Differential expression patterns of two cellulose synthase genes are associated with primary and secondary cell wall development in aspen trees

The quality and quantity of cellulose deposited in the primary and secondary cell walls of plants vary in accordance with their biological function. However, the molecular basis of such cellulose heterogeneity has so far remained unclear. Since enrichment of better-quality cellulose, in terms of increased degree of polymerization and crystallinity, is one of the goals of forest biotechnology, our main objective is to decipher the roles of distinct cellulose synthase (CesA) genes in tree development, with special reference to wood production. Here, we report two full-length CesA cDNAs, PtrCesA3 and PtrCesA4, from an economically important tree aspen (Populus tremuloides). PtrCesA3 is orthologous to the Arabidopsis AtCesA4 gene involved in secondary wall formation, whereas PtrCesA4 is orthologous to the Arabidopsis AtCesA1 gene involved in primary cell wall formation. To define the specific cell types expressing these CesA genes, we explored the natural distribution patterns of PtrCesA3 and PtrCesA4 transcripts in a variety of aspen organs, such as leaves, petiole, stem, and roots, using in situ hybridization with hypervariable region-specific antisense riboprobes. Such a side-by-side comparison suggested that PtrCesA3 is exclusively expressed in secondary-wall-forming cells of xylem and phloem fibers, whereas PtrCesA4 is predominantly expressed in primary-wall-forming expanding cells in all aspen organs examined. These findings suggest a functionally distinct role for each of these two types of PtrCesAs during primary and secondary wall biogenesis in aspen trees, and that such functional distinction appears to be conserved between annual herbaceous plants and perennial trees.