Abstract
Nitrosation of pyridine alkaloids in tobacco generates tobacco-specific nitrosamines (TSNAs), which are notable toxicants in tobacco products and smoke. Burley tobacco, a chloroplast- and nitrogen (N)-deficient phenotype that accumulates high levels of nitrate-nitrogen (NO(3)-N) in its leaves, is particularly susceptible to TSNAs formation. In this study, reciprocal pot and field grafting experiments were conducted using burley tobacco Eyan No.1 and flue-cured tobacco K326 to investigate whether grafting burley tobacco scions on flue-cured tobacco rootstocks could enhance pigment biosynthesis and photosynthesis, while reducing the NO(3)-N level in burley tobacco leaves. Grafting burley tobacco scions on flue-cured tobacco rootstocks significantly increased the total pigment content, photosynthetic rate, biomass, nitrate reductase and glutamine synthetase activities, as well as ammonium-nitrogen (NH(4)-N), total soluble and reducing sugar, and soluble protein levels in burley tobacco leaves compared with burley tobacco self-rooting, while decreasing the NO(3)-N level and nitrate-N to total N ratio. Transcriptomic analysis revealed that grafting resulted in upregulated expression of genes involved in starch, sucrose, porphyrin, chlorophyll, and N metabolism, as well as carbon fixation and carotenoid biosynthesis. The findings suggest that grafting on high N use efficiency rootstock is an exceptionally promising means of decreasing NO(3)-N accumulation by improving photosynthesis and N metabolism in the scion, thereby reducing the levels of harmful TSNAs.