Monolignol glucosides as intermediate compounds in lignin biosynthesis. Revisiting the cell wall lignification and new ¹³C-tracer experiments with Ginkgo biloba and Magnolia liliiflora

Abstract

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.

Hydroxystilbene Glucosides Are Incorporated into Norway Spruce Bark Lignin

“…Glucosides (or, more generally, glycosides) also have weak evidence from NMR spectra (Balakshin et al, 2007), but the assignments are again unauthenticated and there is the additional conceptual problem as to how they result. Logically, they cannot arise from the phenol-glucosylated monolignols, coniferin and syringin, that are known to be derivatives stored in the vacuole (Dima et al, 2015) and which, in the case of coniferin and in softwoods, may be deglucosylated and the usual monomer used for lignification (Terashima et al, 2016); this is simply because a free-phenol is an absolute requirement to enable the generation of the phenolic radical that is used in the radical coupling reactions that typify lignification. If there truly are glucosides in 'normal' lignins, the glucoside must be introduced postpolymerization, and it is difficult to imagine how or why this might happen.…”

Lignification in developing culms of bamboo Sinobambusa tootsik

“…Although monolignols are known to be direct precursors of lignin, whether monolignol glucosides are also involved in lignification is controversial. The biosynthesis and hydrolysis of monolignol glucosides have been investigated in several studies [30][31][32][33][34][35]. A single-gene knockout mutant defective in b-glucosidase, which is responsible for coniferin and syringin hydrolysis, did not show a Table 1 for sampling details.…”

Distribution of coniferin in freeze-fixed stem of Ginkgo biloba L. by cryo-TOF-SIMS/SEM

“…The whole structure of lignin is still unclear, and the controlling mechanism of lignin biosynthesis is also controversial 3 . Monolignol is a monomer unit of lignin, and its glucoside is a promising candidate for the lignin precursor 1 4 5 6 7 8 . Previous studies have showed that administered monolignol glucoside is assimilated into lignin without any alteration to the natural lignification process 9 10 11 12 13 14 15 16 17 .…”