Relationship of dirigent protein and 18s RNA transcript localization to heartwood formation in western red cedar

Western red cedar (Thuja plicata) heartwood contains abundant amounts of structurally complex plicatic acid-derived lignans that help confer protective properties and longevity to this tissue type. Although the lignan biochemical entry point is dirigent protein-mediated, the formation of heartwood and its associated lignans in some species remains poorly understood due to technical difficulties of working with the former. To begin to address such questions, this study therefore focused on the anatomical localization of dirigent protein and 18s rRNA (control) gene transcripts within recalcitrant woody tissues, including heartwood. This in situ mRNA hybridization approach enabled detection of dirigent protein transcripts in cork cambia, vascular cambia and ray parenchyma cells of the sapwood, but not the heartwood under the conditions employed. By contrast, the hybridization of the 18s rRNA (control) transcript resulted in its detection in all tissue types, including radial parenchyma cells of apparently preformed heartwood. Application of in situ hybridization to such recalcitrant tissues thus demonstrates the utility of this technique in identifying specific cell types involved in heartwood formation, as well as the relationship of dirigent protein localization to that of heartwood metabolite generation.