Mutation in shoot-to-root mobile transcription factor, ELONGATED HYPOCOTYL 5, leads to low nicotine levels in tobacco.

Tobacco remains one of the most commercially important crops due to the parasympathomimetic alkaloid nicotine used in cigarettes. Most genes involved in nicotine biosynthesis are expressed in root tissues; however, their light-dependent regulation has not been studied. Here, we identified the ELONGATED HYPOCOTYL 5 homolog, NtHY5, from Nicotiana tabacum and demonstrated that NtHY5 could complement the Arabidopsis thaliana hy5 mutant at molecular, morphological and biochemical levels. We report the development of CRISPR/Cas9-based knockout mutant plants of tobacco, NtHY5, and show down-regulation of the nicotine and phenylpropanoid pathway genes leading to a significant reduction in nicotine and flavonol content, whereas NtHY5 overexpression (NtHY5OX) plants show the opposite effect. Grafting experiments using wild-type, NtHY5, and NtHY5OX indicated that NtHY5 moves from shoot-to-root to regulate nicotine biosynthesis in the root tissue. Shoot HY5, directly or through enhancing expression of the root HY5, promotes nicotine biosynthesis by binding to light-responsive G-boxes present in the NtPMT, NtQPT and NtODC promoters. We conclude that the mobility of HY5 from shoot-to-root regulates light-dependent nicotine biosynthesis. The CRISPR/Cas9-based mutants developed, in this study; with low nicotine accumulation in leaves could help people to overcome their nicotine addiction and the risk of death.