The 'Abnormal lignins': mapping heartwood formation through the lignan biosynthetic pathway

• Main Authors: Gang, D.R. (Washington State University, Pullman, WA.), Fujita, M, Davin, L.B, Lewis, N.G
• Format: Book
• Language: English
• Published: Washington, DC (USA) American Chemical Society 1998
• Subjects: BIOSINTESIS; BIOSYNTHESE; BIOSYNTHESIS; BOIS DE COEUR; DURAMEN; HEARTWOOD; LIGNINAS; LIGNINE; LIGNINS
• ISBN: 084123566X; 9780841235663

A significant portion of 'non-lignin' phenolic extractives in heartwood tissues requires solubilization conditions normally used for lignin dissolution. Although they have incorrectly been characterized as 'abnormal' or 'secondary' lignins, their formation and accumulation differs profoundly from that leading to the lignins in three primary ways: first, they are transported through specialized cells (such as ray parenchyma) and are infused into surrounding pre-lignified cells; second, they are formed via distinct biochemical pathways, affording lignan or (iso)flavonoid-derived substances of various molecular sizes; third, their accumulation begins in the pith and over time extends out centrifugally towards the cambial regions, unless formed locally elsewhere as an inducible response. By contrast, lignification results via direct monomer transport from the cytoplasm of a lignifying cell into its polysaccharide-rich cell wall with subsequent polymerization; this process represents the first and final committed step to lignification, being primarily initiated and completed in maturing cell walls not far from the cambial zone. In this study, western red cedar, western hemlock and loblolly pine were examined to establish how their species-specific, non-lignin heartwood substances are biosynthesized. Specific enzymes and genes were obtained for pivotal steps leading to plicatic acid (western red cedar), alpha-conidendrin (western hemlock) and dihydrodehydrodiconiferyl alcohol and related structures (loblolly pine). In each case, coniferyl alcohol served as the initial precursor, being subsequently metabolized with precise regi- and stereochemical control to first give species-specific lignans from which the 'secondary', or 'abnormal lignins' derive. The results again underscore the need to systematically determine the precise temporal and spatial biochemical processes involved in phenolic coupling, as well as any subsequent metabolic events in planta.