The complex helical texture of the secondary cell wall of Urtica dioica root hairs is not controlled by microtubules: a quantitative analysis

A detailed quantitative analysis on the orientation of the cortical microtubule arrays and the last layer of cellulose microfibrils deposited in the secondary cell wall has been performed on Urtica dioica root hairs. It was found that cortical microtubules of individual root hairs show a preferential orientation, which ranges in the total root hair population from —20 to +20° with respect to the longitudinal cell axis. Immunofluorescence and thin-section preparations are comparable, as long as the individuality of the root hairs and the modal distribution of the microtubules in the root hairs are considered. The cellulose microfibrils in the secondary wall are organized in two steep helices. Quantitatively, the majority of the microfibrils are oriented in an S helix, while simultaneously a smaller group is arranged in a Z helix in the same root hair. It is concluded that microtubules do not directly control the orientation of nascent cellulose microfibrils in this complex texture. The organization of the secondary cell wall texture may be a variant of the organization of the primary cell-wall texture.