Figure 1.

Polyamine biosynthetic pathway in plants. Putrescine is synthesized directly from ornithine by ornithine decarboxylase and indirectly from arginine by arginine decarboxylase, agmatine iminohydrolase and N-carbamoylputrescine amidohydrolase. Spermidine and spermine are, respectively, formed by the subsequent addition of an aminopropyl moiety to putrescine and spermidine by spermidine synthase and spermine synthase, respectively. Two enzymes, diamine oxidase, and polyamine oxidase, participate in degradation of polyamine.