Evolution of lignan and neolignan biochemical pathways

Lignans and neolignans are present in pteridophytes gymnosperms and angiosperms, all of which are putatively derived from simple algal precursors. This progressive evolution from an algal precursor to terrestrial vascular plants required elaboration of the phenylpropanoid pathway. Some lignans and neolignans participate in lignin synthesis and hence have important roles in determining physicochemical and mechanical properties of cell walls, whereas others act as antioxidants, biocides (fungicides, bactericides and antiviral agents) and perhaps even as cytokinins. Based on structural patterns known to date, the simplest lignans/neolignans are in the pteridophytes. As evolution proceeded to gymnosperms and angiosperms, this was accompanied by a progressive increase in the structural complexity of the lignans/neolignans, which apparently peaked in the Magnoliiflorae. Growing evidence links these changes to altered (or improved) defense functions. There are three modes of coupling mechanisms involved in lignan/neolignan and lignin syntheses in pteridophytes and woody gymnosperms/angiosperms, each having evolutionary significance: The first involves monolignol oxidations catalyzed by H2O2-dependent peroxidase(s). These initially catalyze formation of (neo)lignans which undergo further conversion to give lignins. Woody plants also contain O2-requiring laccases which catalyze the coupling of coniferyl/sinapyl alcohols to give racemic (neo)lignans, and which are presumed to be subsequently converted into lignins via cooperative involvement with the peroxidases previously mentioned. By contrast, optically active lignans are formed by stereoselective coupling engendered by hitherto uncharacterized O2-requiring oxidases. There are few examples of lignans/neolignans in the non-woody monocotyledons, except for the formation of dihydroxytruxillic and truxinic acids in certain superorders. These metabolites are apparently formed via photochemical dimerization rather than by enzymatically engendered phenolic coupling i.e., the stereoselective and non-stereoselective coupling enzymes tentatively appear to be absent in the monocotyledons. This decreased reliance upon a full extension of the phenylpropanoid pathway presumably prevents the formation of woody tissue. This, in turn, helps to explain why monocotyledons are generally more readily amenable to biodegradation.