INHIBITION OF GIBBERELLIN BIOSYNTHESIS BY PACLOBUTRAZOL IN CELL-FREE HOMOGENATES OF CUCURBITA-MAXIMA ENDOSPERM AND MALUS PUMILA EMBRYOS

The plant growth retardant paclobutrazol, (PP333) (2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol, inhibits specifically the three steps in the oxidation of the gibberellin-precursor ent-kaurene to ent-kaurenoic acid in a cell-free system from Cucurbita maxima endosperm. The KI50 for this inhibition is 2×10-8 M. The KI50 values for the separated 2S, 3S, and 2R, 3R enantiomers of paclobutrazol in this system are 2×10-8 M and 7×10-7 M, respectively. A cell-free preparation from immature Malus pumila embryos converts ent-kaurene to gibberellin A9, whereas no conversion occurs in a similar preparation from Malus endosperm. The conversion of ent-kaurene by the embryo preparation is inhibited by paclobutrazol with KI50 values for the 2S,3S and 2R,3R enantiomers of 2×10-8 M and 6×10-8 M, respectively. © 1991 Springer-Verlag New York Inc.