Transcriptome Profiling Reveals the Important Role of Exogenous Nitrogen in Alleviating Cadmium Toxicity in Poplar Plants

Cadmium (Cd) is a non-essential metallic element with potentially deleterious effects on plants, animals, and human health. Many studies have indicated that nitrogen (N) application can effectively alleviate Cd toxicity in plants. However, the function of nitrogen in this process remains unclear. In this study, high-throughput RNA sequencing and comparative analysis were carried out on poplar (Populus cathayana Rehd) trees treated with only Cd or co-treated with N and Cd, and both treatments were compared against a negative control (no Cd or N) and plants treated with only N. The results showed that approximately 48045828, 46712688, 46681202, and 48076990 clean reads were obtained in the control, Cd, N + Cd, and N treatments. Gene ontology and pathway enrichment analysis (KEGG) indicated that differentially expressed genes (DEGs) could be classified primarily into 144 pathways, among which 19 were related to heavy metal transportation and cell detoxification. The up- and down-regulated DEGs were predominately involved in glutathione (GSH) synthesis, cysteine (Cys), glycine (Gly), brassinosteroid (BR), and salicylic acid (SA)-related pathways. Real-time quantitative PCR (RT-qPCR) revealed that the expression profiles of DEGs were consistent with the results from RNA sequencing analysis. Eight candidate genes encoding GSH, Cys, Gly, BR, and SA were associated with detoxification of Cd and were selected for further screening. A schematic model to explain the involvement of DEGs and N in the Cd-responsive regulatory network is proposed. This study represents the first comprehensive transcriptome-based characterization of N- and Cd-responsive DEGs in poplar. These results provide fundamental insight into the molecular mechanism of N assistance in Cd detoxification. In addition, the results of this study build a solid foundation for further genetic manipulation of Cd accumulation in plants and for the development of phytoremediation approaches to reclaim Cd-contaminated soils.