It is not known with any degree of certainty as to when the biochemical pathway fully evolved to afford either the monolignols or the monolignol-derived products, the polymeric lignins, and the (oligomeric) lignans. It is presumed to have occurred with the emergence of tracheary elements, with the latter being thought to trace their origins back to about 430 million years ago (1-3). Such a lengthy evolutionary period might seem unlikely, however, to ultimately result in a random assembly process for lignins, particularly as they represent Nature’s second most abundant biopolymer(s).

Yet today, there is currently still a vigorous debate regarding native lignin macromolecular configuration, namely, as to whether it results from proteinaceous control over primary structure with (presumed) template replication (29, 31, 50-52, 284-286), or whether the “random coupling” model of Freudenberg (11,12, 53,54) — speculated to afford lignins with 1066 isomers per 100-mer unit (196) — can be extended to that of “combinatorial chemistry” (175). Since such widely divergent viewpoints are apparently incompatible, determining how native lignin macromolecular configuration is unambiguously achieved in vivo is essential for fully understanding the basis of vascular plant cell wall biochemistry. In this context, we considered it instructive to first briefly discuss the ten or so depictions of proposed “representative” lignin structures, together with either the evidence or lack thereof for same, and the gaps in our knowledge that remain to be filled.