The WUSCHEL-Related Homeobox 4 (BpWOX4) Promotes Secondary Growth Through Lignin Biosynthesis Activation in Betula platyphylla

Secondary growth is a key process in the growth and development of woody plants, influencing biomass accumulation and structural integrity. The WUSCHEL- related homeobox 4 ( BpWOX4 ) gene regulates vascular cambium activity in trees; however, its precise role in secondary growth in Betula platyphylla remains poorly understood. In this study, we functionally characterized BpWOX4 and its role in secondary growth through a combination of transcriptomic, physiological, and histological analyses. Transcriptome analysis among wild-type (WT), overexpression (O), and suppression (SE) lines revealed upregulation of lignin biosynthesis genes, such as PAL , 4CL , CCR , and CAD . Gene Ontology (GO) and KEGG enrichment analyses indicated activation of the phenylpropanoid and lignin biosynthesis pathways. Additionally, key transcription factors involved in secondary growth, including members of the MYB and NAC families, were significantly upregulated in overexpression lines. Furthermore, overexpression of BpWOX4 in B. platyphylla resulted in increased stem diameter and xylem thickness, as well as significantly higher lignin content. Together, these results provide new insights into the molecular mechanisms underlying secondary growth and identify BpWOX4 as a promising genetic target for enhancing biomass production and wood quality in forest trees.