Nitrogen modifies wood composition in poplar seedlings by regulating carbon and nitrogen metabolism

Nitrogen (N) and carbon (C) and N metabolism significantly affect wood formation and composition, a vital C sink of trees. To reveal the mechanism that N supply modulates C-N metabolism to affect lignin and cellulose biosynthesis, poplar seedlings were supplied with N at high or low levels (HN and LN, respectively), and the changes in secondary wall structure, composition, and metabolites involved in C- and N-metabolism during wood development were determined. Results showed that, LN resulted in an increase in lignin, soluble sugar, and starch contents, while a decrease in cellulose, soluble protein, and free amino acid contents, with the thickening of fiber cell walls and a decrease in S/G. Correlation analysis unveiled significant relationship between C/N and lignin and C-N metabolism. Integrating metabolomic and transcriptomic, it identified differential C- and N-metabolic pathways, including multiple amino acid biosynthesis, sucrose metabolism, and phenylpropanoid metabolism pathways. Furthermore, prominent metabolites (cinnamic acid, caffeic aldehyde, p-Coumaroyl shikimate, p-Coumaroyl quinic acid, sucrose, fructose, and glucose) and corresponding genes including PAT, PAL, HCT, CAD, COMT, CAld5H, CesA, SuSy, and INV responsive to N were identified. A hierarchical transcriptional regulation network implicated potential TFs (Potri.005G227900, Potri.003G114100, Potri.005G225800, Potri.017G016700, Potri.014G036600, Potri.008G051200, Potri.010G167900, and Potri.006G267700) modulating wood formation responsive to N supply. WGCNA showed functional genes governing C/N, lignin, and cellulose were distributed in major CHO metabolism, signaling, transport, secondary metabolism, and cell wall pathways, where hub TFs belonged to MYB, HB-other, bZIP, ZF-HD, Nin-like, and G2-like families. This study indicated N can modify lignin and cellulose biosynthesis by regulating C flow within metabolic pathways, as well as functional node genes and key TFs. It offered fresh insights into the regulation of exogenous N on wood composition, providing theoretical reference and genetic resources for optimizing wood composition and improving C sequestration capacity of trees using genetic engineering methods. Meanwhile, a new explanation is proposed for the changes in lignin and cellulose in poplar wood under different N conditions from the view of C utilization. [Display omitted] •The study encompassed wood developmental process from primary to secondary growth.•Carbon-nitrogen b