New Insights on Structures Forming the Lignin-Like Fractions of Ancestral Plants

Abstract

In the present work, lignin-like fractions were isolated from several ancestral plants –including moss (Hypnum cupressiforme and Polytrichum commune), lycophyte (Selaginella kraussiana), horsetail (Equisetum palustre), fern (Nephrolepis cordifolia and Pteridium aquilinum), cycad (Cycas revoluta), and gnetophyte (Ephedra fragilis) species– and structurally characterized by pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) and two-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy. Py-GC/MS yielded marker compounds characteristic of lignin units, except in the H. cupressiforme, P. commune and E. palustre “lignins,” where they were practically absent. Additional structural information on the other five samples was obtained from 2D-NMR experiments displaying intense correlations signals of guaiacyl (G) units in the fern and cycad lignins, along with smaller amounts of p-hydroxyphenyl (H) units. Interestingly, the lignins from the lycophyte S. kraussiana and the gnetophyte E. fragilis were not only composed of G- and H-lignin units but they also incorporated significant amounts of the syringyl (S) units characteristic of angiosperms, which appeared much later in plant evolution, most probably due to convergent evolution. The latter finding is also supported by the abundance of syringol derivatives after the Py-GC/MS analyses of these two samples. Regarding lignin structure, β−O−4′ alkyl-aryl ethers were the most abundant substructures, followed by condensed β−5′ phenylcoumarans and β−β′ resinols (and dibenzodioxocins in the fern and cycad lignins). The highest percentages of alkyl-aryl ether structures correlated with the higher S/G ratio in the S. Kraussiana and E. fragilis lignin-like fractions. More interestingly, apart from the typical monolignol-derived lignin units (H, G and S), other structures, assigned to flavonoid compounds never reported before in natural lignins (such as amentoflavone, apigenin, hypnogenol B, kaempferol, and naringenin), could also be identified in the HSQC spectra of all the lignin-like fractions analyzed. With this purpose, in vitro synthesized coniferyl-naringenin and coniferyl-apigenin dehydrogenation polymers were used as standards. These flavonoids were abundant in H. cupressiforme appearing as the only constituents of the moss lignin-like fraction (including 84% of dimeric hypnogenol B) and their abundance decreased in those of S. Kraussiana (with amentoflavone and naringenin representing 14% of the total aromatic units), and the two ancient gymnosperms (0.4–1.2%) and ferns (0–0.7%).

New Oligomeric Dihydrochalcones in the Moss Polytrichum commune: Identification, Isolation, and Antioxidant Activity

“…It is worth noting that the detection of the oligomers presented in Figure 3 can also shed light on the nature of lignin-like biopolymers in P. commune , which have been previously reported in the literature [ 15 , 16 ]. It can be assumed that the acetone extraction used in our study made it possible to extract only a low molecular weight fraction of the polymeric dihydrochalcone synthesized by the plant by analogy with algal phlorotannins [ 23 ] with the formation of fucol-type aryl–aryl bonds.…”

“…However, Karmanov et al reported lignin isolation from P. commune that was similar to softwood lignins in many characteristics [ 15 ]. The most recent study by Rencoret et al [ 16 ] points to the detection of weak signals belonging to the p -hydroxyphenyl structures typical for lignin in the NMR spectra of a lignin-like substance isolated from P. commune . However, it was noted that flavonoids or other polyphenolic compounds can also be their source.…”

Existence of Syringyl α-Carbonyl-Type Tetrahydrofuran β–β Structure in Hardwood Lignins

“…The observation of the formation of ligninlike polymers in the oxidation of CA using a mushroom laccase was the first demonstration of the nature of lignin as a dehydrogenation polymer derived from monolignols . Furthermore, the occurrences of numerous lignin substructures, including tetrahydrofuran β–β structures from acylated monolignols, benzodioxane structures from catechol-type monolignols, lignin-integrated flavonoids, , and various new lignin substructures produced in transgenic plants, − have been elucidated by the in vitro dehydrogenative polymerization of various lignin monomers.…”

Phylogenomic analyses across land plants reveals motifs and coexpression patterns useful for functional prediction in the BAHD acyltransferase family

“…previous studies, most of the biochemical and genetic pre-requisites for the evolution of true lignin and other derivatives of the general phenylpropanoid pathway were already present in the common ancestor of land plants (Weng and Chapple, 2010;Espiñeira et al, 2011;Renault et al, 2019;Kriegshauser et al, 2021;Rencoret et al, 2021). Our previous results (Kruse et al, 2022) also suggested that the ability to produce caffeoylquinatethe hydroxycinnamoyl CoA quinate transferase (HQT) activityexists in charophytic algae and likely existed in the ancestor of land plants.…”