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Chemistry of Paclobutrazol (PBZ) and its Function in Agriculture: A Review

Dr. Ashok Kumar, Bhuj BD, Shri Dhar

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-precursorent-kaurene toentkaurenoic acid in a cell-free system from Cucurbita maxima endosperm. Paclobutrazol (PBZ) is a plant growth retardant and triazole fungicide. It is a known antagonist of the plant hormone gibberellin. The largest group of plant growth retardants consists of chemicals antagonistic to gibberellins (GA), the hormone that is responsible for plant growth. Commercially used inhibitors of GA biosynthesis are: (a) onium-type compounds, (b) compounds with a N-heterocycle (triazole-type), (c) structural mimics of 2-oxoglutaric acid, and (d) 16, 17-dihydroGAs. It acts by inhibiting gibberellin biosynthesis, reducing internodial growth to give stouter stems, increasing root growth, causing early fruit set and increasing seedset in plants.). It is a synthetic compound [(2 RS, 3 RS) -1- (4-chlorophenyl) - 4, 4-dimethyl-2- (1 H-1, 2, 4-triazol-1-yl) -pentan-3 ol] that inhibits vegetative growth, belonging to the triazole group. Chemical properties of PBZ include: molecular weight 293.8, molecular formula C15H20ClN3O, melting point 165°C–166°C, density 1.22 g ml–1 and water solubility 35 mg L–1. PBZ is a hydrophobic and slightly polar molecule, with hydrophilic parts. Paclobutrazol is an inhibitor of enzymes which use cytochrome P450 as a co-factor. Their active site contains a heme center which activates oxygen from the air to oxidise their substrates. The (2S,3S) isomer inhibits the enzyme ent-kaurene oxidase which is on the main biosynthetic pathway to gibberellins, which are important plant hormones.