Distribution of lignin monomers and the evolution of lignification among lower plants.

Through application of chemical, biochemical and histochemical analyses, we provide new data on the absence/presence of syringyl lignins in the algal species Mastocarpus stellatus, Cystoseira baccata and Ulva rigida, the bryophytes Physcomitrella patens and Marchantia polymorpha, the lycophytes Selaginella martensii, Isoetes fluitans and Isoetes histrix, the sphenophyte Equisetum telmateia, the ferns Ceratopteris thalictroides, Ceratopteris cornuta, Pteridium aquilinum, Phyllitis scolopendrium and Dryopteris affinis, and the angiosperm Posidonia oceanica. Lignins, and especially syringyl lignins, are distributed from non-vascular basal land plants, such as liverworts, to lycopods and ferns. This distribution, along with the already reported presence of syringyl lignins in ginkgoopsids, suggests that syringyl lignin is a primitive character in land plant evolution. Here, we discuss whether the pathway for sinapyl alcohol recruitment was iterative during the evolution of land plants or, alternatively, was incorporated into the earliest land plants and subsequently repressed in several basal liverworts, lycopods, equisetopsids and ferns. This last hypothesis, which is supported by recent studies of transcriptional regulation of the biosynthesis of lignins, implies that lignification originated as a developmental enabler in the peripheral tissues of protracheophytes and would only later have been co-opted for the strengthening of tracheids in eutracheophytes.