PtoNAC90 is a positive regulator for secondary xylem development in Populus

Secondary growth is regulated by numerous transcription factors under multi-level transcriptional control; however, only a few are considered as key regulators of secondary cell walls biosynthesis in Populus. This study identified PtoNAC90 as a transcription factor crucial for xylem development in Populus. PtoNAC90 is specifically expressed in developing xylem tissues. Compared to wild-type plants, transgenic Populus lines overexpressing PtoNAC90 display thickened xylem and enhanced secondary cell walls, demonstrating its positive regulatory role in xylem development. Interestingly, the lignin, cellulose, and hemicellulose content in the secondary walls of stem xylem exhibited distinct increases in PtoNAC90-overexpressing plants. Using electrophoretic mobility shift assay (EMSA) and luciferase reporter assays, we demonstrated that PtoNAC90 is directly activated by PtoWND1A. Furthermore, PtoNAC90 directly activates key secondary cell wall deposition genes, including PtoPAL3, PtoCesA6, and PtoCslA9. These findings demonstrate that PtoNAC90 positively regulates secondary cell wall formation and wood development, providing valuable insights into the molecular mechanisms controlling this process in trees.