Nicotine Biosynthesis, Transport, and Regulation in Tobacco: Insights into the Evolution of a Metabolic Pathway

In Nicotiana tabacum (tobacco), nicotine and related pyridine alkaloids are produced in the roots and accumulate mainly in the leaves. Molecular analyses of nicotine biosynthesis, especially of the steps involved in pyrrolidine and pyridine formation, suggest that this specialized pathway evolved through repeated duplication of primary pathways, followed by the recruitment of the metabolic genes into a regulon. In tobacco, jasmonates elicit nicotine formation via a conserved signaling cascade anchored to the downstream nicotine biosynthesis pathway by master transcription factors of the ERF family, particularly ERF189 and its homolog ERF199. ERF transcription factors upregulate metabolic and transport genes directly involved in the pathway by recognizing cis-elements in the promoters of target genes. A pair of homologous clusters of related ERF genes, including ERF189 and ERF199, occurs in the tobacco genome. ERF189 corresponds to the nicotine-controlling NIC2 locus. A large chromosomal deletion of the cluster that includes ERF189, as found in the nic2 mutant allele, has been exploited to breed low-nicotine tobacco.