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 S1 + S2 + 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 S1, S2, and L layers but not the G-layers. The density of LM11 xylan immunogold labeling in S2 layers increased during the formation of G-layers. This increase was due to the shrinkage of S2 layers during development rather than intrusive deposition of xylan through G-layers. Lignification of the CML, S1, and S2 layers proceeded during G-layer formation. The shrinkage of S2 layers occurred almost simultaneously with the lignification of the S2 layers during G-layer formation, suggesting that the S2 layers shrank with lignification.