Xylan and lignin deposition on the secondary wall of Fagus crenata fibers

The secondary wall ultrastructure of Fagus crenata fibers was examined using field emission scanning electron microscopy (FESEM). Specimens were treated with sodium chlorite and xylanase to remove lignin and xylan, respectively. Microfibrils were clearly visible at the innermost surface of the differentiating fiber secondary wall and there were no globular substances observed in the control specimen. After delignification or xylanase-degradation, microfibrils remained almost the same size and had the same appearance as controls. Anti-xylan antiserum immunolabeling, however, indicated that the microfibrils were coated with very thin layer of xylan(1). Microfibrils were not apparent in the secondary wall of the mature fiber in control specimens. The secondary wall appeared to be a single homogeneous substance. Microfibrils with many globular substances were observed in the delignified specimens and their diameter was larger than that of microfibrils at the surface of the differentiating secondary wall. Following xylanase treatment, the microfibrils had a smooth surface without any globules, indicating that the globular structure is xylan. On the basis of these results, we propose the following mechanism for secondary wall assembly. Cellulose microfibrils synthesized on the plasma membrane are released into the innermost surface of the secondary wall and coated with a very thin layer of xylan that was previously deposited there. Successive deposition of xylan into the cell wall increases the diameter of the microfibrils. The large amount of xylan deposited on the microfibrils has a globular appearance. Lignin deposition occurs simultaneously with xylan deposition and, finally, microfibrils with globular xylan are masked with lignin, resulting in the homogeneous appearance of the cell wall.