NtNAC-R1, a novel NAC transcription factor gene in tobacco roots, responds to mechanical damage of shoot meristem

Topping is the important agronomic measure for flue-cured tobacco, and results in increase of the nicotine content in top leaves. Nicotine content is one of the vitals factors for the quality of tobacco leaves. Nicotine is exclusively synthesized in tobacco roots, and then transported to the leaves through the xylem. To investigate the molecular mechanism of increase in nicotine biosynthesis ability following topping, some responses of tobacco roots to topping were analyzed, and the role of NtNAC-R1 in regulating nicotine synthesis and the development of roots was discussed. The electronic cloning technique combined with RT-PCR was successfully used to clone NtNAC-R1 from tobacco roots. The number of lateral root and nicotine contents in tobacco roots increased following topping. Although spraying MeJA on leaves had no effects on the root phenotype, the nicotine contents and the expression of PMT increased markedly. The miR164 was down-regulated, and NtNAC-R1 was up-regulated in tobacco roots after topping. PMT and ODC were down-regulated in transgenic tobacco with antisense NtNAC-R1, and PMT was up-regulated in transgenic tobacco with over-expressed NtNAC-R1. In conclusion, topping can induce the decrease of miR164 and the increase of IAA content in roots, which up-regulate the expression of NtNAC-R1, resulting in the increase of lateral roots and nicotine contents. Meanwhile, topping is a mechanical wounding which can induce JA signal, and JA can enhance nicotine biosynthesis in tobacco roots. So the increase of nicotine biosynthesis ability would be related to cross-talking of JA and auxin signaling pathway after topping.