Dissection of lignin macromolecular configuration and assembly: Comparison to related biochemical processes in allyl/propenyl phenol and lignan biosynthesis

This comprehensive review describes the current status and knowledge of biochemical and molecular processes involved in allyl/propenyl phenol, lignan, norlignan and lignin biosynthesis. Recent advances made over the last decade are critically discussed, and placed in context with earlier studies largely dating back to the 1950s. Beginning with the recently established formation of phenylalanine in plants, each downstream biochemical conversion is described from the perspective of the mechanistic details known to this point. Particular emphasis is placed upon proteinaceous control of monolignol-derived radical-radical coupling processes, leading to lignans and lignins, as well as apparently related processes affording the various ellagitannins and phenolic terpenoids. The evidence for non-random macromolecular lignin assembly is discussed in detail, this being in contrast to earlier notions that such processes were random. The latter assumptions have largely resulted from a lack of robust analytical procedures and rigorous quantification, as well as a lack of incisive experimental design. In addition, the often-noted severe effects of modulating lignin compositions and contents on plant vascular tissue properties (i.e. in terms of compromised biophysical properties) are described herein, as well as the severe limitations as regards recent claims of compensatory 'combinatorial chemistry' lignin formation. Much of the latter confusion has also resulted from the serious deficiencies in current lignin analytical protocols and quantification, as well as in the general lack of experimental approaches/design to probe lignin primary structure(s).