The recent discovery of dirigent proteins (Davin and Lewis, 1995; Davin et al., 1997; Gang et al., 1999) gives a new perspective into how free radical coupling of monolignol plant phenols is controlled in planta to yield lignans and lignins. With the biochemical pathways to the precursor monolignols essentially fully established (Lewis et al., 1999), this new insight for formation of the lignans and lignins now resolves many if not all of the earlier enigmas associated with phenoxy free radical biochemistry; hitherto, these were considered to lack any defined control in terms of stipulating the outcome of their coupling.

In older textbooks (Sarkanen and Ludwig, 1971), for example, it was suggested that lignin assembly occurs through the passage of monolignol monomers into the cell wall, with polymer formation only requiring oxidative enzymes (such as laccases or peroxidases) to generate the corresponding free radicals, which would then undergo random coupling. If this were correct, then formation of approximately 20% to 30% of all plant organic matter would have been left essentially to chance. This perspective could not, however, explain many biological aspects of lignification, including targeting of specific monolignols into discrete regions within the lignifying cell wall and the observed regiospecificity in coupling resulting in approximately 50% to 70% of all interunit linkages being 8-O-4 bonded. Nor could it explain the optical activities of many of the dimeric lignans (Ayres and Loike, 1990). These observations suggested that some coupling specificity was being exercised in planta, the basis of which was not understood until the discovery of dirigent proteins. From even the very beginnings of the evolution of life, some biochemical mechanisms might have been in place to help “manage” both the generation of free