Studies on the Identification of the SND Gene Family in Pinus massoniana Lamb. and the Function of PmSND4 in Secondary Cell Wall Formation

SND (Secondary wall-associated NAC domain) transcription factors are core regulators of secondary cell wall formation in plants. To identify key genes involved in wood formation in Pinus massoniana Lamb. and elucidate the molecular regulatory mechanisms underlying its wood formation, this study systematically characterized the SND gene family in P. massoniana and performed functional validation of the candidate gene PmSND4. Candidate sequences were obtained by screening the transcriptome database, and SND family members were identified by combining Pfam domain validation. Bioinformatics analysis was used to analyze their phylogenetic relationships and conserved motif characteristics. Tissue expression profiling was employed to screen potential candidate genes, and the molecular characteristics of PmSND4 were analyzed through subcellular localization. Poplar was used as a heterologous expression host to validate its biological function. The results showed that six P. massoniana SND family members (PmSND1-6) with complete NAC domains were identified. PmSND4 belongs to the SND/ANAC075 subfamily, and is preferentially expressed in stems, with relatively lower and comparable expression levels in needles and roots. It is localized in the nucleus. Overexpression of PmSND4 significantly promoted the vegetative growth of transgenic poplar, increased stem xylem cell wall thickness, and significantly enhanced lignin and hemicellulose contents, but had no significant effect on cellulose content. This study completed the systematic identification of the SND gene family in P. massoniana and clarified the effect of PmSND4 on lignin and hemicellulose accumulation in transgenic poplar, providing important gene resources and a theoretical basis for understanding its potential role in wood quality improvement in P. massoniana.