Microscopic distribution of syringin in freeze-fixed Syringa vulgaris stems

Monolignols are precursors of lignin, and their glucosides are often found in plants. Glucosylation creates water-soluble and chemically stable monolignols by protecting the phenolic hydroxyl group. To discuss the role of sinapyl alcohol glucoside, syringin, in planta, the cellular distribution of syringin in the transverse and radial surfaces of quick-frozen stems of Syringa vulgaris L. (lilac) was visualized by cryo-time-of-flight secondary ion mass spectrometry and scanning electron microscopy (cryo-TOFSIMS/SEM) analyses. The amount and rough distribution of syringin were confirmed by high-performance liquid chromatography measurements using serial tangential sections of freeze-fixed lilac stems. The syringin distribution was also discussed with reference to the tissue classification from microscopic observations. Syringin was mainly found in the phloem region. In the xylem region, syringin was evenly distributed irrespective of the cell type from the cambial zone to the early differentiating stage region and selectively distributed in vessels in the later differentiating stage region. After the lignification of wood fibers, syringin was found in rays and some vessels in the initial part of the annual rings. Previously, artificially administered isotope-labeled syringin was shown to be assimilated into lignin in the differentiating xylem region. Based on this, our present data showing syringin storage in the differentiating xylem region and its variation depending on the lignification stage suggest that syringin works as a lignin precursor. Additionally, detection of syringin in vessels and rays indicates intercellular transportation of syringin in lilac stems.