We report on the silencing of codeinone reductase (COR) in the opium poppy, Papaver somniferum, using a chimeric hairpin RNA construct designed to silence all members of the multigene COR family through RNA interference (RNAi). After gene silencing, the precursor alkaloid (S)-reticuline—seven enzymatic steps upstream of codeinone—accumulated in transgenic plants at the expense of morphine, codeine, oripavine and thebaine. Methylated derivatives of reticuline also accumulated. Analysis verified loss of Cor gene transcript, appearance of 22-mer degradation products and reduction of enzyme activity. The surprising accumulation of (S)-reticuline suggests a feedback mechanism preventing intermediates from general benzylisoquinoline synthesis entering the morphine-specific branch. However transcript levels for seven other enzymes in the pathway, both before and after (S)-reticuline, were unaffected. This is the first report of gene silencing in transgenic opium poppy and of metabolic engineering to cause the high-yield accumulation of the nonnarcotic alkaloid reticuline.
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