Immunolocalization of (1,4)-beta-galactan in tension wood fibers of poplar.

The occurrence and distribution of (1,4)-beta-galactan in wood cells of poplar (Populus trichocarpa Torr. & A. Gray x P. koreana Rehder) were studied by immunolabeling with a monoclonal antibody (LM5) specific to this carbohydrate epitope. Immunofluorescence microscopy revealed exclusive binding of the LM5 antibody to tension wood fibers, indicating the specific presence of (1,4)-beta-galactan in cell walls of this wood cell type. Higher magnifications achieved with the fluorescence microscope and additional immunogold electron microscopy showed that the binding of the LM5 antibody was mainly restricted to a narrow cell wall area between the gelatinous G-layer and the secondary cell wall. This labeling pattern strongly suggests a role of (1,4)-beta-galactan in cross-linking the G-layer and secondary cell wall. Furthermore, the exclusive localization of (1,4)-beta-galactan in tension wood strengthens the view that this carbohydrate epitope can be considered a highly specific marker of reaction wood formation in mechanically stressed trees.