Distribution of (1 -> 4)-beta-galactans, arabinogalactan proteins, xylans and (1 -> 3)-beta-glucans in tracheid cell walls of softwoods

Abstract

Polysaccharides were located in the walls of normal and compression wood tracheids of Pinus radiata (radiata pine), Picea sitchensis (Sitka spruce) and Picea abies (Norway spruce) by transmission electron microscopy using immunogold labelling with monoclonal antibodies to (1→4)-β-galactan (LM5), (1→3)-β-glucan, arabinogalactan proteins (AGPs) (MAC207) and heteroxylans (LM10 and LM11). In fully differentiated compression wood tracheids, (1→4)-β-galactan was found in the S2(L) layer and, to a smaller extent, at the interface between the compound middle lamella and the S1 layer. (1→4)-β-Galactan appeared to be displaced from, or modified in, the S1 layer during cell wall formation. (1→3)-β-Glucan (callose) was confined to the helical cavities in the inner S2 layer of severe compression wood. MAC207 AGP glycan epitope was found exclusively in the S1 and S3 layers of normal wood tracheids and in the S1 and inner S2 layers of compression wood tracheids. Binding of LM10, which specifically recognizes unsubstituted or low-substituted xylans, occurred at similar locations to the MAC207 epitope, whereas binding of LM11, which recognizes more highly substituted as well as unsubstituted xylans, occurred throughout the tracheid walls with the exception of the primary wall. Immunogold labelling showed that the different wall layers of softwood tracheids have different polysaccharide compositions which change abruptly during cell wall formation.