Biosynthesis and regulation of tobacco alkaloids

In Nicotiana plants, nicotine and related pyridine alkaloids, such as nornicotine, anabasine and anatabine, are synthesized in underground roots and then translocated via the xylem to aerial parts, where they are mainly stored in vacuoles as defensive toxins against herbivorous insects. A series of structural genes involved in the synthesis and transport of these alkaloids have been isolated, mostly based on their homology or expression profiles, and shown to be expressed in distinct types of root cells. Nevertheless, enzymes catalyzing the late synthetic steps, including ring coupling, have remained elusive. Jasmonate signals, in response to insect hervibory, and cross-talking with auxin and ethylene, coordinately activate the nicotine pathway genes through a signaling cascade consisting of Nicotiana COI1, JAZs, and the bHLH transcription factor MYC2. Two genetic loci, NIC1 and NIC2, mutant alleles of which have been used to breed tobacco cultivars with low nicotine content, specifically control multiple structural genes of the nicotine pathway. A group of closely related ERF transcription factor genes are clustered at the NIC2 locus and deleted in the nic2 mutant. Jasmonate-inducible NIC2-locus ERFs and MYC2 directly up-regulate the transcription of the nicotine pathway genes, recognizing specific cis-elements in the promoters of their downstream target genes.