Dynamic changes of heterogeneous cell wall macromolecules in differentiating conifer xylem using cytochemical...

Tracing dynamic changes of heterogeneous cell wall components during xylem differentiation is essential for understanding the intricate architecture of wood cell walls at the individual secondary cell wall layer level. Here we employ histochemical- and immunological approaches to visualize the deposition of cellular polymers during xylem differentiation in Pinus bungeana. In axial tracheids, deposition of crystalline cellulose and glucomannan preceded xylan and lignin. Lignification was initiated in primary cell wall corners during development of the S₁ layer and intensified with cell wall thickening. Immunofluorescence labeling showed an earlier deposition of glucomannan than xylan with strong presence in S₁ layer corner regions at early stages of differentiation. Quantification of immunogold-labeled xylan and glucomannan showed distinct increasing trends during thickening of tracheid wall layers with xylan labeling of the S₁ and S₂ layers at the S₃ stage greater than the S₂ stage. Differential cell wall polymer deposition was evident in mature tracheid areas with glucomannan absent in warty layers. Pectins were highly concentrated in unlignified primary cell walls but decreased with axial tracheid wall differentiation. The sequence of polymer deposition in ray cells was similar but lagged behind axial tracheids with ray parenchyma remaining unlignified with thinner cell walls than ray tracheids.