Molecular architecture of developing xylem in Norway spruce reveals spatial and temporal patterns of cell wall polymer deposition

Using microarray polymer profiling (MAPP), immunocytochemical staining, and Raman spectroscopy, we conducted a detailed spatiotemporal survey of the deposition of cell wall components during xylem differentiation in Norway spruce. These insights are essential for understanding the intricate architecture of wood cell walls during the initial stages of development immediately following cambial divisions. MAPP was used to obtain an overview of cell wall polysaccharides, glycoproteins, and ferulic acid residues in developing xylem. The most interesting findings were investigated in further detail using immunomicroscopy and Raman spectroscopy. In axial tracheids, deposition of pectic compounds and cellulose only slightly preceded the first deposition of lignin. Lignification was initiated in the middle lamellae and cell wall corners and proceeded during the development of the S1 layer and intensified later during cell wall thickening. Raman analysis revealed an earlier deposition of lignin than shown previously, which coincided with the deposition of methylesterified or partially methylesterified homogalacturonan initially in the radial cell walls and later in the tangential cell walls of developing earlywood. Minimal binding of an antibody against feruloylated compounds indicated that ferulic acid residues do not participate in the initiation of lignification in Norway spruce.