Immunolocalization of a Normal Wood Specific Pectin Methylesterase (CoPME) and Quantification of PME Gene Expression in Differentiating Xylem of Chamaecyparis obtusa

The transverse section of compression wood tracheids has a circular shape and intercellular spaces. The cause has not been determined yet; however, we hypothesized that peeling of the cell wall adhesion would cause cellular intervals, resulting in circularity of the transverse section of tracheids. Homogalacturonan, a type of pectin, functions in cell wall adhesion. Further, pectin methylesterase (PME) is involved in functionalization of homogalacturonan. We quantitated PME gene expression levels in differentiating xylem cells using different degrees of compression wood samples and examined the correlation with circularity of the transverse section of tracheids in each sample. We found that lower gene expression level of the sample corresponded with increasing circularity of the transverse section of tracheids. It is considered that the transverse section of compression wood tracheids becomes circular by suppression of PME gene expression during differentiation. Further, we observed the normal wood specific pectin methylesterase (CoPME) localization in differentiating xylem tracheids by immunolabeling. Labels localized at the entire perimeter of the compound middle lamella in normal wood, whereas sparse labeling was found in compression wood. It suggests that cell walls adhere at sites of CoPME function in differentiating xylem tracheids, but there is inadequate adhesion between cell walls where CoPME does not function. At the end of the expansion zone, the volume of the cell decreases due to a decrease in the turgor pressure of the tracheid. Further, due to moisture shrinkage of the tracheid, the adhesion begins to peel off in places of inadequate adhesion between cell walls, resulting in cell gaps and, thereby, generating a circular cell shape of cell wall formation in compression wood.