Genetic engineering of Nicotiana tabacum for reduced nornicotine content

Nornicotine is an undesirable secondary alkaloid in cultivated tobacco, because it serves as a precursor to N'-nitrosonornicotine (NNN), a tobacco-specific nitrosamine with suspected carcinogenic properties. Nornicotine is produced through the oxidative N-demethylation of nicotine by a nicotine N-demethylase enzyme during the senescence and curing of tobacco leaves. While the nornicotine content of most commercial burley tobacco is low, a process termed "conversion" can bestow considerably increased nornicotine levels in a portion of the plants within the population. Previously, we isolated a nicotine N-demethylase gene, designated CYP82E4, and demonstrated that RNAi-induced silencing of CYP82E4 and its close homologues is an effective means for suppressing nicotine to nornicotine conversion. In this study, we used real-time polymerase chain reaction to confirm the central role of CYP82E4 in nicotine N-demethylation by demonstrating that the transcript accumulation of CYP82E4 is enhanced as much as 80-fold in converter vs nonconverter tobacco. We also show the design of an optimized RNAi construct (82E4Ri298) that suppressed nicotine to nornicotine conversion from 98% to as low as 0.8% in a strong converter tobacco line, a rate of nornicotine production that is about 3.6-fold lower than typically detected in commercial varieties. Southern blot analysis showed that a single copy of the RNAi transgene was as effective in suppressing nornicotine accumulation as multiple copies. Greenhouse-grown transgenic plants transformed with the RNAi construct were morphologically indistinguishable from the empty vector or wild-type controls. These results demonstrate that the genetic transformation of tobacco with the 82E4Ri298 construct is an effective strategy for reducing nornicotine and ultimately NNN levels in tobacco.