Comparative transcriptomic analysis of radial growth regulation in Eucalyptus urophylla × E. grandis and E. cloeziana

Abstract Background Eucalyptus urophylla × E. grandis and E. cloeziana are commercially important eucalypt species with markedly different wood properties, which are closely related to their industrial utilization value. Compared with E. urophylla × E. grandis , E. cloeziana has a significantly greater fiber double wall thickness (DWT) and a smaller fiber lumen width (LW), whereas E. urophylla × E. grandis displays less radial variation in DWT and LW, leading to higher wood uniformity; however, the transcriptomic regulatory patterns underlying the differential regulation of radial growth during wood formation remain unclear. In this study, we used 6-year-old clones of both species to measure fiber DWT and LW across four radial zones from the outer to inner trunk-phloem, outer sapwood, inner sapwood, and transition zones. Moreover, comparative transcriptomic analysis was performed using RNA-seq technology to explore the molecular basis of differences in radial growth. Results The results indicated that E. cloeziana had a significantly greater DWT in all four zones, whereas E. urophylla × E. grandis had a greater LW in all zones. Compared with E. cloeziana , E. urophylla × E. grandis exhibited less variation in DWT and LW across the four zones. However, in contrast to the phenotypic stability of fiber traits along the radial gradient, the number of differentially expressed genes (DEGs) between adjacent zones in E. urophylla × E. grandis was more than twice that in E. cloeziana . Functional enrichment analysis showed that these DEGs were predominantly enriched in the biosynthesis of other secondary metabolites and phenylpropanoid biosynthesis pathways, suggesting that these two pathways are highly coordinated in terms of carbon allocation and lignin deposition, which together drive wood formation in E. urophylla × E. grandis . Through the identification of gene modules significantly associated with DWT and LW, key genes involved in regulating cell wall thickness in Eucalyptus were revealed. Conclusions This study reveals a highly dynamic transcriptional network that buffers phenotypic variation during radial growth in E. urophylla × E. grandis . These findings deepen our understanding of the regulatory mechanisms of radial growth in hardwoods and provide an important theoretical basis for the genetic improvement of wood properties in Eucalyptus .