Oxidation of Coniferyl Alcohol to Lignin-like Products By Tobacco Xylem Cell-walls

Cell walls, isolated from the xylem of tobacco (Nicotiana tabacum cy Samsun) stems, oxidized coniferyl alcohol both in the absence of exogenous hydrogen peroxide and in the presence of catalase. The oxidation was monitored by the reduction of A of coniferyl alcohol at 260 nm and confirmed by the decline of coniferyl alcohol measured by reverse-phase HPLC. Although some water-soluble products were noted in the supernatant during incubation, the products appeared to have greater affinity for the cell walls. After 168 hr incubation, several products were extracted from the walls with methanol. These were retained on the reverse-phase column and were probably more hydrophobic than coniferyl alcohol. The major methanol-soluble product, which may be the dilignol, dehydrodiconiferyl alcohol, was purified by reverse-phase HPLC. The cell walls after complete oxidation of coniferyl alcohol were yellow/orange and had an elevated lignin content. The increased lignin content was probably the result of the deposition of guaiacyl-type ‘lignin-like’ material on to or into the cell walls. The presence of 500 units ml−1 catalase or superoxide dismutase did not prevent the deposition of this ‘lignin-like’ material. The addition of malate, nicotinamide adenine dinucleotides (NAD) and Mn2+ ions did not enhance the deposition of lignin-like material. This suggests a non-peroxidative polymerization of coniferyl alcohol by the cell walls and supports the theory that cell-wall-associated oxidases participate in lignin formation.