Independent origins of syringyl lignin in vascular plants

Abstract

Lycophytes arose in the early Silurian (Ϸ400 Mya) and represent a major lineage of vascular plants that has evolved in parallel with the ferns, gymnosperms, and angiosperms. A hallmark of vascular plants is the presence of the phenolic lignin heteropolymer in xylem and other sclerified cell types. Although syringyl lignin is often considered to be restricted in angiosperms, it has been detected in lycophytes as well. Here we report the characterization of a cytochrome P450-dependent monooxygenase from the lycophyte Selaginella moellendorffii. Gene expression data, crossspecies complementation experiments, and in vitro enzyme assays indicate that this P450 is a ferulic acid/coniferaldehyde/coniferyl alcohol 5-hydroxylase (F5H), and is capable of diverting guaiacylsubstituted intermediates into syringyl lignin biosynthesis. Phylogenetic analysis indicates that the Selaginella F5H represents a new family of plant P450s and suggests that it has evolved independently of angiosperm F5Hs.L ignin is an aromatic heteropolymer that is deposited most abundantly in the secondary cell walls of vascular plants. It provides structural rigidity to the plant body while enabling individual tracheary elements to withstand the tension generated during water transport; it also serves a defensive role against herbivores and pathogens (1). Lignins are derived mainly from the phenylpropanoid monomers p-coumaryl, coniferyl, and sinapyl alcohol, which give rise to p-hydroxyphenyl, guaiacyl, and syringyl subunits when incorporated into the lignin polymer (2). In angiosperms, three cytochrome P450-dependent monooxygenases (P450s) are involved in the biosynthesis of lignin monomers, cinnamate 4-hydroxylase (C4H), p-coumaroyl shikimate/quinate 3Ј-hydroxylase (C3ЈH), and ferulic acid/coniferaldehyde/coniferyl alcohol 5-hydroxylase (F5H) ( Fig. 1) (3). C4H and C3ЈH are responsible for phenylpropanoid 4 and 3-hydroxylation (4, 5), respectively, whereas F5H catalyzes the 5-hydroxylation of coniferaldehyde and coniferyl alcohol, leading to the formation of syringyl lignin (6, 7). Lignin monomer composition has been found to vary among major phyla of vascular plants (2). Generally, ferns and gymnosperms deposit lignins that are derived primarily from guaiacyl monomers together with a small proportion of phydroxyphenyl units, whereas angiosperm lignins are guaiacyl/ syringyl copolymers that also can contain some p-hydroxyphenyl monomers. This distribution suggests that F5H may be a relatively recent addition to plants' biochemical repertoire. Nevertheless, there are older reports in the literature in which syringyl monomers have been detected in lignins from lycophytes, including species of Selaginella (8-12), by using histochemical reagents and by today's standards relatively crude chemical methods. These results have been verified recently by using more modern techniques (13). How species that diverged from angiosperms Ͼ400 Mya (14) acquired the ability to synthesize syringyl lignin is unknown. Results Lignin Composition Analysis in Representativ...

Syringyl lignin biosynthesis is directly regulated by a secondary cell wall master switch

“…Interestingly, no moss or spike moss proteins were found in the higher plant F5H clade. The previously described spike moss (Selaginella moellenodorffii) smF5H is located in a distant clade, consistent with the independent evolution of F5H activity in spike moss (18).…”

“…The apparent absence of F5H genes in lower plants, the absence of green algal homologs of NAC genes, and the limited distribution of algal MYB genes therefore strongly support the evolution of S lignin synthesis as starting after the emergence of land plants, and the expansion and maturation of the regulatory machinery has taken a very long time, starting no earlier than the appearance of angiosperms. However, as indicated previously, the presence of S lignin in spike moss (18) and lower algal plants (22) suggests that there may have been independent, alternative evolutionary routes for the generation of S lignin synthesis and its regulation in lower plants.…”

“…, the biochemical function of which remains to experimentally determined whereas the mosses and algae analyzed do not appear to have F5H homologs, with the exception of Selaginella moellenodorffii, which has an independently evolved F5H (18).…”

“…6A). This indicates that either the green algae included in this study do not have lignins or that algal lignin synthesis genes evolved independently compared with their higher plant equivalents, as seen for smF5H (18). In the MYB phylogeny (Fig.…”

Photodegradation alleviates the lignin bottleneck for carbon turnover in terrestrial ecosystems

“…Lignin in secondary cell walls is the major obstacle affecting the velocity of microbial carbon turnover because enzymes to degrade lignin are energetically costly and nonspecific, and the amorphous nature of lignin makes it resistant to enzymatic degradation (40). In fact, a flurry of recent biotechnological research has focused on opportunities for altering lignin composition and content in second generation biofuel crops, to increase cell wall digestibility and thus improve the efficiency of cellulosic ethanol production (41,42). We show that, in litter of a broad range of plant species, photodegradation can directly reduce lignin concentration (Fig.…”