A Comprehensive Analysis of the Laccase Gene Family of Pinus densiflora Reveals a Functional Role of PdeLAC28 in Lignin Biosynthesis for Compression Wood Formation

In conifers, compression wood (CW) with a high lignin content forms at the base of the stem or branch in response to gravity, which is a good model system for studying lignin-rich wood formation. In this study, we identified and characterized the laccase gene family (PdeLAC) in Korean red pine (Pinus densiflora), which is integral to monolignol polymerization. Phylogenetic analysis of 54 PdeLAC genes with those from gymnosperms (i.e., Pinus taeda and Picea abies) and angiosperms (i.e., Populus trichocarpa, Arabidopsis thaliana, and Oryza sativa) revealed their categorization into five groups, highlighting distinct evolutionary relationships compared to angiosperms. Gene structure and motif analysis showed conserved copper-binding loops and variable substrate-binding loops, suggesting functional diversity. Expression profiling indicated that 23 PdeLAC genes, including three (PdeLAC28, PdeLAC1, and PdeLAC31) homologous to AtLAC17, were upregulated in developing xylem during the growing season, particularly in CW. Transgenic poplars overexpressing PdeLAC28 exhibited increased xylem area, cell wall thickness, and Klason lignin content, underscoring its role in lignin biosynthesis and CW formation. This study provides valuable insights into the molecular regulation of lignin biosynthesis in CW of P. densiflora, setting a foundation for advancing our understanding of wood formation mechanisms in gymnosperms.