Recent advances in gibberellin biosynthesis

Regulation of gibberellin (GA) biosynthesis by endogenous and environmental stimuli is an important factor in the control of plant morphogensis. Recent advances in the molecular biology of GA biosynthesis is enabling these processes to be examined at the molecular level. The biosynthetic pathway to biologically active GAs requires the action of diterpene cyclases, cytochrome P450-dependent mono-oxygenases and 2-oxoglutarate-dependent dioxygenases. The genes or cDNAs for many of these biosynthetic enzyme's have now been cloned from several different species. The dioxygenases include GA 20-oxidase, 3 beta-hydroxylase and PP-hydroxylase; the first catalyses the removal of carbon-20, the second catalyses the final step in the production of the growth-active hormones, which can be deactivated by the PP-hydroxylases. The GA 20-oxidases, and probably also 3 beta-hydroxylases, are encoded by small multigene families, members of which have been shown to be expressed in a developmentally and spacially specific manner. Furthermore, there is evidence for rapid down-regulation of the expression of these genes by GA in a type of feedback control. Gibberellin 20-oxidase gene expression is also regulated by photoperiod in at least some long-day plant species. As a regulatory enzyme, GA 20-oxidase is being investigated as a target for genetic manipulation of GA biosynthesis. Results with model species indicate that considerable changes in GA content and plant morphology can be obtained by overexpressing GA 20-oxidase genes or reducing their expression by introducing anti-sense DNA sequences.