paclobutrazol

Paclobutrazol (PBZ) is the ISO common name for an organic compound that is used as a plant growth retardant and triazole fungicide. It is a known antagonist of the plant hormone gibberellin, acting by inhibiting gibberellin biosynthesis, reducing internodal growth to give stouter stems, increasing root growth, causing early fruitset and increasing seedset in plants such as tomato and pepper. PBZ has also been shown to reduce frost sensitivity in plants. Moreover, paclobutrazol can be used as a chemical approach for reducing the risk of lodging in cereal crops. PBZ has been used by arborists to reduce shoot growth and shown to have additional positive effects on trees and shrubs. Among those are improved resistance to drought stress, darker green leaves, higher resistance against fungi and bacteria, and enhanced development of roots. Cambial growth, as well as shoot growth, has been shown to be reduced in some tree species. == Structure and synthesis == The first synthesis of paclobutrazol was disclosed in patents filed by an ICI group working at Jealott's Hill. 650px 4-Chlorobenzaldehyde and pinacolone are combined in an aldol condensation to form a chalcone which is hydrogenated using Raney nickel as catalyst to give a substituted ketone. This material is brominated and the resulting compound treated with the sodium salt of 1,2,4-triazole in a nucleophilic substitution reaction. The final reduction reaction uses sodium borohydride, which in cold methanol gives almost exclusively the diastereomer pair having the absolute configuration (2R,3R) and its enantiomer (2S,3S), with only about 2% of the alternative (2R,3S) and (2S,3R) isomers. However, this pair of isomers can be produced when the reduction is carried out using butylmagnesium bromide.

In a 1984 study, ICI workers separated the individual enantiomers by chiral resolution and were able to demonstrate that only the (2R,3R) isomer displays substantial fungicidal activity, whereas the (2S,3S) isomer is responsible for the growth regulating properties. However, the commercial product (developed under the code number PP333) was the racemic material, since separation of the isomers was unnecessary when both components had utility in agriculture. == Mechanism of action == 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. A secondary effect arising from the inhibition of ent-kaurene oxidase is that its precursor, geranylgeranyl pyrophosphate accumulates in the plant and some of this is diverted into additional production of the phytol group of chlorophyll and the hormone abscisic acid. The latter is responsible for controlling transpiration of water through the leaves and hence PBZ treatment can lead to better tolerance of drought conditions. The (2R,3R) isomer is a better fit to the active site of the fungal cytochrome P450 14α-demethylase. This inhibits the conversion of lanosterol to ergosterol, a component of the fungal cell membrane, which is lethal for many species. Many other azole derivatives including propiconazole and tebuconazole show this type of activity, so the main commercial opportunity for paclobutrazol was as a plant growth retardant and it was first marketed by ICI in 1985 under the trade names Bonzi, Clipper, Cultar and Parlay. == Usage== As an antagonist of gibberellin biosynthesis, PBZ has a growth retardant effect on most plant species. It is absorbed by plant tissues and transported via the xylem to the growing parts, where the rate of cell division is reduced compared to untreated plants and the new cells do not elongate. ===Ornamental crops=== PBZ is used in horticulture, especially for glasshouse-reared perennial plants. ===Trees and shrubs=== The ability of PBZ to reduce the growth of trees and shrubs means that it has found use in areas where there is a need to moderate such growth, for example under electric power lines and where a right-of-way is to be maintained. A single application of the growth regulator can give season-long control.