NtQPT2 plays critical roles in nicotine biosynthesis and development of tobacco plant

The enzyme quinolinate phosphoribosyltransferase (QPT), encoded by a small gene family in tobacco plant, plays a critical role in the biosynthesis of nicotine, a defensive pyridine alkaloid in Nicotiana species, in addition to its vital function in the NAD(P)(H) synthesis. Previous studies have demonstrated that two NtQPT genes (NtQPT1 and NtQPT2) are present in N. tabacum genome, and it has been believed that NtQPT1 is responsible for NAD(P)(H) synthesis and thus essential for primary metabolism, while NtQPT2 is specifically involved in nicotine biosynthesis. In this study, we generated knockout tobacco lines for NtQPT1 and NtQPT2 respectively using the CRISPR/Cas9-based genome-editing technology and found that knockout of NtQPT2 caused both dramatic reduction of nicotine biosynthesis and a retardation of plant development, indicating that NtQPT2 is important not only to nicotine biosynthesis, but also to the development of tobacco plant. Like NtQPT2, NtQPT1 was also found to contribute to nicotine biosynthesis although to a much lesser extent than NtQPT2. Meanwhile, knockout of NtQPT1 did not significantly affect plant growth. Together with the observation that NtQPT2's expression is remarkably higher than that of NtQPT1 in root, leaf, stem and flower of tobacco plant, it is reasonable to infer that their functional diversification on nicotine biosynthesis and tobacco plant growth may be attributed largely to their markedly different transcript abundance.