Xylan deposition and lignification in differentiating tension wood fibers in Mallotus japonicus (Euphorbiaceae) with multi-layered structure

Xylan deposition and lignification processes were examined in tension wood fibers with gelatinous layers (G-layers) in Mallotus japonicus (Euphorbiaceae). The cell walls consisted of a multi-layered structure of S-1 + S-2 + G + n(L + G), where n indicates the number of repetitions (n = 0-3) and L indicates very thin lignified layers. The formation and lignification processes of the multi-layered structure of tension wood fibers were examined by light microscopy, ultraviolet microscopy, and transmission electron microscopy (TEM) following KMnO4 staining. The deposition of xylan was examined by immunoelectron microscopy with a monoclonal antibody (LM11). Immunolabelling of xylan appeared in lignified cell wall layers, except in the compound middle lamella (CML), i.e., the S-1, S-2, and L layers but not the G-layers. The density of LM11 xylan immunogold labeling in S-2 layers increased during the formation of G-layers. This increase was due to the shrinkage of S-2 layers during development rather than intrusive deposition of xylan through G-layers. Lignification of the CML, S-1, and S-2 layers proceeded during G-layer formation. The shrinkage of S-2 layers occurred almost simultaneously with the lignification of the S-2 layers during G-layer formation, suggesting that the S-2 layers shrank with lignification.

Automated summary

Xylan deposition and lignification processes in tension wood fibers were examined, showing a multi-layered structure of S-1 + S-2 + G + n(L + G) in the cell walls. Immunoelectron microscopy revealed the immunolabeling of xylan in lignified cell wall layers, excluding the G-layers, with the shrinkage of S-2 layers during G-layer formation. Lignification of the CML, S-1, and S-2 layers proceeded during G-layer formation, with the shrinkage of S-2 layers occurring almost simultaneously with their lignification.