OXIDATION OF CONIFERYL ALCOHOL TO LIGNIN-LIKE PRODUCTS BY TOBACCO XYLEM CELL-WALLS

Cell walls, isolated from the xylem of tobacco (Nicotiana tabacum cv S amsun) stems, oxidized coniferyl alcohol both in the absence of exogen ous hydrogen peroxide and in the presence of catalase. The oxidation w as monitored by the reduction of A of coniferyl alcohol at 260 nm and confirmed by the decline of coniferyl alcohol measured by reverse-phas e HPLC. Although some water-soluble products were noted in the superna tant during incubation, the products appeared to have greater affinity for the cell walls. After 168 hr incubation, several products were ex tracted from the walls with methanol. These were retained on the rever se-phase column and were probably more hydrophobic than coniferyl alco hol. The major methanol-soluble product, which may be the dilignol, de hydrodiconiferyl alcohol, was purified by reverse-phase HPLC. The cell walls after Complete oxidation of coniferyl alcohol were yellow/orang e 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 t his 'lignin-like' material. The addition of malate, nicotinamide adeni ne 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 c ell-wall-associated oxidases participate in lignin formation.