Plant alkaloids, one of the largest groups of natural products, provide many pharmacologically active compounds. Several genes in the biosynthetic pathways for scopolamine, nicotine, and berberine have been cloned. making the metabolic engineering of these alkaloids possible. Expression of two branching-point enzymes was engi- neered: putrescine N-methyltransferase (PMT) in transgenic plants of Atropa belladonna and Nicotiana sylvestris and (S)-scoulerine 9-O- methyltransferase (SMT) in cultured cells of Coptis japonica and Eschscholzia californica. Overexpression of PMT increased the nico- tine content in N.sylvestris, whereas suppression of endogenous PMT activity severely decreased the nicotine content and induced abnor- mal morphologies. Ectopic expression of SMT caused the accumula- tion of benzylisoquinoline alkaloids in E. californica. The prospects and limitations of engineering plant alkaloid metabolism are discussed.
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