The advent of genetic engineering has provided a means to manipulate the biosynthesis of plant hormones to the advantage of agriculture. Such manipulation is very dependent on a detailed knowledge of the biosynthetic pathways involved in the production of the hormones, and more precisely about the enzymes involved in this biosynthesis. The knowledge of enzymes assists, but is not always a prerequisite for, the isolation of the genetic material, usually cDNAs, used as the genetic tools to manipulate hormone synthesis.