CHAPTER 16: Gibberellin Metabolism

The gibberellin (GA) plant hormones are diterpenoid carboxylic acids that regulate growth and development throughout the life cycle of flowering plants, but are also present in some species of lower plants, fungi and bacteria. The latter stages of their biosynthesis in flowering plants involve the activities of two main families of 2-oxoglutarate-dependent dioxygenases (ODDs): GA 20-oxidases, which oxidize the C-10 methyl group to the aldehyde and then cleave it to form the C₁₉-GAs, and GA 3-oxidases, which introduce a 3β-hydroxyl group as the final step in the formation of the biologically active hormones. Oxidation on C-2 by other ODD families (GA 2-oxidases) that act on C₁₉-GAs or their C₂₀-GA precursors serves as an inactivation mechanism. A further family of ODDs with a restricted species distribution, the GA 7-oxidases, convert the early precursor GA₁₂-aldehyde to GA₁₂, a reaction that is also catalysed by cytochrome P450 monooxygenases. Members of the ODD gene families are major sites of regulation for GA biosynthesis and catabolism, responding to developmental and environmental signals. The paralogues have distinct but often overlapping expression domains, and differ also in levels of expression and regulation. These enzymes have proved to be useful targets for the introduction of beneficial traits into crop species, while the acylcyclohexanedione inhibitors of the GA 3-oxidases have found important application in agriculture as growth retardants.