Transcriptome analysis provides insights into wood formation during larch tree aging

Tree age affects wood formation and yield. However, the underlying mechanisms are poorly understood, particularly at the molecular level. In this study, we investigated the transcriptomic changes of the uppermost main stems of Larix kaempferi in an entire rotation period using the RNA-Seq method. In total, ∼151 million reads were obtained from the stems of 1-, 2-, 5-, 10-, 25-, and 50-year-old L. kaempferi trees. Combining these with the published Illumina sequencing reads, 299,637 assembled transcripts were generated, of which 161,232 were annotated. Time series expression profiling identified 12,927 transcripts as differentially expressed genes (DEGs); function enrichment analysis of these DEGs showed that 459 gene ontology terms in the biological process category were enriched. These terms were associated with the processes of wood formation, such as cell differentiation, growth and death, and its hormonal regulation. Based on the expression patterns of L. kaempferi homologues of genes associated with ethylene, calcium, and cell wall expansion and synthesis, the regulatory network of tracheid growth was outlined. Altogether, the comparative transcriptomic analysis reported here demonstrated the molecular aspects of aging effects on L. kaempferi wood formation. The identification of genes involved in the regulatory network of tracheid growth provides a means of investigating the regulation of wood formation in gymnosperm trees and also offers potential targets for genetic manipulation to improve the properties of xylem fibers.