Localized laccase activity modulates distribution of lignin polymers in gymnosperm compression wood.

The woody stems of coniferous gymnosperms produce specialized compression wood to adjust the stem growth orientation in response to gravitropic stimulation. During this process, tracheids develop the compression-wood-specific S2 L cell wall layer with lignins highly enriched with p-hydroxyphenyl (H)-type units derived from H-type monolignol, whereas lignins produced in the cell walls of normal wood tracheids are exclusively composed of guaiacyl (G)-type units from G-type monolignol with a trace amount of H-type units. We show that laccases, a class of lignin polymerization enzymes, play a crucial role in the spatially organized polymerization of H-type and G-type monolignols during compression wood formation in Japanese cypress (Chamaecyparis obtusa). We performed a series of chemical-probe-aided imaging analysis on C. obtusa compression wood cell walls, together with gene expression, protein localization, and enzymatic assays of C. obtusa laccases. Our data indicated that CoLac1 and CoLac3 with differential oxidation activities towards H-type and G-type monolignols were precisely localized to distinct cell wall layers in which H-type and G-type lignin units were preferentially produced during the development of compression wood tracheids. We propose that, not only the spatial localization of laccases, but also their biochemical characters dictate the spatial patterning of lignin polymerization in gymnosperm compression wood.