TOWARDS THE SPECIFICATION OF CONSECUTIVE STEPS IN MACROMOLECULAR LIGNIN ASSEMBLY

When Pinus taeda cell suspension cultures are exposed to 8% sucrose so lution, the cells undergo significant intracellular disruption, irregu lar wall thickening/lignification with concomitant formation of an 'ex tracellular lignin' precipitate. However, addition of potassium iodide (KI), an H2O2 scavenger, inhibits this lignification response, while the ability to synthesize the monolignols, p-coumaryl and coniferyl al cohols, is retained. Lignin synthesis (i.e. polymerization) is thus te mporarily correlated with H2O2 generation, strongly implying a regulat ory role for the latter. Time course analyses of extracellular metabol ites leading up to polymer formation reveal that coniferyl alcohol, bu t not p-coumaryl alcohol, undergoes substantial coupling reactions to give various lignans. Of these, the metabolites, dihydrodehydrodiconif eryl alcohol, shonanin (divanillyl tetrahydrofuran) and its apparent a ryl tetralin derivative, cannot be explained simply on the basis of ph enolic coupling. It is proposed that these moieties are the precursors of so-called reduced substructures in the lignin macromolecule. This adds a new perspective to the lignin assembly mechanism.