Wood formation is a complex developmental process driven by the annual activity of the vascular cambium. Conifers usually produce juvenile wood at young ages followed by mature wood for the rest of their lifetime. Juvenile wood exhibits poorer wood quality (i.e., lower density) compared with mature wood and can account for up to 50% of short-rotation harvested logs, thus representing a major challenge for commercial forestry globally. Wood density is an important quality trait for many timber-related products. Understanding the molecular mechanisms involved in the regulation of juvenile wood density is critical for the improvement of juvenile wood quality via marker-aided selection. A previous study has identified several candidate genes affecting mature wood density in Picea sitchensis (Bong.) Carr.; however, genes associated with juvenile wood density in conifers remain poorly characterized. Here, cDNA microarrays containing 3320 xylem unigenes were used to investigate genes differentially transcribed in juvenile wood with high (HD) and low density (LD) in Pinus radiata D.Don. In total, 814 xylem unigenes with differential transcription were identified in at least one of two microarray experiments and 73 genes (45 for HD, 28 for LD) were identified in both experiments, thus representing putative candidate genes for juvenile wood density. Interestingly, cellulose synthases (PrCesA3, PrCesA11) and sucrose synthase (SuSy), which are involved in secondary cell wall formation, had stronger transcription in juvenile wood with HD, while genes functioning in primary wall formation (pectin synthesis, cell expansion and other modifications) were more transcribed in LD wood. Cell wall genes encoding monolignol biosynthesis enzymes, arabinogalactan proteins, actins and tubulins were differentially transcribed in either HD or LD juvenile wood; however, the latter had exclusively greater transcription of genes involved in monolignol polymerization (laccase and peroxidase). The identified candidate genes also included many non-cell-wall genes (transcription factors, environmental- responsive genes, hormone signalling, etc.) and genes with unknown functions, suggesting complex gene pathways in the regulation of juvenile wood density. Interestingly, 19 out of 73 candidate genes for wood density were among the 108 candidate genes previously identified for microfibril angle, and 16 genes appeared to influence both traits in a synergistic manner for wood stiffness. [ABSTRACT FROM AUTHOR]