2'-Isopropyl-4'- (trimethylammoniumii chloride) -5'-
methylphenyl piperidine-l-carboxylate (AAmo 1618),
a plant growth retardant, inhibits the formation of
(-)-kaurene anid (-) -kauren-19-ol but niot transgeranylgeraniol
from mevalonate in honmogenates of
endospermn anid nucellus fromii seed of Ecidnocv7stis
iwiacrocarp(a Greene (wild cuicumber). The enzvmic
cyclizationi of traiis-geranylgeranyl pyrophosphate
was (lirectlv shown to be inhibited by relatively low
levels of this retardant. Several other plant retardanits,
among them tributyl-2,4-dichlorol)enzylammomiuim
chloride (Phosfoin S) and tributvl-2,4-dichloro-
1)en-1ylphosp)honiuni chlori(le (Phosfoni) as well as
several substances closely related structturally to Amo
1618, also were fouln(d to inhibit (-)-kaurene formation
in this system. It is prol)osed that inhibition
of the cyclization reactioni with the resulting decrease
of gibberellin biosynthesis may account for the
growth retarding properties of these suibstanices.