne labeled with C14 in the exocyclic methylene group to C14-gibberellic acid (fig 1, III) labeled in the same positioni (4). Recently we have confirmed this finding with C'4-labeled (-)-kaurene (9). The substrate in this case was derived from 2-C14-mevalonate in the endosperm nucellus of Echinocystis macrocarpa Greene (wild cucumber) seed. Furthermore, Phinney et al. (14) have reported that (-)-kaurene promotes growth of the dwarf-5 mutant of Zea mays, an effect qualitatively indistinguishable from that induced by an added gibberellin. This evidence considered together strongly supports the idea that (-)kaurene is an intermediate in gibberellin biosynthesis. Thus, an inhibition of (-)-kaurene synthesis would result in an inhibition of gibberellin synthesis. The endosperm of E. macrocarpa seed, a source knowln to be relatively rich in gibberellins and gibberellin-like substances (3,8,16), was used in the studies reported in this paper as a source of enzymes which catalyze the formation of (-)-kaurene. (-)kauren19-ol (fig 1, IIb) and trans-geranylgeraniol (fig 1, IV) from mevalonate (7, 9). Since these diterpenoid metabolites are involved in the gibberellin biosynthesis pathway, a study of the effects of Amo 1618 and other growth retardants on their formation was undertaken.

An enzymic site of inhibition of gibberellin biosynthesis by Amo 1618 and other plant growth retardants.