HY5 and COP1 function antagonistically in the regulation of nicotine biosynthesis in Nicotiana tabacum

Nicotine constitutes approximately 90% of the total alkaloid content in leaves within the Nicotiana species, rendering it the most prevalent alkaloid. While the majority of genes responsible for nicotine biosynthesis express in root tissue, the influence of light on this process through shoot-to-root mobile ELONGATED HYPOCOTYL 5 (HY5) has been recognized. CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), a key regulator of light-associated responses, known for its role in modulating HY5 accumulation, remains largely unexplored in its relationship to light-dependent nicotine accumulation. Here, we identified NtCOP1, a COP1 homolog in Nicotiana tabacum, and demonstrated its ability to complement the cop1-4 mutant in Arabidopsis thaliana at molecular, morphological, and biochemical levels. Through the development of NtCOP1 overexpression (NtCOP1OX) plants, we observed a significant reduction in nicotine and flavonol content, inversely correlated with the down-regulation of nicotine and phenylpropanoid pathway. Conversely, CRISPR/Cas9-based knockout mutant plants (NtCOP1^CR) exhibited an increase in nicotine levels. Further investigations, including yeast-two hybrid assays, grafting experiments, and Western blot analyses, revealed that NtCOP1 modulates nicotine biosynthesis by targeting NtHY5, thereby impeding its transport from shoot-to-root. We conclude that the interplay between HY5 and COP1 functions antagonistically in the light-dependent regulation of nicotine biosynthesis in tobacco.