Unravel the Mystery of NIC1-locus on Nicotine Biosynthesis Regulation in Tobacco

Background: Nicotine biosynthesis is mainly regulated by jasmonate (JA) signaling cascade in Nicotiana tabacum. As an allotetraploid species, the regulation of nicotine biosynthesis has been genetically verified via two unlinked NIC loci (named as NIC1 and NIC2) which are possibly originated from its two ancestral diploids. Previously, a N. tomentosiformis originated ethylene response factor (ERF) gene cluster was identified as the NIC2-locus which has been demonstrated positively regulates nicotine accumulation in N. tabacum. Results: Here, we describe the genetic mapping of NIC1-locus, the major nicotine regulatory locus, by using a NIC1-locus segregating population through bulked segregant analysis. We identified two linkage marker TM23004 and TM22038 were delimited the NIC1-locus within a ~34.3-Mb genomic region at pseudochromosome 07 of tobacco genome. Genomic scan within this region revealed a NIC2-like locus ERF gene cluster exist in. To verify this ERF gene cluster is the genetically called 9NIC1-locus9, different functional experiments based on most of the ERFs in regulating nicotine biosynthesis and their influences on alkaloid accumulations have been carried out. Collinearity analysis showed that NIC1-locus ERF genes are originated from N. sylvestris and exclusively expressed in root tissues. In addition, transcriptomic results indicate that NIC1-locus ERF genes are coexpressed with the NIC2-locus ERF genes and other nicotine biosynthetic genes and regulators after JA induction. Furthermore, the suppressed expression of four ERFs of the NIC1-locus genes corresponding with decreased NtPMT and NtQPT expression in NtMYC2-RNAi lines indicates the selected NIC1-locus ERFs function in downstream of NtMYC2 in the JA signaling cascades. In the meanwhile, the alkaloid levels are also determined by the amplitude of the four ERF gene expressions in both wild type and LA mutant. Additionally, in vitro binding assays, transient activation assays, and ectopic expression in transgenic plants demonstrate that these ERF genes are able to bind the GCC-box elements residing in the step-limiting gene promoters (such as NtPMT2, NtQPT2) and functional redundant but quantitatively transactivate nicotine biosynthetic gene expression. For nic1-locus mutation, two different sizes of deletions (nic1-S and nic1-B) were identified which occurred at the surrounding regions of the NIC1-locus gene cluster, which might disrupt, to some extent, chromosomal microenvironment and change gene expression around the deletion regions (including NIC1-locus ERFs), resulting in the decreased expression levels of NIC1-locus ERFs (such as NtERF199) and reduced alkaloid accumulation in the nic1-locus mutant. Conclusions: our findings not only provide insight in to the mechanism of the NIC1-locus ERFs in the regulatory network of nicotine biosynthesis, but also unraveled the theoretical basis of the nic1-locus mutation in low nicotine mutant. These functional verified NIC1-locus ERF genes can be further used as potential target(s) for ethyl methanesulfonate-based mutagenesis to manipulate nicotine level in tobacco variety in tobacco breeding program.