Two poplar cellulose synthase-like D genes, PdCSLD5 and PdCSLD6, are functionally conserved with Arabidopsis CSLD3

Root hairs are tip-growing long tubular outgrowths of specialized epidermal cells, and are important for nutrient and water uptake and interaction with the soil microflora. Here we characterized two poplar cellulose synthase-like D (CSLD) genes, PdCSLD5 and PdCSLD6, the most probable orthologs to the Arabidopsis AtCSLD3/KOJAK gene. Both PdCSLD5 and PdCSLD6 are strongly expressed in roots, including in the root hairs. Subcellular localization experiments showed that these two proteins are located not only in the polarized plasma membrane of root hair tips, but also in Golgi apparatus of the root hair and non-hair-forming cells. Overexpression of these two poplar genes in the atcsld3 mutant was able to rescue most of the defects caused by disruption of AtCSLD3, including root hair morphological changes, altered cell wall monosaccharide composition, increased non-crystalline β-1,4-glucan and decreased crystalline cellulose contents. Taken together, our results provide evidence indicating that PdCSLD5 and PdCSLD6 are functionally conserved with AtCSLD3 and support a role for PdCSLD5 and PdCSL6 specifically in crystalline cellulose production in poplar root hair tips. The results presented here also suggest that at least part of the mechanism of root hair formation is conserved between herbaceous and woody plants.