Regiochemical control of monolignol radical coupling: a now paradigm for lignin and lignan biosynthesis

Background: Although the lignins and lignans, both monolignol-derived coupl ing products, account for nearly 30% of the organic carbon circulating in t he biosphere, the biosynthetic mechanism of their formation has been poorly understood. The prevailing view has been that lignins and lignans are prod uced by random free-radical polymerization and coupling, respectively. This view is challenged, mechanistically, by the recent discovery of dirigent p roteins that precisely determine both the regiochemical and stereoselective outcome of monolignol radical coupling. Results: To understand further the regulation and control of monolignol cou pling, leading to both lignan and lignin formation, we sought to clone the first genes encoding dirigent proteins from several species. The encoding g enes, described here, have no sequence homology with any other protein of k nown function. When expressed in a heterologous system, the recombinant pro tein was able to confer strict regiochemical and stereochemical control on monolignol free-radical coupling. The expression in plants of dirigent prot eins and proposed dirigent protein arrays in developing xylem and in other lignified tissues indicates roles for these proteins in both lignan formati on and lignification. Conclusions: The first understanding of regiochemical and stereochemical co ntrol of monolignol coupling in lignan biosynthesis has been established vi a the participation of a new class of dirigent proteins. Immunological stud ies have also implicated the involvement of potential corresponding arrays of dirigent protein sites in controlling lignin biopolymer assembly.