The latter steps of biosynthesis of non-13-hydroxylated/13-hydroxylated GAs. The steps surrounding production of bioactive GAs in higher plants are catalysed by the 2-oxoglutarate-dependent dioxygenases (2ODDs); 1 GA 20-oxidase, 2 GA 3-oxidase and 3 GA 2-oxidase. In pea the 13-hydroxylated pathway predominates; in Arabidopsis the non-13-hydroxylated pathway generally predominates, however developing seeds contain comparable levels of non-13-hydroxylated and 13-hydroxylated GAs . The sln phenotype of pea arises from a null mutation affecting the predominant GA 2-oxidase (PsGA2OX1) of maturing seeds (steps 3). In this study the activities of a pea GA 2-oxidase (PsGA2OX1) and arabidopsis GA 3-oxidase (AtGA3OX4) are both assayed using the substrate [1β,2β,3β-3H3]GA20. It should be noted that in addition both enzymes are likely to use GA9 as a natural substrate.