Monolignol glucosides as intermediate compounds in lignin biosynthesis. Revisiting the cell wall lignification and new 13C-tracer experiments with Ginkgo biloba and Magnolia liliiflora DOI 10.1515/hf-2015-0224 Received October 17, 2015 accepted February 9, 2016; previously published online March 14, 2016 Abstract: A large amount of monolignol glucosides (MLGs: p-glucocoumaryl alcohol, coniferin, syringin) are found in lignifying soft xylem near cambium and they disappear with the progress of lignification. Recently, it became a matter of debate whether those MLGs are real intermediates in lignin biosynthesis or only a storage form of monolignols outside of the main biosynthetic pathway. The latter is partly based on a misinterpretation of 14 C-tracer experiments and partly on the simple generalization of the results of gene manipulation experiments concerning the flexible and complex lignification. In the present paper, it could be confirmed by the most reliable 13 C-tracer method that MLGs are real intermediates in the pathway from l-phenylalanine to macromolecular ligninpolysaccharides complexes in the cell walls. This pathway via MLGs is essential for transport and programmed delivery of specific monolignols in a stable form from intracellular space to specific lignifying sites within the cell wall. The pool size of MLGs is large in most gymnosperm trees and some angiosperm species that emerged in an early stage of phylogeny, while the pool size is small in most angiosperms. This difference in pool size is reasonably understandable from the viewpoint of plant evolution, in the course of which the role of MLGs changed to meet variation in type of major cells, reaction wood formation, and postmortem lignification.

The structure and biosynthesis of lignin are not yet fully understood, especially the step following the initial dimerization of monolignol. Liquid chromatograph mass spectrometer (LC-MS) was used to analyze the consumption rates of monolignol dimers formed by b-O-4, b-5, and b-b couplings between coniferyl alcohols in efforts to understand the activity of monolignol dimers in enzymatic dehydrogenative polymerization. We investigated the reaction kinetics in single-component and mixedcomponent reaction systems containing one and two species of the dimers, respectively. A difference was observed between the consumption rates of the three dimers we tested, and the consumption rate of one dimer in the singlecomponent reaction was different from that in a mixedcomponent reaction. In qualitative LC-MS analyses, coniferyl alcohol oligomers were detected in the reaction products. Some monolignol tetramers were formed by 5-5 and 5-O-4 coupling between the dimers. The results of this work suggested that monolignol dimers with b-5 and b-b linkages could function as radical mediators in enzymecatalyzed polymerization. 12.0 Hz, H-c1), 3.70 (1H, dd, J = 3.2, 12.0 Hz, H-c2), 3.81 (3H, s, 3-OCH 3 ), 3.85 (3H, s, 3 0 -OCH 3 , erythro isomer), 3.89 (3H, s, 3 0 -OCH 3 , threo isomer), 4.22 (1H, m, H-b), 4.31 (2H, dd, J = 5.4, 1.6 Hz, H-c 0 ), 4.90 (1H, br d, J = 5.2, 6.0 Hz, H-a, threo isomer), 4.90 (1H, br d, J = 5.2, 6.0 Hz, H-a, erythro isomer), 6.28 (1H, dt, J = 15.8, 5.4 Hz, H-b 0 , erythro isomer), 6.30 (1H, dt, J = 15.8, 5.4 Hz, H-b 0 , threo isomer), 6.52 (1H, dt, J = 15.8, 1.6 Hz, H-a 0 , erythro isomer), 6.54 (1H, dt, J = 15.8, 1.6 Hz, H-a 0 , threo isomer), 6.77 (1H, d, J = 8.0 Hz, H-5, erythro isomer), 6.78 (1H, d, J = 8.0 Hz, H-5, threo isomer), 6.86 (1H, d, J = 2.0 Hz, H-2, threo isomer), 6.88-6.90 (1H, m, H-6), 6.93 (1H, d, J = 6.0 Hz, H-5 0 , erythro isomer), 7.05 (1H, d, J = 2.0 Hz, H-2, erythro isomer), 7.09 (1H, d, J = 1.6 Hz, H-2 0 ), 7.12 (1H, d, J = 6.0 Hz, H-5 0 , threo isomer); 13 C NMR d:

Gnetum gnemon L. (Gnetales) forms hardwood-like secondary xylem in its trunks and branches although it is a gymnosperm. The present study tested the surface growth stress in relation to anatomical and chemical properties of the secondary xylem in inclined and vertical stems of G. gnemon using morphological and chemical composition analyses. Secondary growth was promoted on the upper half of the cross section in an inclined stem; at the same time, tensile growth stress increased on the upper side and decreased on the lower side of the inclined stem. However, formation of reaction wood fibers was not detected on either side. The microfibril angle was associated with differences in tensile growth stress. Thus, we conclude that negative gravitropism in G. gnemon is caused by a synergistic effect of increased tensile growth stress as well as the promotion of secondary growth on the upper side of the inclined stem. Our results are comparable to the negative gravitropism observed in the family Magnoliaceae, which does not form gelatinous fibers in its tension wood.