Deposition of glucuronoxylans on the secondary cell wall of Japanese beech as observed by immune-scanning electron microscopy

Glucuronoxylans (GXs), the main hemicellulosic component of hardwoods, are localized exclusively in the secondary wall of Japanese beech and gradually increase during the course of fiber differentiation. To reveal where GXs deposit within secondary wall and how they affect cell wall ultrastructure, immune-scanning electron microscopy using anti-GXs antiserum was applied in this study. In fibers forming the outer layer of the secondary wall (S-1), cellulose fibrils were small in diameter and deposited sparsely on the inner surface of the cell wall. Fine fibrils with approximately 5 nm width aggregated and formed thick fibrils with 12 nm width. Some of these thick fibrils further aggregated to form bundles which labelled positively for GXs. In fibers forming the middle layer of the secondary wall (S-2), fibrils were thicker than those found in S-1 forming fibers and were densely deposited. The S-2 layer labelled intensely for GXs with no preferential distribution recognized. Compared with newly formed secondary walls, previously formed secondary walls were composed of thick and highly packed microfibrils. Labels against GXs were much more prevalent on mature secondary walls than on newly deposited secondary walls. This result implies that the deposition of GXs into the cell wall may occur continuously after cellulose microfibril deposition and may be responsible for the increase in diameter of the microfibrils.