Seasonal reorganization of the xylem transcriptome at different tree ages reveals novel insights into wood formation in Pinus radiata.

*Seasonal wood development produces earlywood (EW) and latewood (LW) with distinct properties. The molecular mechanisms controlling EW and LW formation at different tree ages are poorly understood. *Seasonal reorganization of the xylem transcriptome was investigated in Pinus radiata at four tree ages using cDNA microarrays. Transcriptome profiles were compared with seasonal wood variation measured by SilviScan (CSIRO, Clayton, Australia). *The xylem transcriptome was considerably reorganized during seasonal change, and this reorganization showed a maturation-related pattern. The greater reorganization occurred at the transition (30%) and juvenile (21%) stages, but it declined with tree maturity (11-13%). However, this pattern does not correlate well with maturation-related patterns of seasonal wood variation. In total, 319 seasonal-responsive xylem candidate genes were identified. Many transcripts involved in primary and secondary wall biosynthesis were preferentially accumulated in EW and LW, respectively. A large proportion (45-81%) of the candidate genes are preferentially regulated at a single age and their transcript abundance may influence maturation-related patterns of seasonal wood variation. *Seasonal reorganization of the xylem transcriptome is significantly affected by tree age. Physiological changes at the transition stage may contribute to its greater seasonal transcriptome reorganization. Identified stage-preferential xylem transcripts could influence seasonal wood variation at different tree ages.