Profiling of Oligolignols Reveals Monolignol Coupling Conditions in Lignifying Poplar Xylem

Abstract

Lignin is an aromatic heteropolymer, abundantly present in the walls of secondary thickened cells. Although much research has been devoted to the structure and composition of the polymer to obtain insight into lignin polymerization, the lowmolecular weight oligolignol fraction has escaped a detailed characterization. This fraction, in contrast to the rather inaccessible polymer, is a simple and accessible model that reveals details about the coupling of monolignols, an issue that has raised considerable controversy over the past years. We have profiled the methanol-soluble oligolignol fraction of poplar (Populus spp.) xylem, a tissue with extensive lignification. Using liquid chromatography-mass spectrometry, chemical synthesis, and nuclear magnetic resonance, we have elucidated the structures of 38 compounds, most of which were dimers, trimers, and tetramers derived from coniferyl alcohol, sinapyl alcohol, their aldehyde analogs, or vanillin. All structures support the recently challenged random chemical coupling hypothesis for lignin polymerization. Importantly, the structures of two oligomers, each containing a g-p-hydroxybenzoylated syringyl unit, strongly suggest that sinapyl p-hydroxybenzoate is an authentic precursor for lignin polymerization in poplar.

Naturally p-Hydroxybenzoylated Lignins in Palms

“…Evidence from metabolite profiling of poplar wood for novel β-β-coupled heterodimers between sinapyl alcohol and sinapyl p-hydroxybenzoate [29,32] suggests that it is safe to contend that all lignin p-hydroxybenzoates, across the plant taxa that have them, derive from lignification using monolignol p-hydroxybenzoate conjugates. The evidence is therefore sufficiently compelling that currently unknown transferases are responsible for producing monolignol phydroxybenzoate conjugates that are used as 'monomers' for lignification in palms (as well as in willows, poplars, and aspen).…”

“…The 13 C-NMR chemical shifts in Salix (willow) MWL samples showed good concordance with expected shifts for free-phenolic γ-phydroxybenzoate esters based on model compound studies [28]. Rather than suspecting that the lignin side-chains were acylated after the polymer had already formed, p-hydroxybenzoyl monolignol conjugates were proposed as the precursors to these structures in early research [47]; monolignol acylation was further conjectured [25] and eventually proven by the observation of novel β-β-coupling products in lignins that could only have arisen from pre-acylated monolignols [29,14,48,32]. Recent studies have delivered the transferases p-coumaroyl-CoA:monolignol transferase (PMT) in grasses responsible for lignin p-coumarate esters via p-coumaroylation of monolignols [22,23,49,50] and now an exotic feruloyl-CoA:monolignol transferase (FMT) that feruloylates monolignols prior to lignification [27]; the putative p-hydroxybenzoyl-CoA and acetyl-CoA monolignol transferases have not yet been identified.…”

“…In addition, the palms, which are a large family (Arecaceae) of economically important monocotyledonous plants comprising some 2400 species, also have partially phydroxybenzoylated lignins [34][35][36][37]. Several low molecular mass cross-coupling products γ-acylated with p-hydroxybenzoate were isolated from actively lignifying poplar xylem [29,32]. Enhanced interest in p-hydroxybenzoates has surfaced in conjunction with reduced lignification in aspen and poplar following down-or upregulation of genes in the monolignol pathway [38][39][40][41].…”

“…(family Salicaceae) [28], the poplars and aspens (Populus spp.) (family Salicaceae) [29,24,[30][31][32], and Aralia cordata (family Araliaceae) [33]. In addition, the palms, which are a large family (Arecaceae) of economically important monocotyledonous plants comprising some 2400 species, also have partially phydroxybenzoylated lignins [34][35][36][37].…”

A Key Role for Apoplastic H2O2 in Norway Spruce Phenolic Metabolism

“…To investigate the effects of apoplastic H 2 O 2 scavenging on phenolic metabolism, a time-course sampling of cells and culture medium from the various types of culture (Supplemental Fig. S1A) was analyzed for methanol-soluble phenolic compounds using ultra-HPLC-mass spectrometry (Morreel et al, 2004(Morreel et al, , 2010a(Morreel et al, , 2010b. In cells and culture medium, 505 and 278 compounds were profiled, of which only 31 compounds were in common, providing strong evidence that cross-contamination between the medium and cell extracts was low or even absent.…”