Analysis of the Transcriptome Response to Low Nitrogen in Populus ussuriensis

(1) Background: Nitrogen is a key element that is essential for plant growth, and it is absorbed by roots from the soil. Nitrogen stress severely limits forest tree productivity; therefore, elucidating the molecular mechanisms underlying nitrogen stress tolerance in forest trees is critical for sustainable forestry. (2) Methods: Phenotypic analyses of wild-type (WT) <i>Populus ussuriensis</i> (<i>P. ussuriensis</i>) plantlets grown in vitro were carried out at different time points under both normal and low-nitrogen conditions. Transcriptome analyses of roots were performed at 0, 12, 24, 48, 96, and 336 h under low-nitrogen stress via RNA-seq. A gene regulatory network (GRN) for nitrogen-metabolism-associated DEGs was constructed using a three-gene module framework and a bottom-up Gaussian Graphical Model algorithm. (3) Results: WT <i>P. ussuriensis</i> plantlets grown in vitro exhibited a synergistic response characterized by increased root biomass and suppressed shoot growth. Transcriptome analyses identified 8289 DEGs enriched in nitrogen metabolism, ROS scavenging, root development, and phytohormone signaling. A total of 443 differentially expressed transcription factors (TFs) (mainly MYB, AP2/ERF, and bHLH) were detected. A nitrogen-metabolism-associated GRN comprising 60 nodes was established. (4) Conclusions: Transcriptomic data and nitrogen metabolism pathway predictions from this study establish a systematic foundation for investigating molecular adaptation mechanisms in <i>P. ussuriensis</i> roots under nitrogen stress.