Topping and grafting affect the alkaloid content and gene expression patterns of tobacco ( Nicotiana tabacum L.)

In this study, we aimed to determine molecular mechanisms underlying the effects of topping and grafting in tobacco ( Nicotiana tabacum L.) by comparing the alkaloid contents and enrichment pathways of differentially expressed genes (DEGs) among plants subjected to different combinations of topping and grafting treatments. Plants of the tobacco variety “Zhongyan 100” and eggplant ( Solanum melongena L.) were grafted in four combinations as scions and rootstocks, respectively. The four treatment groups were tobacco with topping, tobacco without topping, topped tobacco grafted onto an eggplant rootstock, and non‐topped tobacco grafted onto an eggplant rootstock. Tobacco leaves were collected on the day of topping, at 7 days after topping, and after flue curing, the alkaloid contents of the collected leaves were determined. Leaves of plants subjected to the different treatments were collected for RNA sequencing and screened for DEGs, which were subsequently subjected to functional enrichment analyses. Analyses revealed reductions in the leaf alkaloid contents of tobacco subjected to combined topping and eggplant grafting. Gene annotation indicated that topping influences biological processes such as starch metabolism and stress response, whereas grafting affected the biosynthesis and metabolic pathways of secondary metabolites. Downregulated DEGs between non‐topped tobacco and eggplant‐grafted topped tobacco and between topped and non‐topped tobacco are mainly involved in inositol phosphate metabolic and biosynthetic processes. Downregulated DEGs between different grafting methods (eggplant‐grafted non‐topped tobacco vs. non‐topped tobacco and eggplant‐grafted topped tobacco vs. topped tobacco) are mainly involved in sesquiterpene synthase activity and photosynthesis. The findings of this study provide important insights into the molecular mechanisms underlying the effects of topping and grafting on tobacco plants.