MAIN CONCLUSION: A 3D model of the tracheid wall has been proposed based on high-resolution cryo-TEM where, in contrast to the current understanding, the cellulose elementary fibrils protrude from the cell wall plane. The ultrastructure of the tracheid walls of Picea abies was examined through imaging of ultrathin radial, tangential and transverse sections of wood by transmission electron microscopy and with digital image processing. It was found that the elementary fibrils (EFs) of cellulose were rarely deposited in the plane of the concentric cell wall layers, in contrast to the current understanding. In addition to the adopted concept of longitudinal fibril angle, EFs protruded from the cell wall plane in varying angles depending on the layer. Moreover, the out-of-plane fibril angle varied between radial and tangential walls. In the tangential S2 layers, EFs were always out-of-plane whereas planar orientation was typical for the S2 layer in radial walls. The pattern of protruding EFs was evident in almost all axial and transverse images of the S1 layer. Similar out-of-plane orientation was found in the transverse sections of the S3 layer. A new model of the tracheid wall with EF orientation is presented as a summary of this study. The outcome of this study will enhance our understanding of the elementary fibril orientation in the tracheid wall.
MAIN CONCLUSION: A 3D model of the tracheid wall has been proposed based on high-resolution cryo-TEM where, in contrast to the current understanding, the cellulose elementary fibrils protrude from the cell wall plane. The ultrastructure of the tracheid walls of Picea abies was examined through imaging of ultrathin radial, tangential and transverse sections of wood by transmission electron microscopy and with digital image processing. It was found that the elementary fibrils (EFs) of cellulose were rarely deposited in the plane of the concentric cell wall layers, in contrast to the current understanding. In addition to the adopted concept of longitudinal fibril angle, EFs protruded from the cell wall plane in varying angles depending on the layer. Moreover, the out-of-plane fibril angle varied between radial and tangential walls. In the tangential S2 layers, EFs were always out-of-plane whereas planar orientation was typical for the S2 layer in radial walls. The pattern of protruding EFs was evident in almost all axial and transverse images of the S1 layer. Similar out-of-plane orientation was found in the transverse sections of the S3 layer. A new model of the tracheid wall with EF orientation is presented as a summary of this study. The outcome of this study will enhance our understanding of the elementary fibril orientation in the tracheid wall.