BBL enzymes exhibit enantiospecific preferences in the biosynthesis of pyridine alkaloids in Nicotiana tabacum L

Plant species can accumulate secondary metabolites in optically pure form or, occasionally, as enantiomeric mixtures. Interestingly, enantiomers of the same molecule can confer different biological activities. In tobacco (Nicotiana tabacum L.), the pyridine alkaloids nicotine, nornicotine, anatabine, and anabasine naturally exist as scalemic mixtures of (R)- or (S)-enantiomers, with the (S)-isoforms predominating. The mechanisms by which tobacco alkaloid enantiomers accumulate remain largely unknown. Experiments were carried out involving tobacco genotypes possessing induced deleterious mutations in three genes coding for nicotine demethylase (NND) enzymes and three genes coding for Berberine Bridge Like (BBL) enzymes that act near the end of the nicotine, anatabine, and anabasine biosynthetic pathways. Data indicate that (R)-nicotine is naturally produced at appreciable levels but is observed in only small amounts due to preferential demethylation by NND enzymes. Data further suggest that BBL-a and BBL-b are preferentially involved in the biosynthesis of (S)-alkaloid enantiomers, while BBL-c is preferentially involved in the biosynthesis of (R)-enantiomers. Gene duplication followed by genetic divergence thus played a role in the evolution of scalemic alkaloid accumulation in tobacco. Through a combination of mutation breeding and transgene overexpression, tobacco genotypes were generated in which the predominant alkaloid enantiomers were reversed from the (S)- to the (R)-isoforms. These results shed light on the genetic control of alkaloid accumulation in N. tabacum and on mechanisms of scalemic mixture formation of secondary metabolites in plants.