The distribution of monolignol glucosides coincides with lignification during the formation of compression wood in Pinus thunbergii

The distributions of monolignol glucosides (MLGs) in compression and opposite woods of Pinus thunbergii were assessed using cryo-time-of-flight secondary ion mass spectrometry to investigate their involvement in lignification. p-Glucocoumaryl alcohol (PG) was identified in the region of the differentiating xylem adjacent to the cambial zone only in compression wood, whereas coniferin (CF) was similarly localized in both compression and opposite woods. Their distribution from the phloem to the xylem was evaluated by high-performance liquid chromatography (HPLC) using serial tangential sections. Variations in storage amounts of CF and PG in the stem of P. thunbergii agreed with lignification stages of the tracheid, supporting the idea that MLGs act as a storage and transportation form of lignin precursors. The imaging of monolignol (ML)-dependent active lignification sites using fluorescence-tagged MLs supported distinct distribution patterns of MLGs for lignification in compression and opposite woods. Methylation-thioacidolysis was applied to compression and opposite wood samples to examine the structural difference between the guaiacyl (G) and p-hydroxyphenyl (H) units in lignin. Most of the H units in compression wood were detected as lignin end groups via thioacidolysis. PG was detected in opposite wood by HPLC; however, the H unit was not detected by thioacidolysis. The differences in ML and MLG distributions, enzyme activity, and resultant lignin structures between the G and H units suggest the possibility of individual mechanisms regulating the heterogeneous structures of G and H unit in lignin.