Multiform biosynthetic pathway of syringyl lignin in angiosperms

To clarify the pathway for biosynthesis of sinapyl alcohol in angiosperms, tracer experiments using stable isotopes were performed on robinia (Robinia pseudoacacia L.), oleander (Nerium indicum Mill.), magnolia (Magnolia kobus DC.) and Arabidopsis thaliana (L.) Heynh. Precursors used in the experiment were ¹³C- and ²H (D)-labeled [8-¹³C, 3-OCD₃]ferulic acid and [8-¹³C, 3,5-OCD₃]sinapic acid. The incorporation of labeled precursor into lignin was confirmed by gas chromatography-mass spectrometry of the products of derivatization followed by reductive cleavage. Crude extracts of differentiating xylem or stems from these plants were also assayed for 4-coumarate-CoA ligase (4CL; EC 6.2.1.12) activity using sinapic acid and ferulic acid as substrates. In robinia and oleander, 4CL activity toward sinapic acid was detected, and labeled sinapic acids were incorporated into syringyl lignin. These results indicate that robinia and oleander have a pathway that produces sinapyl alcohol from sinapic acid via sinapoyl-CoA. By contrast, in magnolia and Arabidopsis, 4CL activity toward sinapic acid could not be detected, and labeled sinapic acid was not incorporated into lignin. These results suggest that syringyl lignin biosynthesis in angiosperms operates via multiple pathways that depend on the species.