Competitive inhibitor-dissected histochemistry of the peroxidase responsible for syringyl lignin biosynthesis in Zinnia elegans xylem

The lignifying xylem from Zinnia elegans hypocotyls gives an intense reaction with the MAule reagent, which is specific for syringyl moieties. The presence of syringyl moieties in the β-O-4 lignin fraction from Z. elegans cell walls was confirmed by thioacidolysis, which gave, as main products, the thioethylated monomers arising from aryl-glycerol-β-aryl ether structures derived from coniferyl and sinapyl alcohols. The guaiacyl/syringyl molar ratio for this lignin fraction was 43/57. Besides β-O-4 syringyl lignins, lignifying Z. elegans hypocotyls also contain a basic peroxidase capable of oxidizing syringyl moieties, such as those present in sinapyl alcohol and syringaldazine, the latter a chromogenic substrate containing the syringyl group specific for angiosperm lignins. Competitive inhibitor-dissected histochemistry of the developing xylem revealed that the stain for syringaldazine oxidase was restricted to the two or three outermost developing (lignifying) xylem cell layers, and which responded to competitive inhibitors, such as ferulic acid and ferrocyanide, and to suicide substrates, such as m-chloroperoxybenzoic acid, in a similar way to the Z. elegans basic peroxidase. From these results, we conclude that this Z. elegans basic peroxidase is found at the precise time and place where lignification is occurring, and emerges as the key enzyme involved in the polymerization of sinapyl alcohol and responsible for syringyl-type lignin assembly.