Towards the specification of consecutive steps in macromolecular lignin assembly

Abstract When Pinus taeda cell suspension cultures are exposed to 8% sucrose solution, the cells undergo significant intracellular disruption, irregular wall thickening/lignification with concomitant formation of an ‘extracellular lignin’ precipitate. However, addition of potassium iodide (KI), an H 2 O 2 scavenger, inhibits this lignification response, while the ability to synthesize the monolignols, p -coumaryl and coniferyl alcohols, is retained. Lignin synthesis (i.e. polymerization) is thus temporarily correlated with H 2 O 2 generation, strongly implying a regulatory role for the latter. Time course analyses of extracellular metabolites leading up to polymer formation reveal that coniferyl alcohol, but not p -coumaryl alcohol, undergoes substantial coupling reactions to give various lignans. Of these, the metabolites, dihydrodehydrodiconiferyl alcohol, shonanin (divanillyl tetrahydrofuran) and its apparent aryl tetralin derivative, cannot be explained simply on the basis of phenolic coupling. It is proposed that these moieties are the precursors of socalled reduced substructures in the lignin macromolecule. This adds a new perspective to the lignin assembly mechanism.